Thermal-Hydraulic Aspects of Progression to Severe Accidents in CANDU Reactors

The progression of events that develop into an accident with severe fuel or core damage in the Canada deuterium uranium (CANDU) reactor is discussed. Such events involve a number of broadly common stages in which the thermal-hydraulic behavior of the reactor fuel, fuel channels, heat transport system, and a number of key process systems governs both the rate at which severely degraded cooling conditions develop and the extent of resultant damage to the reactor core. The quantification of core damage states requires the modeling of the physical phenomena that are active in these accidents, which is a focus of this paper. As discussed in this paper, unique passive features of the CANDU reactor design have a beneficial effect in that they delay the progression of severe accidents, thereby providing ample opportunity for operator actions to stabilize the plant and mitigate the consequences. It is shown that large CANDU reactors are inherently tolerant of a prolonged loss of engineered heat sinks at decay power levels. This is because two large volumes of water (the moderator and shield water) surround the reactor core and act as in situ passive heat sinks in severe accidents. This has significant impacts on severe accident management. The pressure tube reactor design precludes melting of the core at high system pressures; that is, high-pressure melt ejection is physically impossible. In the event that severe undercooling of fuel occurs at high system pressure, a pressure tube will fail well before any significant molten fuel material can accumulate.