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CORROSION OF REACTOR MATERIALS

The two most "modern" fields of technology, Much operational experience and many experi­ nuclear and space engineering, are placing drastic­ mental results have accumulated in recent years re­ ally increased demands on numerous materials; they garding corrosion of reactor materials, particularly have to conform to extremely rigorous specifications, since the 1958 Geneva Conference on the Peaceful in which new physical and chemical properties have Uses of Atomic Energy, where these problems were been added to the old ones. One of the necessary also discussed. It was, therefore, felt that a survey characteristics is increased corrosion resistance. In and critical appraisal of the results obtained during fact, corrosion resistance has often been described this period had become necessary and, in response as the foremost problem in the choice of reactor to this need, IAEA organized a Conference on the materials. Corrosion of Reactor Materials at Salzburg, Austria (4-9 June 1962). It covered many of the theoretical, Reactor materials include nuclear fuels (mainly experimental and engineering problems relating to uranium, but also plutonium and thorium), the clad­ the corrosion phenomena which occur in nuclear re­ ding in which the fuel is contained and pipes and ves­ sels in which circulate (a large variety of actors as well as in the adjacent circuits. Some 200 , , zirconium and other alloys), mod­ experts from 23 countries took part in this meeting. erators (, , etc.), shielding (con­ Behaviour of Steel crete of varying composition), and used for mechanical reactor components. All these are sub­ Several of the papers presented at the confer­ ject to particularly heavy corrosion owing to the fact ence dealt with the behaviour of steel. H. Inouye that they are exposed to coolants at high (USA) reported on studies on high reac­ and pressures and to irradiation, and the fact that tions of certain types of in helium with radiation increases the susceptibility of certain mate­ low concentrations of and carbon mon­ rials to corrosion. . This research was part of the fuel develop­ Heavy corrosion may impair the functioning of ment programme for the Experimental -Cooled reactor parts and reduce their lifetime. This, in Reactor. turn, would increase the shut-down periods of re­ actors for maintenance and repair work, with a cor­ R. Darras, D. Lecleroq and H. Loriers responding rise in operating cost. This factor is of (France) described corrosion effects of carbon di­ considerable importance in view of the large capital oxide on structural steel at average temperatures of between 450°C and 550°C under pressure. The vary­ outlay on reactors. ing compositions of both the steel and the used A second consideration in this connection is the gave revealing but often very widely varying results. possibility of hazards, which may arise from the pol­ H. E. McCoy Jr. (USA) discussed various lution of the primary cooling system with highly radio­ types of gas- interactions and their effect upon active fission products resulting from severe dam­ the service performance of metal. These investi­ age to fuel element claddings. On the other hand, a gations were undertaken to render possible an in­ corrosion-induced in the primary circuit may crease in the operating temperature of gas-cooled to heavy contamination of the adjacent installa­ reactors by selecting suitable structural materials tions or even of the surroundings. Products of such and fuel element claddings. The experiments were corrosion may not only impair the flow of cooling liq­ carried out over a temperature range of 700°"90O"C uids within the heat exchanger and cooling systems with stainless , nickel-based alloys and re­ of reactors; they may also become radioactive them­ fractory metals in environments of argon, , selves and contaminate the whole system. This , carbon dioxide, nitrogen, air and could give rise to considerable radiation hazards. In . Studies on the kinetics of electrode pro­ addition, radioactive contamination may necessitate cesses of low- chrome and chrome-nickel stain­ a prolonged decontamination shut-down of a reactor, less steels and aluminium alloys at temperatures of with corresponding financial loss. up to 300°C and pressures of up to 87 atmospheres Since, broadly speaking, the efficiency of a re­ were described by V. V. Gerasimov (USSR). The actor increases with precisely those factors which data obtained from these studies made it possible to also increase corrosion (primarily rise of tempera­ forecast the corrosion behaviour of structural ma­ ture), corrosion resistance assumes considerable terials at critical temperatures in water of any com­ economic importance. position.

29 V. S. Lyashenko, V.V. Zotov and V. A. Ivanov A group of Soviet scientists, I. L. Rosenfeld, (USSR) reported on experimental work on the resist­ Yu. P. Olkhovnikov and A. A. Sudarikova, reported ance to corrosion of various construction steel sam­ on the effects of the composition of water on corro­ ples in a stream of liquid sodium at temperatures of sion of zirconium alloys at high temperatures and 600°C and 700°C. Various impurities had a signifi­ pressures. Alloys of zirconium with and cant influence on corrosion resistance of these mate­ also with lead, , nickel and niobium have a high rials, either increasing or lowering it. corrosion resistance in pure distilled water at 360° C. The presence of insignificant concentra­ F. M. Lang and P. Magnier (France) gave an tions of certain in the water, which under nor­ account of experiments on the corrosion of graphite mal conditions would be absolutely harmless, in­ by gases. It was found that the corrosion duces intensified corrosion of zirconium alloys at effect on numerous highly purified carbons and high temperatures. On the other hand, certain other of widely different origins was practically ions, which activate reactions under normal condi­ the same. and the presence of hydrogen . tions, have practically no effect on the corrosion of showed a very definite influence; the speed of oxi­ zirconium alloys at the temperatures in question. dation depended on the temperature. The varying nature of the effect of the ions was stat­ ed to be connected with their influence on the char­ The use of carbon-14 for the study of carbon acteristics of the protective films formed on the transport in the radiation-induced reaction between surface of zirconium alloys. carbon and carbon dioxide in a graphite-moderated, Studies on corrosion resistance of a zir­ carbon dioxide-cooled reactor was described by conium-niobium alloy were also dealt with by T. B. Cope stake and F. S. Feates (UK). Carbon-14 S. B. Dalgaard (Canada). He said that because of was incorporated into graphite and the gasification of the improved mechanical qualities which this alloy the graphite was studied by measuring the carbon-14 showed after appropriate heat treatment, it was in­ activity. tended to use it for the fabrication of pressure tubes in Canadian power reactors, in place of the Zircaloy Aluminium and Zirconium Alloys used at present. The corrosion of Zircaloy in various alkaline Corrosion of aluminium alloys in high temper­ media at high temperature - which is of interest in ature water (260°-315°C) depends partly on the con­ pressurized-water reactors - was the object of a ditions under which the tests are run. According to study by H. Coriou, L. Grail, J. Neunier, M.Pelras J. E. Draley and W. E. Ruther (USA), corrosion and H. Willermoz (France). The purpose of using rates largely vary when there is a high rate of flow these media is to make the corrosion products more of water past metal surface. They pointed out that filterable and so to reduce considerably the contam­ seemingly minor variations in conditions cause large ination of the circuits. The authors showed that a changes in corrosion rate. Also, the con­ "critical threshold" exists in regard to hy­ tent of aluminium-nickel-iron alloys was found to droxide concentrations, above which very rapid cor­ have an important influence on their corrosion be­ rosion takes place, and that this threshold is greatly haviour. Alloys with extremely low silicon content influenced by temperatures. For ammonia no such behaved better than those with a more typical silicon threshold was noted. content. The effect of reactor irradiation on the oxida­ On the other hand, K". Videm (Norway) dis­ tion of zirconium alloys in steam was the subject of cussed alloys containing about ten per cent silicon in a paper presented by B. Cox and R. C. Asher (UK). addition to the one per cent nickel normally incorpor­ According to their findings, considerable variations ated in aluminium for high temperature performance. exist in the formation of oxide films, dependent on These alloys were found to be more corrosion- the temperature and on the nature of the neutron . resistant than the ordinary ones. It was observed that the growth of thin oxide films could be attributed to irradiation with gamma rays. M. J. Brabers (Belgium) described investiga­ tions, made by means of a so-called "transito- J. Debuigne and P. Lehr (France) discussed meter", into the formation of oxide layers on high the kinetics of oxidation of non-alloyed zirconium in purity aluminium and its alloys. The "transitometer" a dry oxygen atmosphere at temperatures of between comprises a constant current source and a switch 600°C and 850°C. They showed that the formation which opens the circuit at certain pre-set voltages of and growth of a surface oxide film is accompanied by the electric current and of an electronic timer work­ considerable of the oxygen in the underlying ing simultaneously. metal - a phenomenon which makes the metal more brittle. Zirconium, one of the most important nuclear materials used in producing corrosion-resistant al­ The effect of hydrogen on metals which are loys, was discussed by a number of experts. normally covered and more or less protected by an

30 oxide layer was discussed by J. S. L. Leach and Other Procedures and Summing Up A. Y. Nehru (UK). P. J. Gellings (Netherlands) suggested a pro­ Problems in Advanced Reactors cedure which would make it possible to follow the Since the prospect of economic nuclear power corrosion of test pieces inside a reactor or even of depends to a considerable extent on the possibility of the reactor material itself without the need of their developing breeder reactors, particular importance being removed for inspection. Decontamination of attaches to corrosion effects caused by molten plu- reactors or of parts of the primary system was dealt tonium fuel. B. J. Thamer, R. P. Hammond, with in a paper by J. A. Ayres (USA). Decontami­ R. M. Bid well, C. C. Burwell and J. E. Kemme nation itself is essentially a controlled corrosion (USA) pointed out that molten plutonium alloys may process in which the film is removed along with a combine some of the advantages of fluid fuels with small amount of the underlying metal. Excessive the high breeding ratios that are possible with fast corrosion is suppressed by the addition of suitable Plutonium reactors. The precondition for their inhibitors. Corrosion is caused not only directly utilization is the development of the appropriate con­ by the decontamination process, but also by an en­ tainer material. hanced rate of attack during subsequent operation as a result of the removal of the protective film. The theoretical aspect of corrosion by liquid metals was discussed by J. R. Weeks and C. J. Klamut (USA). M. J. Brabers, chairman of the last session, in a summing-up of the discussions, said: A reactor with the fuel material homogeneously distributed in molten salts is considered to be a promising concept for future power reactors. "In general, results from different countries For the Molten Reactor Experiment at the Oak agreed rather satisfactorily or complemented one Ridge National Laboratory, USA, a high-strength another very well. Great progress has been made nickel-base alloy has been developed. J. H. De Van in the improvement of corrosion resistance to re­ and R. B. Evans (USA) described the results of sev­ actor materials; some years ago, for example, eral tests to demonstrate the excellent corrosion re­ 150°C was considered the maximum temperature for sistance of this material to molten at high aluminium alloys. This week the figure 350°C was temperatures. quoted, representing an increase of 200 degrees which is quite considerable. Zirconium alloys also Lithium has been regarded as a promising show some improvements. We have heard about the coolant for power reactors for some time. beneficial effects of niobium and . It is doubt­ J. R. Di Stefano and E. E. Hoffman (USA) reported ful whether, in this case, increasing the niobium on experiments on the compatibility of various con­ content will further improve the corrosion resistance tainer materials with molten lithium. in view of the enormous effects of the heat treat­ ments. If the improvements in corrosion resist­ Special interest attaches to the use of organic- ance continue at the rate achieved over recent years, cooled and -moderated reactors because of certain it may well be less the corrosion than mechanical superior physical and chemical properties which or­ aspects which will be the determining factor in atom­ ganic liquids possess, as compared to water. One ic energy within several years. This point may al­ of these properties is the low corrosion rate shown ready have been reached in the case of aluminium, by organic coolants when used with common materi­ but I am sure that later on still much higher temper­ als and with metallic nuclear fuels. atures will be given. Irradiation may have many Corrosion effects with organic coolants were effects like face changes, dislocations, etc. In that described by W. E. Parkins (USA), who gave the re­ respect the material presented was still limited, but sults of tests carried out on different types of steel, as somebody remarked, corrosion without any ir­ aluminium, zirconium, and some of their radiation is already difficult enough to understand. alloys. The results showed that all the materials With irradiation as well, a whole series of compli­ tested are essentially inert to the organic coolants cations is added to the problems. " in their pure form; corrosion occurs only as a re­ sult of certain impurities. Magnesium corroded se­ Cestmfr Simrfne, Director of IAEA's Division verely in the presence of small amounts of water; of Technical Supplies, speaking at the closing ses­ hydrogen attacked zirconium. No increase in the sion, said that the papers dealing with technological corrosion rate due to the presence of radiation was aspects had been more impressive than the theoreti­ noted, but it was pointed out that radiation produces cal contributions. The conference, he said, had free hydrogen as well as certain organics which are shown how little we actually knew regarding cor­ then capable of reacting with oxygen to form corro­ rosion, but this very realization was a factor of sive compounds. progress.

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