Nuclear power has been a vital part of the UK's energy mix, providing baseload electricity at around 21% of the UK's total supply, the largest single source of low carbon power. The UK government's commitment for net zero carbon emissions by 2050 will lead to a step increase in electricity demand, as well as the need to decarbonise a range of other energy uses. The Nuclear Innovation and Advisory Board (NIRAB) advises to plan for nuclear energy to provide at least half of the firm low carbon electricity and also that nuclear power can contribute to the decarbonisation of other energy sectors. NIRAB's vision is for three streams of nuclear product development. Large-scale Light Water Reactors (LWR), currently available, for baseload electricity generation. Small Modular Reactors (SMR), based on the same technology for additional flexibility to meet local energy needs. Advanced Modular Reactors (AMR), with higher operating temperatures, to also contribute to the decarbonisation of other energy sectors.
A thorough understanding of thermal-hydraulics is essential in light-water nuclear reactor technology. In fact, key to the design and safe operation of water-cooled nuclear reactors is the effective transfer of heat, which is achieved by convective heat transfer to and from the coolant water. At a fundamental physical level, the process of convective heat transfer is extremely complex and only partially understood at present, despite decades of extensive investigations. As a consequence, even established reactor technologies need to incorporate penalizing safety margins into their design, in order to compensate for the limited understanding of thermal-hydraulics phenomena. Thermal-hydraulics areas crucial to nuclear reactor technology where high-resolution data are particularly needed to advance the fundamental understanding include the following:
1) Flow boiling and two-phase flow: Crucial nuclear reactor technology, here the fundamental understanding is so limited that the analysis and modelling still heavily rely on low-resolution empirical correlations, distilled from steady-state data and arbitrarily extrapolated to transient operating conditions;
2) Conjugate heat transfer: Crucial for convective heat transfer in general and for thermal fatigue prediction in structural components; the high-resolution investigation of the thermal coupling between flows and solid structures is still restricted to laboratory-scale systems and mainly to single-phase flows; high-resolution data are not available at a reactor-representative scale;
3) Flow-Induced vibration: Crucial for fuel durability and profitable operation of nuclear power stations, because the fretting wear that results from flow-induced vibration is responsible of the majority of fuel leaks observed in water-cooled reactors; very few documented studies measure both the mechanical vibration of the rods and the flow field around them, none at reactor-representative scale and operating conditions;
4) Natural convection passive cooling loops: Passive cooling is of vital importance in nuclear reactor technology, yet high-resolution data are not available at a reactor-representative scale, either for single phase or two-phase heat transfer;
These are the knowledge gaps that the present research aims to address.