Meeting 25% of the electricity demand by nuclear energy is one of the pillars of the UK government's strategy for a secure and net-zero UK energy sector by 2050. In the near future, increasing nuclear installed capacity will rely on building new water-cooled fission reactors, which already represents 90% of the worldwide operating fleet.

Water-cooled reactors rely on boiling to efficiently transfer the large amount of heat produced in the core and power the steam turbine generating electricity. The "critical heat flux" (CHF) is a limit on the maximum amount of power that can be safely generated in the reactor. If exceeded, the rate of steam generation is so intense that it can blanket the heating surface (e.g., the fuel rods in the reactor core), compromising the heat transfer capabilities of the system. Temperatures can increase up to the melting of the heating surface, making CHF a major risk to the integrity of the reactor and the safe containment of its radioactive inventory.
However, our knowledge of the physics of boiling is still limited, and we are therefore forced to rely on empirical correlations, developed years ago from full-scale, expensive experimental CHF measurements, for the assessment of the reactor thermal limits. Due to the empirical nature of these models, overly conservative engineering margins are adopted, and reactors are forced to operate at a power that is only ~75% of the predicted CHF limit.

In this project, we will develop higher-fidelity, innovative computational models of boiling built from physical principles and capable of high accuracy. With these models, reactor thermal limits will be established with less conservatism, enabling reactors to operate at higher power levels and provide affordable, reliable and carbon-free electricity to our future society. The project will specifically improve two key areas of nuclear reactor thermal hydraulics: prediction of CHF at pressurized water reactor high pressure (~ 16 MPa) operating conditions, and external passive cooling of the nuclear reactor vessel, a key strategy to mitigate the progression of rare but dangerous reactor accidents.

With heating and cooling applications responsible for around 40% of global CO2 emissions, improvements in heat transfer through boiling will benefit many other sectors, such as cooling and micro-cooling applications in high power density electronics. In these areas, advancement and further improvement of equipment and efficiency will be dependent on the availability of the advanced and reliable modelling capabilities that this project will develop.