Modeling and Validation of Temperature and Void Effects on Reactivity Experiments at the Missouri S&T Research Reactor

To validate an MCNP5 model of the Missouri S&T Research Reactor (MSTR), temperature and void effects on reactivity experiments were simulated and performed. We compared the k_eff of the modeled reactor mirroring the position of all control rods to the actual critical reactor (k_eff = 1.00000). In the simulation we modeled three different scenarios. In the first two scenarios, the reactor is modeled as isothermal at two different temperatures (measured experimentally near the core), and in the third scenario, we split the core into bottom and top parts and used interpolated values for the temperatures of both halves. The model predicted k_eff’s for the “critical reactor” between 1.00234 and 1.00248 (±0.00018) when using as temperature the experimental thermocouple readings at the top of the core and k_eff’s between 1.00296 to 1.00383 (±0.00018) when using the temperature of thermocouple readings at the bottom of the core. In the third experiment, a linear vertical temperature profile was included in the model (only top and bottom of the core), and the model predicted k_eff’s between 1.00218 and 1.00302 (±0.00018). The k_eff modeled and experimental values differed by as much as 0.40%. A void coefficient of the reactivity experiment was also simulated introducing a void tube in the model and the control rods made to mirror the critical experimental reactor with an identical void.