Abstract
An improvement of the IR (Intermediate Resonance) approximation is proposed. The effective resonance integrals of the infinite cylindrical fuel lattices are calculated by the analytical method, efficiently and with reasonably good accuracy. The fuel escape probability Pf(E) is based on the two-term rational approximation with an accurate Bell factor [Abar]. The IR parameters λ, κ and μ are determined taking into account the interference between the resonance and potential scatterings, and the effect of the fuel temperature. The effective resonance integrals of the main resonance levels of 238U were calculated using the improved IR approximations, and the accuracy of the method was ascertained by comparing the results with those obtained by the RICM code, which solves the neutron slowing-down equations numerically. Simple approximate expressions for J(ξ β) and J(ξ, σ, β) functions, which are useful in the IR approximations, are also given together with a table of J(ξ, β) and the errors of the simple approximation.
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