Irradiation Effects in the EGCR Fuel

Post-irradiation examinations have been completed on all but the very long burnup samples for the EGCR fuel evaluation studies. The results have confirmed the reliability of this fuel element design at least up to the burnup thus far attained (2400 Mw-day/metric ton UO₂). Fission gas release was not excessive except for the very high temperature irradiations. Although the pellets did sometimes show considerable cracking, pieces did not fall into the central cavity, and the hollow cylinder pellet design was shown to be stable. In the 1600°F prototype experiments, ridges were formed in the clad at pellet interfaces and evidence of sigma phase formation and some void formation was found in the metallographic examination of the ridges. The UO₂ was found to contain many fine cracks which caused the pellets to break up on handling, but very little fuel shifting occurred before the cans were cut open.