Small modular reactors
Secondly, there are Boiling Water Reactors (BWR) which make up 15% of global reactors. These only have 1 water circuit which flows through the core and the reactor; however, it is at a pressure where it can boil. The steam generated from the water boiling is then fed into the turbine, and electricity is then generated.
Lastly, there are Pressurized Heavy Water Reactors (PWHR) which make up 11% of global reactors.
This design uses heavy water (deuterium oxide). This is water that is composed of deuterium instead of hydrogen, thus making it heavier with a relative molecular mass of 20, instead of 18. PWHRs work similarly to PWRs, but because they use heavy water in the first water circuit, it is possible to use naturally occurring uranium instead of enriched uranium as the heavy water can act as a moderator.
The Rolls-Royce SMR design will use a PWR design and will use industry standard fuel, uranium dioxide (UO 2 ) powder, which has been proven to be safe. The fuel is naturally abundant and will be enriched to no more than 4.95% of Uranium-235. UO 2 is a black powder, which is compressed and inserted into a pellet, which are then loaded into tubes to form fuel rods and placed inside a Reactor Pressure Vessel. During the fission chain reaction in the reactor vessel, heat is produced, raising the temperature in the primary circuit to around 300 o C, but the water is pressurized to around 15.5 megapascals (MPa) so it does not boil. The primary circuit water is then circulated around the reactor before being pumped into a steam generator where it will pass through thousands of inverted U-tubes before its heat is transferred through the tubes to the water in the secondary circuit, which is kept at a lower pressure, 6-7 MPa, allowing it to boil. The boiling water vaporizes into steam and passes across the steam turbine blades, causing it to spin on its bearings. These turbines are connected to an electrical generator, which contains a large electro-magnet, which induces an electric current in the copper wire surrounding it as it spins, providing clean energy for the electricity grid. When the UO 2 fuel is burnt, radiation fission products build up over time and the fuel rods need to be removed from the reactor, after which they are either reprocessed into more fuel or disposed of. ‘ The Low-Cost Nuclear Challenge, proposed by a consortium led by Rolls-Royce, aims to develop an SMR designed and manufactured in the UK capable of producing cost-effective electricity. ’ 3 Rolls Royce, though typically associated with their cars, is leading the charge with the Low-Cost Nuclear Challenge, which is part of the government’s ‘ Ten Point Plan for a Green Industrial Revolution ’. 4 On November 8 th , 2021, the UK government announced up to £210 million funding for the Rolls-Royce SMR program to continue to develop their design for an SMR, one of the world ’ s first. Their proposed SMRs will be able to generate up to 470MW of clean, low carbon energy, which is equal to the power produced by 150 onshore wind farms. The energy created by these SMRs will lessen the UK’s reliance
3 https://www.gov.uk/government/publications/advanced-nuclear-technologies/advanced-nuclear- technologies. 4 https://www.rolls-royce.com/media/press-releases/2021/08-11-2021-rr-announces-funding-secured-for- small-modular-reactors.aspx.
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