No electrical supplies or pumps are required to cool the reactor following an incident, as this is achieved by natural convection and gravity coolant feed. This feature ensures the reactor will remain safe under severe accident conditions.
Natural (Passive) safety systems reduce the capital and maintenance costs compared to large power reactors and fundamentally changes the economic equation in favour of SMR nuclear power generation.
SMRs can be classified into three main types depending on the technology employed:
1. Light water reactors
2. Fast neutron reactors
3. High temperature gas reactors.
SMR-NT will make a final selection of the preferred technology in tandem with the regulatory approval process for design certification in the country of origin.
Set out below is an overview of the key features of the three technology types under review.
The majority (90%) of nuclear reactors worldwide are known as Light Water Reactors because they use water as the primary coolant.
In the Pressurised Water Reactor (a type of light water reactor, originally based on the US naval reactor used for submarines), the primary coolant water is kept under sufficient pressure to prevent it from boiling, and the heat extracted from the nuclear fuel is transferred to a secondary water circuit in a heat exchanger where steam is produced to drive a turbine.
Unlike the thermal neutron Pressurised Water Reactor, where the water slows down (moderates) the neutrons produced in the fission process, a fast reactor has no moderator and is smaller, simpler and has better fuel performance.
Fast reactors only require refuelling at very long intervals – up to 30 years. They operate at or near atmospheric pressure (this minimises plant stresses) and are inherently safe with a negative temperature coefficient which means, if the temperature rises, the nuclear reaction is slowed and the power reduces. They are normally cooled by liquid metals with high conductivity and a high boiling point such as sodium, lead or lead-bismuth. Fast neutron SMRs have outlet temperatures of 500oC, and hence improved thermal efficiency (compared to thermal reactors). This outlet temperature is also suitable for hydrogen production. They typically use natural convection primary cooling systems and have passive back-up cooling systems which means that they do not rely on external power sources for safety.
This technology uses a fuel in the form of TRISO (tristructural isotropic) particles, <1mm diameter, combined with graphite and silicon carbide into pebbles or prisms and is stable to over 1600oC. The preferred coolant gas is helium, with outlet temperatures up to 1,000oC, enabling the reactor to be coupled to a Brayton cycle gas turbine/alternator with up to 50 per cent unit efficiency possible. The VHTR is one of the six reactor types selected by the Generation IV International Forum in 2002 for future nuclear energy systems that would excel in safety, sustainability, cost-effectiveness and avoidance of misuse of nuclear materials (proliferation resistance). The VHTR is a US priority for the next generation reactors and fuel irradiation experiments and qualification of high temperature materials are in progress.
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