Study on short term stress limit in fast reactor fuel pin designs

Abstract

The rationalization of a short term stress limit for primary membrane stress in fast reactor fuel pin design was investigated for the purpose of the advanced designs of fast reactors, using the idea of the structure reliability. Fuel pins made of 20% cold-worked modified 316 stainless steel (PNC316) were tested in transient over power event conducted in EBR-II and no failure occurred in these pins. The stress due to fission gas pressure at the transient over power event was estimated for these pins and compared with the tensile strength of the irradiated PNC316 cladding. The result demonstrated the excess margin included in a currently used design stress limit value. Then, the reliability evaluation was conducted for the tensile strength of PNC316, where the failure probabilities were calculated in the design stress limit with its safety factors varied. The allowable failure probability was determined to prevent any fuel pin failures for 60 years life time of the prototype fast reactor. The safety factors in the design stress limit were investigated so as to maintain the failure probability to be lower than the allowable probability. The results obtained was that the rationalization made possible 30-40% increasing of the currently used design stress limit in the neutron fluence range 0-about 20 x 10 26 n/m 2 (E>0.1 MeV), which was maximum neutron fluence of the irradiation results of PNC316.