Heavy Water Zero Power Reactor (HWZPR) mixed core first criticality, calculation and experiments

Highlights: •Change the core configuration of Heavy Water Zero Power Reactor. •Simulation of the new core and calculation of the physical parameters by different codes. •Substitution thirty six natural U metal fuel rods by natural UO₂ fuel assemblies in the core. •The first criticality operation is done, Measurement of the important physical parameters. •Good agreement between the theoretical and experimental results. -- Abstract: HWZPR original fuel is natural U metal fuel but other kinds of fuel can also be utilized. In a research work on UO₂ fuel, reactor fuel was partially replaced by natural UO₂ fuel and physical parameters of the new core were compared with original core. Thirty six natural U metal fuel rods were substituted by natural UO₂ fuel assemblies. Prior to the first criticality operation with the new core, it was simulated by stochastic and deterministic calculation methods i.e. MCNP-4C and WIMS-CITATON codes, respectively. In order to investigate criticality and safety of the mixed core, important reactor physics parameters such as effective multiplication factor (K_eff) at different water levels, critical water level, reactivity worth of D₂O and reactivity worth of safety and control rods were calculated. The calculated results ensured reactor criticality and satisfied reactor safety criteria. Therefore, with the permission of the reactor safety committee, the first criticality operation was performed successfully. Later, during a series of reactor operations important physical parameters were measured experimentally. There is good consistency between the theoretical and experimental results.