Two-group, three-dimensional model based study of reactivity induced transients in upgraded LEU material test reactors

A two-group, three-dimensional diffusion theory based methodology coupled with one-dimensional single-phase heat transfer calculations has been developed for the transient analysis of typical material test reactors (MTRs). This methodology has been implemented in a FORTRAN based computer program MTRAP3. It uses the CITATION computer program as a subroutine for static neutronic calculations while the group constant generation is performed by employing the WIMS-D/4 code. The MTRAP3 program uses Cranck-Nicolson (CN) based numerical scheme for solution of time dependant neutron diffusion calculations while time-implicit strategy is employed for detailed heat-transfer calculations. The CN-scheme has been found to remain stable for much larger time steps (Δt ∼ 10-5 s) as compared with the time-explicit scheme which is limited to very small time steps only (Δt ∼ 10-10 s). For step as well as for ramp reactivity induced transients, the predicted values of core integrated reactor power and core average temperatures has been found to agree well with the corresponding values found by using the PARET computer program. The assembly-wise power profile as found by the MTRAP3 program has been found consistent with the corresponding experimental measurements