Evaluation of contribution of thermal radiation in transferring decay heat to the moderator, in case of 18 rod bundle facility, simulating advanced nuclear reactors

In case of heavy water moderated nuclear reactors, the decay heat of a fuel bundle can be transferred to the moderator available outside calandria vessel. Radiative heat transfer accounts for significant amount of total energy exchange from the fuel bundle during LOCA (loss of coolant accident) situation. In order to simulate one channel of an advanced reactor comprising 54 nuclear rods of 3.5 m length, scaling analysis has been used to scale it down to an 18 rod bundle facility of 1 m test length. The experimental analysis, in order to examine the contribution of energy transfer by thermal radiation and natural convection in removing decay heat of fuel/heater rods using moderator as a heat sink, is presented. It focuses mainly on the role of moderator in removal of decay heat, for situation with air alone in the annular gap between pressure tube and calandria tube coupled with flowing and standstill/boiling moderator situations. Four experimental tests have been presented in this paper. It is seen that, 80% of heat energy is transferred from the heater bundle of rods to the enclosing pressure tube by means of radiation mode, out of this 20-45% of energy is carried away by the moderator. It depends mainly on the resistances involved in the heat flow path and the test set up conditions.