On the Use of Rossi Alpha Critical Assembly Measurements for Validating and Constraining Nuclear Data

Critical assemblies are exquisitely sensitive to details of the microscopic nuclear reactions that govern neutron multiplication. For this reason experimental studies of critical assemblies represent a cornerstone in the process of validating nuclear data. Several different characteristics of a critical system can be measured. The most commonly considered is the so-called effective k eigenvalue, k_eff. Another well-measured property of these systems is α₀, the inverse e-folding time of the neutron population in the absence of β-delayed neutrons. Through Monte Carlo calculations and appear to perturbation theory they show that for fast critical systems α₀ and k_eff can be viewed as lying on a single straight line for any reasonable assumptions about the underlying nuclear data. This means that the two quantities provide the same constraint on nuclear data. In principle, α₀ could be associated with a very small uncertainty, and this would make the measurements for neutron multiplication rates more useful than k_eff for constraining nuclear data. In practice, though, uncertainties in k_eff and α₀ are dominated by uncertainties in the representation of the critical system, and not by pure measurement errors for these quantities. This, together with the linear relation between α₀ and k_eff, implies that the two measured quantities provide exactly the same constraint on nuclear data. They do not consider other measured or inferred characteristics of critical assemblies, such as neutron generation times or spectral indices, that may be valuable in the validation process