The PBMR design has been put forward by the South African Utility, ESKOM in partnership with an international consortium. It also meets Generation IV design objectives in that it includes passive safety features to meet public acceptance criteria and offers competitive economics. The units are relatively small at 110-120 MWe with good economic and safety characteristics. The PBMR is also flexible in that it can be built virtually anywhere. It operates with a direct Brayton thermo-dynamic cycle, with target efficiency of around 45%. In principle, it can also use a thorium fuel cycle as well as a traditional uranium cycle. The design is modular in order to enable an operating utility to match the size of his station to the demand. The present capital cost is estimated at about $1000US per kWe, the construction period is estimated to be very short at around 2 years.

The PBMR offers a potential complementary service to the energy market in terms of present plant capabilities as both an electrical and non-electrical energy generator. It is of medium size, comparable with current-sized gas plants. It could offer a capability for the co-generation of heat or even dedicated nuclear heating applications, as expanded below.

The PBMR design is based on the HTR-MODULE design previously licensed in Germany for commercial operation. Present activities are aimed at the engineering design, independent safety reviews by participating countries in the ESKOM project and in making provisions for the licensing process.

The HTGRs have desirable features from various safety perspectives. The cores have a large thermal inertia, low power density and a strongly negative Doppler reactivity coefficient. As for most reactor types, the transients can be categorised into two broad categories, reactivity-initiated events and loss of flow events, either with or without depressurisation. For an un-scrammed core heat-up, the maximum core temperatures are reached within 3 days but fuel temperatures do not exceed above 1600°C ensuring that fuel particle integrity is preserved.

One concern with HTGRs is that air could ingress the core resulting in oxidisation of the graphite. This would require a multiple failure scenario of ruptures in the pressure vessel and surrounding concrete. However as noted above, even if such events occurred, there would still be several days to breach the opening of the reactor vessel.

A considerable advantage of gas systems described above is that they are free from the usual problems associated with loss of cooling in LWR systems. Thus there are no phenomena of concern such as ‘Departure from Nucleate Boiling’ loss of heat transfer or ‘Pellet Clad Interaction’ failure.

The reactor has diverse and redundant safety systems. For example, the reactor can be shutdown by three independent control systems. Each system is sufficient in itself to achieve this requirement.

In summary, in addition to electricity generation, HTGRs are being proposed as candidate plants for process heat applications that require high-temperature conditions. These include hydrogen and methanol production in a steam reformer, a process that requires high-temperature heating of steam and methane. Steam could be produced and then utilised for processes such as coal densification and steam injection for the recovery of hydrocarbons. These plants would also be suitable for de-salination processes, which require low-temperature heat. There may be potential to take waste heat from the pre­cooler that would otherwise be wasted. The ways of operating HTGRs in these multi­generation modes would add significantly to the thermal efficiencies that would be achievable with the plant.

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