USING A HEAVY WATER TYPE NUCLEAR POWER PLANT

[0001] The present disclosure relates generally to radioisotopes and more specifically to a method of producing radioisotope sources using a heavy water type nuclear power plant.

BACKGROUND [0002] Radioisotopes are used in various fields such as industry, research, agriculture and medicine. Artificial radioisotopes are typically produced by exposing a suitable target material to neutron flux in a cyclotron or in a nuclear research reactor for an appropriate time. Irradiation sites in nuclear research reactors are expensive and will become even scarcer in future due to the age- related shut-down of reactors. Molybdenum-99 (Mo-99) is particularly useful in the medical field, and it is desired to provide alternative production sites for Mo- 99 and other radioisotopes.

[0003] EP 2 093 773 A2 shows a radionuclide generation system in which short- term radioisotopes having medical applications are generated through nuclear fission in a commercial light water nuclear reactor. Existing instrumentation tubes, within the pressure boundary of the reactor vessel and within the primary coolant loop, conventionally used for housing neutron detectors, are used to generate radionuclides during normal operation of the reactor. Spherical targets are linearly pushed into and removed from the instrumentation tubes. While the axial neutron flux profile of the reactor core is deemed to be known or calculable, optimum position and amount of exposure time of the targets in the reactor core are determined based at least on this parameter. A driving gear system, an actuator or a pneumatic drive can be used for moving and holding the targets. An automatic flow control system maintains synchronicity between all subsystems of this ball measuring system. [0004] Similar systems are also known from US 8 842 798 B2 and US 2013/0170927 A1 , which specifically describes several drive system embodiments (pathways and transport mechanism for the targets), e.g. based on an existing TIP (traversing incore probe) system within the pressure boundary of the light water reactor vessel. A component like a stop valve or a gate valve may be used in connection with dispensing targets at particular times and in particular fashion. US 2013/0315361 A1 suggests a valve for sealing off a base of an instrumentation tube. Alternate paths are provided to preserve access to existing TIP tube indexers, or to provide alternate routing to desired destinations within the pressure boundary of the reactor vessel. In US 2013/0177126 A1 a retention assembly is shown, including a restricting structure like a fork for selective blocking movement of irradiation targets through a pathway and/or into/out from instrumentation tubes.

[0005] The neutron flux density in the core of a some commercial nuclear reactors is measured, inter alia, by introducing solid spherical probes ("aeroballs") of a ball measuring system into instrumentation tubes passing through the reactor core, using pressurized gas for driving the aeroballs. This ball measuring system is for example described in U.S. Patent No. 3,263,081 .

SUMMARY OF THE INVENTION [0006] A method of producing radioisotopes using a heavy water reactor or heavy water type nuclear power plant is provided. The invention is based on the finding that existing or future nuclear power plants, whose main purpose is/will be the generation of electrical power, can be used for producing radioisotopes. The preferred embodiment uses a CANDU (CANada Deuterium Uranium) type pressurized heavy water reactor.

[0007] This method includes inserting targets into a heavy water moderator of the heavy water reactor through a guide tube in a port in a reactivity mechanism deck of the heavy water reactor. The heavy water reactor operates to irradiate the targets to convert the targets into a radioisotope. The method then includes removing the radioisotope via the reactivity mechanism deck. [0008] A heavy water nuclear reactor is also provided. The heavy water nuclear reactor includes a reactor core enclosure; a plurality of pressure tubes in the reactor core enclosure including fuel bundles, heavy water primary coolant flowing from outside of the reactor core enclosure through the plurality of pressure tubes, the reactor core enclosure including heavy water moderator separated from the plurality of pressure tubes; and a reactivity mechanism deck positioned above the reactor core enclosure, the reactivity mechanism deck including a port extending therethrough, the port housing a guide tube including targets configured to convert the targets into a radioisotope upon exposure to radiation emitted by the fuel bundles. The heavy water nuclear reactor can include a pressure tube reactor that is the pressure boundary of the primary coolant loop with a plurality of pressure tubes (a.k.a. fuel channels) in the core including fuel bundles. Heavy water primary coolant flows from feeder pipes through the pressure tubes. The calandria contains the heavy water moderator and is outside of the pressure boundary of the primary coolant loop. The nuclear power plant also includes a reactivity mechanism deck positioned above the pressure tube reactor core enclosure. The reactivity mechanism deck includes a port extending there through. The port houses a new guide tube including targets configured to convert the targets into a radioisotope upon exposure to radiation emitted by the fuel bundles. The new guide tube forms a pressure boundary with the lower temperature and pressure moderator system and not with the high pressure and temperature primary coolant loop that contains fuel bundles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is described below by reference to the following drawings, in which:

[0010] Fig. 1 shows a typical CANDU6 reactor assembly that would be irradiating targets in accordance with an embodiment of the present invention;

[0011] Fig. 2 shows a partial cross-sectional side view of a calandria of the heavy water reactor shown in Fig. 1 . [0012] Fig. 3 shows a typical CANDU6 top view of the heavy water reactor shown in Fig. 1 , showing viewing port locations, schematically illustrating the locations of reactivity control units in a reactivity mechanisms deck positioned above the calandria.

[0013] Fig. 4 shows a typical CANDU6 end view of the heavy water reactor shown in Fig. 1 , showing the viewing port location. [0014] Fig. 5 shows a typical CANDU6 reactivity mechanisms deck of the heavy water reactor shown in Fig. 1 , showing the viewing port location.

[0015] Fig. 6 shows a typical CANDU6 end view of the heavy water reactor shown in Fig. 1 , showing the viewing port location with a new radioisotope production guide tube in place in accordance with an embodiment of the present invention.

[0016] Fig. 7 shows the new radioisotope guide tube assembly shown in Fig. 6 with enlarged views of portions thereof.

[0017] Fig. 8 shows sections of the distributor, ball tube and pressure boundary tube details of the new radioisotope guide tube assembly shown in Fig. 7. [0018] Fig. 9 shows a sectional view of the lower portion on the new radioisotope production guide tube assembly shown in Fig. 7.

[0019] Fig. 10 shows a typical neutron flex density of the CANDU Core.

[0020] Fig. 1 1 shows a cross section for neutron capture in Mo-98 showing resonance peaks. DETAILED DESCRIPTION

[0021] Heavy water type nuclear power plants, specifically CANDU pressurized heavy water reactors, have a very high thermal neutron flux and a high level epi- thermal neutron flux over a wide range of resonance that is capable of activating non-uranium based targets with neutron capture. Such neutron capture considerably reduces the waste created to obtain the radioisotopes as well has the capability to produce significant amounts of radioisotopes such as Mo-99 to replace production from aging research reactors as they are retired. [0022] Several studies have been done, looking at modifying CANDU fuel bundles contained in the pressure tubes of the primary coolant loop primary to include irradiation targets allowing production of isotopes. This involves using the on-line fueling machines to insert and retrieve the modified fuel bundles, which creates on operational risk to the reactor as the fueling functions place restrictions on the operating units as well may increase the risk of a reactor trip due to inadvertent events. The use of modified fuel bundles also requires substantial changes in the plant design to address the modified fuel bundle and getting the fuel bundle out of the Spent Fuel Bay for isotope extraction purposes. [0023] The present disclosure provides a method of inserting and retrieving targets into a heavy water type nuclear power plant that can be done during the operation of the plant without significant impact to the operational risk. A guide tube is provided into the moderator, an area that is outside the pressure tubes of the primary coolant loop primary that is separated from the fuel bundles. [0024] Fig. 1 shows a typical CANDU6 reactor assembly in accordance with an embodiment of the present invention. In this embodiment, it is for a CANDU pressurized heavy water reactor, but in other embodiments it may be another type of heavy water reactor. The typical CANDU6 reactor assembly has separate pressure boundaries categorized as the primary cooling loop where the fuel is contained, the moderator that slows the neutrons which is a separate system, isolated from the primary cooling loop and the end shield which provide radiation shielding and supports the primary cooling loop fuel channels. The primary cooling loop components shown in Fig. 1 consists of the fuel channel end fittings 10 and feeder pipes 1 1 . The moderator system components shown in Fig. 1 are the calandria 1 , calandria shell 2, calandria tubes 3, inlet-outlet strainer 8, moderator outlet 12, moderator inlet pipe 13, pipe to moderator expansion head tank 18, moderator discharge pipes 20, rupture disc 21 , calandria nozzles for reactivity control units 22 and calandria tubesheet 29. The end shield includes the endshield embedment ring 4, fuelling tubesheet 5, endshield lattice tube 6, endshield cooling pipes 7, and steel ball shielding 9. The ports that penetrate the moderator system include ports for horizontal flux detectors units and liquid injection units 14, ion chambers 15, viewing port 23, shutoff unit 24, adjustor unit 25, control absorber unit 26, liquid zone control unit 27 and vertical flux detector unit 28. The assembly is housed in a concrete reactor vault wall 17, with curtain shielding slabs 19, and the overall assembly is protected against seismic events with earthquake restraints 16.

[0025] The reactor core enclosure shown in Fig. 1 is in the form of a calandria 1 , which is delimited by a horizontal cylindrical shell 2. A plurality of calandria tubes 3 are housed inside of calandria shell 2. The heavy water moderator flows into and out of the volume inside calandria 1 via piping 12,13 delimited between the inner surface of calandria shell 2, the outer surfaces of calandria tubes 3 and calandria tubesheet 29. The primary coolant loop, which contains the fuel bundles, is physically separate and flows from the feeder pipes 1 1 , through the fuel channel end fitting 10, and down the pressure tube (a.k.a. fuel channel containing the fuel bundle) and out the opposite fuel channel end fitting 10 and into the opposite feeder pipe 1 1. As schematically shown in the partial cross- sectional view of Fig. 2, heavy water moderator is contained inside the volume delimited between the inner surface of calandria shell 2, the outer surfaces of calandria tubes 3 and calandria tubesheet 29. Each calandria tube 3 surrounds a pressure tube (a.k.a. fuel channel) 44 housing a plurality of fuel bundles 51 therein. Calandria tubes 3, along with a gas filled annular space 48 maintained by garter spring spacers 46, provide a buffer between pressure tubes 44 and the moderator heavy water so heated heavy water primary coolant in pressure tubes 44 does not boil the heavy water moderator . Primary coolant flows into pressure tubes 44 from a cold leg of a primary coolant loop from a feeder pipe 1 1 into an end fitting 10 and flows to receive heat from fuel bundles 51 , then flows out of pressures tubes 44 at the opposite end fitting 10 and out a feeder pipe 1 1 to a hot leg of the primary coolant loop for flowing through a steam generator located downstream in the hot leg. Closure Plugs 52 are on each end fitting 10 to allow for on-line fueling.

[0026] Referring back to Fig. 1 , it further includes moderator inlet pipes 13 for providing cooled water from a moderator main circuit, moderator outlet pipes 12 for providing heated moderator water back to moderator main circuit for cooling and pressure discharge pipes 20 for relieving pressure inside calandria shell 2. A plurality of horizontally extending neutron flux detector units 14 extend horizontally through calandria 1 to monitor the neutron flux in calandria 1 during the operation of reactor. Extending vertically through core are a plurality of reactivity control units therein.

[0027] Fig. 3 shows a top plan view schematically illustrating the locations of reactivity control units in a reactivity mechanisms deck 45 positioned above the calandria 1 . Reactivity mechanisms deck holds all the reactivity control units that extend below reactivity mechanism deck and penetrate calandria 1 from above. From Figure 1 , the reactivity control units include vertically extending neutron flux detector units 28, liquid zone control units 27, adjuster units 25, control absorber units 26 and reactor shutoff units 24, which are all need to be available and capable of operating during the operation. In addition to the reactivity control units, reactivity mechanism deck 45 also includes two viewing ports 23 extending therethrough. A first viewing port 49, i.e., a high flux inspection port, is aligned with a high flux region of the reactor core and a second viewing port 50, i.e., a low flux inspection port, is aligned with a low flux region of the reactor core. Viewing ports 49, 50 are used during the periodic inspection to monitor corrosion and wear of the reactor at two regions exposed to different levels of neutron flux.

[0028] Fig. 4 shows a cross-sectional side view, which illustrates the positioning of reactivity mechanisms deck 45 above calandria 1 with the viewing port 23 location. An existing thimble 53 is in place in the viewing port to allow insertion of a guide tube to monitor neutron flux during the initial startup of reactor when brand new fuel is provided into the reactor. An aluminum guide tube is typically provided with barium fluoride detectors having a very high sensitivity to neutron flux. Once the reactor is started up and neutron flux is detected by the barium fluoride detector, the aluminum guide tube is removed. Leaving the aluminum guide tube during normal operation would lead to permanent damage. After initial startup, viewing ports are available to have radioisotope production guide tubes inserted.

[0029] Fig 5. shows the reactivity mechanisms deck 45 with the viewing port 23 location as well as relative location with respect to, shut off unit 24, adjustor unit 25, control absorber unit 26, liquid zone control unit 27 and vertical flux detector units 28. [0030] Fig. 6 shows a cross-sectional side view, which illustrates the positioning of reactivity mechanisms deck above calandria with the view port 23 location. An existing thimble 53 is in place in the viewing port to allow insertion of a guide tube and the new radioisotope production guide tube 30 is shown inserted. [0031] Fig. 7 shows the overall radioisotope production guide tube 30 assembly including the distributors 36, bulkhead 31 and upper flange 32. The top section is a solid hollow tube 33 with a mid-positioned bearing sleeve 34. In this embodiment the bottom section is perforated with a plurality of radially extending holes 35 to allow for the moderator water to flow in and out of the guide tube 30 along pressure boundary tubes 39 (Figs. 8 and 9), but may be solid and/or may form the pressure boundary tube if an alternative delivery system is used. The bottom has a guide tip 40 to allow positioning within the calandria. The guide tube 30 is approximately 46 feet (14 meters) in length and 3.5 inches (9 centimeters) in diameter. [0032] Fig. 8 shows sections of one of the pressure boundary tube 39 assemblies complete with distributor 36 shown in Fig. 7. The distributor 36 includes a ball tube 38 forming an innermost radial surface thereof and a pressure boundary tube 39 forming an outermost radial surface thereof. The distributor 36 provides for the ability to have the targets 37 delivered into and out of the ball tube 38 via pneumatic actuation 41 and 42 from the delivery system. The view on the left shows a top of the pressure boundary tube 39 complete with distributor 36 and the view on the right shows the bottom of the pressure boundary tube 39. The targets 37 are delivered via the proposed delivery system in U.S. Patent No. 3,263,081 via the distributor 36. The targets 37 go down into port 55 on the distributor 36 and into ball tube 38 by pneumatic pressure 41 pushing the targets 37 down until they stop at the bottom of the ball tube 38 by hitting the ball stop 54. The ball stop 54 has gaps to allow pneumatic pressure to easily pass through ball stop 54 in both upward and downward directions. After the irradiation period, the pneumatic pressure is reversed by applying the pneumatic pressure 42 on the alternative port 56 on the distributor 36 down the pressure boundary tube 39 and then comes back up the ball tube 38 from the bottom, past the ball stop 54 and pushes the targets 37 through the ball tube 38 and up and out of the distributor 36. The individual pressure boundary tube 39 seals against the moderator system pressure boundary, and houses ball tube 38 and ball stop 54 therein. In this embodiment, as shown in Fig. 7, there can be many pressure boundary tubes 39 within one guide tube 30 depending on the anticipated required yield of the radioisotopes. The diameter of the targets is nominally 2 mm, but may vary based on radioisotope in question up to several centimeters. The outer diameters of the targets 37 define the inner diameter of the ball tube 38 with a small clearance to allow ease of movement of the targets 37. The outer diameter of the ball tubes 38 in turn defines the inner diameter of the pressure boundary tube 39, with a radial gap being present between ball tubes 38 and pressure boundary tube 39 to allow air to flow down in the axial direction between ball tubes 38 and pressure boundary tube 39. Targets 37 diameter therefore ultimately limits the maximum amount of pressure boundary tubes 39 per guide tube 30 (see Fig. 7), or that guide tube 30 itself forms the pressure boundary tube 39. [0033] Fig. 9 shows a sectional view of the lower portion of the radioisotope production guide tube 30 showing multiple, five in this example, pressure boundary tubes 39, each including a ball tube 38 therein having an outer diameter sufficiently spaced from the inner diameter of the respective surrounding pressure boundary 39. Two of the pressure boundary tubes 39 are shown from the outside and two of the pressure boundary tubes are shown in full cross section. The fifth pressure boundary tube 39 in shown in partial cross section, illustrating an inner cross section of the respective ball tube 38 with ball stop 54 supporting the targets 37. Also shown is a spacer plate 43 for seismic design that would be appropriately spaced along the length of the guide tube 30. The guide tip 40 is also shown.

[0034] Fig. 10 shows a typical neutron flux density of a CANDU Core. It has a very high thermal neutron flux and a high constant epi-thermal neutron flux over a wide range of resonance that is capable of activating non-uranium based targets with neutron capture. [0035] Fig. 1 1 shows the cross section for neutron capture in Mo-98 showing the resonance peaks well within the wide range of neutron flux of a CANDU pressurized heavy water reactor. [0036] The present disclosure may be used for the production of a radioisotope source, which in a preferred embodiment is Mo-99 for use in the medical field, by inserting targets, which in the preferred embodiment are formed of Mo-98, into the calandria 1 using high flux viewing port 49. Any time after the initial start-up operations, when the plant is operating, and a radioisotope production guide tube 30 is in place, which is shown in Fig. 6 and 7, then targets 37 may be delivered into the guide tube 30 and removed from the guide tube 30 via a delivery system. In a preferred embodiment, the guide tube 30 is formed of a zirconium alloy. In another embodiment the guide tube 30 may be formed of stainless steel. [0037] A target delivery system may also be removably added to reactivity mechanisms deck area for inserting the targets, for example Mo-98. In one embodiment, the target delivery system is the aeroball delivery system disclosed in U.S. Patent No. 3,263,081. The aeroball delivery system utilizes pneumatic power via the distributor 36 to send the targets 37 into guide tube 30 and to extract the irradiated targets 37 upwardly from the guide tube 30 after they have been irradiated and converted into Mo-99. In an alternative embodiment, the targets may be lowered into guide tube 30 by gravity and removed upward out of guide tube 30 by a mechanical drive system. The mechanical delivery system characterized in that mechanical drive system comprises a gate device for discharging the irradiated targets into a collecting container after irradiation. In another alternative embodiment, the delivery system may be portable and attachable to the distributor 36 on an as needed basis, by simply hand feeding, with a commercially available funnel, the targets 37 into the ball tube 38 port 55 of the distributor 36. Then a standard commercially available pneumatic tank with commercially available fittings may be connected to the ball tube 38 port 55 of the distributor 36 and used to supply transport gas into the ball tube 38 to ensure that the targets 37 are fully inserted. After the irradiation time, a standard commercially available shipping flask can be attached on the ball tube 38 port 55 of the distributor 36 and the standard commercially available pneumatic tank with commercially available fittings can be attached to the pressure boundary tube 39 port 56 of the distributor 36. The commercially available pneumatic tank may then be operated to eject the targets 37 from the ball tube 38 and out of the distributor 36 and into the standard commercially available shipping flask. [0038] Utilizing high flux viewing port 49 to provide targets 37 in the form of Mo- 98 into the calandria 1 of CANDU pressurized heavy water reactor advantageously allows targets 37 to be exposed to sufficient radiation to convert into Mo-99 within approximately 6-12 days. In an alternative embodiment, targets 37 may be provided in other forms to produce other radioisotopes such as Lutetium-177 (Lu-177), through alternative delivery systems and into other moderator ports and other periods of time. In preferred embodiments, the moderator ports used for targets 37 are spare ports, specifically viewing ports 23, 49. In other embodiments, other spare ports may also be used, such flux detector ports that are not being utilized or other ports that do not include equipment, e.g., any of the ports shown in Fig. 1 if for some reason they were not housing the respective liquid injection units 14, ion chambers 15, viewing port 23, shutoff unit 24, adjustor unit 25, control absorber unit 26, liquid zone control unit 27 or vertical flux detector unit 28. As an additional advantage, utilizing existing viewing ports 23 or other spare ports to provide targets 37 does not require the removal of any equipment that is frequently used during the operation of plant and thus does not require significant reactor modifications to produce radioisotopes.

[0039] In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.