The criticality startup techniques for MHWRR under low photoneutron level conditions

[Background] The Multi-purposes heavy water research reactor (MHWRR) at the Birine Nuclear Research Center in Algeria had been upgraded. Its instrumentation, control and electrical system had been upgraded and fission molybdenum radiation apparatus were installed in the reactor core. First critical startup at very low photoneutron level after MHWRR upgrade is of great significance to the reactor modernization, marking the completion of the upgrading project. [Purpose] This study aims to fulfill successful critical startup of the reactor without additional neutron source, solve the problem of nuclear measurement blind area, and ensure the startup safety of the MHWRR after long period shutdown. [Methods] Based on the theoretical calculation and analysis of the residual photoneutron source intensity, the nuclear measurement blind area and the critical control rod height, a critical startup experiment scheme was put forward. The experiment was carried out in full accordance with the expectation of theoretical design in the absence of reference data, and nuclear power parameters were effectively monitored. [Results] The measuring data range of startup monitoring devices are well connected with the original monitoring ionization chambers outside the reactor core, and the error between theoretical value of critical control rods position and experimental value is less than 0.84%. [Conclusions] Successful critical startup and the agreement between the experimental results and the theoretical results show that the experiment technique for critical startup of MHWRR is reasonable and feasible. (authors)