Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
METHOD OF SLUDGE REMOVAL IN PRESSURIZED WATER NUCLEAR REACTORS
Document Type and Number:
WIPO Patent Application WO/2001/011935
Kind Code:
A3
Abstract:
A novel method of removing metal-oxide sludge deposits from the secondary side of a pressurized water nuclear steam generator as the nuclear steam generator is brought off-line, at a temperature of about 100 °C to 177 °C (212 °F to 350 °F) is disclosed. The method comprises injecting into the water of the secondary side of a pressurized water nuclear steam generator, as it is brought off-line, a polymeric dispersant represented by structural formula (I): wherein R1 is a lower alkyl of from 1 to about 6 carbon atoms, and x = OH or OM wherein M is a cation.

Inventors:
BURGMAYER PAUL R (US)
Application Number:
PCT/US2000/021391
Publication Date:
February 21, 2008
Filing Date:
August 04, 2000
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
BETZDEARBORN INC (US)
BURGMAYER PAUL R (US)
International Classes:
C02F5/14; C11D3/37; G21C19/307; C11D11/00; F22B37/48; F28G9/00; G21D1/00; G21F9/00; G21F9/28
Foreign References:
US5158684A1992-10-27
US4446028A1984-05-01
US4446046A1984-05-01
US5512183A1996-04-30
US5594084A1997-01-14
US5864596A1999-01-26
US5898002A1999-04-27
US5294686A1994-03-15
Attorney, Agent or Firm:
BOYD, Steven, D. et al. (4636 Somerton RoadTrevose, PA, US)
Download PDF:
Claims:

CLAIMS 1. A method of removing metal-oxide sludge deposits within the secondary side of a pressurized water nuclear steam generator via off-line cleaning techniques wherein the improvement comprises the step of injecting a polymeric dispersant represented by the structural formula: wherein R, is a lower alkyl of from 1 to about 6 carbon atoms, and X = OH or OM where M is a cation, into the secondary side of the pressurized water nuclear steam generator as the pressurized water nuclear steam generator is brought off line at temperatures from about 100° C to 177° C (212° F to 350° F) whereby metal oxide sludge deposits are dispersed.
2. The method of claim 1 wherein R, is CH3 and X is OH.
3. The method of claim 1 wherein said polymeric dispersant is injected at a concentration of from about 1 part per million to about 1000 parts per million of the water in the secondary side of the nuclear steam generator.
4. The method of claim 1 wherein said polymeric dispersant is injected at a concentration of from about 5 parts per million to about 100 parts per million of the water in the secondary side of the nuclear steam generator.
Description:

METHOD OF SLUDGE REMOVAL IN PRESSURIZED WATER NUCLEAR REACTORS FIELD OF THE INVENTION The present invention relates to additives for the secondary side of pressurized water nuclear reactor which enhances sludge removal as the reactor is brought off line.

BACKGROUND OF THE INVENTION The present invention relates to novel methods and materials for removing sludge deposits from steam generator tubes in the secondary side of pressurized nuclear steam generators by utilizing specific polymer dispersants.

The consequences resulting from the buildup of metal oxides/sludge within the secondary side of a steam generator are reduced steam output thereby resulting in lost electrical output from the generating plant, increased water level fluctuations within the steam generator thereby resulting in lower steam and electrical output, and the

initiation of corrosion deposits within the heat exchanger through the concentration of the dissolve chemical species from the secondary water.. These sludge deposits are primarily various forms of iron oxide although other metals like copper, chromium, nickel, and zinc can also be present. The corrosion within the secondary side of a pressurized nuclear steam generator ultimately may result in tube plugging and sleeving and the eventual loss of electrical output because of lost heat transfer or flow imbalances unless the steam generators themselves are replace at a cost of approximately $200,000,000 per plant.

The major technique utilized for the removal of suspended and dissolve impurities from the secondary side of the recirculating steam generator involves removing a portion of the water from the steam generator during operation on a continuous or periodic basis through a blowdown system. Typically, the blowdown system only removes up to 10 percent of the total metal oxides or impurities which enter the recirculating nuclear system generator during operation, with the remaining metal oxides or impurities continuing to build up and to be deposited within the secondary side of the recirculating nuclear steam generator. This deposition may result in pressure loss, level fluctuations, and corrosion of the secondary side of the nuclear steam generator.

Several mechanical and chemical methods have been suggested for removing metal oxides or impurities from within the secondary side of nuclear steam generators when the system is near or at shutdown conditions. One of these methods utilizes sludge lancing at shutdown which employs high-pressure water to flush loosely adhered oxide deposits and sludge from the lower tube sheet of the nuclear steam generator. This process typically does not address deposition of corrosion in the upper tube support plates and does not clean any clogged crevices on the secondary side of the nuclear steam generator.

The percentage of metal oxides or corrosion products removed by this process is about two percent of the total oxides entering the nuclear steam generators over a typical 18-month fuel cycle. The cost of completing a sludge lancing is approximately $350,000 for each 18-month fuel cycle in a typical four-loop plant.

Another method suggested for removing metal oxides/sludge after shutdown from the secondary side of a nuclear steam generator is the bundle-flush process. This process entails directing flush water from the upper part of the recirculating nuclear steam generator to remove the loose sludge from the upper tube support plates. The cost of the bundle flush process is approximately $500,000 per application; however, the process only removes the soft, loosely adhered sludge, and does not remove sludge which is strongly adhered to the heat transfer surfaces.

Additionally, the small crevices within the heat transfer structure are not cleaned at all by this process. Accordingly, this process is of limited value and does not overcome the problem of removing strongly adhered deposits or impediments within the heat-transfer structure.

Crevice flush techniques have been suggested in an attempt to open or clean closed or packed crevices by heating the secondary side of the nuclear steam generator above a boiling point with an inert atmosphere overpressure and then releasing this overpressure. The crevice flush process results in a boiling action which purportedly flushes the impurities from the crevices in the nuclear steamed generator.

However, this method has only demonstrated limited effectiveness and is very time consuming, thereby prolonging downtime, an added cost in the electrical industry.

Chemical-soak techniques have been suggested for use during shutdown to promote removal of loose sludge and loosely adhered deposits within the nuclear steam generator. The chemical soaks employ amines such as dimethylamine and morpholine. These soaks have exhibited limited effectiveness in removing loosely adhered deposits, and the amount or percentage of metal oxides removed is less than acceptable. The advantage of this process is that the cost is low; but the disadvantages of this method are that the process is time consuming, and the effectiveness and the amount of metal oxides removed is less than satisfactory.

Pressure-pulse cleaning or water lapping are mechanical methods which are utilized during an outage or shutdown for removing loosely adhered sludge from the upper tubes or the tube support plates of the nuclear steam generator. The sludge or deposits are removed by raising the water on the secondary side to a desired level and then injecting a high-pressure gas such as nitrogen into the water. The bursting of the bubbles as the gas approaches the surface of the water partially removes limited amounts of the loosely adhered sludge or oxide deposits. This technique may increase the amount of metal oxides removed from 5-15 percent of the total amount of metal oxides deposited within the nuclear steam generator; however, this method does not remove hard deposits and does not open crevices packed with metal oxides/sludge or other corrosion. The cost of a pressure pulse cleaning is typically $200,000 to $600,000 per unit. It is recommended that such a cleaning be employed every one-to-four refueling cycles.

Finally, the methods of chemical cleaning at low or high temperatures and the use of chemically enhanced pressure pulse

cleaning are processes utilizing specific organic materials that dissolve the metal-oxide deposits within the nuclear steam generator. The cleaning solution dissolves the metal-oxide deposits, and the spent cleaning solution must be processed and properly disposed. The chemical-cleaning processes may be selected to remove specific metal oxides contained within the nuclear steam generator. Variations of the chemical cleaning process include the heating of the cleaning solution above the liquid-boiling temperature under an inert atmosphere and then releasing the pressure to force boiling in the cracks and the crevices and the use of pulse-cleaning techniques to promote circulation and movement of the cleaning solution. The chemical cleaning processes remove virtually one-hundred percent of the metal-oxide/sludge deposits within the secondary side of the recirculating steam generator, but at a cost of between $5,000,000-$10,000,000 per cleaning. Many of the nuclear generating plants in operation may require chemical cleaning at least once during their lifetime.

Naturally occurring polymer dispersants have been used to minimize deposition of sludge deposits in fossil steam generators since the early 1900's, and synthetic polymers have been recently utilized for metal-oxide dispersing and sludge conditioning in fossil steam generators. The utility of these polymers for iron-based sludge has been less than optimal (see HTP-2, A New Iron Dispersant for Medium to High Pressure Industrial Boilers, S. Boyette and F. Elliot, International Water Conference, October 1982). Thus, there exists a need for a dispersant which would provide substantial improvement in the amounts of iron- based sludge removed from a steam generator without the expense of elaborate mechanical methods.

Although phosphonate-containing polymers have been used for many years in fully powered fossil steam generators as dispersants, they have not been used in nuclear steam generators because of concerns about the corrosion reaction of phosphates on mild steel surfaces at operating temperatures. These phosphate corrosion effects have been shown to be of concern only at temperatures above 177° C (350° F) (see Investigation of Phosphate-Sludge Interaction, EPRI Report NP-2963, March 1983).

SUMMARY OF THE INVENTION It is one object of the present invention to provide a method and process of removing sludge, corrosion, or metal-oxide deposits within the secondary side of nuclear steam generators as the generators are brought off line.

The present invention relates to the utilization of selected dispersants for removing deposits of metal oxides and sludge within the secondary side of a nuclear steam generator as the generator is brought off line. The polymer dispersant is selected from a group consisting of homopolymers and copolymers, having a repeat or copolymer unit represented by the following structural formula:

whereby R, = lower alkyl of from 1 to about 6 carbon atoms, and wherein X = OH, or OM where M is a cation. The preferred polymer is poly (isopropenylphosphonic acid) i. e., R, = CH3 and X = OH.

It has been discovered that these phosphonate-based reagents are ineffective metal oxide and sludge dispersants at temperatures below 100° C (212° F). However, if they are applied during shutdown at temperatures above 100° (212° F) but below 177° C (350° F), they are much more effective iron dispersants than conventional carboxylate- based dispersants while not causing corrosion of the mild-steel surfaces in the generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In a pressurized water nuclear power station, a reactor is connected to a steam generator wherein heat from the reactor is directed through a heat exchanger within a steam generator. The reactor heats feedwater entering the secondary side of the steam generator in the steam-drum portion of the steam generator to produce saturated steam which is directed from the secondary side to drive a turbine and generator. The spent steam exiting the turbine is directed to a condenser under vacuum where the circulating steam/feedwater is cooled. The cooled feedwater exits the condenser and passes through a series of extractors and low and high pressure heaters for heating the recycled feedwater. A blowdown exits the lower secondary side of the nuclear steam generator for facilitating and permitting removal of impurities that build up within the nuclear steam generator. The blowdown material exiting the secondary side of the nuclear steam generator is filtered by a series of filtering means and returned to the condenser under vacuum for further

purification and returned as feedwater to the secondary side of the nuclear reactor. In an alternative, the blowdown material may be directed to various filter means where it is treated in order to permit discharge.

The blowdown method of removing oxides/sludge and impurities from the secondary side of a steam generator only removes up to about 10 percent of the total metal oxides/sludge that build up within the secondary side of a nuclear steam generator. The present invention utilizes select polymer dispersants which, when added to the feedwater entering the secondary side of a nuclear steam generator as the generator is brought off line significantly enhance the removal of metal oxides/sludge from the secondary side of a nuclear steam generator. The polymer dispersants are added while the secondary side of the nuclear steam generator is within the temperature range of from 100° C to 177° C (212° F to 350° F).

The polymer dispersants and sludge conditioners utilized in the present invention are homopolymers having a repeat unit represented by the structural formula: wherein R, is lower alkyl of from 1 to about 6 carbon atoms, and wherein X = OH or OM where M is a cation. The preferred polymer is poly (isopropenylphosphonic acid) (PIPPA), i. e., R = CH3 and X = OH.

Addition of the polymer dispersants and sludge conditioners of the present invention to the secondary side of a nuclear steam generator as the generator is being brought off line as for routine maintenance will significantly enhance the removal of sludge from the surface in the secondary water system. The polymer dispersants and sludge conditioner are added to the secondary side of the generator as it is brought off line and is at a temperature of between 100° C to 177° C (212° F to 350° F). Within this temperature range, the polymer dispersants of the present invention have been found t be effective at dispersing metal oxide sludge while not contributing to corrosion of mild steel surfaces.

Addition of the sludge conditioner of the present invention to the secondary side of the reactor as the reactor is brought off line, that is at a temperature of from about 100° C to 177° C (212° F to 350° F), provides for conditioning of both loose and hard metal oxide sludge to make removal by conventional techniques considerably more effective. The sludge conditioner of the present invention can be added at level from about 1 ppm up to about 1000 ppm preferably from about 5 ppm to about 100 ppm of the water in the secondary side.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art.

The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.