PRELIMINARY REPORT ON SODIUM TEMPERATURE COEFFICIENTS IN LARGE FAST REACTORS

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