On-line quantification of delayed neutron precursor drift reactivity in molten salt reactors: A shadow-precursor observer verified against closed-form benchmarks in Modelica/TRANSFORM

In molten salt reactors the delayed neutron precursors drift with the fuel salt. The steady-state consequences — a reduced effective delayed neutron fraction — are well captured by system codes, but the reactivity of the drift has been available only on a single exponential mode or, on-line, only at constant power, where it equals the inventory reading β 0 − β eff ( t ) . In power-varying transients that reading conflates the flow effect with ordinary precursor lag, and the built-in reactivity of the codes deliberately omits the drift channel. This paper derives a shadow-precursor observer that isolates the drift component as an on-line equivalent reactivity for arbitrary power histories — one differential equation per precursor group, non-intrusive — together with an exponential-mode closed form and the circulating-fuel inhour relation for the core–loop geometry. A three-tier, experiment-free benchmark set executed in Modelica/TRANSFORM verifies all three against closed-form references in the code’s own discrete equations: the observer recovers the constant-power reading exactly and matches the inhour and exponential-mode references in free-power flow transients. Under a ＋50 pcm step at constant flow, the observer exposes a drift modulation of + 67.1 pcm, and the common static- β eff substitution overestimates the period of that insertion by + 28 % .