Method of decontaminating metal surfaces in a cooling system of a nuclear reactor

Abstract

A method of decontaminating metal surfaces in a cooling system of a nuclear reactor comprises: an oxidation step, comprising at least one acidic oxidation step and at least one alkaline oxidation step wherein metal oxides and radioisotopes on the metal surfaces are contacted with aqueous permanganate oxidant solutions; followed by a decontamination step wherein an aqueous solution comprising oxalic acid, formic acid, citric acid, tartaric acid, picolinic acid, gluconic acid glyoxylic acid or mixtures thereof is used to dissolve at least part of the metal oxides and radioisotopes; and a cleaning step wherein radioisotopes are immobilized on an ion exchange resin; wherein at least one treatment cycle includes a high temperature oxidation step, wherein the permanganate oxidant solution is kept at a temperature of at least 100 C.

Claims

1. A method of decontaminating metal surfaces in a cooling system of a nuclear reactor, wherein the metal surfaces are coated with metal oxides including containing radioisotopes, and wherein the cooling system comprises one or more primary loops including at least one reactor coolant pump, and a residual heat removal system, the method comprises conducting a plurality of treatment cycles, with each of the treatment cycles comprising: a) an oxidation step wherein the metal oxides containing radioisotopes are contacted with an aqueous solution of a permanganate oxidant; b) a decontamination step wherein the metal oxides subjected to the oxidation step are contacted with an aqueous solution of an organic acid selected from the group consisting of oxalic acid, formic acid, citric acid, tartaric acid, picolinic acid, gluconic acid, glyoxylic acid and mixtures thereof, so as to dissolve at least part of the metal oxides and the radioisotopes; and c) a cleaning step wherein at least the radioisotopes are immobilized on an ion exchange resin; wherein the oxidation step comprises at least one acidic oxidation step and at least one alkaline oxidation step carried out one after another in either the same or different treatment cycles, and wherein the plurality of treatment cycles comprises at least one treatment cycle including a high temperature oxidation step, during which high temperature oxidation step the permanganate oxidant solution is kept at a temperature of at least 100 C. and wherein the at least one reactor coolant pump is used to circulate and heat the oxidation solution inside the one or more primary loops and the residual heat removal system is used to control the temperature of the oxidant solution during the high temperature oxidation step.

2. The method according to claim 1, wherein the permanganate oxidant is selected from the group consisting of HMnO.sub.4, HMnO.sub.4/HNO.sub.3, KMnO.sub.4/HNO.sub.3, KMnO.sub.4/KOH and KMnO.sub.4/NaOH.

3. The method according to claim 1, wherein the aqueous solution of the permanganate oxidant has a pH value of less than about 6 in the at least one acidic oxidation step.

4. The method according to claim 1, wherein the aqueous solution of the permanganate oxidant has a pH value of at least 8 in the at least one alkaline oxidation step.

5. The method according to claim 3, wherein the permanganate oxidant in acidic oxidation step comprises HMnO.sub.4, HMnO.sub.4/HNO.sub.3 or KMnO.sub.4/HNO.sub.3 or other metal salts of permanganate.

6. The method according to claim 4, wherein the permanganate oxidant in the alkaline oxidation step comprises KMnO.sub.4/NaOH or KMnO.sub.4/KOH.

7. The method according to claim 1 wherein the plurality of treatment cycles comprises an alternating sequence of treatments cycles wherein a first treatment cycle comprising an acidic oxidation step is followed by a second treatment cycle comprising an alkaline oxidation step, or vice versa.

8. The method according to claim 1 wherein all of the plurality of treatment cycles comprise a high temperature oxidation step wherein the oxidant solution is kept at a temperature of at least 100 C.

9. The method according to claim 1 wherein during the high temperature oxidation step the oxidant solution is kept at a temperature in a range of from 120 to 150 C.

10. The method according to claim 1 wherein at least one acidic oxidation step comprises a high temperature oxidation step wherein the oxidant solution is kept at a temperature of at least 100 C.

11. The method according to claim 1 wherein at least one alkaline oxidation step comprises a high temperature oxidation step wherein the oxidant solution is kept at a temperature of at least 100 C.

12. The method according to claim 1 wherein the organic acid is oxalic acid.

13. The method according to claim 1 wherein the oxidant solution is kept at a pressure of more than 1 bar during the high temperature oxidation step.

14. The method according to claim 3, wherein the aqueous solution of the permanganate oxidant has a pH value of less than about 4 in the at least one acidic oxidation step.

15. The method according to claim 4, wherein the aqueous solution of the permanganate oxidant has a pH value of at least 10 in the at least one alkaline oxidation step.

16. The method according to claim 8 wherein during each of the high temperature oxidation steps the oxidant solution is kept at a temperature in a range of from 120 to 150 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the decontamination system of the present invention;

(2) FIG. 2 shows a graph illustrating the increase of the decontamination factor according to the invention; and

(3) FIG. 3 shows a graph comparing the decontamination factor of a low temperature process to a high temperature process.

DETAILED DESCRIPTION OF THE INVENTION

(4) According to the method of the present invention, oxide layers containing radioisotopes are effectively removed from metal surfaces in the cooling system of a nuclear reactor. The reactor cooling system is understood as comprising all systems and components which are in contact with the primary coolant during reactor operation, including but not limited to the primary loop(s) or circuit(s) including the reactor pressure vessel, reactor coolant pumps and steam generator(s), and auxiliary systems such as the residual heat removal system, chemical volume control system and reactor water clean-up system.

(5) Referring to the embodiment shown in FIG. 1, the reactor cooling system of a pressurized water reactor comprises at least two primary loops 10, 12 for circulating a primary coolant through the reactor pressure vessel 14 and steam generators 16 and 18. The primary coolant is circulated by means of reactor coolant pumps 20 and 22.

(6) Residual heat removal (RHR) systems 24 and 26 including RHR system pumps (not shown) are coupled to the primary loops 10, 12. The coolant system further comprises a chemical volume control system (CVCS) 28 and a reactor water clean-up system 30 which are connected to the primary loops 10, 12 and which are also in contact with the primary coolant during power generating operation of the reactor.

(7) An external decontamination equipment loop 32 is connected to at least one of the primary loops 10, 12 and/or the RHR systems 24 and 26. The decontamination loop 32 preferably is of a modular design and comprises a UV reactor 34 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 external decontamination equipment loop 32 may be connected to different components of the cooling system at different positions, wherein one possibility is connecting at two different RHR systems, as shown in FIG. 1. The UV reactor 34 is used for UV decomposition of decontamination chemicals, the sampling devices will be used during the treatment cycles for process control, and mechanical filtration may be performed during the decontamination step.

(8) It is understood by those skilled in the art that the reactor design schematically shown in FIG. 1 may vary and is not limiting to the present invention.

(9) The method of the present invention is suitable for a full system decontamination wherein the contaminated metal oxide layers are removed from all surfaces in the reactor cooling system that are in contact with the primary coolant during reactor operation. Typically a full system decontamination involves all parts of the primary circuit as well as the RHR systems, the chemical volume control systems and possibly other systems which are contaminated to a certain extent.

(10) The decontamination method of the present invention is particularly useful for the decontamination of the cooling system in a pressurized water reactor (PWR), preferably a PWR comprising steam generator piping having metal surfaces of nickel alloys such as Inconel 600, Inconel 690 or Incoloy 800.

(11) For removing metal oxides contaminated with radioisotopes from the metal surfaces in the reactor cooling system, the decontamination method comprises conducting a plurality of treatment cycles, wherein each of the treatment cycles comprises an oxidation step wherein the metal oxides including radioisotopes are contacted with an aqueous solution of a permanganate oxidant. The oxidation step is carried out in order to oxidize insoluble chromium(III) present in the metal oxide layer to soluble chromium(VI).

(12) For carrying out the oxidation step, the components of the cooling system to be decontaminated are filled with the aqueous solution comprising the permanganate oxidant, and the oxidant solution is circulated through the cooling system. The oxidant solution can be introduced into the cooling system by means of the reactor CVC system 28 or the external decontamination equipment loop 32.

(13) Preferably, the oxidant is selected from the group consisting of HMnO.sub.4, HMnO.sub.4/HNO.sub.3, KMnO.sub.4/HNO.sub.3, KMnO.sub.4/KOH and KMnO.sub.4/NaOH or other metal salts of permanganate and/or metal hydroxides. These oxidants are able to oxidize chromium(III) to chromium(VI).

(14) After a residence time of, for example, a plurality of hours, the oxidant solution is replaced or treated in such a way that it can be used in the subsequent decontamination step. Preferably, the oxidation step is terminated when no further increase in the chromium(VI) concentration can be determined.

(15) Following the oxidation step, a decontamination step is carried out wherein the metal oxide layers are contacted with an aqueous solution of an organic acid selected from the group consisting of oxalic acid, formic acid, citric acid, tartaric acid, picolinic acid, gluconic acid, glyoxylic acid and/or mixtures thereof, so as to dissolve at least part of the metal oxides and the radioisotopes thereby forming a decontamination solution containing the radioisotopes and metal ions originating from the metal oxide. A residue of oxidant still present in the solution of the oxidation step is neutralized by an appropriate excess of the organic acid.

(16) Preferably, the organic acid is oxalic acid.

(17) The decontamination step is terminated as soon as no activity increase can be determined in the decontamination solution.

(18) In a cleaning step following the decontamination step, the metal ions and radioisotopes leached from the oxide layer and dissolved in the decontamination solution are removed from the solution and immobilized on an ion exchange resin.

(19) Preferably, the cleaning step includes a decomposition of the organic acid by photocatalytic oxidation while simultaneously passing the decontamination solution through an ion exchanger column. The photocatalytic oxidation of the organic acid preferably comprises the step of exposing the organic acid to UV radiation whereby the organic acid is reacted to form carbon dioxide and water.

(20) According to the method of the present invention, the plurality of treatment cycles preferably comprises at least one treatment cycle comprising an acidic oxidation step and another treatment cycle comprising an alkaline oxidation step. In the acidic oxidation step, the pH value of the aqueous solution of the oxidant is controlled to be less than about 6, preferably less than about 4, more preferably 3 or less. In the alkaline oxidation step, the pH value of the aqueous oxidant solution is controlled to be at least 8, preferably at least 10.

(21) The order of the treatment cycles is not particularly limited. That is, the treatment cycle comprising the acidic oxidation step may be performed after the treatment cycle comprising the alkaline oxidation step, or vice versa. Moreover, there may be a number of subsequent treatment cycles each using an acidic or alkaline oxidation step, without a change between acidic and alkaline, followed by one or more subsequent treatment cycles using the other of an acidic or alkaline oxidation step.

(22) Preferably there is at least one change between a treatment cycle comprising an acidic oxidation step and a treatment cycle comprising an alkaline oxidation step. The effect of changing between an acidic oxidation step and an alkaline oxidation step is that an increase in the decontamination factor is observed when compared to the decontamination factor of the preceding cycle.

(23) The change between an acidic oxidation step and an alkaline oxidation step can also be carried out in one and the same treatment cycle. If a pH change is carried out within a single treatment cycle, for example by carrying out an oxidation step in acidic solution after an oxidation step in alkaline solution by substituting the acidic solution for an alkaline solution containing the oxidant or converting the alkaline oxidant solution in situ into an acidic solution, or vice versa, an increase in the decontamination factor is also achieved compared to a treatment cycle in which a plurality of oxidation steps are carried out without a pH change.

(24) However, preference is given to carrying out a treatment cycle including an oxidation step in acidic solution and a subsequent treatment cycle including an oxidation step in alkaline solution, or vice versa.

(25) The temperature of the oxidant solution in one or more of the oxidation steps may be in the range of from 60 to 95 C.

(26) According to the method of the invention, at least one of the plurality of treatment cycles comprises a high temperature oxidation step wherein the oxidant solution is heated and kept at a temperature of at least 100 C., preferably at least 120 C., and more preferably to a temperature in the range of from 120 to 150 C.

(27) In one embodiment, the high temperature oxidation step is an acidic oxidation step wherein the pH value of the aqueous solution of the permanganate oxidant is less than about 6, preferably less than about 4, more preferably 3 or less.

(28) In other embodiments, the high temperature oxidation step is an alkaline oxidation step wherein the pH value of the aqueous permanganate oxidant solution is controlled to be at least 8, preferably at least 10, or both of the acidic oxidation step and the alkaline oxidation step are carried out as a high temperature oxidation step.

(29) More preferably, more than one of the plurality of treatment cycles comprises a high temperature oxidation step, and most preferably, all of the treatment cycles comprise the high temperature oxidation step.

(30) For carrying out the high temperature oxidation step, the external decontamination loop 32 is separated from the coolant system, and the oxidant solution is circulated through the cooling system by operating at least one of the pumps of the RHR systems 24, 26 and/or the reactor coolant pumps 20, 22 in the primary loops 10, 12.

(31) The waste heat generated by the reactor coolant pumps is used to heat the solution of the oxidant to the desired process temperature of at least 100 C. or more. The RHR systems 24, 26 are operated to control and keep the process temperature of the oxidant solution at the predetermined value. Accordingly, the process temperature of the high temperature oxidation step can be controlled to be in the range of from 120 to 150 C. easily by operating only power plant system equipment without raising any safety issues.

(32) After termination of the high temperature oxidation step, the oxidant solution is cooled down and the external decontamination equipment loop 32 can be (re-)connected to the reactor coolant system. The decontamination step is then started to reduce excess oxidant and dissolve the oxide layer in the organic acid solution, as described above, thereby forming the decontamination solution containing radioisotopes and metal ions originating from the metal oxide layers on the metal surfaces. Alternatively, the organic acid solution can be fed into the cooling system using the CVCS system 28.

(33) The treatment cycle is completed by immobilizing at least the radioisotopes and preferably other metal ions on an ion exchanger (not shown).

(34) The following laboratory examples further illustrate the invention but shall not be understood in a limiting sense.

Example 1

(35) In this experiment, sections of contaminated tubing from a steam generator of a pressurized water nuclear reactor were used. Each section was cut longitudinally to provide two samples having dimensions of 43.5 cm and a surface area of 14 cm.sup.2. The tubing and the samples consisted of Inconel 600. The initial surface activity of the samples was 2.710.sup.3 Bq/cm.sup.2.

(36) The samples were placed in separate containers and subjected to a total of ten (10) treatment cycles including alternating acidic and alkaline oxidation steps. The acidic oxidant solution was an aqueous solution of permanganic acid HMnO.sub.4 having a concentration of 0.15 g/l and a pH less than 3. The alkaline oxidant solution was an aqueous solution of 0.2 g/l of potassium permanganate and 0.2 g/l sodium hydroxide. The samples were kept in the oxidation solutions for about 17 hours with agitation.

(37) After each oxidation step, the samples were placed in a solution of oxalic acid having a concentration of 1 g/l in deionized water. The samples were kept in the organic acid solution for about 5 hours at a temperature of 95 C.

(38) The oxidation steps of the first seven treatment cycles were carried out at a temperature of 95 C. In order to determine the effect of a high temperature oxidation, wherein the oxidant solution is heated to a temperature beyond the boiling point of the solution, one of the samples was subjected to a treatment cycle comprising an oxidation step still at 95 C., followed by two treatment cycles comprising a high temperature oxidation at 125 C. in an autoclave, whereas the other sample was subjected to three treatment cycles comprising high temperature oxidation at 125 C.

(39) The following Table 1 gives the results of testing the samples using different temperature conditions during the oxidation steps.

(40) TABLE-US-00001 TABLE 1 Sample Size Sample 1 4 3.5 cm Oxidation Oxidation Decontamination Surface Sample Surface pH value Temperature Factor Activity 14 cm.sup.2 [] [ C.] [] [Bq/cm.sup.2] Initial Activity 2.74E+03 1st Cycle 8 95 1.3 2.13E+03 2nd Cycle 3 95 1.4 1.93E+03 3rd Cycle 8 95 2.0 1.38E+03 4th Cycle 3 95 4.1 6.64E+02 5th Cycle 8 95 5.3 5.21E+02 6th Cycle 3 95 6.9 3.99E+02 7th Cycle 8 95 7.5 3.64E+02 8th Cycle 3 95 11.0 2.48E+02 9th Cycle 8 125 20.8 1.31E+02 10th Cycle 3 125 61.4 4.46E+01 Sample Size Sample 2 4 3.5 cm Oxidation Oxidation Decontamination Surface Sample Surface pH value Temperature Factor Activity 14 cm.sup.2 [] [ C.] [] [Bq/cm.sup.2] Initial Activity 2.73E+03 1st Cycle 8 95 1.3 2.11E+03 2nd Cycle 3 95 1.5 1.87E+03 3rd Cycle 8 95 2.0 1.34E+03 4th Cycle 3 95 4.0 6.81E+02 5th Cycle 8 95 5.6 4.84E+02 6th Cycle 3 95 7.1 3.84E+02 7th Cycle 8 95 7.6 3.60E+02 8th Cycle 3 125 20.4 1.34E+02 9th Cycle 8 125 26.3 1.04E+02 10th Cycle 3 125 151.0 1.81E+01

(41) The effect of the high temperature oxidation step is apparent from a comparison of the decontamination factors of the 8.sup.th treatment cycle. Using the high temperature oxidation step in this cycle, about twice the amount of the surface activity was removed as compared to an oxidation step carried out below the boiling point of the oxidant solution.

(42) The 9.sup.th and 10.sup.th treatment cycle were carried out with a high temperature oxidation step for both samples to confirm the results found for sample 2 and on sample 1. The increase of the decontamination factor for both samples is evident.

(43) The results of Example 1 are also illustrated in FIG. 2 showing the development of the decontamination factor after each treatment cycle for samples 1 and 2.

Example 2

(44) A similar experiment was conducted to show the efficiency of treatment cycles comprising high temperature oxidation steps with respect to a reduction of the number of treatment cycles. Two samples as described in Example 1 were subjected to a total of three treatment cycles under the same conditions as shown in Example 1, with the exception that all treatment cycles were carried out using a high temperature oxidation step. Further, sample 1 was subjected to a first treatment cycle comprising an alkaline oxidation step followed by two treatment cycles each comprising an acidic oxidation step. Sample 2 was subjected to treatment cycles using alternating alkaline and acidic oxidation steps, starting with a treatment cycle comprising an oxidation of the metal oxides under alkaline conditions, followed by a treatment cycle comprising an acidic oxidation step and a subsequent treatment cycle comprising an alkaline oxidation step.

(45) The results of this experiment are given in the following Table 2.

(46) TABLE-US-00002 TABLE 2 Sample Size Sample 1 4 3.5 cm Oxidation Decontamination Surface Sample Surface Temperature Oxidation Factor Activity 14 cm.sup.2 [ C.] pH value [] [Bq/cm.sup.2] Initial Activity 2.58E+03 1st Cycle 125 8 1.4 1.86E+03 2nd Cycle 125 3 2.2 1.17E+03 3rd Cycle 125 3 4.6 5.63E+02 Sample Size Sample 2 4 3.5 cm Oxidation Decontamination Surface Sample Surface Temperature Oxidation Factor Activity 14 cm.sup.2 [ C.] pH value [] [Bq/cm.sup.2] Initial Activity 2.64E+03 1st Cycle 125 8 1.4 1.87E+03 2nd Cycle 125 3 2.3 1.16E+03 3rd Cycle 125 8 5.7 4.66E+02

(47) A comparison of the test results for Sample 2 of Example 2 with Sample 2 of Example 1, both of which are using alternating alkaline and acidic oxidation conditions, shows the efficiency of a high temperature oxidation. The high temperature oxidation used in Example 2 resulted in an overall decontamination factor of 5.7 after only three treatment cycles. Sample 2 of Example 1 required about 5-6 treatment cycles in order to achieve a comparable result, using low temperature oxidation conditions of below 100 C. The above comparison of the results of Examples 1 and 2 is also illustrated in FIG. 3.

(48) The test results show that using the high temperature oxidation step according to the present invention may divide the number of treatment cycles required for full system decontamination in half. A rough calculation shows that the elimination of one treatment cycle results in a waste reduction in the order of between 2 I and 38 I of ion exchange resin per cubic meter of system volume. Depending on the reactor design, the total system volume may range from 120 to 800 m.sup.3. It is immediately apparent that a reduction of the number of treatment cycles results in lower process costs as well as a reduction of the amount of radioactive waste.

(49) 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.