Post-LOCA Ductility of Cr-coated cladding and its Implications on Accident Coping Time

Coated cladding has been developed to extend accident coping time by suppressing steam oxidation during accidents. There is a time period between the post-LOCA ductility limit and either melting or significant loss of coolable geometry upon reflood quenching which marks an initiation of severe accidents[1]. An accident coping time may refer to the time period before the initiation of severe accident. Violation of the Design Basis Accident (DBA) criteria (i.e., Emergency Core Cooling System – ECCS Criteria) can be considered an initiation of severe accident phase, although its extent on the accident progression may still be limited. Hence, it is important to quantify the increase in accident coping time of coated-Zircaloy cladding in terms of time required to reach the ECCS limit. It is noteworthy Accident Tolerant Fuel (ATF) candidates may as well comply with the current Emergency Core Cooling System (ECCS) criteria based on post-LOCA ductility in order to main the consistency in regulatory framework. In that, the extension of accident coping time can be evaluated in terms of time extended to reach the post-LOCA ductility limit. Such definition may be taken as the minimum, yet most conservative, extension of accident coping time. Single-sided coating on the outer cladding surface leads to the internal oxidation upon cladding rupture followed by ballooning at ~ 600-700°C[2]. Hence, it is anticipated that the coated Zircaloy cladding undergoes appreciable embrittlement due to internal oxidation. In this study, high-temperature steam oxidation and water quenching experiments were conducted to quantitatively assess the effect of coating on postLOCA ductility, and discusses its implications on extension of accident coping time.