In any safety text a summary of previous accidents in similar systems is most important, because it is only by our experience that we learn. An excellent survey of all reactor accidents up to 1964 is contained in the work of Thompson and Beckerley, referred to in the Preface. The following section merely highlights those main operating experiences and occurrences which have taken place in fast reactors. No systematic classification is intended; rather it is hoped to show a general picture of the multitude of kinds of problem that arise and that have to be accounted for in a com­prehensive safety evaluation.

4.6.1 Survey

No really large fast reactors have yet been built; the largest which are presently under construction are the Prototype Fast Reactor (PFR) in Britain, Phenix in France, and BN 350 and BN 600 in the USSR, all of which are above 550 MWt. In the USA, the Fast Flux Test Facility (FFTF) will include a 400 MWt reactor. Table 4.7 shows a listing of the main para­meters of the most important reactors already built or in the planning stage.

Apart from CLEMENTINE which was cooled by mercury, all fast — reactor systems to date have been sodium — or NaK-cooled. There have been no gas-cooled versions.

Some of these systems have suffered operational teething problems before and after start-up, and others have experienced accidents and incidents dur­ing operation. These accidents have never resulted in any risk to the public, and only two have resulted in considerable material damage and delays. The following sections outline some of the major problems.