The inner and outer breeder sections of the DFR were originally loaded in 1958 with natural uranium elements clad in stainless steel. Early in 1965 it was found that a few of the lightly irradiated elements in the outer breeder were difficult to remove, although the inner breeder elements were in good condition. A comprehensive survey of the outer breeder was carried out in September 1965, and a number of elements were found to be distorted or swollen. Investigation showed that this had been caused by higher than normal uranium temperatures due to abnormal coolant flow conditions in some regions of the breeder. This will not occur in future fast reactors since coolant flow conditions will be different, and the breeder fuel itself will be ceramic and therefore not subject to the temperature limitations of natural uranium. It was decided to remove 500 breeder elements, and to carry out the work. Special cutting tools and removal equipment had to be manufactured.

The work was completed by the end of December and the reactor went critical again on 23 January after loading new experiments.

Such incidents in no way weakened AEA confidence in the concept of the fast breeder. On the contrary: while they pressed on with detailed designs for the PFR they had already satisfied themselves that the prospects were excellent:

The design study of a 2×1000 MWe fast reactor power station in general endorsed the conceptual design of the prototype fast reactor as representing the most likely features of the first commercial fast reactors. A capital cost estimate for this study indicates a cost similar to that of the best thermal reactor available at the same time, with potential for further reductions. (AEA Annual Report 1965-66, paragraph 157)