DECONTAMINATION OF PRESSURIZED WATER REACTORS

Abstract: A method of decontaminating radionuclide-contaminated acid insoluble corrosion products from primary system surfaces in pressurized water reactors by oxidation and subsequent dissolution in an acidic decontamination solution of the corrosion products which have been made acid-soluble by the oxidation. The characterizing feature of the method is that the oxidation is carried out with a water-based oxidation agent having a pH below 7 and containing permanganate, chromic acid and ozone. In accordance with a preferred embodiment of the method the oxidation is carried out at relatively low temperature, e.g. under about 60<o>C and often below about 25<o>C.

Description: TITLE OF INVENTION DECONTAMINATION OF PRESSURIZED WATER REACTORS TECHNICAL FIELD • The present invention relates to a method by which radi active coatings on the walls of the primary heating system in nuclear reactors of the pressurized water type can be removed More specifically, the invention relates to the decontaminat¬ ion of in acid insoluble or sparingly soluble corrosion pro¬ ducts from these primary system surfaces. In this respect the invention is a development of the technique that includes a first step wherein the contaminated surfaces are contacted with an oxidation agent, for oxidation of the insoluble pro¬ ducts to acid-soluble oxidation products, whereupon in a sub¬ sequent step the oxidized products are dissolved and removed by means of an acidic decontamination solution. BACKGROUND ART . Corrosion products stemming from the primary heating system, which to a major extent comprises the tubes and pipe¬ lines of the steam generators, are conveyed into the reactor core where they are deposited on the fuel elements. After some time, the corrosion products, which are now radioactive after the neutron irradiation, are liberated from the fuel elements and are subsequently deposited on the parts of the primary system in contact with water which lie outside the reactor core. Then the radioactive corrosion products giv rise to radiation fields outside the core and thereby to radiation doses to the operational personnel. Another cause of the occurrence of radiation fields is fuel element leakage. In case of leakage in the encapsulating material of the fuel elements fission products are leached out by the circulating water. These products are then incor¬ porated in the oxide layers on the parts of the system (pri¬ marily the steam generators) lying outside the reactor core. The.radiation doses received by personnel must be kept within prescribed limits. For reasons of health and operational economy, the doses should of course be kept as low as is reasonably possible. Before undertaking major work on the primary system, it can thus be desirable to remove the radioactive corrosion and fission products which have been deposited on the primary system surfaces. By a partial or complete dissolution of the oxide layers, a substantial portion of the radioactive isoto¬ pes can be removed from the system surfaces. In nuclear reactor terminology this process is denoted decontamination. Most of the known processes within this technology have been described in detail in J.A. Ayres, Ed., Decontamination of Nuclear Reactors and Equipment, the Ronal Press Company, N.Y. (1970). During the years from about 1961 and up to the first years of the seventies, only a very small number of deconta¬ minations of reactor systems were carried out. The most dis¬ cussed decontaminations during this period were those of the Shippingport PWR (PWR = Pressurized Water Reactor) in the. USA and the PWR plant at Greifswald in the GDR. Modified versions of the APAC method developed during 1961 in the USA were used in these decontaminations. There -were two steps in this method, namely a first oxidizing step with alkaline permanganate followed by a secon dissolving step with an acidic decontamination solution con- taining ammonium citrate. Common to all modifications of the APAC process is that the contents of chemicals must be relatively high for accept¬ able decontamination factors to be achieved. The decontaminat ion factor (Df) is defined in the following way: Df radiation field before decontamination radiation field after decontamination In occasional cases where the APAC process has been used, it has been necessary to repeat the decontamination a number of times to obtain a satisfactory result. The radioactive solutions of chemicals from this pro- cess have either been purified by ion exchangers or been treated in special evaporators. The greatest disadvantage wit ^0 " R OM the APAC process is the large volumes of waste occurring in the form of radioactive ion exchangemasses or evaporator residues . The above-mentioned disadvantages have resulted in that during 1970 work was started in several quarters on developin new processes. The aim then was to achieve processes which: provide an acceptable decrease of the radiation field during a treatment time of maximum 36 hours, only require low concentrations of chemicals in the final step by the utilization of continuous regenerat ion of the chemicals with cation exchange, are possible to perform at temperatures below 100°C, give a final waste in the form of ion exchange masses containing all chemicals present, including metals an radio isotopes released or liberated during the pro¬ cess. As to the processes which began to be developed during the seventies and which are used today, it has been found necessary to include a pre-treat ent step with oxidizing reactants. In said pre-treatment step essentially the following ox dizing agents are used : permanganate in an alkaline or nitric acidic environ¬ ment (in the latter case the pH is about 2.5) - potassium hexacyanoferrate in an alkaline environment In the subsequent treatment step there are used almost exclusively organic acids (citric or oxalic acids and ammoniu salts of these) and some strong complex forming agent, e.g. EDTA (ethylenediaminetetraaceticacid) . Additives in the form of reducing agents such as aldehydes or ascorbic acid can also be present in the acid treatment step. The conditions (reducing, high pH) prevailing in a pressurized water reactor are such that the oxide layers form ed will to a large extent have relatively high contents of chromium, partially together with nickel, in the form of oxid or spinel phases. To have these oxide layers dissolved at all in organic acids, it is thus necessary to carry out the OMPI pre-treatment in an oxidizing environment. At present the completely dominating oxidation agent in this respect is permanganate. The reaction sequence for the oxidation step is substantially as follows: 3Mn0 ~ + Cr 3+ + 8H 2 0 "*■ * ■ 3Mn 2+ + 5Cr0 4 2" + 16H + In order to illustrate more in detail the decontami¬ nation effect, which may be obtained by the processes avail¬ able today, reference is made to the following. In all processes available today in Europe, USA and Canada there are at least two treatment steps, one of which is always the above-mentioned pre-oxidation step. All these processes have been tested, partly on a laboratory scale, partly at half or full scale in some cases. The processes worked out in Europe have been tested in two international decontamination projects. These are the Agesta decontaminat¬ ion project in process in Sweden, and the project in process at the Pacific National Laboratories, Richland, Washington, USA. The tests in the USA have been carried out in an authen¬ tic steam generator taken from the Surry-1 ' PWR plant after an approximate operation time of 6 years. In the Agesta pro¬ tect, laboratory tests have been carried out on samples taken from the steam generators in Ringhals-2 (Sweden) , Biblis A (Germany) , Millstone 2 (USA) and from the inlet chamber in one of the steam generators in the Borssele reactor in Holland. The Swedish laboratory tests have been carried out with so-called "soft" processes (i.e. processes where low contents of chemicals are used) developed at: Studsvik Energiteknik AB (Sweden) - Kraftwerk Union (Germany) EIR (Switzerland) BNL (CEGB) (England) The samples from the above-mentioned PWR were of the following materials: - Ringhals-2 Inconel 600 Millstone-2 Inconel 600 Biblis A Incoloy 800 Borssele AISI 304. CMPI In this connection it may be mentioned that the compo¬ sitions of these materials in percent per weight are: Material C Si Mn Cr Ni Mo Fe AISI 304 0.04 0,4 1.2 19 9.5 0.2 residue Incoloy 800 0.02 0.6 0.6 21 33 residue Inconel 600 0.02 0.3 0.8 16 73 residue The results of these tests can be summarized as follows The samples of Inconel 600 were difficult to deconta¬ minate. Decontamination factors exceeding 3 (the lowe acceptable value) could only just be achieved by thre of the four processes. The samples of Incoloy 800 and AISI 304 reached satis factory decontamination factors by a good margin. In the tests in the steam generator from Surry-1 PWR, a process was tested which had been developed in Canada as well as a process similar to the one tested by BNL (CEGB) in the Agesta project. The resultsof the tests show.ed here as well that surfa¬ ces of Inconel 600 were very difficult to decontaminate. Acceptable decontamination factors could be achieved only after several treatment cycles. It should be noted in this connection that a pre-oxidation step with permanganate is in¬ cluded in both these processes. As prior art in this area, even if this art is not utilized in practice today, the art disclosed in the Swedish Patent Application Serial No. 8001827-8 (based on US Serial No. 028 200 filed on April 9, 1979) may also be mentioned. Said patent application describes a decontamination method where the pre-oxidation step is carried out by means of ozone as the oxidation agent. In the subsequent acid dissolving step organic acids and complex forming agents are used at high temperatures such as 85 C and 125 C. In the patent application there are described decontamination attempts on samples pre-oxidized for 7 days (PWR environment at 350°C) and thereafter exposed for 3 months at 250 C in a PWR trial plant. In the trials, decontamination factors with a mean of about 2.7 were obtained for samples of Inconel 600, which o must be regarded as a low value. DISCLOSURE OF THE INVENTION In accordance with the present invention it has surpris ingly been found possible to substantially eliminate the dis- advantages of the previously known art, above all large amoun of secondary waste, low decontamination factors, high content of chemicals and high treatment temperatures, which in turn lead to increased corrosion and high costs, etc. The method according to the invention more specifically involves the utilization of the combination of permanganate, chromic acid and ozone as oxidation agent in the introductory oxidation step. Thus, surprisingly this combination has been found to give a synergistic effect which could not be predicted against the background of the known properties of these oxidation agents taken individually. In the method according to the invention, the contami¬ nated surfaces are brought into contact with the above-mentio ned oxidation agent in an aqueously based form and with an acidic pH, i.e. a pH be-low 7 . This may mean, for example, tha the oxidation agent is present in the form of an aqueous solution of permanganate and chromic acid, and ozone prefe¬ rably in a saturated solution and dispersed form. In accord¬ ance with another embodiment of the method the oxidation agen can however be utilized in the form of a two-phase ozone gas- -water mixture, where ozone in gaseous form is dispersed in water with added permanganate and chromic acid. This in turn means that the ozone addition per se can take place substan¬ tially in accordance with the same principles as in the Swed¬ ish Patent Application Serial No. 8001827-8, which therefore do not need to be repeated here. In addition to the above-mentioned advantages with the invention in relation to the prior art, it has furthermore been found, surprisingly, to be possible to achieve the favourable results at room temperature already, and while using low proportions of the chemicals utilized. This signi¬ fies, of course, an extremely vital contribution to the art in the area, since it is thus possible to save costs thanks to the use of smaller amounts of chemicals, thanks to savings in energy and thanks to reduced corrosion. A particularly pre¬ ferable embodiment of the method in accordance with the inven ion thus means that the oxidation step or the acid dissolving step or preferably both these steps are carried out at room temperature or lower, i.e. at a temperature below about 25 C and preferably below about 20 C. However, very favourable effects in relation to the known art are already obtained in the oxidation step already at a temperature below about 60 C. The oxidation step according to the invention means tha the contaminated surfaces are contacted with the new oxidatio agent for a sufficient period of time to oxidize insoluble oxi¬ des, so as to make these soluble in the acidic decontaminatio solutions used in subsequent steps. The period of time requir ed in each individual case is of course easily determined by one skilled in the art against the background of utilized concentrations of oxidation reagents as well as subsequent acid solutions, utilized treatment temperatures etc. The choice of permanganate is made according to the prior art, i.e. the permanganate is preferably an alkali meta permanganate and particularly potassium permanganate. The origin of the chromic acid is preferably dosed chromium tri- oxide, and the ozone is suitably utilized in the form of an ozone-enriched oxygen gas or air. The concentrations or proportions of the chemicals in¬ cluded in the oxidation agent are determined by one skilled in the art from case to case, so as to obtain the desired results, inter alia depending on the materials which are to be decontaminated and the desired decontamination effect, but generally the concentrations are usually within the range of 0.01-50 g/1, preferably 0.5-2 g/1, of the permanganate, with¬ in the range of 0.01-50 g/1, preferably 0.05-0.2 g/1, of the chromic acid and within the range of 0.001-1 g/1, preferably 0.005-0.015 g/1 of the ozone. The water-based or aqueous oxidation agent ' has prefer¬ ably been made acidic by nitric acid or an aliphatic satur- ated monobasic carboxylic acid, suitably to a pH of about 3, an example of the latter type of acid being acetic acid. Although the concentration of the acid used in the acid dissolving step can be kept low in accordance with the presen invention, even in spite of the fact that the temperature is about or below room temperature, the type of acid is selected in accordance with the prior art. As examples of utilizable acids citric acid and oxalic acid can thus be mentioned. The method in accordance with the invention is generall utilizable for the decontamination of all those different types of materials which are present in these connections. However, the invention has been found to give extremely good results in the decontamination of chromium (III) oxide from a chromium-nickel-iron alloy, such a decontamination there- fore representing an especially preferable embodiment of the invention. The invention will now be described in conjunction with some non-limiting examples. EXAMPLES In a number of decontamination tests carried out on samples of Inconel 600 taken from a steam generator in a PWR after an operation time of about 8 years, which samples were the most difficultly decontaminable ones to be obtained, the samples were treated at room temperature (about 20°C) for 21 hours in an oxidizing solution in accordance with the invent¬ ion. This solution consisted of an aqueous solution, made acidic to pH 3 with acetic acid or nitric acid, of 1 g/1 of potassium permanganate, 0.2 g/1 of chromic acid and 12 g/1 of boric acid to which ozone was continuously supplied. The second step treatment was carried out in an aqueous solution containing 12 g/1 of boric acid and solely citric acid in an amount of about 2 g/1 at a pH of 3,5 (adjusted with NH,) and room temperature (about 20 C) for 6 hours. Contamination factors of 15-20 were obtained at these experiments. In comparative tests carried out on similar samples, i.e. tubes of Inconel 600, with the most effective of the sof processes tested at the Agesta and Surry-1 projects, both requiring an operational temperature of 80-90°C, decontaminat ion factors of merely 2-6 were obtained. In addition to the decontamination tests reported above corrosion tests have been carried out on blank, non-preoxidiz samples of Inconel 600. The test pieces- were 3 pieces of stea generator tubes with lengths of 5 cm. To simulate the con¬ dition in the rolled zone in the tubes of the tube plate thes samples had been rolled internally to half the lengths thereo The cold deformation obtained was about 5%. The three samples were exposed in parallel in a 100 ml glass container in an aqueous solution containing 12 g/1 of boric acid, 1.0 g/1 of KMn0 4 and 0.2 g/1 of Cr0 3 in 1% acetic acid at a pH of about 3. Oxygen gas with about 2.5 percent by volume of ozone was bubbled into the same container at a rate of about 0.1 1/min. The temperature was 20 C and the exposure time was 21 . hours. The corrosion .test was terminated after a subsequent • exposure of up to 6 hours to an oxygen-free 2 % citric acid solution at 20 C and a pH of about 3. The weight losses during this exposure are accounted for in the Table below. A further exposure cycle, identical with the first one was carried out. The weight losses at this exposure are also presented in the Table. The total material loss after two cycles is well below 1 ym, which must be regarded as extre¬ mely satisfactory. No signs of local corrosion have been observed. TABLE Corrosion tests carried out on 3 identical rolled tube samples of Inconel 600 exposed in parallel to each other Exposure times: oxidizing step time hours reducing step time ho.urs 1a 23 2a 6 1b 21.5 2b 33 OMPI Tube Material loss (ym) Material losses samp1e Decont. cycle 1 Decont. cycle 2 during 2 cycles No. Step la Step 2a Step 1b Step 2b (ym) 0 0.4 0.16 0.003 0.56 1 0.36 0.005 0.18 0.003 0.55 2 0.43 0.003 0.21 0.006 0.65

Claims: CLAIMS 1. A method of decontaminating radionuclide-contamina ted acid-insoluble corrosion products from primary system sur faces in pressurized water reactors, especially of decontami¬ nating chromium (III) oxide from a chromium-nickel-iron alloy by contacting the contaminated surfaces with an oxidation agent and dissolving in an acidic decontaminating solution th corrosion products which have been made acid-soluble by the oxidation, characterized by performing the oxidation with a water-based oxidation agent having a pH below 7 and contain¬ ing permanganate, chromic acid and ozone in such proportions that decontamination is obtained. 2. A method according to claim 1, characterized in that the oxidation agent is an acidic aqueous solution of permanganate and chromic acid and ozone in a saturated solution and dispersed form. 3. A method according to claim 1, characterized in that the oxidation agent is a two-phase ozone gas-aqueous mixture, where ozone in- gaseous form has been dispersed in an acidic aqueous solution of permanganate and chromic acid. 4. A method according to any one of the preceding claims, characterized in that the permanganate is potassium permanganate. 5. A method according to any one of the preceding claims, characterized in that the oxidation is carried out ' at a temperature below about 60 C, preferably below about 25°C and especially below about 20°C. 6. A method according to any one of the preceding claims, characterized in that the dissolution of the oxidized products with the acidic decontamination solution is carried out at a temperature below about 25°C, preferably below about 20 C. 7. A method according to any one of the preceding claims, characterized in that the water-based oxidation agent has been made acidic with nitric acid or aliphatic saturated monobasic carboxylic acid to a pH of below 7, preferably to a pH of about 3. "gjREX; O PI • 2 8. A method according to claim 7, characterized in that the aliphatic saturated monobasic carboxylic acid is acetic acid. 9. A method according to any one of the preceding claims, characterized in that the acidic decontamination solution is an aqueous solution of citric acid or oxalic acid 10. A method according to any one of the preceding claims, characterized in that the concentration of permanga¬ nate is 0.01-50 g/1, preferably 0.5-2 g/1, the concentration of chromic acid is 0.01-50 g/1, preferably 0.05-0.2 g/1, and the concentration of the ozone is 0.001-1 g/1, preferably 0.005-0.015 g/1. -gJRE O PI

Inventors: ARVESEN JAN, HERMANSSON HANS-PETER, ARVESEN, JAN, HERMANSSON, HANS-PETER
Applicants: STUDSVIK ENERGITEKNIK AB
Application year: 1984
Main classification: G21F9
IPC codes: G21F9