Investigation of Zr-Based Alloy Cladding Burst Mechanisms Under Station Blackout Using Bison

摘要

Fuel rod behavior during accidents andtransients is one of the major safety issues for a lightwater reactor (LWR). As a result, fuel rod behavior underdesign basis accidents (DBA) such as loss of coolantaccident (LOCA) and reactivity initiated accident (RIA)has been extensively studied. After the Fukushima Daiichinuclear power plant accident, station blackout (SBO) hasbeen widely recognized as one of the most severepostulated events and the fuel rod behavior should be wellunderstood under such conditions. However, studies onfuel rod performance such as cladding failure under SBOare scarce. In this work, cladding burst mechanisms in ashort-term postulated SBO scenario without anymitigation measures have been investigated using theadvanced fuel performance code Bison developed atIdaho National Laboratory (INL). In particular, thesimulations were carried out for a full-length PWR fuelrod in which the cladding is Zry-4 with UO_2 fuel in a17x17 fuel assembly design. Four fuel rod burst criteriaare used in this comparative study: overstrain, plasticityinstability (PI), overstress (OS), and the combination ofOS/PI. It is found that all four rod burst criteria predictsimilar results in fuel rod failure time. As SBO starts afterreactor scram, fuel temperature drops rapidly. The fueltemperature reduces to about the same temperature as thatof the cladding for about 5000 seconds, and eventually thecladding and fuel temperatures increase quickly as thecoolant boils off. Meanwhile, significant fuel-claddingcontact is observed due to outward fuel thermal expansionand inward cladding creep resulting from higher externalpressure in the reactor primary system. The time to burstfor the cladding is about 6000 seconds, which is muchlonger than the typical few hundred seconds for a LOCAscenario. The fuel burst spot is in the high temperatureregion, accompanied by significant phase transformationfrom the hcp α-phase to the bcc β-phase around the burstzone. However, cladding ballooning is not observed underSBO, which is in contrast to the LOCA event. Therefore,the cladding burst mechanisms under SBO are apparentlydistinct from those under LOCA.