Explicit and simplified modeling of in-core instrument for emitter region reaction rate calculation using STREAM/RAST-K

Abstract

This study proposes an efficient and accurate method to model the In-Core Instrumentation (ICI) that utilizes Self-Powered Neutron Detectors (SPNDs) for real-time monitoring of neutron flux and power distribution in reactor cores. Traditional core analyses have often neglected the detailed ICI effects, leading to potential inaccuracies in reactivity and power predictions. The newly developed ICI models, based on simplified geometry, explicitly consider the resonance self-shielding effects on the emitter nuclide cross sections, irradiation behavior during burnup, and reactivity perturbations. The use of a two-term rational approximation significantly improves the accuracy of absorption cross sections of emitter materials compared with reference data. The reactivity effect of ICI is approximately 276 pcm for a typical Westinghouse-type fuel assembly, causing critical rod position shifts of 2-6 cm in full-core simulations. In boron-free SMR conditions, absorption reaction rates of emitters were evaluated using critical rod search calculations. Results indicate that pseudo modeling shows deviations up to 20% in end-of-cycle power predictions, whereas explicit and simplified ICI approaches consistently match with actual assembly power. Therefore, these improved ICI modeling techniques are essential for precise instrumentation and reactivity evaluation in advanced boron-free SMR cores.