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 concurrent 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 at relatively low temperatures with a water-based oxidation agent having a pH below 7 and containing cerium nitrate, chromic acid and ozone.

Description: TITLE OF INVENTION DECONTAMINATION OF PRESSURIZED WATER REACTORS TECHNICAL FIELD The present invention relates to a method by which r active coatings on the walls of the primary heating system nuclear reactors of the pressurized water type can be remo More specifically, the invention relates to the decontamin ion of in acid insoluble or sparingly. soluble corrosion pr ducts from these primary system surfaces. In this respect invention is a development and simpli ication of the techn that includes a first step wherein the contaminated surfac contacted with an oxidation agent, for oxidation of the in le products to acid-soluble oxidation products, whereupon subsequent step the oxidized products are dissolved and re by . means of an acidic decontamination solution. BACKGROUND ART OF THE INVENTION Corrosion products stemming from the primary heating system, which to a major extent comprises the tubes and pi lines of the steam generators, are conveyed into the react core where they are deposited on the fuel elements. After some time, the corrosion products, which are n radioactive after the neutron irradiation, are liberated f the fuel elements and are subsequently deposited on the pa of the primary system in contact with water which lie outs the reactor core. Then the radioactive corrosion products rise to radiation fields outside the core and thereby to radiation doses to the operational personnel. Another cause of the occurrence of radiation fields fuel element leakage. In case of leakage in the encapsulat material of. the fuel elements fission products are leached out by the circulating water. These products are then inco 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 ke 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, can thus be desirable to remove the radioactive corrosion a fission products which have been deposited on the primary system surfaces. By a partial or complete dissolution of th oxide layers, a substantial portion of the radioactive isot 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 bee described in detail in J.A. Ayres, Ed., Decontamination of Nuclear Reactors and Equipment, the Ronal Press Company, N. (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 th Shippingport PWR (PWR = Pressurized Water Reactor) in the U and the PWR plant at Greifswald in the GDR. Modified version of the APAC method developed during 1961 in the USA were us in these decontaminations. There were two steps in this method, namely a first oxidizing step with alkaline permanganate followed by a sec dissolving step with an acidic decontamination solution con taining ammonium citrate. Common to all modifications of the APAC process is th the contents of chemicals must be relatively high for accep able decontamination factors to be achieved. The decontamin ion factor (Df) is defined in the following way: nf - ra at ^ on ^ e -'- c ^ ^ e f° re decontamination radiation field after decontamination In occasional cases where the APAC process has been used, it has been necessary to repeat the decontamination 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 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 th during 1970 work was started in several quarters on develop new processes. The aim then was to achieve processes which: provide an acceptable decrease of the radiation fie during a treatment time of maximum 36 hours, only require low concentrations of chemicals in the final step by the utilization " of continuous regener 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 mass containing all chemicals present, including metals radio isotopes released or liberated during the pro cess. As to the processes which began to be developed durin the seventies and which are used today, it has been found necessary to include a pre-treatment step with oxidizing reactants. In said pre-treatment step essentially the following dizing agents are used : permanganate in an alkaline or nitric acidic enviro ment (in the latter case the pH is about 2.5) - . potassium hexacyanoferrate in an alkaline environme In the subsequent treatment step there are used almos exclusively organic acids (citric or oxalic acids and ammon salts of these) and some strong complex forming agent, e.g. EDTA (ethylenediaminetetraacetic acid) . ' Additives in the for 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 fo ed will to a large extent have relatively high contents of chromium, partially together with nickel, in the form of ox or spinel phases. To have these oxide layers dissolved at al in organic acids, it is thus necessary to carry out the 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: 3MnO~ + Cr 3+ + 8H 7 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 whic 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 decontamina ion project in process in Sweden, and the project in proce at the Pacific National Laboratories, Richland, Washington, USA. The tests in the USA have been carried out in an auth tic steam generator taken from the Surry-ϊi PWR plant after an approximate operation time of 6 years. In the Agesta pr ject, laboratory tests have been carried out on samples ta 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 w so-called "soft" processes (i.e. processes where low conte 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:s were of t following materials: - Ringhals-2 Inconel 600 Millstone-2 Inconel 600 Biblis A Incoloy 800 Borssele AISI 304. In this connection it may be mentioned that the comp 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 0.5 0.8 16 73 residue The results of these tests can be summarized as foll The samples of Inconel 600 were difficult to decon minate. Decontamination factors exceeding 3 (the l acceptable value) could only just be achieved by t of the four processes. The samples of Incoloy 800 and AISI 304 reached sa factory decontamination factors by a good margin. In the tests in the steam generator from Surry-II PWR process was tested which had been developed in Canada as w as a process similar to the one tested by BNL (CEGB) in th Agesta project. The resultsof the tests showed here as well that sur 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 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 Swedi Patent Application Serial No. 8001827-8 (based on US Seria 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 oz 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 trials on samples pre-oxidized for 7 days (PWR environment at 350°C) and thereafter exposed for 3 months at 250°C in a PWR tria plant. In the trials, decontamination factors with a mean about 2.7 were obtained for samples of Inconel 600, which must be regarded as a low value. DISCLOSURE OF THE INVENTION In accordance with the present invention it has surpr ingly been found possible to substantially * eliminate the di 5 advantages of the previously known art, above all large amo of secondary wastes, low decontamination factors, high conte of chemicals and high treatment temperatures, which in turn lead to increased corrosions and high costs, etc. The method according to the invention which is preferably performed in " ■• ) step only, more specifically involves the utilization in an dic, preferably nitric acidic, aqueous solution of the comb ion of ceriunι(4)nitrate, chromic acid and ozone. Thus, surpri ly this combination has been found to give a synergistic effect which c not be predicted against the background of the known proper 15 of these oxidation agents taken individually. In the method according to the invention, the contami nated surfaces are brought into contact with the above-ment ned oxidation agent in an aqueously based form and with an acidic pH, i.e. a pH below 7. This may mean, for example, t 20 the oxidation agent is present in the form of an aqueous solution of cerium nitrate 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 ag can however be utilized in the form of a two-phase ozone ga 25 -water mixture, where ozone in gaseous form is dispersed in water with added cerium nitrate and chromic acid. This in tur means that the ozone addition per se can take place substan tially in accordance with the same principles as in the Swe ish Patent Application Serial No. 8001327-8, which therefor 30 do not need to be repeated here. In addition to the above-mentioned advantages with th 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 35 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 thank to the use of smaller amounts of chemicals, thanks to savi in energy and thanks to reduced corrosions. A particularly ferable embodiment of the method in accordance with the in ion thus means that the decontamination is carried out at temperature or lower, i.e. at a temperature below about 25 and preferably below 20 C. However, very favourable effect relation to the known art are obtained in the decontaminat already at a temperature below about 60°C. The decontamination according to the invention means the contaminated surfaces are contacted with the acidic so ion with the new oxidation agent for a period of time su ficient to oxidize insoluble oxides, so as to make these soluble in the same solution. The period of time required each individual case is of course easily determined by one skilled in the art against the background of utilized conc trations of oxidation reagents, utilized treatment tempera res etc. A water-soluble cerium salt has oxidizing properties only when the cerium ion is present in its highest oxidati stage, viz. Ce , while the pH of the solution is preferab about 1. When selecting the cerium salt Ce(5)nitrate is pr ferably used which in contact with the ozone is immediatel oxidized to Ce(4)nitrate. The origin of the chromic acid is preferably dosed chromium trioxide, and the ozone is suitably utilized in t form of an ozone-enriched oxygen gas or air. The concentrations or proportions of the chemicals i cluded in the oxidation agent are determined by one skille in the art from case to case, so as to obtain the desired results, inter alia depending on the materials which are t be decontaminated and the desired decontamination effect, generally the concentrations are usually within the range 0.01-50 g/1, preferably 0.5-2 g/1, of the cerium nitrate, within the range of 0.01-50 g/1, preferably 0.05-0.2 g/1, the chromic acid and within the range of 0.001-1 g/1, pre¬ ferably 0.005-0.015 g/1 of the ozone. The water-based or aqueous oxidation agent has pre ' fe ably been made acidic by nitric acid, preferably to a pH o about 1. The method in accordance with the invention is gener 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 go results in the decontamination of chromium (III) oxide fro a chromium-nickel-iron alloy, such a decontamination there fore representing an especially preferable embodiment of t invention. The invention will now be described in conjunction w some non-limiting examples. EXAMPLES In a number of decontamination tests carried out on samples of Inconel 600 taken from a number of different po ions of steam generator in a PWR after an operation time o 'about 8 years, which samples were the most difficultly dec taminable ones to be obtained, the samples treated at temperature (about 20°C) for 48 hours in an oxidizing solu in accordance with-the invention. This solution consisted an aqueous solution, made acidic to pH 1.4 with nitric aci of 1.5 g/1 of cerium(3)nitrate, 0.1 g, 1 of chromic acid an 12 g/1 of boric acid to which ozone was continuously suppl The decontamination factors obtained at these experi ments were 20-300. In comparative tests carried out on similar samples, tubes of Inconel 600, with the most effective of the soft cesses tested at the Agesta and Surry-II projects, both re quiring an operational temperature of 80-90°C, decontamina factors with an average of merely 6.2 were obtained. In addition to the decontamination tests reported a corrosion tests have been carried out on blank, non-preoxi samples of Inconel 600. The test pieces were 3 pieces of s generator tubes with lengths of 5 cm. To simulate the con tion in the rolled zone in the tubes of the tube plate th samples had been rolled internally to half the lengths th of. The cold deformation obtained was about 5".. The three samples were exposed in parallel in a 100 glass container in an aqueous solution containing 12 g/1 boric acid, 1.5 g/1 of Ce(3)nitrate and 0.1 g/1 of .CrO, i nitric acid at a pH of about 1.4. Oxygen gas with about 2 percent by volume of ozone was bubbled into the same con¬ tainer at a rate of about 0.1 1/min. The temperature was 20°C and the exposure time was hours. The weight losses during this exposure are accounte for in the Table below. A further exposure cycle, identical with the first was carried out. The weight losses at this exposure are a presented in the Table. The material loss after each expo cycle is on an average well below 1 urn, which must be reg ed as extremely satisfactory. Xo signs of local corrosion have been observed. TABLE Corrosion tests carried out on 5 identical rolled t samples of Inconel 600 exposed in parallel to each other Exposure times Cvcle Time hours; 1a 48 1b 48 B

Claims: CLAIMS 1. A. method of decontaminating radionuclide-contam ted acid-Insoluble corrosion products from primary system surfaces in pressurized water reactors, especially of deco taminating chromium (III) oxide from a chromium-nickel-iro alloy, by contacting the contaminated surfaces with an oxi ation agent in an acidic solution and dissolving the corro ion products which have been made acid-soluble by the oxid ion, characterized by performing the oxidation ith a wate based oxidation agent having a pH below 7 and containing cerium nitrate, chromic acid and ozone in such concentrat¬ ions that decontamination is obtained. 2. A method according to claim 1 , characterized in that the oxidation agent is an acidic aqueous solution of cerium nitrate and chromic acid and ozone in a saturated solution and dispersed form. 5. 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 cerium nitrate and chromic a 4. , A method according to any one of the preceding claims, characterized in that the oxidation and dissolutio are performed in one and the same step. 5. A method according to any one of the preceding claims, characterized in that the oxidation and dissolutio are carried out at a temperature below about 60°C, prefera ly 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 water-based oxidation agent has been made acidic with nitric acid. 7. A method according to claim 6, characterized in that the water-based oxidation agent has been made acidic a pH value of about 1. " 8. A method according to any one of the preceding claims, characterized in that the concentration of cerium nitrate is 0.01-50 g/1, preferably 0.5-2 g/1, the concentr ion of chromic acid is 0 01-50 g/1, preferably 0.05-0.2 g/ and the concentration of the ozone is 0.001-1 g/1, prefer¬ ably 0.005-0.015 g/1.

Inventors: ARVESEN JAN, ARVESEN, JAN
Applicants: STUDSVIK ENERGITEKNIK AB
Application year: 1985
Main classification: G21F9
IPC codes: G21F9