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What is a Supercritical Water Reactor?
A Supercritical Water Reactor (SCWR) promises an exciting next generation in reactor design. Imagine scientists asking, “What if we use water, but not just any water — super-hot and pressurised water — to create electricity?”. This inventive idea started gaining attention in recent years, sparking excitement across the nuclear community.
Neutrons - fast or slow?
A Supercritical Water Reactor is designed to be operated as a thermal reactor, using light water as a moderator to ensure neutron energies remain in equilibrium with their surroundings, typically around 0.025 eV.
Some experimental SCWRs have the potential to use fast neutrons with energy exceeding 1 MeV, hence the reactor does not require a moderator to slow neutrons down.
Supercritical Water
It’s in the name! A supercritical water reactor uses supercritical water as a coolant, as well as a moderator.
But what does this actually mean? Water becomes supercritical if it is heated and pressurised beyond what is known as its critical point. At such extremes of pressure and temperature (approximately 22.1 MPa and 374°C), gaseous steam and liquid water have very similar properties, meaning that the water no longer behaves as either a distinct gas or liquid: there is no phase boundary and the water behaves as a super-dense fluid.
The use of supercritical water eliminates the need for pressurisers and steam generators in an SWCR, as would be required in a Pressurised Water Reactor (PWR).
Fuel
Due to the flexibility of supercritical water reactor designs, a variety of fuel forms are possible. Most commonly, SCWRs use low-enriched uranium fuel (similar to that used in pressurised water reactors). However, there is also the potential to use plutonium-based fuels and close the fuel cycle or use next-generation TRISO fuel (as is being designed for use in high temperature gas-cooled reactors).
Development of Supercritical Water Reactor Technology
History of the Supercritical Water Reactor
As a concept, the supercritical water reactor evolved from decades of experience with light water reactors such as the pressurised water reactor and boiling water reactor designs.
The aim of a supercritical water reactor? To combine the best of both worlds. Research began in the 1950s in both the USA and Soviet Union then has progressed further since the 1990s under the Generation IV initiative.
What is happening today?
What sets a supercritical water reactor apart is its use of water in a state that is significantly hotter, denser and more pressurised than the norm.
Countries like Canada, China, and Russia are taking the lead in supercritical water reactor design – collaborating to explore and enhance SCWR technology. While SCWRs are not yet powering our cities, scientists across these nations and more are building smaller prototypes to better understand their capabilities. Ongoing international collaboration pushing the concept toward commercial readiness.
Explore Further
Choose from the articles below to continue learning about nuclear.
Supercritical Water Reactor (SCWR) – Steam, Speed and Sustainability
Lead-cooled Fast Reactor (LFR) – from Metal to Megawatts
Molten Salt Reactor (MSR) – The Future of Clean Energy?
Advanced Modular Reactor (AMR) – The Future of Nuclear Energy Explained
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