Calculations of AXIAL Variations in Sheath Strains During a Hypothetical LOCA

Abstract

The thermal and mechanical responses of a fuel element were calculated for a hypothetical loss-of-coolant-accident (LOCA) in a CANDU-6O0 nuclear reactor. In this paper, we present calculations for strains in fuel sheaths, during an 80% break in the outlet header. Offsite power is assumed unavailable following the LOCA and reactor trip. The objective of this study is to assess the coolability of the fuel for this event. The study focused on conditions when large axial variations are expected in sheath strains. The stress/strain codes ELESTRES and ELOCA-A were used. Use of ELOCA-A permitted modelling of axial variations along element length, due to effects like end-flux-peaking, and variations in the microstructure of Zircaloy due to brazing and welding. The results show that end-flux-peaking increases hoop strains near endcaps. For the above conditions, the maximum plastic hoop strain is 2.5%. The peak temperature in the sheath is 1050 degrees C. Oxidation of the sheath is negligible, as are a thermal strain and the probability of sheath failure due to beryllium-assisted cracking. These results demonstrate that the fuel is coolable, with no large change in flow area of the coolant. The risk of embrittlement is also low.