Fuel Fragmentation during LOCA simulated transients: investigation of irradiated MOX vs UO_2 fuel

摘要

Potential fuel fragmentation occurring during a LossOf Coolant Accident (LOCA) has been studied in the past20 years by testing UO_2 fuel rodlets in semi-integralLOCA tests either in-pile (IFA-650 test series in theHBWR of the Halden Reactor Program (HRP)) or out-ofpile(Studsvik LOCA test series of the Studsvik CladdingIntegrity Project (SCIP)). Among other separate effecttests (e.g. NFIR – heating tests), these experiments enableto define a Burnup threshold from which fuel finefragmentation occurs during a prototypical LOCAtransient, i.e. a significant amount of the fuel isfragmented into particle sizes below 1 mm.The limited data available on MOX fuel fragmentationdoesn’t include tests on irradiated MOX fuel segments. Asa result, EDF initiated in IFE hot cells a series ofparametric out-of-pile LOCA tests involving irradiatedMOX and UO_2 fuel rodlets, with M5 cladding. The goal ofthis test series is to evaluate and compare the MOX fuelfragmentation behavior with standard UO_2. To enrich theapproach, tests with various boundary conditions havebeen performed on both types of fuel (i.e. impact of initialrod internal pressurization or initial free volume).The different tests performed in the LOCA test furnace inKjeller hot cells include 4 segments from PWR rods (2UO_2 ～65 GWd/tM local BU and 2 MOX ～57 GWd/tM).The rodlets experienced temperature transients up to900°C, with ～3 to 4°C/s ramp rates and various holdingtimes (in order to obtain cladding ballooning and burst orsignificant ballooning). MOX fuel rodlets have beenremoved from fuel rods irradiated in the EDF commercialreactor Tricastin 1, France, and the reference UO_2rodlets from Vattenfall commercial reactor Ringhals,Sweden, (actually, the latter was the sibling of the UO_2rodlets used in IFA-650.15 test / Studsvik counterparttest). The test campaign has been split in 2 test series,each of them containing one UO_2 and one MOX segmentwith the same test design and test conditions. As such, thetests series aimed at studying a possible MOX fuel effecton fuel fragmentation (as compared to standard UO_2) andthe impact of the initial test conditions on fuelfragmentation phenomena (on both irradiated UO_2 andMOX).Results on fine fragmentation are presented and discussedwith the impact of the initial segment design (i.e. initialrod internal pressure, initial free volume) and the fueltype (MOX versus standard UO_2).