Loss of emergency core cooling

The pressure tube geometry means that fuel elements are always within a few centimeters of the moderator, which can act as an emergency heat sink for postulated severe accidents such as a loss of coolanf accidenf (LOCA) combined with loss of emergency core cooling (LOECC). This also provides an inherenf limit to metal-water reactions in a severe accident because the fuel bundle is close to fhe emergency heat sink. 

Inherent in the concept of heavy water moderated reactors is the separation between heat transport coolant and moderator coolant. This has significant safety advantages for low probability events such as postulated dual failures when normal heat transport systems becomes ineffective, e.g. loss of coolant accident coupled with loss of emergency core cooling system. For this postulated event, the pressure tubes balloon, contact the calandria tubes and thereby provide a heat transfer path to the moderator. Design improvements to the moderator heat sink will be made for these low probability events. 

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