Hydrogen generation aspects during accidents in pressurised heavy water reactors

One of the limits for accepting the performance of Emergency Core Cooling Systems (ECCS) is that hydrogen generation during the accident condition should not exceed a particular limit. For light water reactors the limit is 0.01 items the hydrogen that would be generated if all the zircaloy present in the core were to react and produce hydrogen. Although the basis of this acceptance limit is not very clear from the literature, it appears that it has been derived from the considerations of limits of hydrogen beyond which it would explode in the containment. If this were the basis, then containment volume, hydrogen distribution and ambient temperature etc. should also be the parameters in the acceptance limit statement. In practice this criteria hardly ever puts a demand on the ECCS. The other criteria viz. coolability criteria are often the limiting criteria. But in the current standards of safety where many severe accidents are becoming a part of design, the hydrogen criteria needs further attention. In Pressurised Heavy Water Reactors, which are pressure tube type horizontal reactors, the amount of zircaloy present in the core is large. This is due to the presence of zircaloy pressure tubes. In view of this the earlier stated criteria is likely to be met if coolability criteria are met. However, attention is to be paid to identify any additional constraints. One consideration is that presence of hydrogen inhibits flow. While for vessel type of reactors the hydrogen gets separated and collected in the upper part of the vessel during the transient and hence the flow is not significantly affected, for pressure tube type of reactors there is no such room available in the coolant path where it can get separated and not affect the coolant flow significantly. Under low flow conditions the hydrogen would get collected at the top of steam generator tubes and disturb the natural circulation which is often the ultimate means of decay heat removal. This paper discusses in detail the hydrogen generation aspects mentioned above and presents a proposal for additional criteria for Pressurised Heavy Water Reactors. (author)