The adverse effect of excess oxygen on the irradiation performance of UO² was demonstrated empirically. Subsequent in-reactor experiments confirmed that finding and helped to explain it.

The thermal conductivity of nonstoichiometric uranium oxide during irradiation has been deduced from thermocouple measurements, and from the observed amount of central melting for known heat ratings. The conductivity of UO^2.12 is approximately 0.8 times that of UO^2.0; for UO^2.19 the corresponding figure is 0.4. Observed differences in the diametral expansion of the sheaths supported these deductions.

The consequences of accepting nonstoichiometric uranium oxide for highly rated fuel elements have been estimated. It is concluded that an oxygen: uranium atomic ratio greater than 2.01 should be avoided.