Fundamental-mode sources in approach-to-critical experiments

An equivalent fundamental-mode source is an imaginary source that is distributed identically in space, energy, and angle to the fundamental-mode fission source. Therefore, it produces the same neutron multiplication as the fundamental-mode fission source. Even if two source distributions produce the same number of spontaneous fission neutrons, they will not necessarily contribute equally toward the multiplication of a given system. A method of comparing the relative importance of source distributions is needed. A factor, denoted as g* and defined as the ratio of the fixed-source multiplication to the fundamental-mode multiplication, is used to convert a given source strength to its equivalent fundamental-mode source strength. This factor is of interest to criticality safety as it relates to the 1/M method of approach to critical. Ideally, a plot of 1/M versus {kappa}{sub eff} is linear. However, since 1/M = (1 {minus} {kappa}{sub eff})/g*, the plot will be linear only if g* is constant with {kappa}{sub eff}. When g* increases with {kappa}{sub eff}, the 1/M plot is said to be conservative because the critical mass is underestimated. However, it is possible for g* to decrease with {kappa}{sub eff} yielding a nonconservative 1/M plot. A better understanding of g* would help predict whether amore » given approach to critical will be conservative or nonconservative. The equivalent fundamental-mode source strength g*S can be predicted by experiment. The experimental method was tested on the XIX-1 core on the Fast Critical Assembly at the Japan Atomic Energy Research Institute. The results showed a 30% difference between measured and calculated values. However, the XIX-1 reactor had significant intermediate-energy neutrons. The presence of intermediate-energy neutrons may have made the cross-section set used for predicted values less than ideal for the system.« less