**Abstract**

The energy released from the nuclear fission process drives thermal expansion and mechanical interactions in nuclear reactors. These phenomena cause changes in the neutron chain reaction which results in further changes in thermal expansion and mechanical interactions. Coupling finite element analysis with Monte Carlo neutron transport analysis provides a pathway to simulate the thermal expansion and mechanical interaction to determine a fundamental parameter, namely, thermal expansion temperature coefficient of reactivity. Knowing the coefficient value allows predictions of how a reactor will behave under transient conditions. Using the coupling of finite element analysis and Monte Carlo neutron transport analysis, the thermal expansion temperature coefficient of reactivity was determined for the Godiva-IV reactor (−2E−05 Δk/k/°C) and the Experimental Breeder Reactor-II (EBR-II) (−1.4E−03 \$/°C). The Godiva-IV result is within 3% of the measured result. The thermal expansion and mechanical interactions within EBR-II are sufficiently complex that experimentally measuring the isolated coefficient of reactivity was not possible. However, the calculated result fits well with the integral EBR-II reactivity coefficient measurements. Coupling finite element analysis with Monte Carlo neutron transport analysis provides a powerful technique that gives reactor operators and designers greater confidence in reactor operating characteristics and safety margins.

**Keywords:** FEA, Monte Carlo, reactivity, temperature coefficient, reactor
