3. DESCRIPTION OF TEST FACILITY
The CABRI REP Na test data provide a very consistent basis for the assessment of fuel behaviour mechanisms relevant for RIA transients. For the CABRI REP Na rods that did not fail, valuable information on fission gas release and especially on cladding deformation have been generated in the test programme. REP Na cladding strain data have been used as a start point for deriving an admittedly simple RIA failure criterion. This is based on a maximum strain which can be tolerated by the cladding, i.e. failure can occur only when this strain level is exceeded.
- For cladding that still retains ductility, failure at high burn-up (50-60 MWd/kg) is predicted beyond a 1% (permanent) diameter strain.
- For cladding that has been embrittled due to e.g. large corrosion, spalling and hydriding, a zero ductility is assumed, i.e. the failure threshold is at onset (0%) of permanent strain.
The conditions under which the ductility decreases from 1% to zero are provisionally set in the paper, taking into account the evidence from the CABRI tests and from ANL laboratory tests. Based on the above, one derives that the lowest failure limit at 60 MWd/kg is ~65-70 cal/g, which applies to heavily corroded/hydrided fuel, i.e. for oxide thickness of ~80 mm and in presence of oxide spalling. For corrosion resistant fuel, i.e. for oxide thickness well below 80 mm and in absence of spalling, the failure threshold at the same burnup is ~100 cal/g.
This failure criterion predicts well three of the four failed REP Na tests, but not the REP Na-1 test. For this, the predicted failure threshold is 63 cal/g, whereas the reported experimental value is 30 cal/g. For CABRI unfailed rods, the predicted failure limit is very close to the enthalpy achieved during the test. When used at its higher degree of conservatism (i.e. ductility = 0), the proposed failure threshold predicts reasonably well also the failed NSRR PWR and BWR rods. In general, more data at prototypical conditions are needed in order to arrive to comprehensive, best-estimate failure predictions. The PIE data of unfailed REP Na rods show a remarkable similarity between the trend exhibited by the cladding strain and by the fission gas release. This is briefly discussed in this paper with regard to a possible FGR mechanism (i.e. it is implied that the gas may mostly be released upon cooling, when cladding constraints are reduced). In any case, it appears that FGR as such has little to do with the mechanism of failure when this occurs at low enthalpy.
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