Instant release fractions for 14 C, 60 Co, and 125 Sb from irradiated Zircaloy oxide film
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.486
Instant release fractions for 14C, 60Co, and 125Sb from irradiated Zircaloy oxide film Tomofumi Sakuragi1 and Yu Yamashita2 1
Radioactive Waste Management Funding and Research Center, Akashicho 6-4, Chuo city, Tokyo, 1040044, Japan
2 Toshiba Energy Systems & Solutions Corporation, Ukishimacho 4-1, Kawasaki-ku, Kawasaki city, 2100862, Japan
Abstract
The oxide films formed on spent fuel claddings are regarded as a potential source of the instantaneous release of radionuclides, such as 14C, after waste disposal. We investigated the instant release fraction using the irradiated oxide exfoliated from a Zircaloy-2 water rod, whose bundle burnup was 53.0 GWd/MTU. We performed a rapid leaching test in a dilute NaOH solution (pH of 12.5) for 10 min in an ultrasonic bath to ensure the release of radionuclides. The activity ratios of the leached amount to the total amount for 14C, 60Co, and 125 Sb were extremely low at approximately 10-4 to 10-3, among which the maximum value was 2.65 × 10-3 for 125Sb. These ratios were higher than that predicted from the thermodynamic solubility of ZrO2, i.e., less than 10-6. However, given the low ratios, it is too conservative to regard the inventory of all radionuclides in the Zircaloy oxide as instantaneous release. A small part of the released 14C was found as volatile species.
INTRODUCTION Spent fuel cladding is highly activated and strongly contaminated, and it is disposed of in underground repositories. A typical activation product in activated metal waste is 14C, which is mainly generated by the 14N(n,p)14C reaction and potentially produces a significant exposure dose owing to large inventory, long half-life (5730 years), rapid release rate, and the mobile speciation in repository environments. In the
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preliminary safety case in Japan, the release of radionuclides from a metal matrix (bulk Zircaloy) is 80% of cladding inventory and is regarded as corrosion-related congruent release. Whereas cladding oxide films having 20% inventory are regarded as a source of the instant release fraction (IRF) [1,2]. In the 1990s, researchers conducting a study using spent pressurized water reactor (PWR) cladding with an 80 μm thick oxide film found that the oxide inventory of 14C contains 17% of the remaining radionuclides which they suggested was constitutive of the IRF [3]. Guenther et al. [4] estimated 15% oxide inventory for US cladding with a 50 μm oxide film. In contrast, recently, the IRFs for the 14C inventory of oxide films have been reported as approximately 9% for Japanese PWRs and 0.8% for boiling water reactors (BWRs) based on the ORIGEN calculation performed using the oxide thickness data of Japanese claddings for different types of fuels with varying burnup
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