(0.02%), the TRISO coatings provide complete fission product retention. The graphite matrix and the ZrC fuel element surface coatings provide additional barriers to fission product release. The Support Element The ZrH moderator is contained in the support elements, illustrated in Fig. 3. There is one support element for every two fuel elements. This contrasts with a ratio of 1/6 in NERVA and 1/3 in the Pewee. The high ratio of support-to-fuel elements in the NDR permits the accommodation of a high ZrH loading. The ZrH is not clad. Hydrogen loss is precluded by keeping the ZrH at temperatures not to exceed 420 K. This temperature is maintained by introducing a stream of 35 K hydrogen to cool the tie tubes. The unclad ZrH permits ready hydrogen depletion in the event of inadvertent core overheat as in loss-of-flow or loss-of-coolant accidents. This also permits a passive means for permanent irreversible ultimate reactor shutdown.
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