Experimental study on the safety of Kyoto University Research Reactor at natural circulation cooling mode

• The natural circulation cooling capacity of Kyoto University Research Reactor (KUR) was experimentally investigated.
• The distributions of the outlet temperature of the fuel elements under natural circulation operations were measured.
• The average temperature rise and the average natural circulation flow velocity in core were calculated.
• The safety of KUR under all of the normal operations with natural circulation cooling mode has been analyzed.
• The natural circulation flow after the reactor shutdown was confirmed.

In this study, the natural circulation cooling capacity of Kyoto University Research Reactor (KUR) is experimentally investigated by measuring the inlet and outlet temperatures of the core under natural circulation operation at various thermal powers ranging from 10 kW to 100 kW and the shutdown state. In view of the uneven power distribution and the resultant inconsistent coolant outlet temperature in the core, eight measuring points located separately in the outlet of the fuel elements were chosen to investigate the distribution of the outlet temperature of the core. The natural circulation cooling capacity represented by the average natural circulation flow velocity in the core is calculated from the temperature difference between the outlet and inlet temperature of the core. The measured outlet temperature of the fuel elements shows a cross-sectional distribution agreeing with the distribution of the thermal output of the fuel elements in the core. Since the measured outlet temperatures decrease quickly in the flow direction in a small local region above the outlet of the core, the mixing of the hot water out of the core with the cold water around the core outlet is found to happen in the small region not more than 5 cm far from the core outlet. The natural circulation flow velocity in the core increases non-linearly with the thermal power. The safety of KUR has been analysed by conservatively estimating the highest coolant temperature, the highest fuel cladding temperature, the highest fuel meat temperature and the minimum DNBR under all of the normal operations with natural circulation cooling mode. The natural circulation operation at the thermal power of 100 kW is the most serious operating condition. The average natural circulation flow velocity at the thermal power of 100 kW is 3.13 cm/s. All of these estimated parameters under normal natural circulation operation meet the safety criterion of research reactor.