The Transient Reactor Test facility (TREAT) at Idaho National Laboratory (INL) is a unique reactor capable of stressing test fuel during a power pulse transient. TREAT fuel is composed of high enriched uranium (HEU) heterogeneous fuel; microscopic UO₂ fuel grains with an average radius of 10 μm are randomly distributed in a graphite moderator matrix. Microscopic heterogeneity separates the fuel grains, which produce heat from fission, from the graphite moderator that provides the majority of temperature feedback control (spectral hardening promoting core leakage) during a pulse. The proposed conversion of TREAT to a low enriched uranium (LEU) heterogeneous fuel fundamentally changes feedback mechanisms during a pulse. LEU fuel grains would provide a significant amount of Doppler feedback within the fuel grains, in addition to feedback provided from the graphite moderator. With TREAT providing conditions to stress a test fuel during a pulse, a method to simulate feedback mechanisms of TREAT during a transient pulse is necessary in order to provide boundary conditions to impose on a model of a test sample, both for the current HEU and proposed LEU conversion fuel concepts. The demonstrated method couples the heat energy and neutron diffusion transport equations taking into account heat diffusion from fuel grain to graphite moderator (termed “time lag” in literature) and varying feedback mechanisms (local Doppler broadening within fuel grains and larger scale spectral hardening from the graphite moderator). The method is implemented in the MAMMOTH code. A simplified reflected cube reactor based on TREAT fuel contents is simulated; HEU and LEU fuel concepts are simulated with 20, 10, and 5 μm radius fuel particle sizes assuming a spherical shape. Initial reactivity used to initiate transients is varied from 0.98 %Δk/k (weaker) to 4.56 %Δk/k (stronger) for each enrichment and particle radius. Lower resolution homogeneous models for HEU and LEU fuels are also simulated for each reactivity showing the effect the method produces on pulse simulations. The demonstrated method is shown to increase fidelity of a model by accounting for feedback from heterogeneity at the local grain-level scale. LEU fuel requires more inserted reactivity to achieve similar characteristics to a HEU baseline model. LEU fuel particles do not achieve as high average temperatures as HEU fuel particles during pulses with similar reactivity insertions. With increasing initial reactivity insertion, fuel grain average temperatures move away from moderator average temperatures until temperature peaking occurs. The method also shows that HEU fuel is indeed dominated by moderator temperature while LEU fuel has a significant portion of its feedback driven by fuel grain temperature.