DESCRIPTION OF FUEL INTEGRITY PROJECT METHODOLOGY PRINCIPLES

TN International and International Nuclear Services (INS) have started the Fuel Integrity Project (FIP) in early 2000s, which goal is the development of a methodology to evaluate, as a safety requirement, the nature and the extent of fuel assemblies (FA) damage during accident drops of a packaging. From TN International previous knowledge acquired from fresh FA behaviour during drop tests, a mechanical tests programme including testing on fresh and used fuel rod samples has been planned by both companies and executed by INS. Tests results analysis has led to the elaboration of FIP methodology by TN International. Experimental knowledge on fuel was collected from the tests programme and the main mechanical phenomena arising from a drop have been identified and quantified. As a result, the FIP methodology, structured in flow charts, gives guidelines to study the effects of a lateral or axial drop of a packaging loaded with fresh or used FA of PWR or BWR types. The flow charts of methods have the same philosophy: several pessimistic mechanical evaluations based on direct calculations or dimensionless comparisons with appropriate reference tests permit to determine FA damage, gradually increasing with acceleration. First, elastic models distinguish the null or slight damage cases; then, plastic models permit to rule out cases with extreme FA damage that lead to unacceptable criticality hypotheses; finally, other plastic models quantify the extent of fuel rods deformations in moderate FA damage cases. FIP methodology application to a given case leads to the following output, used as criticality hypotheses for the safety analysis: existence or not of fuel rods rupture, their number, their location, the associated amount of released fuel material, and the extent of fuel rods array deformation and sliding. All the knowledge arising from the FIP is synthesized in the Technical Guide, which presents extensively the methodology and builds up all background experimental data. The methodology is applicable to fresh and used FA provided that brittle fracture risks are excluded.