Rolls-Royce Small Modular Reactor Contract Signed for Wylfa

The Rolls-Royce SMR is a pressurised water reactor, or PWR, the most widely deployed reactor type in the world. Its operating principle is one that A-level Physics students will find familiar from nuclear fission. Uranium-235 nuclei absorb slow-moving, or thermal, neutrons and split into two smaller nuclei, releasing energy and additional neutrons. Those neutrons are slowed down by a moderator, in this case ordinary water, and go on to trigger further fission events in a self-sustaining chain reaction.

The energy released by fission appears as kinetic energy of the fission fragments, which heats the surrounding fuel. In a PWR, water is pumped through the reactor core at high pressure, typically around 155 bar, preventing it from boiling even at temperatures above 300 degrees Celsius. This hot, pressurised water passes through a steam generator, a large heat exchanger where it transfers its thermal energy to a secondary water circuit at lower pressure. The secondary water boils, producing steam that drives a turbine connected to an electrical generator. The principle is identical to a coal or gas power station from the turbine onwards. The difference is in the heat source.

What makes the SMR design distinctive is its compactness. In a conventional large PWR, the reactor vessel, steam generators, pressuriser, and coolant pumps are separate components connected by large-bore piping. In the Rolls-Royce SMR, many of these are integrated into a more compact arrangement, reducing the overall footprint and simplifying construction. The entire nuclear island, meaning the reactor and its immediate support systems, fits within a building roughly the size of two football pitches.

Factory fabrication changes the economics and the quality control. In a factory setting, components can be manufactured using automated welding, precision machining, and standardised inspection processes. Welds can be X-rayed and ultrasonically tested in controlled conditions. Workers are not exposed to weather, and logistics are simpler. The finished modules are then transported to the site by road, where they are lifted into position and connected. This approach is well established in other industries; shipbuilding and offshore oil platform construction both extensively use modular fabrication, but it is relatively new in nuclear.

Safety is designed in rather than bolted on. The SMR uses passive safety systems that rely on natural physical processes, gravity, convection, and stored energy, rather than active pumps and operator intervention to keep the reactor safe during abnormal conditions. If the cooling pumps fail, for example, natural water circulation driven by temperature differences is sufficient to remove decay heat from the core. This is a direct application of the principle that hot fluids rise and cool fluids sink, driven by density differences. It means the reactor can reach a safe shutdown condition without any human action or external power supply.

The fuel itself is standard enriched uranium dioxide, the same type used in the existing global fleet of PWRs. Each fuel assembly contains fuel pellets, each about the size of a pencil eraser, stacked inside zirconium alloy cladding tubes. Zirconium is chosen because it has a very low neutron absorption cross-section, meaning it does not soak up the neutrons needed to sustain the chain reaction, while providing a strong, corrosion-resistant barrier between the fuel and the coolant.

For students interested in nuclear, mechanical, or materials engineering, the SMR programme offers a glimpse of how established physics is being repackaged through modern manufacturing techniques to solve a contemporary problem: how to build low-carbon power stations on time and on budget.