Development of a new convergence criterion for Monte Carlo simulation with thermal-hydraulics feedback

Coupled multi-physics approach plays an important role in improving computational accuracy. Compared with deterministic neutronics codes, Monte Carlo codes have the advantage of higher resolution level. In the present paper, a three-dimensional continuous-energy Monte Carlo reactor physics burnup calculation code, Serpent, is coupled with thermal-hydraulics safety analysis code, RELAP5. A new convergence criterion for the coupled simulation is developed based on the statistical uncertainty in power distribution in Monte Carlo code, rather than an arbitrarily chosen criterion in previous research. The coupled simulation is based on the OECD-NEA/NRC PWR MOX-UO₂ Core Transient Benchmark. The convergence criterion of normalized axial power distribution is tested on both UO₂ and MOX single assembly models. Compared with previously implemented convergence criteria based on temperature, eigenvalue or flux (or power), it takes into account both the local and global convergence. It does not use a pre-set tolerance limit and is decided by the statistical accuracy of the Monte Carlo code itself. This new convergence criterion is shown to be stable, more stringent and direct, equally convenient to use but may need a few more steps to converge. (author)