Dry fuel handling station experience and Ontario Hydro (CNS) programs

Irradiated CANDU fuel is normally kept under water when handled at Ontario Hydro stations. It is sometimes necessary, however, to handle fuel dry. Dry fuel exposure limits have to specified to prevent sheath failure, U02 oxidation causing element distortion and contamination. Central Nuclear Services (CNS) of Ontario Hydro has been engaged over several years in studies to improve the understanding of fuel behaviour under dry conditions and analyzing actual cases. A fundamental study on U02 oxidation behaviour and dimensional stability using irradiated CANDU fuel was carried out. A time-temperature criterion was derived using a limit of 2% diametral expansion. Fission product release during oxidation was measured for three month old fuel at temperatures up to 900 degrees Celsius and was found to be insignificant (less than 10-6 of inventory). Bruce NGS Bundle G70551W was left without sufficient cooling for about five hours in a transfer mechanism commencing two hours after discharge. The top half of the bundle was severely oxidized and U02 fuel fragments became dislodged from failed sheaths. Minimal U02 oxidation was found and no pellet/sheath interaction observed