Penetration rates of uranium and uranium 5wt% fissium fuels through Type 304 stainless steel cladding have been measured in the TREAT reactor using a new electrical failure-detection method. Penetration through a 0.009 in. clad takes about 1 sec in the 1100 to 1200 C temperature range. These results agree very well with out-of-pile laboratory experiments performed earlier on the same materials. This agreement indicates that the idealized, basic laboratory experiments can give reliable safety information, but that they should be substantiated by the more realistic in-pile experiments for specific applications.