In order to transform the current energy supply to low-carbon technology, the trade-off between sustainability, energy security and affordability has to be considered. The path forward lies between two alternatives, reducing the storage costs for the intermittent renewables or developing an affordable and more flexible nuclear power. One of the possible solutions proposed in this thesis is developing a Small Modular Boiling Water Reactor (SMBWR) combined with external superheaters.

The SMBWR is a BWR-type small modular reactor. It is designed to adopt natural recirculation of coolant within its primary system. The SMBWR is also combined with the external superheater system. The system consists of 3 pieces of equipment: a superheater, reheater and economiser. The heat for the external superheaters could be supplied by a conventional gas boiler, waste heat from gas turbines or heat stored in molten salt from Concentrated Solar Power (CSP) plant. By having the external superheaters, the SMBWR power conversion cycle efficiency could be substantially improved, which means more electric power could be generated, improving the economics of the reactor. Furthermore, it offers the possibility for the SMBWR to follow the load only by adjusting the external heat provided to the superheaters, while keeping the reactor power continuously at its maximum nominal level, which would be another major economic advantage of the SMBWR. The objectives of this thesis are to demonstrate that the concept is practical and to quantify a number of hypothesised benefits of the SMBWR with external superheaters.

The investigation on the effect of SMBWR operating pressure showed that increasing the SMBWR operating pressure from 6.5 to 10 MPa has no significant effect on the neutronic performance. It is also found that increase in pressure would reduce the core pressure drop but increase the minimum chimney height required to develop natural circulation. In terms of thermodynamics, it is found that increasing the SMBWR operating pressure from 6.5 to 10.0 MPa will improve its thermal efficiency slightly by Δη of about 1.2%, which is small but not negligible. In order to investigate the trade-off between neutron leakage (neutronics), chimney height requirement for natural circulation (thermal-hydraulics), and dimensions of the core, three different geometry configurations, accounting for different length to diameter ratios were studied. The investigation on the power manoeuvring capability of the SMBWR found that the combined system can reduce its load down to 65% by only reducing the external heat provided to the superheaters, while keeping the reactor operation at full rated power.