Reducing fuel temperature during depressurized loss of forced coolant transients, using neutron poisons in the external reflector in a Once-Through-Then-Out PBMR-200 DPP core
Kgomotshwane, Duduetsang Reneilwe
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Nuclear power plant safety is an important topic worldwide. It is therefore essential to continue to improve the safety of nuclear power plants. HTR technology improves safety and quality through its inherent design, ensuring that no internal or external incident results in the excessive release of radioactive material. Previous studies have been carried out successfully to reduce maximum Depressurised Loss of Forced Coolant (DLOFC) fuel temperatures for the PBMR-400 MW core to about 300 °C below the chosen safety limit of 1600 °C. Such reductions have been achieved through the introduction of neutron poison into its central reflector, in the centre of its annular fuel core. This strategy was successful both for the standard six-pass fuel pebble recirculation scheme and the Once-Through-Then-Out (OTTO) refuelling schemes. The aim of this study is to extend these gains to pebble bed reactors with cylindrical fuel cores, i.e. cores that does not have a central reflector, such as the PBMR-200. For such cores the only option would be to place the neutron poison in the external reflector, which can be expected to be less effective than placing it in the central reflector. Furthermore, it was decided to opt for the OTTO refuelling scheme, as many recent designs have selected this option. For technical reasons, it was decided to mimic the lack of a central reflector in the PBMR-200 by placing the poison only in the external reflector of the PBMR- 400, with an OTTO refuelling cycle, rather than to develop the required models to test this option for a PBMR-200 core. In practice, it is normally a DLOFC temperature hotspot in only a relatively small part of the fuel core that results in the upper limit for the maximum DLOFC temperature being exceeded. Therefore, the objective was to optimise the neutron poison placement in the external reflector to suppress DLOFC temperature hotspots by greatly flattening the axial profile of the DLOFC temperature. So, while the average DLOFC temperature might not be reduced substantially, the maximum DLOFC temperature can be expected to be greatly reduced. The value of this strategy was assessed by comparing the results to the case where optimised neutron poison distributions were placed in both the internal and external reflectors of the PBMR-400 core. The optimisation attempts were greatly frustrated by instabilities in the simulated power distribution, induced by the neutron poisons. From previous studies, it was known that a largely flat axial profile for the DLOFC temperature could be obtained by inducing a large power peak near the top of the core, followed by a suppression in power lower down and then again, a smaller power peak towards the bottom of the core. However, when the poisons were used to induce such a power distribution, the Very Superior Old Program (VSOP) diffusion simulation code apparently experienced instabilities in its calculational algorithm. This resulted in an unstable oscillation, where one refuelling cycle produced a large power peak at the top and almost no power at the bottom, while the alternating cycle produced a large power peak at the bottom and almost no power at the top. After much trial and error, good stable power profiles were obtained for both cases, i.e. where poison was placed only in the external reflector and where it was placed in both the internal and external reflectors. Switching from the 6-pass to the OTTO refuelling cycle increased the maximum DLOFC temperature for the PBMR-400 from its typical value of 1880 °C to 1979 °C. However, placing a tailored poison distribution in only the external reflector reduced the maximum DLOFC fuel temperature 1584 °C. Placing the poison in both the central and external reflectors reduced the maximum DLOFC temperature substantially to only 1519 °C. However, the more serious problem was that the poison in the external reflector substantially reduced the thermal neutron flux in the external reflector, which reduced the number of neutrons that were available to be absorbed by the control rods, which were placed in the external reflector. This meant that placing poisons in the external reflector reduced the ability of the control rods to shut the reactor down safely. For this reason, it was concluded that reducing DLOFC temperatures by means of poison placement in the external reflectors of pebble bed reactors with cylindrical cores was not an optimal strategy. Based on these results, it was concluded that for the future development of this technique it would be best to revert to core designs with a central reflector and to place the bulk of the poison in the central reflector.
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