An Analytical Model for Heat Transfer Within a CANDU Fuel Bundle Residing in Air

Abstract

There are a number of different situations in a CANDU fuel cycle when the irradiated fuel resides in air. The typical examples include operating during fuel transfer, shipping, potential dry storage. In abnormal situations, dry fuel may be found in a reactor channel, end fitting, fueling machine, etc. As soon as a fuel bundle is withdrawn from its normal D₂O (or H₂O) environment, its sheath temperature starts to rise until it reaches the equilibrium (maximum) value. This fuel heat up is caused by decay power. UO₂, as well as Zircaloy-4 oxidation rates depend on the temperature. Hence, the knowledge of the fuel temperature is essential in assessing the behaviour of dry fuel. A new analytical model has been developed to estimate the maximum sheath temperature of an irradiated fuel bundle residing in air. Obtained numerical results show excellent agreement with the existing experimental values using 19- and 28-element electrically heated bundles. The maximum temperature is always attained at the surface of a central element. The calculations also reveal that the thermal radiation is the dominant heat transfer within a CANDU fuel bundle even for low temperatures.