Quantitative analysis of MMRTG power decay due to thermal inventory loss

Abstract

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) experiences electrical power decline due to Pu-238 decay and thermoelectric (TE) degradation. In this work, the contribution of Pu-238 decay is isolated and quantified by modeling thermal inventory loss as the sole aging mechanism. A full-scale three-dimensional coupled thermal-electrical finite-element model of the MMRTG is constructed and evaluated over a 17-year operating period, with temperature-dependent thermoelectric material properties retained in each quasi-steady solution, but with no time-dependent aging or degradation of thermoelectric properties, contact resistance, or radiative properties. The results indicate that fuel decay alone reduces the electrical output from approximately 124 W_e at beginning-of-life to ∼98.4 W_e after 17 years, corresponding to a cumulative reduction of ∼20.6%. This behavior is equivalent to an effective compounded electrical decay rate of approximately 1.3-1.35%/yr, exceeding the commonly assumed 1.1%/yr decay-only approximation and leading to a ∼4% overprediction of end-of-life power in simplified models. Comparison of this decay-only reference with published Curiosity MMRTG flight telemetry enables a quantitative decomposition of total power loss. Over 17 years, approximately 45% (∼25 W_e) of the cumulative loss is attributable to Pu-238 decay, while the remaining ∼55% (∼30 W_e) appears as a lumped non-decay residual, interpreted here primarily as the consequence of thermoelectric aging together with other non-decay loss mechanisms and model–flight mismatch effects. The decay-only trajectory defined here provides a physically constrained reference for interpreting MMRTG performance data and separating unavoidable nuclear decay effects from thermoelectric aging.