The history of fast breeder reactors is quite dissimilar from that of thermal reactors. From the earliest days after the Second World War the development of different types of thermal reactor was pur­sued in different countries: light-water reactors in the United States, heavy-water reactors in Canada and gas-cooled reactors in the United

Kingdom, for example. Only towards the end of the 20th century did the various nationally based lines of development converge.

In contrast virtually the same path was followed in all the countries where work on fast reactors was done. The reason for this seems to have been that until the 1960s at least fast reactors were seen to be commercially valuable only well into the future, so that the advant­ages of cooperation appeared to outweigh the disadvantages of aiding possible competitors. Thermal reactors on the other hand were com­mercially important from the start and were developed in competition, which restricted the exchange of ideas and allowed different concepts to flourish.

International cooperation played a major role in fast reactor devel­opment for two main reasons. Firstly the nuclear data on which designs had to be based were inadequate until the 1960s. There was a lot to be gained from worldwide cooperation in measuring neutron cross­sections to the required accuracy and exchanging and comparing the results. Secondly cooperation to ensure the safety of fast reactors was desirable even when there was competition in other areas.

This need to exchange information resulted in, among other things, a series of international conferences on fast reactors that were addressed mainly to the problems of reactor physics and safety and were held in the United States and various European countries throughout the 1960s and 1970s. These, together with the continual publication of information in the scientific press, kept the think­ing in different countries from diverging and encouraged parallel development.

In one respect it is not altogether certain that this was an advant­age. The use of liquid metals as coolants acquired a great deal of momentum, mainly because “everyone did it”, and the search for alternatives was discouraged. Gas has certain advantages as a coolant, but at the time of writing no group has been able to develop a gas — cooled fast reactor to the point where it can be assessed fairly in comparison with a liquid-metal-cooled reactor.