C-14 Release Behavior and Chemical Species from Irradiated Hull Waste under Geological Disposal Conditions

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C-14 Release Behavior and Chemical Species from Irradiated Hull Waste under Geological Disposal Conditions Yu Yamashita1, Hiromi Tanabe2, Tomofumi Sakuragi2, Ryota Takahashi1 and Michitaka Sasoh1 1 2

Toshiba Corporation, 4-1 Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan Radioactive Waste Management Funding and Research Center, Pacific Marks Tsukishima, 115-7 Tsukishima, Chuo-ku, Tokyo, 104-0052, Japan

ABSTRACT C-14 contained in Hull waste is one of the most important radionuclides in the safety assessment of transuranic (TRU) waste disposal. For more realistic safety assessment, it is important to clarify the release mechanism and chemical species of C-14 from Hull waste. In this research, leaching tests were conducted using an irradiated Zry cladding tube from a boilingwater reactor (BWR) to obtain leaching data and to investigate the relationship between Zry metal corrosion and C-14 release behavior. Both organic and inorganic C-14 compounds existed in the the liquid phase, and some C-14 moved to the gaseous phase. The release rate of C-14 obtained from the BWR cladding tube after two-year leaching tests was lower than the release rate from a pressurize water reactor (PWR) cladding tube. It is considered that the BWR cladding tube used in this test did not easily corrode since it used a comparatively new material. The release rate of C-14 was slightly lower as compared with the corrosion rate of unirradiated Zry. This is thought to be the result of improved corrosion resistance conferred by neutron irradiation, which encouraged the dissolution of grain boundary precipitation elements, such as Fe, Cr, and Ni, into the crystal grains. The leaching tests will be continued for 10 years. INTRODUCTION C-14 contained in Hull and End-pieces is an important radio nuclide in the safety assessment of transuranic (TRU) waste. Various studies have been conducted on the C-14 release behavior from such radioactive metal waste. However, there are insufficient long-term leaching data for evaluating C-14 release and migration behavior in the geological environment, C-14 chemical species and their adsorption action, and the relationship between C-14 release and metal corrosion. For realistic safety assessment, it is necessary to clarify the C-14 inventory of radioactive waste, the release mechanism of C-14, and the C-14 chemical form. Regarding the C14 inventory, Sakuragi et.al. have reported the total amount and concentration of C-14 in various kinds of metal waste [1]. Although it is assumed that the C-14 release behavior is related with metal corrosion, there are few studies that considered the relationship over a long period of time. There is also a report that the chemical species of released C-14 has a superior organic form [2]. Adsorption of C-14 in an organic form into cement, which is the main material used to construct disposal facilities, cannot be expected, and this leads to too conservative input condition for safety assessment. Against this background, in the research described here, we examined the C-14 releas