A preliminary study on neutronic effect of local burn-up distribution in MHTGR-350 single assembly - KF170

For the design and safety assessment of the VHTR reactor, the burn-up analysis should be performed with appropriate core division. Also, it is well known that the fission source convergence must be considered for the Monte Carlo criticality calculation to avoid the calculation bias. However, the correlation between the axial core division strategy and the fission source distributions as the progress of the fuel burn-up has not been studied for the VHTR reactor. As a preliminary study to establish a core division strategy with Monte Carlo burn-up calculation, the neutronic effect of core division on burn up step was evaluated by dividing a single assembly MHTGR-350 into axial sub regions. The burn up evaluations were carried out using the MCNPX2.7 code with the ENDF 7.0 and the endf70sab thermal cross-section libraries. Also, the Shannon Entropy was evaluated with MCNP5 code. Although the fission source distributions at initial time steps were converged, the power and burn-up distributions were evidently changed as the core division strategy. Analysis shows that the differences of the power and burn-up distributions were occurred by the accumulations of the burn-up bias caused by the Monte Carlo calculations performed without the fission source convergence as the progress of the burn-up steps. The results show that the decision of the active cycle for the burn-up calculation of the VHTR reactor must be separately established as the core division strategy. Also, it was analyzed that all fission source convergences at each burn-up step must be considered for the Monte Carlo burn-up calculation. (authors)