TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of decontaminating a heavy water cooled and moderated nuclear reactor, and in particular to a method of decontaminating metal surfaces in the primary circuit and/or the moderator circuit of a heavy water nuclear reactor wherein the metallic surface has a radioactive oxide layer.

BACKGROUND OF THE INVENTION

The piping of a nuclear reactor is usually made of stainless steel, carbon steel and/or Co alloys. The steam generator tubes and main surfaces inside the primary circuit may include Ni alloys. Under operational conditions of a nuclear reactor metal ions are leached out of the alloys of the piping and are dissolved and transported into the coolant. When passing the reactor core during operation, some of the metal ions are activated to form radioisotopes. During operation of the reactor these metal ions and radioisotopes are deposited as a radioactive oxide layer on metal surfaces inside the reactor cooling system. The removal of these radioactive metal oxide layers is necessary to reduce the level of personnel radiation exposure prior to carrying out inspection, maintenance, repair and dismantling procedures on the reactor cooling system. Many procedures are described to remove the oxide layers containing radioisotopes from metal surfaces of the cooling system in a light water cooled nuclear reactor. A commercially successful method comprises the steps of treating the oxide layer with an oxidant such as permanganate in order to convert Cr(lll) to Cr(VI), and subsequently dissolving the oxide layer under acidic conditions using a decontamination solution of an organic acid such as e.g. oxalic acid. The organic acid additionally serves to reduce a possible excess of oxidant from the preceding oxidation step, and to reduce the dissolved Cr(VI) to Cr(l ll) in the decontamination solution. An additional reducing agent can be added to remove the oxidant and convert Cr(VI) to Cr(lll). The decontamination solution containing metal ions and radioisotopes originating from the oxide layer, such as Fe(ll), Fe(lll), Ni(ll), Co(ll), Co(lll) and Cr(lll), is then passed through an ion exchange resin to remove the radioisotopes and some or all of the metal ions from the decontamination solution. The organic acid in the decontamination solution is decomposed by photocatalytic oxidation to form carbon dioxide and water.

However, this decontamination method is not directly applicable to a heavy water cooled and moderated reactor. The operation of a heavy water cooled and moderated nuclear reactor requires that the concentration of heavy water is maintained at >99.8 percent. Replacing the heavy water by light water prior to decontamination is too costly and would also require large storage capacities for the low level contaminated heavy water, as well as sophisticated test methods to control the heavy water concentration for re-starting a power generating operation in the nuclear reactor.

U.S. Patent 3,737,373 discloses that a heavy water cooled and moderated reactor can be decontaminated by employing a heavy water solution of deuterated oxalic acid. The deuterated oxalic acid is produced by dissolving oxalic acid anhydride in heavy water. The radioactive substances eluted in the deuterated oxalic acid solution are removed by decomposing the deuterated oxalic acid by means of irradiation with gamma-rays. The radioactive substances are precipitated and removed by filtration. The ions in the filtered heavy water are removed by ion exchange resin techniques.

CA 1 062 590 is directed to a method of decontaminating a heavy water moderated and cooled nuclear reactor or a light water cooled reactor. A relatively small quantity of acidic reagent composition is injected into the circulating coolant of the reactor, which is shut down but not defueled, so as to provide a dilute solution of reagent which dissolves radioactive contaminants in the system. The coolant is then passed through cationic exchange resin to remove the contaminant and leave the regenerated reagent which is returned to the cooling system. When the cationic resin stops removing contaminants it is removed and discarded. The reagent is finally removed from the system by anionic exchange resin. Suitable reagents include mixtures of certain organic acids such as oxalic acid, acetic acid, and citric acid with or without chelating agents such as EDTA or hydrazine. In case of heavy water moderated and cooled reactors, a preferred cationic resin is in D ⁺ form, and preferred anionic resin is in OD ^" form so that the original coolant composition is restored. CA 1 136 398 A is an improvement over the method disclosed in CA 1 062 590 A. A metal surface contaminated with radioactive materials is decontaminated by circulating an aqueous solution of decontaminating reagents comprising oxalic acid, formic acid, citric acid and EDTA. The efficacy of the organic acid decontaminating reagents is prolonged under ionizing radiation by the inclusion of formic acid therein. Where heavy water coolant or moderator is used in the decontamination, the ion exchange resins are converted to the D ⁺ and OD ^" forms in order to avoid downgrading the deuterium content.

These prior art decontamination methods suffer from the fact that a high number of treatment cycles are necessary in order to completely remove the metal oxide layer and to achieve a satisfactory reduction of activity on the metal surfaces, thus resulting in a high amount of radioactive waste produced therewith.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide an effective decontamination method for a heavy water cooled and moderated reactor which prevents the entrainment of light water into the primary heavy water coolant during the decontamination treatment and which reduces the number of treatment cycles and minimizes the amount of radioactive waste resulting from the decontamination treatment. According to the invention, the object is solved by a method of decontaminating a metal surface in a heavy water cooled and moderated nuclear reactor, wherein the metal surface has a coating comprising one or more metal oxides and radioisotopes, and wherein the metal surface is in contact with a heavy water coolant or moderator, the method comprising one or more treatment cycles each comprising:

a) an oxidation step wherein the metal surface is contacted with a

solution of an oxidant in heavy water; b) a decontamination step wherein the metal surface subjected to the oxidation step is contacted with a decontamination reagent in heavy water for dissolving at least part of the one or more metal oxides and to form a decontamination solution containing the decontamination reagent, one or more metal ions dissolved from the metal oxides and the radioisotopes, and wherein the decontamination solution is passed over an ion exchange resin to immobilize the metal ions and the radioisotopes; and

c) a decomposition step wherein the decontamination reagent in the decontamination solution is decomposed;

wherein the oxidant, the decontamination reagent and the ion exchange resin are provided in a deuterated form and/or are free of active hydrogen.

The decontamination method of the present invention avoids a dilution of the heavy water coolant and moderator by light water during the decontamination treatment cycles since all decontamination chemicals are provided in their deuterated form and no light water is formed as a byproduct.

Therefore, the primary coolant and moderator itself can be used as the solvent for the decontamination chemicals. No further cleaning of the heavy water coolant and moderator after decontamination is necessary to remove light water impurities. The process also saves the costs involved with replacing the heavy water in the reactor cooling and moderating system by light water just for performing a decontamination of the primary coolant and moderator circuit of the heavy water reactor.

Heavy water is widely available at the facilities of a heavy water cooled and moderated reactor, and can be used for preparing a bulk of deuterated ion exchange resins. The deuterated ion exchange resins can then be used to produce a stock solution of the deuterated oxidant in heavy water for use in the oxidation step, as well as a stock solution of the deuterated decontamination reagent in heavy water for dissolving the metal oxide coating in the decontamination step.

The decontamination solution containing metal ions and radioisotopes from the dissolved metal oxide coating is passed over the deuterated ion exchange resin to immobilize radioactive components and metal ions dissolved in the decontamination solution. Thus, the decontamination solution is depleted of the radioisotopes and metal ions, while the decontamination reagent is regenerated. In the following decomposition step, the decontamination reagent is decomposed.

The side products of the decontamination treatment are only carbon dioxide and heavy water so that the heavy water coolant will not be diluted with light water during the decontamination treatment. Oxidation of the metal surfaces prior to the decontamination step is effective to reduce the number of treatment cycles. Use of deuterated oxidants and decontamination reagents also results in a reduction of radioactive waste. According to a preferred embodiment, the oxidation step is performed at a temperature of from 20 to 120 °C and optionally under higher than atmospheric pressure to avoid boiling of the heavy water coolant and moderator in low pressure parts of the primary circuit. Preferably, the temperature is in a range of from 80 to 95 °C. Oxidation treatment at high temperatures is effective to facilitate the formation of pores in the metal oxide layer.

In a further embodiment, the oxidation step may be performed at a temperature of from 95 to 120 °C under elevated pressure.

During the oxidation step, chromium(lll) in the metal oxide coating is converted into soluble chromate (Cr(VI)) and is dissolved in the oxidant solution. Additionally, a certain amount of nickel(ll) is solubilized by mechanisms not necessarily involving a change of oxidation state of the nickel.

The dissolution of Cr(VI) and Ni(ll) can be shown by analyzing the oxidant solution during the oxidation step. The oxidation step is terminated as soon as no increase of the amount of chromium(VI) in the oxidant solution can be determined.

Preferably, the oxidant is deuterated permanganic acid, DMn0 ₄ , which is preferred over alkali metal permanganate salts because less waste is produced.

More preferably, the deuterated permanganic acid is controlled at a concentration of from 10 to 800 mg/l during the oxidation step, preferably 100 to 200 mg/l. Still more preferably, the deuterated permanganic acid is prepared by ion exchange reaction between an alkali metal permanganate salt and a cationic ion exchange resin in deuterated form.

The deuterated permanganic acid DMn0 ₄ can be provided as a stock solution in heavy water, D ₂ 0, having a concentration of from 1 to 45 g/l preferably a concentration of from 30 to 40 g/l.

According to a further preferred embodiment, the decontamination reagent is selected from the group consisting of deuterated oxalic acid, linear, fully deuterated alkyl dicarboxylic acids having active acidic deuterium atoms, alkali metal salts of oxalic acid, alkali metal salts of fully deuterated straight-chain alkyl dicarboxylic acids, and mixtures thereof.

Preferably, the decontamination reagent is a deuterated dicarboxylic acid selected from at least one of deuterated oxalic acid and a linear alkyl dicarboxylic acid having active deuterium atoms, and most preferably deuterated oxalic acid. More preferably, the decontamination reagent is provided as a stock solution in heavy water having a concentration of from 25 to 150 g/l, preferably from 25 g/l to 120 g/l, and more preferably from 25 g/l to 100 g/l.

Still more preferably, the decontamination reagent is prepared by ion exchange between an alkali metal salt of the decontamination reagent, preferably an alkali metal salt of the dicarboxylic acid, and a cationic ion exchange resin in deuterated form.

The decomposition step preferably comprises the step of decomposing the decontamination reagent to form carbon dioxide and heavy water. More preferably, the decontamination reagent is decomposed by reaction of the decontamination reagent with ozone and exposure to UV radiation.

Preferably, the temperature during the decomposition step is between 20 and 95 °C.

The chemicals used for the decontamination treatment can be injected into the primary heavy water coolant and moderator at a dosing station located in the low-pressure part of the coolant and moderator circuit of the heavy water reactor. Preferably, the decontamination treatment can be applied using external decontamination equipment to monitor the decontamination treatment and achieve the decontamination targets. The process temperatures are preferably kept below the boiling point of heavy water to eliminate the need of using complex and expensive pressure-proof components for the external decontamination equipment.

Accordingly, a further aspect of the invention is a heavy water cooled and moderated reactor adapted to perform the above decontamination method wherein the reactor comprises a primary coolant circuit having a low-pressure section and a high pressure section, a moderator circuit and an external decontamination device connected to the low-pressure circuit of the primary coolant circuit and/or the moderator circuit, wherein the oxidant and/or the decontamination reagent are injected into the primary coolant circuit and/or the moderator circuit by means of the external decontamination device. Preferably, the low-pressure section of the primary circuit comprises a high pressure pump, a volume control system upstream of the high pressure pump, and a pressure reducing station upstream of the volume control system, wherein the decontamination device is connected to the primary coolant circuit at a position upstream of the high pressure pump, preferably upstream of the volume control system and downstream of the pressure reducing station.

The construction and method of operation of the invention together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

- Figure 1 is a schematic diagram of a heavy water cooled and moderated nuclear reactor; and

- Figure 2 is a schematic diagram showing a decontamination device connected to a primary coolant circuit of the heavy water reactor. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the method of the present invention, a metal oxide coating containing radioisotopes is effectively removed from metal surfaces in the cooling and moderator system of a heavy water nuclear reactor. The cooling and moderator system is understood as comprising all systems and components which are in contact with the heavy water coolant and moderator during reactor operation, including but not limited to the primary cooling and moderator circuits including the reactor vessel, reactor coolant and moderator pumps, pipework and steam generators, and auxiliary systems such as the volume control system, pressure reducing station and reactor water clean-up system.

Referring to the embodiment shown in Fig. 1 , a heavy water reactor 10 comprises a primary coolant circuit 12 for circulating a primary heavy water coolant under high pressure through fuel bundles 14 and steam generator 16. The primary coolant is circulated by means of reactor coolant pump 18 and pressurized by pressure tank 20.

The fuel bundles 14 are located in a low-pressure vessel or calandria 22 containing the heavy water moderator surrounding the fuel bundles 14. The moderator circuit 23 includes a moderator pump 24 and heat exchanger 26 for cooling the heavy water moderator. Adjustment rods 28 are provided for controlling neutron flux in the fuel bundles 14.

As shown in Fig. 2, the primary coolant circuit 12 further comprises a low- pressure section including a pressure reducing station 30, a reactor water cleanup system 32 including anionic ion exchanger 34 and cationic ion exchangers 36, a volume control system 38 and a high pressure pump 40 downstream of the volume control system 38 which are also in contact with the primary heavy water coolant during power generating operation of the reactor.

A decontamination circuit 42 including external decontamination device 44 is coupled to primary coolant circuit 12 by connecting decontamination circuit 44 to the low-pressure section of the primary coolant circuit 12 downstream of the pressure reducing station 32 and the intake side of high pressure pump 40, preferably upstream of volume control system 38. In alternative embodiments, the decontamination circuit 42 may be connected to other components in the low- pressure section of the primary coolant circuit 12 at different positions, depending on the specific reactor design.

For decontaminating the moderator circuit 23 operated under low pressure, the external decontamination device 44 can be operated in parallel to a moderator cleaning system (not shown) and connected to any part of the moderator circuit 23.

The decontamination device 44 preferably has a modular design and may comprise an UV reactor and at least one circulation pump, heaters, ion exchange columns, filters, sampling modules, monitoring systems, automation and remote controls and chemical injection equipment (not shown).

The UV reactor 34 is used for photocatalytic decomposition of the decontamination reagent. The sampling devices will be used during the treatment cycles for process control, and mechanical filtration may be performed during the decontamination step. It is understood by those skilled in the art that the reactor design schematically shown in Figures 1 and 2 may vary and is not limiting to the present invention.

The decontamination method of the present invention is particularly useful for decontamination of the cooling and moderator system in a boiling water reactor or a pressurized water reactor such as CANDU or KWU heavy water reactors, and preferably a heavy water reactor comprising steam generator piping having metal surfaces of nickel alloys such as Inconel™ 600, Inconel™ 690 or Incoloy™ 800.

The decontamination treatment can be carried out on reactor subsystems or as full system decontamination, without degrading the heavy water concentration by entrainment of light water. During full system decontamination the contaminated metal oxide coating is removed from all metal surfaces in the reactor cooling and moderator system that are in contact with the heavy water coolant and moderator during reactor operation. Typically, full system decontamination involves all parts of the primary coolant circuit and the moderator circuit as well as the volume control system, the pressure reducing station and possibly other systems which are contaminated to a certain extent. Ion exchange resins and chemicals used in the decontamination method of the present invention are prepared specifically for use in the heavy-water reactor. Since heavy water has limited availability worldwide, the deuterated ion exchange resins and decontamination chemicals are preferably prepared directly at the nuclear power plant facilities where heavy water is widely available.

A) Preparation of deuterated ion exchange resins and decontamination chemicals

Preparation of deuterated cationic and anionic exchange resins

Deuterated ion exchange resins are required for preparing the decontamination chemicals as well as for removing and immobilizing of the radioisotopes from the decontamination solution during the decontamination step. Therefore, the decontamination method preferably comprises the step of providing a bulk of deuterated ion exchange resins.

Cationic and anionic exchange resins are commercially available in regenerated form. Cationic exchange resins have sulfonic acid groups and anionic exchange resins have quaternary amine groups. These commercial ion exchange resins cannot be used directly in the primary coolant circuit of a heavy water reactor because both types of ion exchange resins would emit hydrogen ions into the heavy water coolant thereby causing a dilution of the heavy water with light water. This process is shown in following equations 1 and 2 wherein "Polymer" denotes the inert resin backbone of the ion exchange resins:

Cationic exchange resin

D ₂ 0 + Polymer - S0 ₂ - OH → DHO + Polymer - SO ₂ - OD

Equation 1

Anionic exchange resin

D ₂ 0 + Polymer - NR ₃ OH → DHO + Polymer - NR ₃ OD

Equation 2

For preparing the bulk of deuterated ion exchange resins, the resins are poured into an ion exchange column filled with heavy water, and are left overnight. The heavy water is then removed from the ion exchange column, and the amount of light water in the eluate is determined. The ion exchange column is again filled with heavy water, and the process is repeated until the amount of light water in the eluate has reached a predetermined threshold. Preferably, the amount of light water in the eluate is 1 weight percent or less. It has been found that a degree of deuteration of > 99% is sufficient for use.

The ion exchange column may be part of the facilities of the nuclear power plant, such as the reactor water cleaning system or is provided as an external mobile ion exchange column. Preferably, the ion exchange column is connected to the external decontamination device 42.

Flushing of the ion exchange resins with heavy water will also exchange light water bound in the resin skeleton and/or used for swelling the ion exchange resin.

Preferably, the anion exchange resin has functional groups consisting of tertiary amino end groups such as trisalkyl ammonium groups so that only one hydrogen atom is exchanged per functional group, as shown in Equation 2.

Anionic exchange resins having primary or secondary amino end groups bear an additional one or two hydrogen atoms, and would therefore be able to elute additional hydrogen atoms into the heavy water, as shown in the following Equations 3a to 3c.

Secondary amino groups

D ₂ 0 + Polymer - NHR ₂ OD → DHO + Polymer - NDR ₂ OD

Equation 3a

Primary amino groups:

D ₂ 0 + Polymer - NRH ₂ OD → DHO + Polymer - NRDH OD

Equation 3b

D ₂ 0 + Polymer - NRDH OD → DHO + Polymer - NRD ₂ OD

Equation 3c The reactions according to Equations 3a to 3c would consume additional amounts of heavy water. Therefore, anionic exchange resins having primary and secondary amino end groups are less preferred.

The above process is suitable for the removal of light water from anionic exchange resins and cationic exchange resins and providing a bulk of deuterated ion exchange resins.

The bulk of deuterated cationic exchange resins obtained by this process are also used to prepare the decontamination chemicals such as the deuterated oxidant and the deuterated dicarboxylic acid. Both of the deuterated anionic exchange resin and deuterated cationic exchange resins are used in the decontamination step of the decontamination treatment cycle for immobilizing the radioisotopes and metal ions.

Preferably, the bulk of ion exchange resins have a light water content of less than 1 weight percent. It has been found that a degree of deuteration of > 99% is sufficient for use. Therefore, the light water content of the decontamination chemicals produced from these deuterated ion exchange resins can also be controlled to be less than 1 percent.

In cases were a degree of deuteration of 99% would exceed the concentration limit of light water, such as for decontamination of sub-systems with small volumes, the degree of deuteration of both the anion and cation ion exchange resin may be adapted accordingly. In order to increase the degree of deuteration, the above described preparation process is extended for some additional days. For any case of the application it is assumed that the total content of light hydrogen stored on each deuterated ion exchange resin, which will be used during the chemical decontamination process, is dissolved in the final heavy water filling of the subsystem. According to this assumption the degree of deuteration of each ion exchange resin being used is adapted by calculation. The above described mechanism for leaching out light water can be easily controlled by atomic mass spectroscopy. Preparation of the oxidant

The preferred oxidant used in the oxidation step is deuterated permanganic acid. Alkali metal salts of permanganate, free of crystal water, such as potassium permanganate can also be used but are less preferred because the alkali metal ions would increase the mass of the waste ion exchange resin to be disposed.

Preferably, deuterated permanganic acid is prepared from dry potassium permanganate by ion exchange using a deuterated cationic exchange resin, as prepared above. An alkali metal salt of permanganate is dried, and the water free salt is dissolved in heavy water, as shown in the following Equation 4:

KMn0 ₄ = K ⁺ (solv) + Mn0 ₄ (solv) Equation 4

The solution of potassium permanganate in heavy water is passed over the deuterated cationic exchange resin and transformed into deuterated permanganic acid to provide a stock solution of deuterated permanganic acid in heavy water. This reaction is shown in the following Equation 5 wherein "Polymer" denotes the backbone of the ion exchange resin:

cooling

Κ ⁺ + ΜηΟ + Polymer - S0 ₂ - OD > D ⁺ + MnO^ +

Polymer— S0 ₂ — OK

Equation 5

Preferably, the stock solution of deuterated permanganic acid in heavy water has a concentration of from 1 to 45 g DMn0 ₄ per liter of heavy water, more preferably a concentration of from 30 to 40 g/l. Less concentrated solutions of the oxidant are inefficient because of the high amount of heavy water to be transported. Oxidant solutions having a concentration of deuterated permanganic acid of greater than 40 g/l are difficult to obtain because of the limited solubility of the alkali metal permanganate salt in heavy water. In addition, stock solutions of deuterated permanganic acid having a concentration of greater than 40 g/l tend to decompose autocatalytically even at room temperature, as shown in the following Equation 6.

Room temperature

4 D ₃ 0 ⁺ + 4 ΜηΟ » 6 D ₂ 0 + 4 Mn0 ₂ + 3 0 ₂

Equation 6

The concentrated stock solution of deuterated permanganic acid prepared in accordance with Equation 5 can be used in the oxidation step of the decontamination treatment cycle without changing or degrading the heavy water concentration in the primary coolant circuit. The concentrated stock solutions of deuterated permanganic acid obtained in accordance with Equation 5 are stable at room temperature for several weeks, preferably at least four weeks. The stock solution can therefore be prepared in sufficient time prior to performing the chemical decontamination treatment. Preparation of the decontamination reagent

Preferably, the decontamination reagent used in the decontamination step is a deuterated dicarboxylic acid selected from at least one of deuterated oxalic acid, D0 ₂ C-C0 ₂ D, and fully deuterated linear alkyl dicarboxylic acids used for dissolving the metal oxides and radioisotopes deposited on the metal surfaces during operation of the nuclear power plant. Examples for the linear alkyl dicarboxylic acids are deuterated malonic acid and deuterated succinic acid.

The deuterated dicarboxylic acid can also act as a complexing agent or chelating agent by forming metal complexes with the metal cations from the metal oxides and radioisotopes and keeping the metal complexes in solution. Deuterated oxalic acid and linear, fully deuterated alkyl dicarboxylic acids which do not have a secondary or tertiary carbon atom are preferred because these reagents can be decomposed to form water and carbon dioxide without any intermediary products. Secondary and tertiary carbon atoms cannot be oxidized to tetravalent carbon and result in intermediary products which have to be removed from the primary coolant using anionic exchange resins. This would result in an additional consumption of anionic exchange resin during decontamination and cause additional costs for preparing the required deuterated anionic exchange resin. For example, deuterated malonic acid is suitable for use as a decontamination reagent in the process of the present invention, whereas deuterated 2-trisdeuteromethyl malonic acid will not be useful because it has a secondary carbon atom and is not completely decomposed in the cleaning step.

In an alternative embodiment, the decontamination reagent is an alkali metal salt of oxalic acid such as disodium oxalate, or an alkali metal salt of the linear, fully deuterated alkyl dicarboxylic acid. This embodiment is less preferred because the decontamination process will then introduce alkali metal ions into the primary coolant which could lead to corrosion. Moreover, the additional alkali metal ions may increase the amount of secondary waste because the alkali metal ions must be removed from the heavy water coolant using additional deuterated cationic exchange resin.

For preparing the deuterated dicarboxylic acid, an alkali metal salt of the dicarboxylic acid is dissolved in heavy water and passed over a deuterated cationic exchange resin to obtain a stock solution of the deuterated dicarboxylic acid by ion exchange.

Still more preferably, the ion exchange process is carried out at room temperature, as illustrated in the following Equation 7 showing the preparation of deuterated oxalic acid.

Room Temperature

2 Na ⁺ + C ₂ 0¾- + 2 Polymer - SO ₂ - OD » 2 D ⁺ +

C ₂ Ol ^~ + 2 Polymer - S0 ₂ - ON a

Equation 7

The solubility of sodium oxalate in heavy water at room temperature is about 37 g/l. Preferably, the deuterated oxalic acid used as the decontamination reagent has a concentration of 25 g/l in heavy water, after the ion exchange process.

The stock solution of the decontamination reagent in heavy water can be further concentrated to a concentration of 100 g/l to 200 g/l by evaporating the heavy water solvent. Preferably, the solution of the decontamination reagent is evaporated in vacuum at temperatures of about 100 °C in a rotary evaporator until the concentration of the decontamination reagent is about 100 g/l or more. The concentrated stock solution so obtained can be stored at room temperature for several weeks, preferably at least four weeks.

A stock solution of the decontamination reagent in heavy water at a concentration of about 100 g/l or more has been found useful in the decontamination method of the present invention. For example, the decontamination of a primary coolant circuit having a volume of about 100 m ³ requires about 2000 I of the decontamination reagent such as deuterated oxalic acid having a concentration of 100 g/l in heavy water. This volume of the decontamination reagent fits well into the technical capacities of a nuclear power plant. If deuterated oxalic acid is used as the decontamination reagent, the temperature of the evaporation process for preparing the concentrated stock solution must not exceed 140 °C. Otherwise, a thermal decomposition of deuterated oxalic acid will take place at temperatures above 150 °C. Similar conditions apply to deuterated malonic acid. Deuterated succinic acid was found to be stable up to 200 °C.

B) Chemical Decontamination Treatment

Metal surfaces in the primary coolant and moderator circuit of an operating nuclear power plant are coated with metal oxide deposits including radioactive isotopes such as Co-60 during reactor operation. Chemical decontamination of the metal surfaces dissolves these metal oxide coatings together with the radioisotopes incorporated therein. The metal surfaces are cleaned, and metallic bright surfaces without oxide deposits are obtained. The method of the present invention is directed to the decontamination of metal surfaces in the primary coolant and moderator circuit of a heavy water reactor. Similar to a pressurized water reactor, the metal surfaces in the primary coolant and moderator circuit of the heavy water reactor will have chromium containing metal oxide deposits. The inventors therefore contemplate that the decontamination treatment cycle must include an oxidation step.

The decontamination method is suitable for use with any CANDU reactors as well as other heavy water reactors, but is not limited to these reactor types. The inventors also contemplate use of the decontamination method in a boiling water reactor operated with heavy water. In this case, the oxidation step could be omitted if the metal oxide deposits on the metal surfaces in the primary coolant circuit include iron oxides having a chromium content of less than 1 weight percent.

Under operational conditions of a nuclear reactor at temperatures of up to 330 °C, metal ions are leached out of the alloys of the piping in the primary coolant and moderator circuit and are dissolved and transported into the heavy water coolant and moderator. When passing the reactor core during operation, some of the metal ions are activated to form radioisotopes. During operation of the reactor these metal ions and radioisotopes are deposited as an oxide layer on metal surfaces inside the reactor cooling and moderator system.

Depending on the type of alloy used for a component or system, the oxide layers which are formed contain mixed iron oxides with divalent and trivalent iron as well as other metal oxide species including chromium(lll) and nickel(ll) spinels. Especially the oxide deposits formed on the metal surfaces of the steam generator tubes have a high chromium(lll) or Ni(ll) content which makes them very resistant and difficult to remove from the metal surfaces.

Over extended reactor operation periods, the amount of the radioisotopes, such as Co-60, Co-58, Cr-51 , Mn-54 etc., deposited together with the metal oxides on the inner metal surfaces of the reactor cooling system accumulates. This results in an increased surface activity or dose rate of the components of the reactor cooling and moderator system. The removal of this radioactive matter results in a measurable reduction of personnel radiation exposure. In general, one or more decontamination treatment cycles are carried out in order to achieve a satisfactory reduction of activity on the metal surfaces. The reduction of surface activity and/or the dose reduction correlating to surface activity reduction is referred to as "decontamination factor". The decontamination factor is calculated either by the surface activity in Bq/cm ² before decontamination treatment divided by the surface activity in Bq/cm ² after the decontamination treatment, or by the dose rate before decontamination treatment divided by the dose rate after decontamination treatment.

Preferably, the decontamination factor of a technically satisfying decontamination treatment is greater than 100. A decontamination treatment cycle of the present invention comprises an oxidation step, wherein the metal surface is contacted with a solution of an oxidant in heavy water; a decontamination step wherein the metal surface subjected to the oxidation step is contacted with a decontamination reagent in heavy water for dissolving at least part of the one or more metal oxides and to form a decontamination solution containing the decontamination reagent, one or more metal ions dissolved from the metal oxides and the radioisotopes, and wherein the decontamination solution is passed over an ion exchange resin to immobilize the metal ions and the radioisotopes; and a decomposition step wherein the decontamination reagent in the decontamination solution is decomposed.

Preferably, the decontamination treatment cycle comprises a reduction step wherein the oxidant is reacted with the decontamination reagent.

In the decomposition step, the decontamination reagent is decomposed to form carbon dioxide and heavy water.

More preferably, only deuterated cationic ion exchange resins are used for cleaning the decontamination solution in the decomposition step and during decomposition of the decontamination reagent.

As soon as the concentration of the deuterated dicarboxylic acid in the decontamination solution is less than 50 mg/kg, a cleaning step can be performed wherein anionic and cationic ion exchange resins are used to further remove the deuterated dicarboxylic acid and remaining metal ions. Preferably, the anionic ion exchange resin is operated downstream of the cationic ion exchange resin. The concentration at which the cleaning step will be started can vary according to local waste regulations. Some countries have limits for the concentration of organic acids on waste resin. Therefore the the decomposition step can be extended to reach concentrations less than 10 mg/kg. This extension is technically possible. It will extend the decomposition time but lower the concentration of dicarboxylic acid on the resulting waste from spent anionic exchange resin.

A preferred embodiment of the decontamination method for a heavy water cooled and moderated reactor may comprise the following treatment cycles: First decontamination treatment cycle: a) Oxidation of metal oxide coating using a solution of deuterated permanganic acid in heavy water; b) Reducing the deuterated permanganic acid and dissolving metal oxide coating using a solution of deuterated dicarboxylic acid in heavy water; c) Photocatalytic decomposition of deuterated dicarboxylic acid including exposure to UV radiation; d) Performing an intermediate Cleaning step Second decontamination treatment cycle: a) Oxidation of metal oxide coating using a solution of deuterated permanganic acid in heavy water; b) Reducing the deuterated permanganic acid and dissolving metal oxide coating using a solution of deuterated dicarboxylic acid in heavy water; c) Photocatalytic decomposition of deuterated dicarboxylic acid including exposure to UV radiation; d) Performing an intermediate Cleaning step

Last decontamination treatment cycle: a) Oxidation of metal oxide coating using a solution of deuterated permanganic acid in heavy water; b) Reducing the deuterated permanganic acid and dissolving metal oxide coating using a solution of deuterated dicarboxylic acid in heavy water; c) Photocatalytic decomposition of deuterated dicarboxylic acid including exposure to UV radiation; d) Final cleaning step.

Preferably, the decontamination method comprises two to four decontamination treatment cycles. It has been found that a sufficient decontamination factor could be achieved with this number of treatment cycles in full system decontamination and/or decontamination of subsystems or components of a heavy water cooled and moderated reactor. However, the number of decontamination treatment cycles is not limited to the numbers given above, but may also depend on reactor design and level of radioactive contamination. According to the invention, the oxidant, the decontamination reagent and the ion exchange resin are provided in a deuterated form and/or are free of active hydrogen, preferably free of any hydrogen. "Active hydrogen" is understood as acidic hydrogen atoms which are reactive against a Grignard reagent such as methyl magnesium bromide. Thus, the decontamination method may comprise a plurality of treatment cycles without dilution of the heavy water coolant and moderator by light water during the decontamination treatment.

The various steps of the decontamination method are described in greater detail below

Oxidation step

For carrying out the oxidation step, a stock solution of the deuterated oxidant such as deuterated permanganic acid is injected into the primary coolant and moderator circuit or the subsystem which is to be decontaminated, and the oxidant solution is circulated through the cooling and moderator system. The oxidant solution can be introduced into the cooling and moderator system by means of the external decontamination device 42.

Preferably, the deuterated oxidant is injected into a low-pressure section of the cooling and moderator system. Examples for suitable injection positions are the volume control system, the reactor water cleaning system and/or a residual heat removal system.

The deuterated permanganic acid reacts with spinell-type metal oxides in the metal oxide coating which are inert to organic and mineral or acids by oxidizing Cr(lll) to soluble Cr(VI).

Preferably, the oxidation step is carried out at a temperature of between about 20 to 120 °C, more preferably at a temperature of from 80 to 95 °C. The oxidation step is faster at higher temperatures.

Accordingly, higher oxidation temperatures are preferred. Moreover, the boiling point of a solution of deuterated permanganic acid in heavy water under atmospheric pressure is higher than 95 °C so that the oxidant solution can easily be circulated through the cooling and moderator system using external pumps of the decontamination device. However, it is also possible to carry out the oxidation step at temperatures of up to 120 °C at a higher than atmospheric pressure. Thus, the temperature of the oxidation step is selected depending on the pressure conditions in the decontamination device. Generally, the temperature is selected as high as possible within the temperature range of from 95 to 120 °C, but is controlled to be at least 5 K below the boiling point of the heavy water solution as calculated for the actual hydrostatic pressure within the decontamination system. Thus, the heavy water solution is prevented from boiling to protect the circulating pumps against cavitation. Preferably, boiling graphs for light water can be used to calculate the boiling point of the heavy water solution due to the negligible difference between the boiling points of light water and heavy water, as a function of the ambient pressure.

Preferably, the concentration of the deuterated oxidant in the cooling and moderator system is controlled to be in the range of from 10 to 800 mg/kg during the oxidation step, and preferably to range from 100 to 200 mg/kg. If the concentration of the deuterated oxidant in the oxidation solution is lower than 10 mg/kg, and preferably lower than 100 mg/kg, the reaction rate of the oxidation is too low. If the concentration of the oxidant in the oxidant solution exceeds 800 mg/kg, a large excess of the oxidant will be present at the end of the oxidation step. Preferably, the amount of the oxidant is controlled to be as low as possible at the end of the oxidation step because the deuterated oxidant is expensive, and removal of excess deuterated oxidant will increase the amount of secondary waste.

Preferably, the amount of the deuterated oxidant in the oxidation solution during the oxidation step is controlled by monitoring the concentration of Cr(VI) in the oxidation solution. As long as the oxidation reaction continues and the oxidation of the metal oxide layer is incomplete, the concentration of Cr(VI) increases, as shown in Equation 7:

24 D ₂ O + 5 Cr ⁱ⁺ + 3 n0 ₄ ^" → 5 CrO¾ ^~ (solv) + 16 D ₃ 0 ⁺ + 3 Mn

Equation 7 The residence time of the oxidant solution in the cooling and moderator system during the oxidation step may comprise a plurality of hours, preferably up to 30 hours. It is desired that the oxidation of the metal oxide layer is substantially complete so that as much as possible of the oxide coating thickness is reacted during the oxidation step. Preferably, the oxidation step is terminated when no further increase in the Cr(VI) concentration can be determined.

Instead of monitoring the concentration of Cr(VI), it is also possible to monitor the presence of the radioisotope Cr-51 in the oxidant solution by means of gamma spectroscopy. As soon as the oxidation step is terminated, preferably a reduction step is started.

Reduction step

The oxidation step may be followed by a reduction step which is the shortest step of the treatment cycle. The reduction step comprises reducing an excess of the oxidant remaining in the oxidant solution at the end of the oxidation step by reacting the oxidant with the decontamination reagent. Preferably the oxidant is deuterated permanganic acid, and the decontamination reagent is a fully deuterated dicarboxilic acid such as deuterated oxalic acid, as shown in the following Equation 8:

16 D ₃ 0 ⁺ + IMnO^ + 5 C ₂ 0| ^" → 2 Mn ²⁺ + 24 D ₂ 0 + 10 C0 ₂

Equation 8

A stock solution of the decontamination reagent is injected into the primary coolant and moderator circuit or the subsystem which is to be decontaminated, and the solution containing the decontamination reagent is circulated through the cooling and moderator system. The stock solution of the decontamination reagent can be introduced into the cooling and moderator system by means of the external decontamination device 42 at the same position as described above with respect to the oxidant solution. Mangenese cations generated by the reduction of permanganic acid are dissolved in the decontamination reagent solution as a manganese(ll) oxalate complex. Carbon dioxide is dissolved in the heavy water solution under high pressure and will be released into the environment in the low-pressure part of the cooling and moderator system such as in the volume control system.

All metal ions dissolved from the metal oxide coating during the reduction step solution as well as Cr(VI) generated in the oxidation step will be reduced by the decontamination reagent to a lower oxidation step. Cr(VI) will be converted to Cr(lll), most of the iron will be present as iron(ll), and nickel and manganese will be present as nickel(ll) and Mn(ll).

The duration of the reduction step is dependent on the excess of deuterated permanganic acid in the oxidant solution. Accordingly, it is desired to terminate the oxidation step at a concentration of the deuterated permanganic acid as low as possible. The duration of the reduction step is further influenced by the effectivity of removal of the carbon dioxide dissolved in the heavy water solution, because the carbon dioxide must be removed in the low-pressure part of the cooling and moderator system without damaging the pumps in the decontamination circuit due to cavitation. Moreover, the duration of the reduction step is also influenced by the injection rate of the deuterated oxalic acid which is preferably injected into the heavy water coolant circuit at a dosing station located in the low-pressure part of the heavy water coolant circuit. The reduction step is controlled by monitoring the removal of carbon dioxide and the concentration of permanganic acid in the decontamination solution solution. As soon as the reaction between deuterated permanganic acid and deuterated oxalic acid is completed, the decontamination step is started. However, it is understood that the reduction step may also be considered part of the decontamination step.

Decontamination of the metal surfaces

The decontamination step comprises the step of contacting the metal oxide layer subjected to the oxidation step with the decontamination reagent to dissolve metal ions and radioisotopes incorporated in the metal oxide coating and to form a decontamination solution containing the decontamination reagent, one or more metal ions dissolved from the metal oxides and the radioisotopes, and passing the decontamination solution over an ion exchange resin to immobilize the metal ions and the radioisotopes.

Preferably, the decontamination solution is passed over a deuterated cationic exchange resin which is prepared as described above by flushing with heavy water. Accordingly, no light water is entrained into the decontamination solution during the ion exchange reaction. Equation 9 shows an example of the ion exchange reaction using nickel ions:

Ni ²⁺ + 2 Polymer - S0 ₂ - OD → 2 D + Ni(Polymer - S0 ₂ - 0) ₂

Equation 9

Similar to nickel ions, all other cations dissolved in the decontamination solution, including manganese(ll) generated from the deuterated permanganic acid, as well as the radioisotopes dissolved in the decontamination solution are removed from the decontamination solution and immobilized on the cationic ion exchange resin.

The progress of the decontamination step and the ion exchange reaction can be monitored by measuring the concentration of selected radioisotopes and metal ions. Samples can be taken from the decontamination solution and analyzed by spectroscopic methods such as atom absorption spectroscopy. The amount of radioisotopes dissolved in the decontamination solution can be determined by gamma spectroscopy or by using a gamma counter.

The decontamination step is terminated as soon as no substantial increase of the amount of metal ions removed from the decontamination solution and immobilized on the cationic ion exchange resin is determined and/or no further increase of the activity of the radioisotopes immobilized at the ion exchange resins is determined.

The deuterated dicarboxylic acid in the decontamination solution is regenerated by release of deuterium ions during the ion exchange reaction as exemplified above in Equation 9. Therefore, the deuterated dicarboxylic acid is not depleted in the decontamination step. Rather, the decontamination of the metal surfaces is only limited by a decrease of the solubility of metal ions dissolved from the metal oxide coating. The reason for the decrease of the solubility of the metal oxides in the decontamination solution is found in the fact that the metal oxide layer reacted in the oxidation step is completely removed at the end of the decontamination step, and a further oxidation of the remaining metal oxide coating is required to dissolve further metal ions into the decontamination solution.

Decomposition step

In order to start a further oxidation step, the decontamination reagent must be removed from the decontamination solution. Theoretically, the decontamination reagent such as deuterated dicarboxylic acid could be reacted with deuterated permanganic acid as shown in Equation 8 above. For example, this process can be used for decontamination systems having small volumes, e.g. during the decontamination of isolated heat exchangers and the like.

However, this reaction would require a substantial amount of permanganic acid and also generate additional secondary waste in the form of manganese ions and/or manganese oxide. Therefore, the decontamination method of the present invention includes a decomposition step comprising a photocatalytic oxidation of the decontamination reagent. The photocatalytic oxidation of the decontamination reagent such as fully deuterated dicarboxylic acid does not generate additional waste but results in the formation of heavy water and carbon dioxide.

Preferably, the decontamination reagent is reacted with ozone. Use of oxygen would also be possible but is less preferred. The byproducts of the photocatalytic oxidation of the decontamination reagent are carbon dioxide and heavy water. No light water is produced since no hydrogen-containing reagents are used. The reaction of ozone and deuterated dicarboxylic acid such as deuterated oxalic acid is shown in the following Equation 10. Use of ozone as the oxidant in the photocatalytic oxidation reaction is preferred since six electrons per molecule of ozone are available for the oxidation reaction. Thus, three moles of oxalate can be reacted with one mole of ozone to form carbon dioxide and heavy water.

0 ₃ + 3 C ₂ 0| ^" + 6 D ₃ 0 ⁺ → 6 C0 ₂ + 9D ₂ 0

Equation 10 Preferably, the photocatalytic oxidation reaction is conducted at a temperature of from 20 to 95 °C.

Use of ozone as the oxidant in the photocatalytic oxidation reaction further has the advantage that no hydrogen atoms are introduced into the reaction solution, as it would be the case if hydrogen peroxide was used.

Preferably, the ozone is generated using pure oxygen. If air was used for generating ozone by means of an electrical ozone generator, a small amount of nitrogen oxides NOx would be generated and converted into nitrate in the decontamination solution. The nitrates must be removed from the decontamination solution by passing the solution over an anionic ion exchange resin, which would increase the amount of secondary waste.

Preferably, a UV reactor is immersed into the decontamination solution, and the ozone is injected into the decontamination solution by means of a Venturi mixer upstream of the UV reactor. Thus, the ozone is thoroughly mixed with the decontamination solution.

The injection of ozone into the decontamination solution is controlled so that no dissolved ozone is determined downstream of the UV reactor.

Preferably, the ozone concentration in the decontamination solution is determined by measuring the oxidation potential against a standard electrode Ag/AgCI, and more preferably by controlling the oxidation potential of the decontamination solution subjected to ozone treatment to be less than +200 mV downstream of the UV reactor.

Alternatively or simultaneously, the ozone concentration can be measured indirectly by monitoring the concentration of iron(ll) in the decontamination solution. If the concentration of iron(ll) downstream of the UV reactor is greater than 2 mg/kg, ozone is completely eliminated. Otherwise, ozone would immediately react with iron(ll) to form iron(lll), as shown in the following Equation 1 1 .

0 ₃ + 6 Fe ²⁺ + 6 D ₃ 0 ⁺ → 6 Fe ³⁺ + 9 D ₂ 0

Equation 1 1 Preferably, the concentration of ozone downstream of the UV reactor is measured continuously, and the rate of the ozone injection is adjusted continuously.

Preferably, the UV reactor comprises a medium pressure mercury lamp. A power of 10 kW was found to be sufficient for a volume flow rate of 15 to 50 m ³ /h of the decontamination solution. The minimum total amount of iron, including iron(ll) and iron(lll), in the decontamination solution should preferably exceed 10 mg/kg in order to enable a reliable measurement of the ozone concentration.

In a further preferred embodiment, the concentration of ozone is determined by means of an ion selective electrode sensitive to ozone.

The reaction rate of the photocatalytic oxidation of the fully deuterated dialkyl carboxylic acid is a first order reaction, if sufficient ozone is present. The progress of the decomposition reaction of the deuterated dicarboxylic acid can therefore be determined as shown in the following Equation 12.

N t) = N ₀ * e ^~n ^

Equation 12 wherein

N ₀ denotes the initial concentration of the deuterated dicarboxylic acid [mg/kg] N(t) denotes the concentration of the deuterated dicarboxylic acid at time t t denotes the decomposition time [h] n denotes the number of UV reactors operated in parallel

F denotes the flow rate per UV reactor [m ³ /h]

V denotes the volume of the decontamination solution [m ³ ]; and k denotes the reaction constant specific to the deuterated dicarboxylic acid.

During the photocatalytic decomposition of the decontamination reagent, dissolved metal ions and radioisotopes are removed from the decontamination solution and are immobilized on the cationic ion exchange resins. The removal of the metal ions and radioisotopes in the decomposition step and/or the decontamination step may take place in a bypass conduit in the low-pressure part of the reactor, most preferably using cationic ion exchange columns present in the water cleaning system of the heavy water reactor. Alternatively or in addition, external ion exchange columns can be operated in parallel to the ion exchange columns of the reactor water cleaning system.

The decomposition step is terminated if the decontamination solution is depleted of the decontamination reagent and the concentration of the decontamination reagent such as deuterated dicarboxylic acid in the decontamination solution is 50 mg/kg or less.

Intermediate and final cleaning step

After terminating the decomposition step, when the concentration of the decontamination reagent in the decontamination solution is 50 mg/kg or less, an intermediate or final cleaning step is performed wherein the decontamination solution depleted of the decontamination reagent is cleaned by further removal of metal ions and deuterated dicarboxylic acid by means of deuterated cationic exchange resins and deuterated anionic exchange resins operated downstream of the cationic exchange resins.

If a further oxidation step is to be carried out, the concentration of the decontamination reagent in the decontamination solution is preferably controlled to be less than 10 mg/kg so that the consumption of deuterated permanganic acid in the initial phase of the oxidation step is as low as possible.

In a final cleaning step, the conductivity of the heavy water coolant is controlled to be 10 μ8/θΓη at 20 °C. Preferably, the final cleaning step is conducted at a temperature of 60 °C or less, more preferably 30 °C or less.

The decontamination method of the present invention is preferably applied to the decontamination of both, the primary coolant circuit, and the moderator circuit of the heavy water reactor. The primary coolant circuit is provided for cooling of the reactor core including the fuel bundles and for transferring the hot heavy water to the steam generator where energy is transferred from the primary coolant to a secondary light water circuit passing through the steam generator. The moderator circuit comprises the reactor vessel filled with heavy water surrounding the fuel elements and is provided with a separate cooling and cleaning system.

Since the heavy water moderator circuit is operated at low pressure, a decontamination device for injection of the decontamination chemicals can be coupled to the moderator circuit at any suitable position and operated in parallel to the moderator circuit.

A decontamination of the primary coolant circuit requires that the decontamination device is connected to a low-pressure section of the primary coolant circuit using the volume control system as described above. In the high-pressure section of the primary coolant circuit, the heavy water is circulated under a pressure of 100 bar or higher. Thus, connecting the decontamination device to the high-pressure section of the primary coolant circuit may damage the pumps of the decontamination device and/or requires use of expensive pressure-proof equipment. Preferably, the decontamination device is connected to the low-pressure section of the primary coolant circuit for injecting the decontamination chemicals into the primary coolant upstream of the intake side of the high-pressure pump and downstream of a pressure reducing station for transferring the decontamination solution out of the primary coolant circuit back into the decontamination device.

The decontamination method of the present invention avoids a dilution of the heavy water coolant and moderator by light water during the decontamination treatment cycles since all decontamination chemicals are provided in their deuterated form and no light water is formed as a byproduct. No further cleaning of the heavy water coolant and moderator after decontamination is necessary to remove light water impurities. The heavy water primary coolant and moderator itself can be used as the solvent for the decontamination chemicals. The process also saves the costs involved with replacing the heavy water in the reactor cooling and moderating system by light water just for performing a decontamination of the primary coolant and moderator cycle of the heavy water reactor. Oxidation of the metal surfaces prior to the decontamination step is effective to reduce the number of treatment cycles. Use of deuterated oxidants and decontamination reagents prepared from bulk deuterated ion exchange resins also results in a reduction of radioactive and secondary waste. Although the invention is illustrated and described herein as embodied in a method for surface decontamination, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the scope of the appended claims.