JP6547224B2 - Chemical decontamination method - Google Patents

Description

  The present invention relates to a method of chemical decontamination.

  In a nuclear power plant, decontamination work is performed to remove radioactive materials attached to equipment, piping, etc. at the time of maintenance work during operation and disassembly work at the time of decommissioning. In particular, since the piping members (including mainly stainless steel and Inconel) that make up the nuclear power plant system are exposed to the high temperature and high pressure environment during operation, an oxide film is formed on the surface, and radioactive substances in the film are formed. Is captured. A method called chemical decontamination is known as a method of removing this oxide film and decontaminating a member.

  Representative examples of chemical decontamination include a process of dissolving and removing an oxide film containing iron and nickel formed on the surface of the object to be decontaminated using a reduction reaction, and chromium containing an oxidation reaction. A combination of processes for dissolving and removing the oxide film has been put to practical use. Specifically, the HP method using permanganic acid as the above-mentioned oxidizing agent is known. As an example of the decontamination method which improved this HP method, the technique described in the following patent document 1 is mentioned.

  The chemical decontamination method and decontamination apparatus described in Patent Document 1 mainly contain chromium and iron by using permanganic acid and tetravalent cerium as an oxidizing agent and using oxalic acid as a reducing agent. It is said that a good decontamination effect can be obtained for the decontamination target area made of an alloy (stainless alloy).

  In addition to the above-described stainless steel alloys, nickel-based alloys such as inconel mainly composed of nickel are widely used in various devices and pipes in nuclear power plants. In this case, in the decontamination by the HP method represented by Patent Document 1, there is a possibility that the decontamination effect can not be sufficiently obtained as compared with stainless steel.

  On the other hand, by adding potassium permanganate in an oxidizing agent and setting the decontamination environment on the alkali side using an alkaline solution, a technology for obtaining the decontamination effect on the oxide film of a nickel-based alloy (for example, AP method) Is also applied to the actual machine. By alternately repeating the steps using the plurality of techniques (HP method and AP method), it is believed that decontamination effects can be expected for both stainless steel and nickel base alloy.

JP, 2013-88213, A

  When potassium permanganate is used as the oxidizing agent, a large amount of potassium ions ionized in the treated water are generated as secondary waste. On the other hand, there is a problem that permanganic acid is more expensive than potassium permanganate, and it is possible to repeatedly apply the HP method and the AP method as described above with process delay, increase of secondary waste and cost increase. There is sex.

  The present invention has been made in consideration of such circumstances, and it is an object of the present invention to provide a chemical decontamination method in which a sufficient decontamination effect can be obtained and at a low cost.

In order to solve the above-mentioned subject, the present invention adopts the following means.
The chemical decontamination method according to one aspect of the present invention comprises oxidizing chromium from the object to be decontaminated by adding permanganic acid as an oxidizing agent containing permanganate ion to treated water in which the object to be decontaminated is immersed. A first addition step of elution, and a second addition step of changing the pH to the alkaline side by adding an alkalizing agent to the treated water without replacing the treated water after the first addition step; and oxidation step having, after oxidation step, a reduction step the iron and nickel is reduced eluted from the decontamination object by adding an organic acid to the processing in water without replacement of the treated water, including.

  According to the method as described above, in the first addition step in the oxidation step, chromium contained in the object to be decontaminated (mainly stainless steel) is oxidized and eluted in the treated water. Furthermore, in the second addition step, the alkalinizing agent is added to the treated water, whereby the pH of the treated water changes to the alkali side to promote the dissolution of the chromium of the nickel-based alloy. In the subsequent reduction step, the oxides of iron and nickel are reduced and eluted from the object to be decontaminated by the addition of the organic acid acting as a reducing agent in the treated water. That is, chromium, iron and nickel in the object to be decontaminated (stainless steel and nickel base alloy) can be eluted without replacing the treated water. This can reduce the working time.

  In the chemical decontamination method according to one aspect of the present invention, in the oxidation step, the temporal change rate of the chromium ion concentration contained in the treated water is less than or equal to a predetermined reference value after the first addition step. In the determination step, the reduction step may be performed when it is determined in the determination step that the time change rate is equal to or less than the reference value.

Here, the rate of change with time of the chromium ion concentration contained in the treated water gradually decreases, and finally becomes approximately a constant value. At this time, it can be considered that the oxidation and elution of chromium contained in the object of decontamination progressed sufficiently.
In the above-described method, the reduction step is performed after it is determined that the temporal change rate of the chromium ion concentration is below the reference value. This can reduce the possibility of proceeding to the reduction step while the elution of chromium from the object to be decontaminated in the oxidation step is insufficient.

  In the chemical decontamination method according to one aspect of the present invention, in the second addition step, the alkalizing agent may be added until the pH of the treated water becomes 10 or more and 12 or less.

  According to the method as described above, since the pH of the treated water after the second addition step is 10 or more and 12 or less, iron and nickel can be sufficiently reduced and eluted in the treated water, and Waste generation can be reduced.

  In the chemical decontamination method according to one aspect of the present invention, the alkalizing agent may contain at least one of sodium hydroxide and potassium hydroxide.

  According to the method as described above, in addition to the fact that sodium hydroxide and potassium hydroxide are strong bases, the solubility in water is relatively high. It can be changed.

  In the chemical decontamination method according to one aspect of the present invention, the object to be decontaminated may include SUS and Inconel.

  In the chemical decontamination method according to each aspect described above, chromium contained in SUS and iron and nickel contained in inconel can be sufficiently eluted in treated water. Thereby, even if the object to be decontaminated contains SUS and Inconel, a sufficient decontamination effect can be obtained.

  According to the present invention, a sufficient decontamination effect can be obtained, and a low-cost chemical decontamination method can be provided.

It is a systematic diagram showing an example of the nuclear installation to which the chemical decontamination method concerning each embodiment of the present invention is applied. It is process drawing which shows an example of the chemical decontamination method which concerns on 1st embodiment and 3rd embodiment of this invention. It is process drawing which shows an example of the chemical decontamination method which concerns on 2nd embodiment of this invention. It is a graph which shows the decontamination performance by the chemical decontamination method which concerns on each embodiment of this invention.

First Embodiment
A first embodiment of the present invention will be described with reference to the drawings.
First, the decontamination object Z which is the target of the present embodiment will be described.
The object of decontamination Z targeted by the present embodiment is a component such as a pipe, a container, various devices, etc. constituting the nuclear power plant, and is a component which the reactor water contacts.

  Here, as a nuclear power plant, for example, as shown in FIG. 1, there is a nuclear power plant P provided with a pressurized water reactor 50.

  The nuclear power plant P includes a pressurized water reactor 50 in which fuel rods 51 and the like are housed, and a pressurizer 52 that pressurizes primary cooling water to suppress boiling of primary cooling water (light water) in the pressurized water reactor 50. A steam generator 53 for converting the secondary cooling water into steam by the heat of the primary cooling water, a coolant pump 54 for returning the primary cooling water from the steam generator 53 to the pressurized water reactor 50, and the steam generator 53. A steam turbine 56 driven by the generated steam, a generator 57 generating electricity by driving the steam turbine 56, a condenser 58 for returning the steam from the steam turbine 56 to water, and steam generation of the water from the condenser 58 And a water supply pump 59 for returning to the vessel 53.

  The pressurized water reactor 50 and the steam generator 53 are connected by primary cooling water pipes 55a and 55b, the steam generator 53 and the steam turbine 56 are connected by the steam pipe 55c, and the condenser 58 and the steam turbine 56 are connected. Are connected by a water supply pipe 55d.

  In the nuclear power plant P configured as described above, a pressurized water reactor 50 is used as a primary cooling system member (hereinafter, referred to as “decontamination target object Z”) that is a member in contact with reactor water, that is, primary cooling water. There are a pressurizer 52, a steam generator 53, a coolant pump 54, primary cooling water pipes 55a and 55b connecting these, various valves provided in the primary cooling water pipes 55a and 55b, and the like. The object to be decontaminated Z is formed of stainless steel mainly containing iron and containing chromium and nickel, Inconel or the like which is a nickel base alloy.

  The metal element constituting the object to be decontaminated Z slightly dissolves in the reactor water, and a part thereof adheres to the surface of the fuel rod 51 in the pressurized water reactor 50. The metal element attached to the surface of the fuel rod 51 causes nuclear reaction by irradiation of neutron beam from the fuel, and becomes a radionuclide such as chromium, nickel, cobalt and the like. Most of these radionuclides remain attached to the surface of the fuel rods 51 in the form of oxides, but some of them are dissolved in reactor water or released as insoluble solids. The radionuclide eluted or released into the reactor water adheres to the reactor water contact surface of the object to be decontaminated Z, and forms an oxide film mainly composed of iron on the reactor water contact surface. For this reason, the worker who works in the vicinity of the object Z to be decontaminated is exposed to the radiation from the radionuclide in the oxide film formed on the part.

  The present embodiment relates to the system decontamination of the nuclear power plant P described above, and the decontamination object Z is circulated while circulating the treated water W which is a chemical solution inside the piping etc. which is the decontamination object Z. The chemical decontamination method M1 of removing and discarding the oxide film containing the radionuclide attached to the inner surface of the

  As shown in FIG. 2, the chemical decontamination method M1 according to the first embodiment of the present invention comprises an oxidation step S10 in which chromium is oxidized and eluted in treated water W, and a reduction step S20 in which iron and nickel are reduced and eluted. including.

First, the oxidation step S10 is performed. Oxidation process S10 which concerns on this embodiment contains 1st addition process S11, measurement process S13, determination process S14, and 2nd addition process S12.
In the first addition step S11, an oxidizing agent is added to the treated water W, and the treated water W having the oxidizing agent added to the inside of the object of decontamination Z is supplied and circulated. In the present embodiment, permanganic acid is added as an oxidizing agent. In the first addition step S11, the chromium in the oxide film attached to the object to be decontaminated Z containing stainless steel or nickel base alloy by circulating the treated water W to which permanganic acid is added inside the object to be decontaminated Z Is oxidized and eluted as hexavalent chromium ion. As a result, in the first addition step S11, primary treated water W1 containing chromium, which is a radionuclide, is generated.

  Specifically, the concentration of permanganic acid (oxidant) added to the treated water W in the first addition step is preferably 50 to 500 ppm, and more preferably 150 to 300 ppm. Furthermore, the temperature of the treated water W is preferably maintained at 80 to 150 ° C., and more preferably 90 to 100 ° C. Most preferably, the treated water W is maintained at 95 ° C. In addition, in the present embodiment, this first addition step is preferably performed for 8 to 12 hours. However, depending on the thickness of the oxide film on the object to be decontaminated Z, it is also possible to carry out the first addition step for up to about 24 hours.

  Subsequently, the second addition step S12 is performed. In the second addition step S12, an alkalizing agent is added to the primary treatment water W1. Specifically, the alkalizing agent is a chemical solution containing either potassium hydroxide or sodium hydroxide, or both of them. By adding such an alkalinizing agent, the pH of the above-mentioned primary treatment water W1 changes to the alkali side, and becomes secondary treatment water W2.

  In this secondarily treated water W2, elution of chromium that has not been eluted in the first addition step further proceeds. That is, the chromium contained in the object of decontamination Z is sufficiently eluted by executing the first addition step S11 and the second addition step S12 described above. Thus, the oxidation step S10 is completed.

  In the present embodiment, it is preferable that the above-mentioned alkalizing agent be added until the value of pH in the secondarily treated water W2 becomes 10 or more and 12 or less. Furthermore, as pH value of secondary treated water W2, 10 or more and 11 or less are more preferable, and 10.5 is the most preferable.

  Next, the reduction step S20 is performed. At this time, the secondary treatment water W2 is alkaline after passing through the second addition step S12 in the above-mentioned oxidation step S10. For example, oxalic acid is added as an organic acid to the secondary treated water W2. Thereby, the oxides of iron and nickel contained in the object of decontamination Z are reduced and eluted as iron ions or nickel ions in the secondary treated water W2. As a result, in the reduction step S20, tertiary treatment water W3 containing iron and nickel which are radionuclides is generated. Thus, the reduction step S20 is completed.

  In addition, it is desirable to perform removal process S30 as shown below following said reduction process S20. In the removal step S30, for example, ion exchange resin is used to remove radionuclides such as chromium, iron, nickel and the like contained in the above-mentioned tertiary treated water W3 and ions such as permanganate polluted by these radionuclides Perform decontamination. The decontaminated tertiary treated water W3 can be used again as the treated water W in the next and subsequent cycles.

Next, the operation of the chemical decontamination method M1 according to the above embodiment will be described.
According to the chemical decontamination method M1 as described above, in the oxidation step S10 (the first addition step S11), permanganic acid as an oxidizing agent is added to the treated water W, whereby the primary treated water W1 which is acidic is treated. It is generated. The chromium contained in the object of decontamination Z is oxidized and eluted as hexavalent chromium ions by the primary treatment water W1.

  Furthermore, in the subsequent second addition step S12, the pH of the primary treatment water W1 changes to the alkali side by the addition of an alkalizing agent to the treatment water, and the secondary treatment water W2 is obtained. As a result, elution of chromium that has not been eluted in the first addition step proceeds. That is, the chromium contained in the object of decontamination Z is sufficiently eluted by executing the first addition step S11 and the second addition step S12 described above.

  In the subsequent reduction step S20, oxalic acid as an organic acid is added to the secondarily treated water W2, and the above-mentioned reduction reaction proceeds. By this reduction reaction, oxides of iron and nickel contained in the object of decontamination Z are reduced and eluted. As described above, in the chemical decontamination method M1 according to the present embodiment, any of chromium, iron, and nickel can be eluted without replacing the treatment water. This makes it possible to reduce the number of operation steps and the operation cost.

  Here, a graph showing the relationship between the number of operation steps (number of decontamination cycles) and the decontamination performance (DF) for inconel in the above-mentioned chemical decontamination method M1 and the conventional decontamination method is shown in FIG. In this graph, the decontamination cycle number is taken on the horizontal axis, and the decontamination performance is taken on the vertical axis. Furthermore, the solid line graph in the figure represents the decontamination performance by the chemical decontamination method M1 according to the present embodiment. The alternate long and short dash line graph indicates the decontamination performance when decontamination is alternately performed using permanganic acid and potassium permanganate as oxidizing agents. The two-dot chain line graph shows the decontamination performance when permanganic acid alone is used as the oxidizing agent.

  As shown by the solid line graph and the alternate long and short dash line graph in the same figure, according to the chemical decontamination method M1 according to the present embodiment, permanganate and potassium permanganate are used as oxidizing agents in an arbitrary number of decontamination cycles. Decontamination performance equivalent to that of the conventional case can be obtained.

  In particular, the conventionally known decontamination methods include an oxidation step (HP method) for dissolution of chromium oxide in stainless steel and a reduction step for dissolution of oxides of iron and nickel, The oxidation step (AP method) and the reduction step are alternately repeated to expect dissolution of Inconel's chromium oxide. However, in the chemical decontamination method M1 according to the present embodiment, as described above, it is possible to achieve the target decontamination performance Ft with a smaller number of cycles than the conventional method.

  Furthermore, in the second addition step S12 in the above-mentioned chemical decontamination method M1, an alkalizing agent is added until the pH of the secondary treated water W2 becomes 10 or more and 12 or less. Thereby, iron and nickel can be sufficiently reduced and eluted in the secondary treatment water W2, and the amount of secondary waste generated can be reduced. That is, the processing load of the ion exchange resin used in the subsequent removal step S30 can be reduced.

  In addition, in the second addition step S12, at least one of sodium hydroxide and potassium hydroxide as a strong base is employed as the alkalizing agent. In addition to the fact that all of these chemical species exhibit strong basicity, since the solubility in water is relatively high, the pH of the treated water W can be sufficiently changed to the alkali side in a short time. In addition, this alkalizing agent may contain both sodium hydroxide and potassium hydroxide.

The chemical decontamination method M1 according to the first embodiment of the present invention has been described above. However, various modifications are possible without departing from the scope of the present invention.
For example, in the above-mentioned embodiment, the example which circulates treated water W in piping, such as a temporary cooling system member in nuclear power plant P, was explained as decontamination object Z. However, the mode of the decontamination object Z is not limited to the above. Even when chemical decontamination is performed by immersing parts generated by the dismantling of the nuclear power plant P or the like as the decontamination object Z in a container (pool) storing the treated water W, the chemical decontamination is performed as described above. It is possible to apply the decontamination method M1.

Second Embodiment
Subsequently, a chemical decontamination method M2 according to a second embodiment of the present invention will be described with reference to FIG. In addition, detailed description is abbreviate | omitted about the structure and process similar to each above-mentioned embodiment.
As shown in FIG. 3, the chemical decontamination method M2 according to the present embodiment is a measurement step S13 executed after the first addition step S11 in the oxidation step S10 in which chromium is oxidized and eluted in the treated water W. And a determination step S14.

In measurement process S13, the time change rate V of the said chromium ion concentration contained in primary treated water W1 is calculated | required. An example of a method of determining the time rate of change V of the chromium ion concentration will be described below.
First, the chromium ion concentration in the primary treated water W1 is measured a plurality of times (n times) with a constant measurement interval. As a result, a plurality of (n) measurement values are obtained as the chromium ion concentration. By calculating the quotient of the difference between the nth measurement value and the n-1st measurement value among the n measurement values and the above measurement interval (time), the chromium ion concentration The time rate of change V is determined.

  Next, the determination step S14 is performed. In the determination step S14, the time change rate V of the chromium ion concentration is compared with a predetermined reference value Vc. Specifically, it is determined whether the temporal change rate V of the chromium ion concentration calculated in the above-mentioned measurement step S13 is less than or equal to a reference value Vc. It is desirable that the reference value Vc be determined in advance based on experiments, operation results, and the like.

  If it is determined in the above determination step that the temporal change rate V of the chromium ion concentration is equal to or less than the reference value Vc, the second addition step S12 is performed as in the first embodiment described above. In the second addition step S12, an alkalizing agent is added to the primary treatment water W1. As a result, the pH of the primary treatment water W1 changes to the alkali side, and becomes the secondary treatment water W2.

On the other hand, if it is determined in the determination step that the temporal change rate V of the chromium ion concentration is not less than or equal to the reference value Vc, it can be determined that the oxidation elution of chromium is still progressing, so the first addition again Step S11 and measurement step S13 are performed (FIG. 3).
Thus, the oxidation step S10 is completed.

  As described above, the chemical decontamination method M2 according to this embodiment determines whether or not the temporal change rate V of the chromium ion concentration contained in the primary treated water W1 is less than or equal to a predetermined reference value Vc. A determination step S14 is included. When it is determined in the determination step S14 that the time change rate is equal to or less than the reference value Vc, the subsequent second addition step S12 and the reduction step S20 are performed.

  Here, the temporal change rate V of the chromium ion concentration contained in the treated water W eventually becomes approximately a constant value while gradually decreasing. At this time, it can be considered that the oxidation and elution of chromium contained in the object of decontamination Z proceeded sufficiently. In the above-described determination step S14, the reduction step S20 is performed after determining that the temporal change rate V of the chromium ion concentration is less than or equal to the reference value Vc. Thereby, the possibility of proceeding to the reduction step S20 can be reduced while the elution of chromium from the object of decontamination Z in the oxidation step S10 is insufficient. In other words, according to the above method, chromium can be sufficiently eluted from the object of decontamination Z.

Third Embodiment
Subsequently, a third embodiment of the present invention will be described. In addition, detailed description is abbreviate | omitted about the structure and process similar to the above-mentioned 1st embodiment.
The chemical decontamination method M3 according to the present embodiment includes the same steps as the chemical decontamination method M1 shown in FIG. That is, the chemical decontamination method M3 includes an oxidation step S10 in which chromium is oxidized and eluted in the treated water W, and a reduction step S20 in which iron and nickel are reduced and eluted. Here, in the present embodiment, a solution containing potassium permanganate and nitric acid is used as the oxidizing agent to be added to the treated water W in the oxidation step S10, and the chemical decontamination method M1 is It is different. Thereby, chemical characteristics (pH etc.) similar to the above-mentioned primary treatment water W1 can be obtained.

  Also by such a method, the same decontamination effect as the chemical decontamination method M1 according to the above-described first embodiment can be obtained. In addition, since potassium permanganate used as the oxidizing agent in the present embodiment is generally inexpensive compared to the permanganic acid described in the above first embodiment, the chemical elimination according to the present embodiment can be omitted. According to the dyeing method M3, the operation cost required for the decontamination work can be further reduced.

P: Nuclear power plant 51: Fuel rod 50: Pressurized water reactor 52: Pressurizer 53: Steam generator 54: Coolant pump 56: Steam turbine 57: Generator 58: Condenser 59: Water supply pump 55a, 55b ... Primary cooling water piping 55c ... Steam piping 55d ... Water supply piping M1, M2, M3 ... Chemical decontamination method S10 ... Oxidation process S11 ... First addition process S12 ... Second addition process S13 ... Measurement process S14 ... Determination process S20 ... Reduction process S30: Removal step V: Time change rate of chromium ion concentration Vc: Reference value W: Treated water W1: Primary treated water W2: Secondary treated water W3: Tertiary treated water Z: Decontamination target

Claims (5)

A first addition step of oxidizing and eluting chromium from the object to be decontaminated by adding permanganate as an oxidizing agent containing permanganate ion to treated water in which the object to be decontaminated is immersed, and the first addition step After the step, an oxidation step having a second addition step of changing the pH to the alkali side by adding an alkalizing agent to the treated water without replacing the treated water ;
After the oxidation step, a reduction step the iron and nickel is reduced eluted from the decontamination object by adding an organic acid to the processing in water without replacement of the treated water,
Chemical decontamination methods including: The oxidation process is
After the first addition step, the method further includes a determination step of determining whether the temporal change rate of the chromium ion concentration contained in the treated water is below a predetermined reference value,
The chemical decontamination method according to claim 1, wherein the reduction step is performed when it is determined in the determination step that the time change rate is equal to or less than the reference value.   The chemical decontamination method according to claim 1 or 2, wherein in the second addition step, the alkalizing agent is added until the pH of the treated water becomes 10 or more and 12 or less.   The chemical decontamination method according to any one of claims 1 to 3, wherein the alkalizing agent contains at least one of sodium hydroxide and potassium hydroxide.   The chemical decontamination method according to any one of claims 1 to 4, wherein the object to be decontaminated includes SUS and inconel.

Images