The fast ‘breeder’ reactor is the system on which the long term prospects of nuclear energy generation are based … Work on this system has been increasing steadily for some ten years and the greatest effort of the

AEA’s research and development programme is now devoted to this type of reactor. Expenditure in 1966-67 was approximately £12 million and there will be increasing capital expenditure over the next few years as the construction of the DFR prototype proceeds. This system is regarded as likely to provide a very cheap source of electricity. Building costs (at 1967 prices) of fast reactor stations are expected to be as low as £50 per kilowatt installed and generating costs to be reduced ultimately to 0.3d (old pence) per kilowatt hour. The prototype, a large station producing 250 MWe, is expected to be on power in 1971.

By 1968, the AEA was looking to have at least 15 gigawatt electric (GWe) of fast breeders in operation by 1986. On the basis of "another bold decision" by government, exploitation of the fast breeder would be "the major event of the rest of the century". By 1969 the AEA was asserting that "the UK has the firm intention of introducing fast reactors as rapidly as possible after the operation of our 250 MW prototype."

Meanwhile, in May 1967, the primary cooling circuit of the DFR sprang a leak of molten-sodium-potassium. The reactor was shut down in July 1967 for nearly a year. The DFR was also manifesting other engineering problems. No reactor hitherto in operation had subjected its structural materials to intense high energy neutron radiation for lengthy periods. To find materials able to withstand the demanding environment in the core of a fast reactor was a daunting challenge.

While these practical problems occupied the attention of the staff at Dounreay, the AEA was linking up with the Central Electricity Generation Board (CEGB) and the two reactor building consortia for further design studies on commercial fast breeder power stations. On 14 October 1970, introducing the AEA annual report, chairman Sir John Hill characterized the outcome thus:

…we have had a most useful study of the fast reactor by a group consisting of engineers of the CEGB, the industrial design and construction firms and the Authority… we have now an agreed programme… which could lead to the CEGB being able to start the construction of the first civil fast reactor, possibly of 1300 MW, by early 1974 … seeing how the prototype fast reactor performs in 1972 and 1973.

As it turned out, the PFR did not perform at all in 1972 or 1973. Nevertheless, with the PFR falling steadily farther behind schedule, the 1971 AEA annual report was still confident. The cooperative study had resulted in:

the formulation of a strategic plan for the introduction of fast reactors to the CEGB network; this assumes that construction of a first commercial station will start in 1974 as a ‘lead’ station, following operation of the PFR. This would be followed by other stations after an interval of perhaps two years. This plan assumes that the technical and economic results from the development programme confirm present expectations; it will be reviewed each year in the light of progress achieved.