Adaptive coupling of transport and depletion using the neutronically weighted inventory deviation (NWID) metric

Most depletion engines solve transport at every depletion interval (fully transport-coupled depletion), this is the highest-fidelity option but impractical for many-interval histories. Some engines also offer a cheaper transport-independent mode that loses accuracy as the spectrum drifts. This paper introduces adaptive coupling as a new option that applies to both within-interval and across-interval depletion problems, triggering transport recalculation only when the Neutronically Weighted Inventory Deviation (NWID) tracking metric exceeds a user-defined tolerance. NWID quantifies inventory drift weighted by neutronic importance and is computed after each depletion interval at negligible cost. Demonstrations on PWR and SFR pin cells depleted at an irregular schedule to 45 and 131 MWd/kg, respectively, show that end-of-life baseline NWID differs by an order of magnitude between the two systems (145% for the PWR versus 14% for the SFR), mirroring the gap in their fully independent inventory errors and consistent with the spectral rigidity of fast systems. At 3% NWID tolerance simulations for both cases, the PWR recovers 0.61% maximum actinide error, 0.58% maximum fission product error, and - 45 pcm k eff deviation throughout the depletion history all relative to the fully coupled reference. While the SFR achieves 0.11% actinide error, 0.10% fission product error, and a k eff deviation of only - 19 pcm. Benchmarking results presented above were achieved while reducing the number of transport solves by 2.3 x and 14.3 x for PWR and SFR, respectively, with proportional reductions in runtime.