JPS59164998A - Method of processing oxidation film in pipe circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to the treatment of chromium(I)-containing films, layers or deposits of corrosion products formed on the inner surface of chromium-containing steel piping systems, such as heat exchange systems in nuclear reactors. The present invention relates to a method of oxidizing chromium compounds contained in corroded coatings and the like by oxidizing them with dilute solutions of iron (Vl) salts to render them more susceptible to the action of conventional cleaning and decontaminating agents.

During nuclear reactor operation, high temperature, high pressure cooling water corrodes the wet surfaces of piping, valves, heat exchangers, and core elements. The transport of molten and pulverized materials into and out of the reactor core, bombarded by neutrons, involves radioactive isotopes of certain metals, most notably iron-59, cobalt-58, cobalt-60, chromium-51, and manganese. 54. These radioactive isotopes are mixed into the growing oxide film along with the radioactive fission products resulting from fuel defects and the uranium oxide, and the radioactive isotopes become distributed over the entire surface of the cooling pipe.

This accumulation of radionuclides on the inner surface of pipes exposes those working near areas where cutting or grinding is required and increases the risk of air contamination. Typically, repair or maintenance requires decontamination of the piping system, in which case almost all corrosion products must be removed along with any contaminant and integrated radionuclides to achieve an acceptable decontamination rate. It is necessary. The decontamination rate is defined as the ratio of radioactivity before decontamination to radioactivity after decontamination.

Several studies have been conducted on the composition and structure of oxide films found on the inner surfaces of nuclear reactor piping. The nature of the deposits depends on the composition of the piping and the chemistry of the cooling water.

In a light water-cooled pressurized water reactor (PWR), the total internal surface area is usually comprised of piping of which 10-20% is made of stainless steel type 604. Zircaloy (trademark) fuel cladding and Inconel 600 (trademark)
The remainder of the internal surface area of the steam generator tube can be made up of y equal parts.

The chemical conditions maintained during operation in a PWH are typically reducing. When the base metal material corrodes, metal ions are released into the cooling water and then redeposited on the metal surface to form oxides. Typical PWH corrosion films are generally composed of magnetite, nickel ferrite, and iron chromite (Fe
Contains OCr203). The amount of chromium in the coating is generally between 30 and 40 parts by weight. This type of oxide containing chromium is highly insoluble. The presence of a chromium-rich film severely limits the effectiveness of decontamination solutions. In order to solubilize the chromium-rich film,
It is necessary to oxidize the substantially insoluble chromium (Jl) to the more soluble chromium (Vl). This is accomplished by treating the oxide film in the reactor piping with a strong oxidizing agent prior to the use of conventional cleaning agents.

In a boiling water reactor (BWR), half of the total internal surface area consists of primary piping, typically made of stainless steel type 604, and the other half is made of Zircaloy fuel cladding. Most BWRs operate with slightly oxidizing coolant (up to 200 ppb oxygen). Typical WH corrosion films are generally composed mainly of hematite (Fe203) and some macnetite (Fe203).
a O4) and some nickel ferrite (NiO
Fe203), but very little chromium-containing oxide.

Chromium from basic metal materials is mostly □chromium (■1),
i.e. it is oxidized to a soluble form of chromium. This chromium (Vl) is then removed from the system in a reactor cleaning system with an ion exchange column. On some metal surfaces of the BWR, chromium-rich bands were detected close to the base metal material where the oxygenated coolant does not reach. Up to 20% of the radionuclide concentration in the coating is contained within this chromium-rich layer, and therefore it is essential to eliminate this band to achieve high decontamination rates. Therefore, this treatment of the cooling system with an oxidizer is applicable to both types of light water cooled reactors, and is also applicable to, for example, CANZU (C
ANDU: Trademark) type pressurized heavy water reactor (PHWR)
) can also be applied to coatings of corrosion products in other water-cooled nuclear reactors.

A considerable portion of the plant of the Cyanzu-type heavy water-cooled reactor is constructed from chromium-containing alloys. Both the Inconel 600 steam generator and stainless steel type 410 pressure tube linings contain about 15% chromium in their metals. The reducing conditions of the coolant lead to chromium-rich oxide deposits on metal surfaces.

The radioactivity of the corrosion products in a nuclear reactor makes disposal of the corrosion products difficult once they have been removed from the metal surface. Thus, it is important that all methods of dissolving and removing corrosion coatings require the addition of very small amounts of reagents and provide the removed radioactive corrosion products in concentrated form, preferably in solid form.

In this way, the amount of radioactive waste can be kept to a minimum and energy-consuming concentration operations thereof for dilute solutions can be avoided.

Additionally, it is important that the reagents used in dissolving and removing corrosion products must not be unduly corrosive to the piping systems in which they are used.

Numerous methods have been proposed for removing oxide films from the inner surfaces of piping systems, particularly those of reactor cooling systems. However, only a few of these methods are effective in removing oxide films containing a high proportion of chromium.

A conventional method for removing corrosion products containing oxides of chromium (JI) consists of a two-step process.

The first step involves the use of highly alkaline heated permanganese, acid potassium. Typical concentrations are 4% (w/v) potassium permanganate, 10 parts (w/v) sodium or potassium hydroxide at 80-120C. This treatment is effective in oxidizing chromium (J[) oxide present in the layer to soluble chromium (Vl).

Once the chromium is removed, the remaining iron and nickel oxides can be removed using one of a number of acidic decontamination treatments.

There are several drawbacks to using this alkaline potassium permanganate method. These drawbacks include:

a) The reactor piping system needs to be drained prior to application of the alkaline potassium permanganate solution, ￥)) Reacts with radionuclides during the oxidation and decontamination process, or Either large quantities of chemicals are required that become contaminated with nuclides and therefore require concentration before disposal; C) The system is replaced with several times the volume of fresh water before the second stage. d) The reagent has to be flushed, thus resulting in further waste liquid.
: Trademark) is extremely corrosive to certain alloys.

The present invention treats chromium-containing corrosion products found on internal surfaces of metals, such as those in nuclear reactor cooling systems, with a dilute solution of iron (VD salts, also called ferrates (Vi)) to form a corrosion coating. makes the chromium compounds contained in the chromium compounds more soluble, thus also making conventional cleaning and decontaminating agents such as the agents described in Canadian Patent No. 1.062,590 to Hatcher et al. This treatment involves oxidizing with a dilute aqueous solution of the chromium (II) compound ferrate (Vl) contained in the deposits of these corrosion products. It will be done.

The nature of such corrosion products depends on a) the materials of which the piping is constructed, b) the conditions inside the piping, including the fluidizing medium, pH, temperature, radiation, etc., and c) the age of the piping system. In many nuclear reactors, the piping system is made of P-containing steel. The radioactive corrosion coating present on all internal surfaces of this type of reactor is 1, especially about 100C and about 500C.
When operating at temperatures between p, it naturally contains chromium(II) oxides among other metal compounds.

Such corrosion films are particularly rich in chromium(1) compounds when the chemical conditions within the piping system are reducing. Chromium (jI) oxides are substantially insoluble in conventional cleaning and decontamination agents and cannot be removed by known decontamination methods, such as those described in the Batcher patent. .

Batcher's method is as described below and is referred to as the CAN-DECON (trademark) method. This method involves the addition of an acidic agent to a coolant circulating in a contaminated piping system of a nuclear reactor. The resulting dilute reagent solution solubilizes most of the corrosion products deposited on the internal surfaces of the piping system, especially precipitated iron salts and oxides. The acidic reagent solution is passed through a cation exchange resin to remove dissolved cations, including radionuclides, and the regenerated reagent solution is recycled each time it is needed. Once this decontamination step is complete, the reagent solution is passed through a mixed bed of ion exchange resin to remove the reagent from the coolant.
The coolant is thus regenerated.

Typically, enough reagent is added to the coolant to give a concentration of: o, i% (wt/vol) and the resulting reagent solution is circulated for 6 to 24 hours at 1'2 DC. be done.

Under these conditions, the chromium (ffi) compounds contained in the corrosion product deposits are virtually insoluble.

To remove chromium-containing deposits, chromium(II)
An oxidation treatment is required to convert the chromate to a more soluble chromate.

The treatment of chromium-containing corrosion products with ferrate (Vl) as an oxidizing agent according to the invention has several advantages over known oxidation methods. That is, ferrate (Vl) is a strong oxidizing agent and its dilute solution oxidizes chromium (IN) to chromium (Vl ) in basic or neutral medium, whereby ferrate mainly contains iron (1 ) was found to be reduced to Ferrates can be added directly to aqueous fluids that are normally circulated through the piping system, such as the coolant in the heat exchange system of a nuclear reactor. According to the invention, the products formed in the oxidation step and unreacted ferrates can be removed from the aqueous fluid by passing it through ion exchange resins and, if necessary, filtration means. , the fluid can be regenerated on the spot. Thus, the steps of draining the fluid, replacing the fluid with an oxidizing solution, and latching the piping system after oxidation and solubilization have occurred can be avoided. As a result, it becomes possible to reduce the downtime of the system.

This is particularly important in the case of nuclear reactors. Pretreatment of a nuclear reactor piping system according to the present invention requires shutting down the reactor and depressurizing and cooling the coolant. However, it does not require the removal of reactor fuel and the replacement of the coolant with an oxidizer bath liquid. Therefore, the method of the present invention does not require the use of radioactive oxidizing and cleaning solutions as well as cleaning solutions. Because there is no need for contact, it not only reduces reactor out-of-operation times, but also reduces the volume of radioactive waste.Dissolved deposits and associated radioactivity are retained in the resin and in the filter. Ru.

According to one aspect of the present invention, a method is provided for oxidizing chromium-containing corrosive compounds deposited on the interior surfaces of piping systems in which aqueous fluids circulate. This method uses a circulating aqueous fluid with pH
ferrate ('VI) to form a dilute ferrate solution. Ferrates react with chromium compounds contained in corrosion products. The dilute ferrate solution can be circulated until the chromium salt concentration in the solution approaches a stable value. The fluid is purified by passing the dilute ferrate solution through an ion exchange resin and filtration means.
can be manufactured.

According to a second aspect of the invention, a method is provided for decontaminating a nuclear reactor piping system in which an aqueous coolant is circulated. This method involves adding an acidic detergent to the circulating coolant to form a dilute detergent solution, which is then circulated to react with deposits of corrosion products on the interior surfaces of the piping system, resulting in corrosion formation. It consists of regenerating the cleaning agent solution by removing the material, recycling the regenerated cleaning agent solution, and then removing the cleaning agent from the coolant. The improvement of this invention consists of pre-treating deposits of corrosion products in the piping system with ferrate (l) prior to addition of the acidic detergent. ferrate < while keeping between 7 and 14.
Vl > to form a dilute ferrate solution and continuing to circulate the dilute ferrate solution to oxidize chromium compounds contained in the corrosion product deposit.

The ferrate is placed in the circulating fluid at a temperature of about 80C or less;
Preferably it is added at a temperature between about 15C and about 8DC, more preferably between 45° and 60C. The fluid is adjusted to a pH of between 7 and 14, preferably between about 9 and 10, and most preferably about 10. At this acidic pH range and higher temperatures, the appropriate ferrate decomposes according to the formula 2.

The concentration of ferrate in the fluid should be at least about 0.01% (weight/volume) calculated as FeO4''-, with a preferred range between about 0.01% and about 0.5
%, with a more preferred range between 0.05% and 0.05%.
2, with the most preferred concentration being about 0.1. The ferrate-containing fluid is generally recycled until the rate of solubilization of the chromium compounds is close to zero.

This takes about 10 minutes to about 10 hours. Under preferred conditions, IEJi times of between about 6 hours and about 6 hours are usually suitable. Additional amounts of ferrate and/or acid or alkali may be required from time to time during the reaction to maintain the desired ferrate concentration and pH.

Ferrate 171, which is soluble in aqueous fluids, is suitable for this oxidation treatment. Examples of preferred ferrates include potassium ferrate and other alkali metal ferrates and alkali metal ferrates.

Most preferred is heptalium ferric oxide (K 2 Fe O4).

The circulating fluid may also contain compounds that tend to increase the stability of ferrates, such as certain carbonates and phosphates, and/or promote reactions between ferrates and oxide deposits. It may contain a compound that

The products formed in the oxidation process according to the invention, which are primarily ferric oxides and chromates, as well as unreacted ferrates, are removed by passing the fluid through filtration means and ion exchange means as described above. It can be removed by
The coolant can thus be regenerated. If desired, unreacted ferrate can be converted to iron(1) oxide by heating or by addition of acid.

The fact that very small amounts of ferrate have to be added facilitates the regeneration of the fluid, reduces the amount of radioactive solids formed, and at the same time reduces the cost of the process.

A further decontamination step, such as a can-decon process, may be performed after regeneration of the fluid.

Alternatively, decontamination agents, such as those used in the can-decon process, can be added directly to the spent ferrate solution containing oxidation products.

The Can-Decon process reagents react with the coating of corrosion products in the furnace piping system, dissolve salts and oxides precipitated during the oxidation pretreatment, and destroy excess ferrate. Cation exchange resins can be used to remove solubilized iron salts, etc., and anion exchange resins or mixed bed ion exchange resins can be used to remove all other contaminants, including the reagents themselves. They can be used to regenerate fluids.

Using this preferred method of coupling, there is no waste; instead, all dissolved deposits, as well as any contained, integrated radioactivity, are retained in the ion exchange column and on piping surfaces, pump and valve elements, and wicks. Keep the internal body and coolant clean (.

This ion exchange waste can be handled by conventional operations known to those skilled in the art.

When using the ferrate process according to the invention in combination with the can-decon process, it is possible to achieve decontamination rates of over 100, but in most cases contamination removal rates are in the range of about 5 to 25.

The effectiveness of the total decontamination process depends on the composition of the corrosion film. The proportion of chromium in the oxide film varies widely depending on, for example, operating conditions, materials, and age of the piping system.

Visual inspection of surfaces treated with ferrate according to the invention as well as measurements of the corrosion rate of the surfaces showed that corrosion was very low due to the treatment with ferrate.

According to a preferred embodiment of the invention, deposits of chromium-containing corrosion products on the internal surfaces of a PWR--its heat transport system is constructed from stainless steel and Inconel 600--remove the furnace from operation and pressure and about 6
It can be removed by cooling to 0C. With operation of the main recirculation pump, a concentrated solution of potassium ferrate is added via the chemical injection pump directly to the main coolant until a reagent concentration of approximately 0.1% Fe042- (wt/vol) is reached. The pH of the dilute aqueous solution can be maintained at a constant value of about pHID. Additional reagents, acids or alkalis, are added from time to time as needed to keep both ferrate concentration and pH constant.

After a period of up to 10 hours - during which time the chromium concentration in the coolant is periodically checked - the amount of solubilized chromium reaches an approximately constant value, i.e. the rate of removal of chromium from the corrosion film becomes zero. Get closer. The most effective decontamination is generally achieved when the preferred F e O4'' concentration is maintained throughout the process.

The coolant is first passed through a filter to remove particulate matter such as iron (Jl) oxide, and then passed through a mixed bed ion exchange resin to remove chromate, unreacted ferrate, etc. In this way, the coolant can be regenerated, while the piping system can be directly subjected to further cleaning methods such as can-decon treatment. There is no need to flush the piping system or replace the coolant.

From the foregoing description, it will be seen that the present invention provides a simple and rapid oxidation pretreatment method for the decontamination of piping systems, particularly those of heat transfer systems in nuclear reactors.

The invention will be further illustrated by the following experimental results.

It should be noted that these examples are merely illustrative and should not be construed as limiting the present disclosure.

Example 1 Can-decon process 2 operating BWRs and 6 operating P
A sample fragment was taken from the piping of the primary cooling system of the WH. BWR
The samples from BWR, designated as (A) and BWR (B), are pieces of stainless steel type 604, and also PWR (G), PWR (D) and PWR (E
) The sample from the PWR was a fragment of Incononel 600 steam generator tubing. Test specimen BWR (
The corrosion deposits in specimen BWR (B
The corrosion deposits in ) contained chromium-rich bands adjacent to the base metal material.

The corrosion deposits on all PWR specimens contained large amounts of chromium. The PWR (c) test specimen was manufactured by Compassion Engine 1 (Combustion Engine 1).
ering Inc,
The test specimen for E) was manufactured by Westinghouse.
It was obtained from a nuclear reactor related to the construction of GHUSE. The main difference between the two types of specimens was the relative thickness of their oxide coatings and the radioactivity associated with these coatings. The PWR (D) and (E) specimens are more radioactive and account for differences in the operating time of each reactor and possible slight differences in the chemical conditions maintained in the reactor during that operating time. As a result, the corrosion film was generally thicker and more firmly fixed than the PWR (G) specimen.

Sample sections of piping were subjected to various decontamination treatments in a test loop. The loop was made of stainless steel tubing and contained approximately 10Δ deionized water as the circulating fluid. The loop was equipped with a pump to circulate water and dissolved reagents into the closed loop. The test equipment is capable of measuring the flow rates, pressures, temperatures, and p
It was designed to reproduce H and conductivity extremely well.

The radioactivity of the sample fragments was measured by holding the sample 10-20 cIIL away from a native germanium gamma counter. The signal from the counter is Can is La Series 8 (C
The samples were analyzed using a nuclear analyzer called Anberra 5eries'8 (trademark), and then processed using a computer PO-11 (trademark). The combinator was programmed to deliver radioactivity of the appropriate isotope in microcuries.

After measuring the radioactivity of the samples, the four types of specimens were processed according to the Candecon method. LND-101 (trademark) was used as the acidic agent in each case.

LND-1[)1 contains about 40% ethylenediaminetetraacetic acid, 60% oxalic acid, and 60% citric acid.

This acidic agent was added to water until its concentration reached 0.1%. For the PWR (c) and (D) specimens, the temperature was kept at 1200 and the treatment continued for 6 hours. The BWR (A) test specimen in Table 1 was heated at a temperature of 125C for 6 hours.
The test specimens of BWR (B) in Table 1 were each held at 135C for 24 hours. The fluid is cation exchange resin Amberlit)IR-120(H,)('Amberlit)
e: Trademark) during the 6 hour period. The reagents were then removed using a mixed bed ion exchange resin (Amberley) IRN-150. The final radioactivity was measured and the decontamination rate was determined. The results are shown in Table 1.

BWR is achieved by treatment with acidic chemicals using this Candecon method.
It is believed that samples of the material are much more effectively decontaminated than samples of PWR material. Test specimen (A) shows the highest contamination removal rate. The decontamination rate of specimen (B) is lower, mainly due to the fact that this sample contains chromium-rich bands. The contamination removal rates obtained for two different samples of PWR material were very low.

These results indicate that the Can-Decon method reagent alone produces chromium (Il[)!] under reducing conditions in the cooling system of most PWHs. It does not remove much of the J-notch coating, thus proving that the can-decon method clearly cannot decontaminate PWH materials as effectively as it removes corrosion coatings from BWR materials.

Table 1 Comparative example of the effect of can-decon treatment on PWR materials and BWR materials 2 Comparison of ferrate pretreatment and permanganate pretreatment PW
To determine the effectiveness of the oxidation pretreatment in removing chromium-rich corrosion deposits, it is possible to include a second treatment step that can dissolve the iron oxides and associated radionuclides. is necessary.

The can-decon method can be used for this purpose. As shown in Table 1, when used without pretreatment,
Can-decon process agents and most other non-oxidizing agents are ineffective in removing rom-rich corrosion coatings such as the deposits that form in PWR cooling systems. It follows that the improvements in decontamination rates of pipes that have been subjected not only to can-decon treatment but also to oxidation pretreatment are directly attributable to the oxidation pretreatment. Tables 1 and 1 show the effect of oxidation pretreatment on samples from multiple PWRs. The radioactivity of ticR samples pwn(c) and pwR(D) (Example 1) from two different PWRs was measured. Subsequently, the sample is pretreated with ferrate according to the invention (method) or with J.
Ni Aires (J, A.

Ayres) is ``Decontamination of Nuclear Reactors and Equipment (D
eco-contamination of nuclei
ar Reactors and Equipment
t)J (The Ronald Press)
Press Go, New York
ork) 1970] with alkaline permanganate (method B).

In Methods A and B, the samples were placed either in a test loop with circulating fluid (see Example 1) or in a glass beaker equipped with a stirrer to stir the fluid. Deionized water was used as the fluid.

In Method A, the fluid was maintained at the temperature shown in Table 1 and column 6 of Table 1 for each sample.

For K2FeO4, the final reagent concentration in the fluid was 0.01% (sample 1 and 2 in Table 1) or 0.1% in weight/volume concentration.
(Samples 6 to 5 in Table 1 and Samples 1 to 4 in Table 1). The pH of the dilute aqueous solution is constant p
It was kept at H10. Additional acid or alkali was added from time to time to maintain the pH. The concentration of ferrate did not remain constant and decreased over time. After the time specified in Table 1 and column 4 of Table 1, the fluid is passed through a mixed bed ion exchange resin (Amberley) IRN-150 to remove chromate, unreacted ferrate, etc. or, for convenience, drain the loop or beaker and
Filled with water again.

In method B, the fluid was heated to 100C.

Potassium permanganate and sodium hydroxide were added until their concentrations reached 6% (w/v) and 10% (w/v), respectively. After the time specified in Table 1 and column 4 of Table 1, the loop was drained, flushed with fresh water, and filled with fresh water.

New fluid (Method B) or regenerated fluid (Method A)
Add the can-decon method reagent to the PWH
Samples were treated at 120C for 6 hours by the can-decon method described in Example 1. All sample sections were placed into the test loop for can-decon processing.

The final radioactivity of each sample was measured and the contamination removal rate was determined.

For sample 4 in Table 1, after completion of the can-decon process, the purified fluid was cooled to about 60C and the process repeated, followed by a second can-decon process.

Samples 6 and 7 in Table 1 were not pretreated. Sample 6 was subjected to the can-decon method once, and sample 7 was subjected to the can-decon method twice.

As can be seen from Table 1, in the case of PWR(c) material - the radioactivity was relatively low - both the ferrate-pretreated and permanganate-pretreated samples They show a significant improvement in their decontamination rates compared to that of the test specimen PWR(G). Even sample 2, which was pretreated with a very dilute ferrate solution, shows a very high decontamination rate. These results indicate that ferrate pretreatment is extremely effective, especially considering that only very low concentrations (0.01-0.1%) and relatively low temperatures (60 C) are required. It is shown that. In contrast, permanganate pretreatment requires a 16% solution and a temperature of 100C.・As shown in Table 1, P
The WR(D) material showed extremely high initial radioactivity.

When compared to Samples 6 and 7 and the PWR(D) specimens of Table 1, Samples 1-5 exhibit improved contamination removal rates.

The total decontamination rate for the PWR (D) specimen is PWR (
lower than that of the test specimen in a). This is PWR(D)
This may be because the corrosion deposits on the specimen are thicker than those on the PWR(C) specimen. Oxidizing agents dissolve chromium deposits in the surface layer, but cannot dissolve iron oxides. These are removed using the can-decon method. Therefore, the effectiveness of the oxidation treatment is limited to the first few micrometers of the corrosion film. As a result, a thick corrosion film makes it advantageous to subject it to two successive ferrate/candecon treatments. The surface layer of the corrosion film is oxidized and removed in the first ferrate/can-decon treatment, and the remaining corrosion film undergoes the effects of oxidation in the second ferrate pretreatment step and oxide removal in the second can-decon treatment. Make it easier to receive. This will be explained using samples 2 and 4. Both samples
Sample 4 was processed under identical conditions except that sample 4 was subjected to a second ferrate/candecon treatment. The contamination removal rate of sample 4 obtained was 50% higher than that of sample 2.

Table 5 and Table 1 show that for PWH materials, pre-treatment of the sample with dilute ferrate solution considerably improves the contamination removal rate compared to that which can be obtained by treatment with the can-decon method alone. It clearly indicates that Furthermore, these results demonstrate a significant effect of ferrate treatment compared to treatment with much higher concentrations of alkaline permanganate.

Because of its high concentration, alkaline permanganate is much more difficult to remove from fluids than ferrate. Thus, in order to perform alkaline permanganate treatment in conjunction with a cleaning method such as the Can-Decon process, the fluid must be passed through a large volume of ion exchange resin (approximately 100 times the volume required for ferrate removal). Otherwise, the system must be drained and flushed, resulting in large amounts of radioactive waste.

Example 6 Measurement of corrosion rate A specimen of Inconel 600 and a specimen of stainless steel type 304 (304SS) were weighed and subjected to one of the following treatments.
I attached it to.

(1) Candecon treatment according to Example 1 under the conditions of 0.6% reagent concentration and 135C for 124 hours; (2) The method of Example 2 under the conditions of ferrate 0.1% and 6CI for 6 hours. ferrate treatment with <1) followed by can-decon treatment with (3) potassium permanganate 4 parts, NaOHI 0%,
Alkaline permanganate treatment according to method B of Example 2 under conditions of 100C 16 hours, followed by candecon treatment in (1).

After treatment, the scale is removed, the specimen is reweighed, the weight loss per treatment time and surface area is determined, and the corrosion rate (
μ7n/hour) was calculated. The results shown in Table 2 are the average of 4 samples.

From the results in Table ■Table ■, 304SS and Inconel 600
It can be seen that the corrosion rates of both samples are virtually the same whether the samples were pretreated with ferrate or not. Naturally, the small amount of corrosion that occurs is mostly due to the Keyyan decon treatment.

Example 4 In Example 2, pretreatment of PWR specimens with ferrate according to Method A included potassium ferrate in the circulating fluid, typically an initial Fe4-9 degree of 0.1% ( weight/volume). In this method, the effective ferrate concentration after 1-2 hours is much lower than the initial concentration (...this is mainly due to oxidation reactions and decomposition of the reagents.

In the following series of experiments (results are shown in Table 7):
New reagents were added as needed to maintain the desired ferrate concentration throughout the ferrate treatment.

PW referred to as PWR(E) specimen (see Example 1)
The radioactivity of samples from R was measured. Subsequently, the sample is subjected to a pretreatment with ferrate according to the invention or to J.N.
It was subjected to a pretreatment with alkaline permanganate as described by Aires (see Example 2).

The sample was placed in a test loose with fluid circulating as described in Example 1.

For ferrate pretreatment, the fluid was maintained at the temperature indicated in column 6 of Table 7 for each sample. In this fluid, 2Fe0
4, the final reagent concentration was 0.1% in weight/volume concentration (Sample 2
．． 6 and 4) or 0.5% (sample 5). The pH of this dilute aqueous solution was kept constant at pH 10. Additional acid or alkali was added from time to time as necessary to maintain this pH, and additional ferrate was added to maintain the desired ferrate concentration. After the time indicated in column 4 of Table 7, the fluid was passed through a mixed bed of ion exchange resin, or, for convenience, the loop was drained and refilled with water. The fluid was heated to 10 DC for permanganate pretreatment. Potassium permanganate and sodium hydroxide are added when the concentration of potassium permanganate is 4.
% (weight/volume), and the sodium hydroxide concentration is 10% (
weight/volume). After 6 hours, the loop was drained, flushed with fresh water, and filled.

To this new or regenerated fluid was added the reagents of the Can-Decon method until its concentration reached 0.6%, and the sample of PWR(E) was treated with the Can-Decon method described in Example 1 for 24 hours at 165C. did.

The final radioactivity of each sample was measured and the contamination removal rate was also determined. The corrosion rate was determined using the same method as in Example 6.

Sample 6 was not pretreated before being subjected to the can-decon process.

The initial radioactivity, final radioactivity and corrosion rate shown in Table 7 are the average of two samples.

Table 7 shows that pretreatment with dilute ferrate solutions at substantially constant ferrate concentrations decontaminates radioactive PWR(E) samples very effectively. Ferrate pretreatment was performed along with can-decon treatment to reduce radioactivity by approximately 95 to 99 times for each sample (Samples 2-4).
. The decrease in radioactivity due to Candecon treatment was less than 20% (Sample 6). PW at 0.5% ferrate concentration
Treatment on the R(E) material did not improve contamination removal rates (Sample 5), but tended to be slightly less efficient than treatments at lower ferrate concentrations. Concentrated alkaline permanganate pretreatment is performed in conjunction with candecon treatment to reduce the radioactivity of the sample to approximately 99-99%.
5% (sample 1).

Thus, a pretreatment of PWR(E) material for 6 hours at 45 tl' with a ferrate solution held at a concentration of 0.1:10 qb permanganate in a 10qb sodium hydroxide solution is 1ooot:' It was virtually as effective as the 6-hour treatment.

As can be seen from Table 7, column 8, there is no substantial effect on the overall corrosion rate of the PWR(E) material.

The small amount of corrosion that occurs is due to the Ca/Decon treatment on each sample.

Patent applicant: Ronton Nookdoor Limited 9 (4 others)

Claims (1)

[Claims] (1) A dilute ferrate solution is formed by adding ferrate (Vl '') to an aqueous fluid for circulation in a piping system while maintaining the pH between 7 and 14; A method for oxidizing chromium-containing corrosion products deposited on the inner surface of a piping system in which an aqueous fluid is circulated, characterized by reacting chromium compounds contained in the corrosion products deposited on the inner surface of the piping system. (2) The dilute ferrate solution is continued to be circulated while maintaining the ferrate in it at an effective concentration until the chromium concentration in the solution approaches a stable value, and the aqueous fluid is passed through an ion exchange means and an f-filter. A method according to claim 1, further comprising the step of purifying the dilute ferrate bath by passing it through a means. (3) maintaining the temperature of the dilute ferrate bath at 80C or less; The dilute ferrate solution is at least 4
ferrate according to claim 1, having a F e O4''-concentration of 0.01% (weight/volume) and wherein the ferrate is selected from water-soluble ferrates (Vl). Method: (4) Maintaining the temperature of the dilute ferrate solution at or below 80C, wherein the dilute ferrate solution has a temperature of at least 0.
2. A method according to claim 2, wherein the ferrate is selected from water-soluble ferric (Vl') salts. (5) The method of claim 1, 2, or 3, wherein the dilute ferrate solution further comprises a stabilizing compound. (6) the dilute ferrate solution has a FeO,i''-concentration between 0.01% and 0.5% (w/v); Section (2) or Section (
The method described in section 3). A method according to claim 1, 2 or 3, wherein the pH of the dilute ferrate solution is maintained between 9 and 10. A method according to claim 1, 2 or 3, wherein the temperature is maintained between 45 C and 60 C. (9) The dilute ferrate solution is % and 0.2% (weight/volume) of FeO4"-concentration between
The method described in section. aψ The method according to claim 1, 2 or 3, wherein the ferrate is selected from the group consisting of sodium ferrate and potassium ferrate. I The dilute solution contains 0.1% (wt/vol) FeO
The method according to claim 1, 2 or 3, wherein the ferrate is potassium ferrate. The method of claim (1), (2), or (3), wherein: (2) the rate of chromium removal from corrosion products deposited with said dilute ferrate solution approaches zero; The method according to claim 1, wherein the dilute ferrate solution is circulated continuously for 10 minutes to 10 hours. ) or (4).αω Next step a) Water-soluble ferrate (Vi) is added to the circulating coolant of the reactor piping system, and the pH of the resulting dilute ferrate solution is 7 and 1
4 and while maintaining its temperature at or below 80C to form a dilute ferrate solution having a FeO4'' concentration of at least 0.01% (wt/vol), and b) The dilute ferrate solution is continued to be circulated until the rate of removal of chromium from corrosion products deposited on the surfaces of the piping system approaches zero. Chromium (
1) Method of oxidizing components. (161) of said claim further comprising the step of regenerating said coolant by passing said coolant through an ion exchange means and a filter means to remove fines and dissolved oxidation products and unreacted ferrates. The method according to paragraph αω: (17) the ferrate is selected from the group consisting of sodium ferrate and potassium ferrate, and the dilute ferrate solution contains between 0.01% and 0.5% F. eO4 concentration, the pH is maintained between 9 and 10, and the temperature is between 15 tl' and 8
A method according to claim 0 or 00, wherein the method is maintained between 0C and 0C. (c) said patent wherein said ferrate is potassium ferrate, said dilute ferrate solution is maintained at a F e O4 concentration of 0.1%, and said temperature is maintained between 45C and 60C. The method according to claim 19 or 16. The method according to claim 15, wherein the dilute ferrate solution further contains a compound that increases the stability of ferrates
The method according to paragraph 1 or paragraph 00. (2DI) An acidic detergent is added to the circulating aqueous coolant of the reactor piping system to form a dilute detergent solution, and the detergent solution is circulated to form chromium-containing corrosion on the internal surfaces of the piping system. regenerating a detergent solution by reacting with material deposits and removing corrosion products from the detergent solution, recycling the regenerated detergent solution, and then removing the detergent from the coolant. in a method for decontaminating a piping system of a nuclear reactor having deposits of chromium-containing corrosion products on its internal surfaces and in which an aqueous coolant is circulated, prior to the addition of said acidic detergent. The pretreatment process consists of pre-treating the deposits of corrosion products in piping systems with ferrate (Vl) while maintaining the pH between 7 and 14 in the circulating coolant. ) to form a dilute ferrate solution and continuing to circulate the dilute ferrate solution to oxidize chromium compounds contained in the corrosion product deposit. (21) The method according to claim 2, further comprising the step of regenerating the coolant prior to adding the acidic detergent to the coolant. (c) The temperature of the dilute ferrate solution is continued until the rate of removal of chromium from the deposited corrosion products approaches zero. Claims 120) and (2) in which the
The method described in paragraph υ or paragraph (2). Claims 2, 00 or C1, wherein the dilute ferrate solution has a FeO4"-concentration of at least 0.01% (wt/vol). (c) The method according to claim (2), wherein the temperature of the dilute ferrate solution is maintained between 15C and 80C. (c) The dilute ferrate solution is [] , O0% and 0.5% (
(27) The ferrate salt comprises sodium boferrate and potassium ferrate. Claim No. (23) selected from the group
(c) The method according to claim 1 or (2e), wherein the circulation of the dilute ferrate solution is continued for 1D minutes to 12 hours. (c) Regeneration step of the cooling liquid (passing the dilute ferrate solution through a mixed bed ion exchange resin system to remove corrosion products as described in Section Cυ, Section (2a) or Section (20)). Method. (To) Adding an acidic detergent to a circulating aqueous coolant in a nuclear reactor piping system to form a dilute detergent solution, and circulating the detergent solution to eliminate chromium on the internal surface of the piping system. reacting with deposits of corrosion products contained therein, passing the detergent solution through a cationic ion exchange resin to remove dissolved corrosion products and regenerating the detergent solution, and recycling the regenerated detergent solution. , then passing the detergent solution through a mixed bed ion exchange resin system to remove the detergent from the coolant, with deposits of chromium-containing corrosion products on the internal surface, and the aqueous coolant being circulated. A method for decontaminating a piping system of a nuclear reactor in which corrosion product deposits in the piping system are pretreated with ferrate (v■) prior to addition of the acidic detergent. The pretreatment method consists of adjusting the circulating coolant to a pH of 9.
and 10C, and while keeping the temperature between 45C and 60C, add potassium ferrate to 0.1% (wt/vol).
forming and maintaining a dilute ferrate solution having a FeO4"-concentration of 01) Claims (2), 00 or 00, wherein the dilute ferrate solution further contains a stabilizing compound. The method described in.