Fast reactors are gradually attaining a very prominent place in the research and development programs of reactor research centers throughout the world. It is appropriate, therefore, that international conferences be held periodically to foster direct communication among fast reactor specialists. Subject-wise, this first conference was limited to the topics of breeding, economics, and safety of large fast reactors, with particular emphasis on the interrelation among them. Many other topics remain to be covered in future meetings.

Present practice in fast reactor design is to eliminate any known positive temperature coefficients of reactivity. This approach has been feasible for the fast power reactors now being constructed. In recent considerations of advanced fast reactor designs at APDA, some cases were encountered in which the sodium temperature coefficient of reactivity was positive. Because of the serious implications of this result. a study of the sodium temperature coefficient of a design in which the problem appears to be most severe was made. The principle objective of the study was to obtain a better understanding of the problem. Simply stated, reducing the sodium density causes spectral reactivity effects which can, in some situations, be positive and larger in magnitude than the negative leakage effect, thereby causing a positive temperature coefficient of reactivity. In addition. some attention was also given to possible methods of influencing the sign of the sodium coefficient. Although the results indicate that these methods couid be successfully applied. there are penalties involved with their use. The results given are believed to be quaiitatively correct, but the uncertainties in the cross section data on which they are based cast some doubt on the quantitative accuracy. (auth).

Critical mass, spectrum, breeding ratio, and coolant removal coefficients were calculated for a series of large Pu-U238-fueled sodium-cooled fast-breeder power reactors, using a new 16-group cross-section set based in part on recent microscopic cross-section measurements. The parameters studied include reactor size, plutonium isotopic content, and type of structural material. Reactors cooled with Pb-Bi eutectic and those containing U233-Th fuel were examined.

BS>Methods for predicting the stability of fast reactors in the presence of both positive and negative reactivity coefficients are surveyed. Experimental methods of determining reactor stability are reviewed: excursion, transient, and oscillator. Sources of reactivity coefficients are discussed for both zero power and nonzero power operation. Representative reactivity coefficient values are presented, and positive coefficients due to Doppler effect, fuel bowing, and sodium coolant expansion are discussed. (D.L.C.).