Virtually all existing nuclear reactors in the United States will be shut down by 2100, while China and other nations continue to build them. See, U.S. Nuclear Demise Amid Increases Elsewhere.
Small nuclear reactors have been proposed, where their modular design and ability to scale reactor size to demand are major benefits, but where the cost remains around $6,000 per KW, essentially the same as traditional boiling water reactors, including Gen-3 reactors, such as the four being built in Georgia and South Carolina. See, Small Modular Reactors.
Recently, another nuclear engineering company, X-Energy, announced a joint effort with South Carolina Electric & Gas Co. (SCE&G) to investigate the deployment of an Xe-100 pebble bed nuclear reactor (PBR).
The proposed Xe-100 design is for a 50 MW unit that could easily fit in available acreage at existing power plant sites, with the potential for upgrading coal-fired and nuclear power plants.
X-Energy and SCE&G claim the reactor is less costly to build than traditional reactors, but it’s not known whether they will cost less than $6,000 per KW.
PBRs are not a new concept. Germany was an early leader in PBRs, operating a 20 MW AVR unit for several years. Germany also built a 300 MW unit, but shut it down after two years, mainly due to political issues.
A pebble bed reactor, HTR-10, has been built in China, but another PBR proposed for South Africa was never built.
Pebble bed reactors (PBR) operate at high temperatures, using helium as a gas coolant and heat transfer agent. PBRs cannot melt down, which was the case with the core of the Fukushima reactors. PBRs could also be used to produce hydrogen.
The billiard-ball size pebbles are encased in graphite that cannot melt, and can provide stability for the long-term storage of spent pebbles.
Another advantage of PBRs is that new pebbles can be added to the unit while it’s in operation, thereby eliminating the need to shut down the reactor for refueling.
Referring to the schematic, pebbles are added at the top of the reactor, with spent pebbles removed at the bottom. The nuclear reaction within the pebbles generates heat that heats the helium to a high temperature. The helium flows through a heat exchanger where water is turned to steam. The steam is then used to drive a steam turbine generator.
The main technical and cost issue is whether PBRs can be built at costs well below $6,000 per KW. With natural gas power plants being built at a cost of $1,100 per KW, and the potential to build ultra-supercritical coal-fired power plants at around $2,800 per KW, PBRs may find that the economics are not in their favor.
Even so, PBRs have major advantages, including safety, easier disposal of pebbles, ability to operate at high temperatures, no need to shut down while the unit is being refueled and their modular design.
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