Subcriticality Measurement by Neutron Source Multiplication Method Swith a Fundamental Mode Extraction

Abstract

A new neutron multiplication method has been proposed for an accurate measurement of subcriticality. The proposed method consists of two correction processes for (1) extraction of the fundamental mode from measuring data of a neutron detector that contains higher modes as well as the fundamental mode feeding from an external neutron source and (2) spatial corrections for perturbations induced by a reactivity addition in the distributions of the fundamental mode and a neutron importance field. Feasibility of the proposed method has been verified from a numerical study, although under some limitations such that the neutron multiplying system to be analyzed is small-sized and a reactivity change takes place homogeneously in a fuel loaded region. With extraction of the fundamental mode and the spatial corrections, the subcriticality can be estimated accurately even with measuring data highly contaminated with higher modes due to a detector position near to an external point neutron source. For a future application to measurement of control rod bank worth of a nuclear power plant from measuring data during a reactor physical testing, some useful guidelines have been obtained.

Examination of Actual Neutron Count Rate Data during a Control Rod Drop Testing for Estimation of Control Rod Worth in PWRs

“…In a previous study, 3,4) we proposed the MNSM method for estimating control rod worth using neutron count rate data during a control rod drop testing in PWRs and showed the result of numerical simulation. The basic theory is explained briefly for convenience.…”

“…The basic theory is explained briefly for convenience. The conventional NSM method 3) estimates the subcriticality using the following simple formula as to a subcriticality defined as 1 À k eff :…”

“…Detailed derivation of these correction factors will be omitted because it was described in previous papers. 2,3) Definitions of the correction factors are summarized in Table 1 with remarks of the relevant parameters. In Table 2, the correction factors for each control rod configuration are listed.…”

“…From this point of view, the subcriticality measurement has been investigated. 2,3) In a previous paper, 4) the MNSM method was applied to subcriticality estimation in a numerical simulation based on the neutron count rate data during a control rod drop testing. This test is required before the criticality to prove that each control rod group can drop the full stroke of the axial length from full withdrawal to the full insertion position within the allowable time interval.…”