Appendix 1 Thermal Hydraulics, Draft for Comment

Several different fuel failure criteria have been used in previously NRC-sponsored SFP accident studies. Benjamin, et. al. used the onset of runaway fuel clad oxidation as the fuel failure criterion in NUREG/CR-0649 [Ref. 1]. This criterion was criticized because clad rupture can occur at a relatively low temperature causing a gap release. The consequences of gap release can be significant if the radioactive iodine has not yet decayed to insignificant amounts. SHARP calculations [Ref. 2] used the onset of clad swelling as an acceptance criterion for prevention of fuel failure. The onset of clad swelling leading to gap release occurs at approximately 565 0C, which corresponds to the temperature for 10-hour creep rupture time [Ref. 3]. A cladding temperature of 570 0C is used as a thermal limit under accident conditions for licensing of spent fuel dry storage casks.