[en] A method introduced in the U.K. during 1969 for estimating the cladding temperature of fast reactor fuel elements depends on the experimental observation that there is a relationship between the operating temperature of 316 steel cladding and the temperature of release during a post irradiation heating of the Krypton 85 fission product which had been implanted by recoil into the cladding. Such a method for determination of fuel element cladding temperatures applied to the Rapsody reactor has given results which diverge significantly from temperature values predicted by calculation even though there was no lack of agreement between the individual results themselves. On the other hand, a very satisfactory agreement has been obtained with the cladding of an experimental fuel element when comparison was made with temperatures measured direct during irradiation using thermocouples. Analysis of the different results indicates that the temperature of release of Krypton during the test depends not only on the irradiation temperature but also on the post irradiation thermal history (quenched or recovered) of the fuel element during handling and storage. (Auth.)