Patent Grant
6973154
The present invention relates to a nuclear power plant of water cooling type and, more particularly, to a chemical decontamination method and a chemical decontamination system by which radioactive nuclides are chemically removed from metallic material surfaces of primary cooling system components and pipes and a system including the component and the pipes which are contaminated with radioactive nuclides.
As conventional technologies in connection with chemical decontamination, Japanese Patent publication No. 3-10919 discloses a method in which components of a nuclear power plant made of metals are chemically decontaminated using permanganic acid as an oxidizing agent and dicarboxylic acid as a reducing agent. As methods of decomposing the above-mentioned organic acids, PCT/JP97/510784 discloses a method of decomposing the acid into carbon dioxide and water using an iron complex and ultraviolet rays. According to this method, since hydrogen peroxide of the oxidizing agent and the organic acid react by acting the iron complex as a catalyst to produce carbon dioxide and water, the organic acid can be prevented from becoming waste products.
Although oxalic acid is used as the above organic acid, oxalic acid has strong solvency for iron. Accordingly, when the decontaminating solution is allowed to flow through a system made of carbon steel which is easy to be corroded compared to stainless steel, a large amount of iron ions are dissolved from the carbon steel to increase an amount of produced waste products, or the oxalic acid is precipitated in the form of iron oxalates. Therefore, sufficient effect can not be obtained in decontamination using oxalic acid of a system having low corrosion resistant materials such as carbon steel.
In order to apply the method to the system containing the low corrosion resistant materials, it is considered that hydrazine is added to oxalic acid in order to adjust so as to increase the pH of the decontaminating agent. However, since hydrazine is trapped in a cation exchange resin column (hereinafter, referred to as cation resin column), load of the cation resin column is increased when the decontaminating solution is allowed to directly flow into the cation resin column. Therefore, an amount of hydrazine exceeds an exchanging capacity of the cation resin column to cause hydrazine to flow out. As a result, the amount of hydrazine flowing out is increased as load of metallic ions increases to excessively increase the pH of the decontaminating agent and accordingly to decrease the decontaminating effect. In order to avoid this problem, it is necessary to control the concentration of the hydrazine appropriately. It is preferable that the control means preferably decomposes into nitrogen and water. Although hydrazine can be decomposed by irradiating ultraviolet rays onto the hydrazine using a UV column (ultraviolet ray irradiation apparatus), the oxalic acid as well as the hydrazine is decomposed. It is difficult to selectively decompose only the hydrazine, and it is insufficient to reduce the load of the cation resin column because the ratio of decomposing hydrazine is low to produce ammonia. SFEA ┌Actes de la Conférence Internationale Proceedings of the International Conference┘, 24-27 Apr. 1994, Nice-F rance, page 203–210 “A FULL SYSTEM DECONTAMINATION OF THE OSKARSHAMN 1 BWR” by Johan Lejon and Åsa Hermansson.
A first object of the present invention is to provide a chemical decontamination method and a chemical decontamination system comprising a chemical decontaminating agent decomposing apparatus for selectively decomposing hydrazine which are components of load to the cation resin column. Further, after completion of decontamination process, it is important that the decomposing agent does not become waste products by decomposing not only the components to be trapped by the cation exchange resin but also components to be trapped by an anion exchange resin. However, there is a problem in that provision of a plurality of the decomposing apparatuses increases the cost of system. A second object of the present invention is to provide a chemical decontamination method which moderates corrosion of material by using a chemical decontaminating agent decomposing apparatus capable of decomposing not only the components trapped by the cation exchange resin but also components trapped by an anion exchange resin at a time.
Key points of the present invention are as follows.
The present invention provides the chemical decontamination method in the above-mentioned item (1), wherein in the process of decomposing the reductive decontaminating agent using the decomposing apparatus, the at least two kinds of chemical substances in the reductive decontaminating agent are decomposed at a time.
Further, the present invention provides the chemical decontamination method, wherein when the apparatus for decomposing at least two kinds of chemical substances in the reductive decontaminating agent cleanses radioactive nuclides from the decontaminating agent using a cation resin column during decontaminating, a composition trapped by the cation resin column at an inlet side of a cleaning apparatus is selectively decomposed.
Furthermore, the present invention provides the above chemical decontamination method, wherein in the above-mentioned decomposing apparatus for the reductive decontaminating agent, a composition trapped by the cation resin column at the inlet side of the cleaning apparatus is selectively decomposed when the radioactive nuclides in the decontaminating agent are cleansed using the cation resin column during decontaminating, and at least two kinds of compositions are decomposed at a time by controlling an adding amount of hydrogen peroxide after completion of the decontaminating step, and the reductive decontaminating agent includes oxalic acid and hydrazine as the compositions.
The present invention provides the chemical decontamination method of the above-mentioned items (1) and (2), wherein the reductive decontaminating agent contains oxalic acid and hydrazine, and is a reductive acid solution of which a concentration of oxalic acid is 0.05 to 0.3 wt % and a pH is 2 to 3. Otherwise, the chemical decontamination method further comprises an oxidative dissolving process for oxidatively dissolving chromium in a metal oxide on the metallic material surface contaminated by the radioactive nuclides into hexadic chromium using permanganate before or after the reductive dissolving process for dissolving and removing the metal oxide.
Further, the present invention provides the chemical decontamination method in the above-described item (2), wherein the reductive dissolving process and the oxidative dissolving process are alternatively performed, and the reductive dissolving process is performed at least twice.
Furthermore, the chemical decontamination method in the above-described items (1) and (2), wherein a catalyst decomposition column is used as the decomposing apparatus for the reductive decontaminating agent, and at least one element selected from the group consisting of platinum, ruthenium, vanadium, palladium, iridium and rhodium is used as a catalyst filled in the catalyst column and an oxidizing agent is supplied in an inlet side of the catalyst column.
Further, the present invention provides the chemical decontamination method in the above-mentioned items (1) and (2) wherein a quantity of hydrogen peroxide added is less than an equivalent weight of the components trapped in the cation resin column when components trapped in the cation resin column is selectively decomposed, and a quantity of hydrogen peroxide added is more than an equivalent weight react with the components trapped in the cation resin column when components trapped in the cation resin column and components trapped in the anion resin column are decomposed at a time.
The present invention provides the chemical decontaminating system in the above item (3), which further comprises a gas-liquid separating apparatus for separating decomposed gas in a downstream side of the catalyst decomposition column and in an upstream side of the ion exchange resin.
1 . . . decontaminated part, 2 . . . circulation line, 3 . . . circulation pump, 4 . . . heater, 5 . . . cooler, 6 . . . catalyst decomposition column, 7 . . . cation resin column, 8 . . . agent tank, 9 . . . agent injection pump, 10 . . . pH adjusting agent tank, 11 . . . pH adjusting agent injection pump, 13 . . . hydrogen peroxide injection pump, 14 . . . mixed-bed resin column, 15 . . . gas-liquid separating tank, 16 . . . UV column, 31 to 45 . . . valve (a solid valve indicates closed, and a hollow valve indicates opened).
The present invention will be described below in detail, referring to embodiments.
[Embodiment 1]
Initially, heat-up mode in the first cycle of
Predetermined quantities of oxalic acid from the agent tank 8 and hydrazine from the pH adjusting tank 10 are injected into the portion 1 to be decontaminated using pumps 9 and 11, respectively. After starting the injection, water is allowed to flow through the cation resin column 7 in order to collect metallic ions mainly composed of radioactive nuclides and iron dissolved out of the portion 1 to be decontaminated.
Since hydrazine of the pH adjusting agent is trapped to the cation resin column 7, hydrazine is decomposed in the catalyst decomposition column 6 while hydrogen peroxide is being injected before water is allowed to flow through the cation resin column 7. The injecting amount of hydrogen peroxide is controlled so as to become a molar number twice as large as a molar concentration of the hydrazine.
By doing so, decomposition of the oxalic acid component can be suppressed and only the hydrazine can be selectively decomposed. After adjusting the oxalic acid concentration in the system to 2000 ppm and an indication value of the pH meter in the outlet side of the portion 1 to be decontaminated to 2.5, the reducing agent decontamination mode (the first cycle of
Since the concentration of oxalic acid in the system is decreased every moment, the injecting amount of hydrogen peroxide is decreased by controlling an opening degree of the valve 39 based on an indication of a conductometor in an outlet side of the portion 1 to be decontaminated utilizing that the concentration of oxalic acid is nearly in a proportional relationship to the conductivity. It is confirmed by analyzing the sampling water sampled through a sampling line in an outlet side of the heater 4 that the concentration of oxalic acid in the system becomes below 10 ppm and the concentration of hydrazine becomes below 5 ppm, and then the reductive decontaminating agent decomposing process (the first cycle of
After that, cleaning mode shown in
Next, the process is entered to the second cycle of
After completion of the oxidizing agent decontamination, the oxidizing agent decomposing mode of
After the decomposition is completed and the system water becomes transparent, the second reducing agent decontamination mode, the second reducing agent decomposition mode and the final cleaning mode showing the second cycle of
The processing after that is the same as that in the first reducing agent decontamination process, that is, decontamination is performed by repeating the oxidizing and the reducing agent decontamination processes necessary times, the final cleaning is performed after decomposing the reducing agent following to sufficient removing of radioactivity of the portion to be decontaminated, cleaning is performed using the mixed-bed resin column 14 until the conductivity of the system water becomes below 1 μs/cm, and thus the decontamination is completed.
In order to obtain information on the removed radioactivity and the removed amount of metals, sample water is sampled from sampling lines arranged in the inlet and the outlet of the resin columns 7 and 14 to analyze radioactive nuclides and metallic concentrations in the sample water, and load to the cation resin column 7(or the mixed-bed resin column 14) can be calculated using a water flow rate and a water flowing time to the resin column 7 (or the resin column 14).
The above will be described below in more detail, assuming that a reductive decontaminating agent adjusted to pH 2.5 by adding hydrazine to oxalic acid of 0.2% and an oxidative decontaminating agent of potassium permanganate of 0.03% are used as the decontaminating agents. In the reducing agent decontamination process, the water is heated up using the circulation pump 4 and the heater 4 as shown in
In the step of decomposing the reductive decontaminating agent after completion of the reducing agent decontamination process (4 hours to 15 hours), operation of the pH adjusting agent injection pump is stopped to increase an adding amount of hydrogen peroxide supplied to the catalyst decomposition column and to change the operating mode so that decomposition of oxalic acid as well as hydrazine is progressed. The concentration of hydrogen peroxide at that time is within the range between a molar concentration equal to a value of the sum of twice of a molar concentration of hydrazine and a molar concentration of oxalic acid as the lower limit and three times of the value as the upper limit, but operation near the lower limit is preferable. The reason why the upper limit is set to the hydrogen peroxide concentration is as follows. That is, although hydrogen peroxide not contributing to the reaction in the catalyst decomposition column is decomposed into oxygen and water by the catalyst, a large amount of partially un-decomposed hydrogen peroxide flows out to the downstream of the catalyst decomposition column 6. In such a case, because the ion exchange resin is deteriorated by the hydrogen peroxide, it possibly happens the radioactive nuclides and so on trapped to the ion exchange resin are released. Since the concentration of hydrogen peroxide in the system is decreased as decomposition of the reductive decontaminating agent is progressed, the injecting amount of hydrogen peroxide is gradually decreased by continuously or intermittently measuring the concentration of decontaminating agent. By doing so, almost all the reductive decontaminating agent in the system is decomposed and accordingly load to the ion exchange resin caused by the un-decomposed reductive decontaminating agent can be suppressed.
After completion of decomposing the reductive decontaminating agent, water is allowed to flow through the mixed-bed resin column 14 (or the anion resin column) to remove chromic acid ions remaining in the system water, and potassium permanganate of the oxidative decontaminating agent is injected into the system from the agent injection tank 8 using the agent injection pump 9 to adjust the concentration to a predetermined value (0.05%). At that time, the catalyst column 6 and the resin column 7 are isolated by closing valves. This is because the catalyst and the ion exchange resin are prevented from being deteriorated by the oxidizing agent.
After completion of the oxidizing agent decontamination process (4 hours to 8 hours), oxalic acid and hydrazine are again injected in order to decompose and reduce permanganate ions into bivalent manganese ions. After completion of the decomposition, water is re-started to flow through the cation resin column 7 to remove radioactivity and manganese ions, potassium ions released from the cation resin column 7 while hydrogen peroxide is added to the catalyst column 6 by an amount necessary for decomposing the hydrazine, as similarly to in the initial reducing agent decontamination process.
After completion of the second reducing agent decontamination process, the reducing agent is decomposed in the same procedure as that in the first reducing agent decomposition process, and after completion of the decomposition the final cleaning is performed using the mixed-bed resin. Although the process in
Catalysts capable of being used for decomposing the reductive decomposing agent are noble metal catalysts such as platinum, ruthenium, rhodium, iridium, vanadium, palladium catalysts and the like. A measured result of decomposition ratio at a certain time after adding the catalyst into a beaker. It can be understood from the result that ruthenium catalyst is preferable from the viewpoint of decomposition ratio. Further, it is known that ruthenium catalyst is also effective to decomposition of hydrazine. The decomposition efficiency of ruthenium catalyst to hydrazine is, however, extremely decreased when oxalic acid is mixed in the decontaminating solution, but the decomposition can be progressed by adding hydrogen peroxide to the decontaminating solution.
A test was conducted to study decomposition ratios for hydrazine and oxalic acid in the catalyst decomposition column 6. The test was conducted by using 0.5% ruthenium-carbon particles made by N. E. Chemcat Co., and a pre-heated decontaminating solution added with hydrogen peroxide was allowed to flow at a speed of SV 30 to the catalyst decomposition column 6 set the outer surface temperature to 95° C. of the upper limit temperature of the decontaminating agent. The test result is shown in
Since nitrogen is produced when hydrazine is decomposed and carbon dioxide gas is produced when oxalic acid is decomposed, these gases need to be exhausted outside the system. Although any apparatus for removing the gases is not shown in
Although trivalent iron complex and bivalent iron ions are produced by the decontamination, the bivalent iron ions can be removed by the cation resin column 7 in the reducing agent decontamination process. Nearly one-half amount of the trivalent iron complex is removed by the cation resin column 7 in the reducing agent decontamination process. The residual amount of the trivalent iron complex becomes iron hydride by hydrogen peroxide injected in the reducing agent decontamination process and removed by the catalyst.
According to the present embodiment, the pH is moderated to 2.5 because hydrazine is added, and consequently the base material of the portion 1 to be decontaminated is suppressed to be dissolved. Therefore, the amount of produced radioactive waste products can be reduced and thinning of the base material can be suppressed. Particularly, when the base material of the portion 1 to be decontaminated is low anti-corrosion carbon steel, the effect of reducing corrosion is very large.
[Embodiment 2]
Although in Embodiment 1 the vent mechanism is arranged in the catalyst decomposition column 6 in order to remove the produced gas, a gas-liquid separating tank having a vent cooler for separating the gas may be arranged downstream of the catalyst decomposition column 6 and upstream of the cation resin column 7. In this case, there is an advantage in that the gas-liquid separating tank 13 can also be used as a buffer for receiving a volume of liquid increased by injection of the agent.
[Embodiment 3]
The main process in the present embodiment of the chemical decontamination method is shown in
In Embodiment 3, the cooler 5, the cation resin column 7 and the mixed-bed resin column 14 are arranged in the upstream side of the catalyst decomposition column 6.
An advantage of the system configuration shown in Embodiment 3 is that the concentration of radioactivity in the water flowing to the catalyst decomposition column 6 is low because the water flows into the catalyst decomposition column 6 after flowing through the cation resin column 7, and consequently accumulation of radioactivity in the catalyst decomposition column 6 can be substantially suppressed. Further, it is unnecessary to decompose hydrazine by the catalyst decomposition column 6 until hydrazine breaks through the cation resin column 7.
On the other hand, after hydrazine breaks through the cation resin column 7, injection of hydrazine is unnecessary, and an excessive amount of hydrazine flowing out corresponding to an amount of metallic ions trapped to the cation resin column 7 is decomposed in the catalyst decomposition column 6. The water flow rate to the catalyst decomposition column 6 may be controlled so as to maintain the pH of the decontamination solution to 2.5. The procedure of the other processes is basically the same that of Embodiment 1 (
That is, in this embodiment, each of the modes of the main process shown in
[Embodiment 4]
The main process in the present embodiment of the chemical decontamination method is shown in
The system of Embodiment 4 is constructed by adding a UV column (ultraviolet ray irradiation apparatus) 16 to the configuration of Embodiment 3 and arranging the UV column in parallel to the catalyst decomposition column 6. The piping route is branched at the exit of the flowmeter F1 into a route from the exit of the flowmeter F1 to the UV column 16 and the gas-liquid separating tank 15 through a valve 45 and a route from the exit of the flowmeter F1 to the catalyst decomposition column 6 and the gas-liquid separating tank 15 through a valve 44. During the reducing agent decontamination in the first and the second cycles under water flow operation to the cation resin column 7 (the valve 44 is closed and the valve 45 is opened), the water is allowed to flow though the UV column 16, and trivalent iron complex is reduced to bivalent iron ions to be removed by the cation resin column 7. Because the trivalent iron complex can not be removed by the cation resin column 7 due to an anion type, the decontaminating solution with an iron concentration keeping high proceeds to the next process of decomposing the reductive decontaminating agent. In such a case, iron deposits on the catalyst to decrease the catalyst power. The system of Embodiment 4 has an effect to suppress decrease of the catalyst power. Life time of the catalyst can be lengthened and an amount of catalyst disposed as radioactive products can be reduced. The processing and opening and closing of the valves in the other processes in the main process shown in
[Embodiment 5]
Embodiment 5 shows the different operation process after hydrazine breaks through the cation resin in embodiment 3.
The difference between
When the cation resin column 7 is changed before using it in the second cycle, the hydrazine injection is stopped when hydrazine breaks through the cation resin in the second cycle just like in the first cycle.
The main process of the chemical decontamination method in the case of no changing cation resin between the first cycle and the second cycle is shown in
Although
According to the present invention, since increase in the amount of waste products caused by adding hydrazine can be suppressed, it is possible to increase the pH of the decontaminating solution a value higher than that of a decontaminating solution using solely oxalic acid and it is possible to perform decontamination of a system including a low corrosion resistant material. Further, since hydrazine can be selectively decomposed by only one catalyst decomposition column and oxalic acid can be decomposed together with hydrazine, cost in regard to the decontaminating agent decomposition apparatus can be reduced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-274128 | Sep 1998 | JP | national |
This is a continuation-in-part (CIP) application of U.S. Ser. No. 09/405,217 filed Sep. 27, 1999 now U.S. Pat. No. 6,335,475, now allowed, the entire disclosure of which is hereby incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 20020099252 A1 | Jul 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09405217 | Sep 1999 | US |
| Child | 10000338 | US |
