Prediction of Fuel Cladding Performance for Ultra-long Cycle Fast Reactor Application

Abstract

As a part of R&D activities for the design of advanced fast reactors, the feasibility of ultra-long cycle fast reactors (UCFR) based on the assessment of key technical issues is investigated. The concept for UCFR is designed to be operating with no refueling during the overall operation period, so the design requirement for the fuel cladding such as creep rupture and swelling would be more challengeable than that in conventional fast reactors. The total operation period of UCFRs is varied from 30 to 60 years, the peak cladding temperature is 650℃ or higher, and the maximum neutron damage can go up to 300 dpa (displacement per atom) or higher depending on the specific core design. In this study, several key design parameters for UCFR fuel claddings including the internal pressure cause by fission gas release, thermal creep, irradiation creep and swelling are technically evaluated on the basis of UCFR concepts proposed. Considering the overall operation time of UCFR from 30 to 60 years, a large amount of fission gas is expected to be up inside of fuel cladding. This may raise a several technical issues on the safety of the cladding. There are various equations to formulate and model the fission gas release (FGR) in each type of nuclear metallic fuels. The definition of irradiation creep is the difference in dimensional changes between a stressed and an unstressed samples irradiated under identical conditions. Also irradiation creep occurs when external non-hydrostatic stresses are applied during irradiation. And thermal creep is severe issue in materials that exposed to high temperature environment for long time. This study briefly summarizes especially with respect to their possible inter-correlation between irradiation creep and thermal creep. When high neutron dose by long operation time with no refueling (30years), high temperature and internal pressure (about 400dpa, 600℃ and 470MPa, respectively) applied to UCFR, either high irradiation creep or thermal creep can occur. Therefore this part should be thoroughly examined and evaluated. Swelling is mainly caused by the increase of volume and decrease of density of materials subjected to intense neutron radiation. The operation environment of UCFR is high neutron dose (near 400dpa). It causes serious problem in cladding material because of swelling. After the threshold fluence of 1022n/cm2 is achieved, the swelling is exponentially increased. After the fluence threshold of 1022n/cm2 is attained, early experience characterized the increase of swelling in terms of an exponential rise. The candidate cladding materials include ferritic-martensitic steels (FM steels), oxide dispersion strengthened steels (ODS steels), and SiC/SiCf composite. Among these materials, the result of this study shows that the SiC/SiCf composite is the most promising cladding material which would meet the fuel cladding design criteria for UCFR because of favorable material properties under high burnup and radiation environment.