Long-Term Corrosion of Zircaloy Hull Waste under Geological Disposal Conditions: Corrosion Correlations, Factors Influen
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Long-Term Corrosion of Zircaloy Hull Waste under Geological Disposal Conditions: Corrosion Correlations, Factors Influencing Corrosion, Corrosion Test Data, and Preliminary Evaluation Hiromi Tanabe, Tomofumi Sakuragi, Hideaki Miyakawa, and Ryota Takahashi Radioactive Waste Management Funding and Research Center (RWMC), Pacific Marks Tsukishima, 1-15-7 Tsukishima, Chuo-ku, Tokyo, 104-0052, Japan Tel.: 81-3-3534-4533; E-mail: [email protected] ABSTRACT The carbon-14 generated in Zircaloy (Zry) hull waste is considered an important radionuclide in the TRU waste geological disposal concept in Japan. Given that the metal Zry is highly corrosion-resistant in the anaerobic and low-temperature conditions of the repository, and that the C-14 release rate is assumed to be controlled by the corrosion rate, a variety of corrosion and leaching tests have been performed. However, since the Zry corrosion rate is extremely slow, it is not possible to predict long-term corrosion behavior through low-temperature corrosion tests conducted in a reasonable time period. A vast amount of testing has been conducted in the higher-temperature range of 523 to 633 K, and corrosion correlations have been obtained from these tests. Corrosion correlations have been used to predict the corrosion rate of Zry in a tuff repository. Long-term Zry autoclave corrosion data have been analyzed to develop new corrosion correlations. Extrapolating these correlations to a lower temperature range requires verification that the mechanisms do not change over the range of testing and extrapolation. Factors that influence corrosion rates under geological disposal conditions, such as material and environmental factors, should also be examined. Corrosion correlations, factors influencing corrosion rates, the results of corrosion and leaching tests, and a preliminary evaluation are discussed. INTRODUCTION A safe disposal concept of Transuranic (TRU) wastes generated from reprocessing and MOX fuel fabrication plants has been studied in Japan by the Federation of Electric Power Companies of Japan, Japan Atomic Energy Agency, and Japan Nuclear Fuel Limited with the support of research organizations [1]. This report is named as the second TRU report. Based on the results of the TRU waste disposal safety assessment, the I-129 in spent absorbents and C-14 in hull waste are considered nuclides which contribute substantially to radiation dose rate. To develop the safety concept, hot tests using simulated hulls generated from PWR spent fuel reprocessing have been pursued to evaluate C-14 inventories in Zry metal and Zry oxide film, C-14 release rates, chemical forms of C-14 released into simulated groundwater, and distribution coefficients of C-14 in cement [2]. Similar tests using BWR spent fuel and non-irradiated Zry metals were also conducted later [3, 4, 5, 6, 7]. However, it is not possible to predict long-term Zry corrosion behavior and the associated C-14 release rate solely through
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low-temperature corrosion tests conducted in a reasonable time period be
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