A study on post-discharge decay heat evolution & isotopic contribution of uranium, thorium, and TRU-based zirconium hydride fuels in TRIGA Mark II reactor, Bangladesh

This computational study investigates the decay heat characteristics and isotopic source term evolution of three nuclear fuel cycles using OpenMC particle transport and depletion: Conventional 235U cycle (Model 1), 233U–Thorium cycle (Model 2), and transuranic (TRU)-loaded cycle (Model 3). Simulations modeled an 800-day irradiation period at 3 MWth (discharge burnup ∼ 50 MWd/kg) across a 1,000-year cooling horizon. During operation, all models reached saturation within 5 days, led by Model 1 in peak decay heat (∼190 kW), followed by Model 3 (∼160 kW) and Model 2 (∼150 kW). Post-discharge decay heat increased with burnup and peaked immediately after discharge before decreasing over time. While all models decayed rapidly for 10 days, Model 3 maintained a long-term thermal burden above 10 3 W after 10,000 days, whereas Models 1 and 2 converged below 10 2 W. Isotopic analysis showed short-lived nuclides (134I, 138Cs, 91Rb) dominating immediately post- discharge. At 2–4 years, Models 1 and 2 consolidated around 90Y and 137mBa, while Model 3 remained driven by alpha-emitting actinides (244Cm, 238Pu, 241Am). By 100 years, Model 3 carried a significantly higher thermal burden dominated by 241Am (∼600 W), whereas Model 2 retained a unique 233U signature. The Wigner–Way approximation proved accurate for conventional cycles (within ± 5.22%) but failed for the TRU cycle (deviations exceeding −50.03% by day 10) due to uncharacterized minor actinide alpha-decay heat. Additionally, multi-percentile sensitivity analysis revealed a secondary isotopic diversity peak near 10^5 days in Model 2 from 4n + 1 neptunium series progeny.