The proposed Molten Uranium Breeder Reactor (MUBR) is a radical new reactor design with potentially significant consequences even though it has some common elements with existing reactors such as molten-salt reactors and Canadian heavy-water reactors (CANDU). The reactor has large fuel tubes where the molten uranium fuel circulates as in some molten salt reactors. This increases the rate of fast fission of U-238 and decreases the amount of U-238 which needs to be converted to Pu-239 which increases the conversion ratio. Like Canadian CANDU reactors it uses heavy water as the moderator and reflector (reducing neutron loss and increasing the conversion ratio) but is cooled by circulating the molten uranium metal fuel through a heat exchanger. Some fission products are continuously removed from the circulating molten fuel, mostly by evaporation into a cover gas which circulates through a condenser where the fission products are condensed out of the inert cover gas and collected and stored until the reactor is decommissioned. The reactor is a breeder reactor (or near breeder in some smaller configurations), so it operates on a breed-and-burn fuel cycle, and the fuel lasts longer than the life of the reactor. (See Neutrons for a theoretical discussion of thermal breeders). Hence there is not a new batch of used nuclear fuel (i.e., nuclear waste) to deal with every few years. Much of the reactor's initial fuel can be the existing used nuclear fuel from other reactors, greatly reducing or eliminating the need to store used fuel. While it may require some new uranium fuel when it is initially built, it will not use any more new fuel over the reactor life, greatly reducing the need to mine and process uranium for fuel.
The patented control system of the reactor has low neutron loss and a wide control range of around 15,000 pcm, which allow the reactor to operate with an extensive range of initial fuel fissile content and fission product load and enables a long fuel life. It uses negative feedback with moderator steam to displace liquid heavy water between the moderator and the reflector areas. It also safer than traditional control rods.
The nuclear reactor produces heat, which a secondary coolant transfers to a separate electricity generation complex. The generation complex has a number of high-efficiency air-Brayton cycle turbine generators and a heat-storage system using ordinary salt. The generation complex can store some of the heat in the salt instead of generating electricity when other producers (notably wind and solar) are producing lots of electricity and the price is low. When other producers are producing less power and the price is high the generators can produce extra power by using the heat stored in the salt and sell all of the electricity at high prices. This provides higher profit margins for the plant operator and provides price stability and reliable power which benefits both other producers and the electricity users on the grid.
Numerous possible configurations of the MUBR design have been simulated with both MCNP and SCALE to determine the best configurations, to verify that they will operate as intended, and to provide some estimates of the potential fuel life. These configurations include a 3-tube micro reactor, a 19-tube small modular reactor, and a 80-tube large, grid scale reactor. These simulations also provide guidance for future research. In 2020, Oak Ridge National Laboratory ran a number of MCNP6 burnup simulations on a provided MUBR configuration, confirming that from a neutronics perspective, the reactor could work.
The MUBR reactor concept can provide safe, reliable, low-carbon electric power with more flexibility and at lower cost than other proposed reactors. It operates on a uranium-plutonium breed-and-burn fuel cycle so most of its energy comes from fission of the plentiful uranium-238. Some energy comes directly from fast fission of uranium-238 but most comes indirectly by converting uranium-238 to plutonium-239. It can use the existing stockpiles of used nuclear fuel (UNF) as part of its fuel, replacing existing and proposed UNF storage systems (unlike other proposed reactors, it uses all of the metal in UNF, not just the small amount of plutonium). It can greatly reduce the need to mine and process uranium for fuel. With its high fuel temperature it can provide higher temperature process heat than other reactors. It can store heat energy in higher capacity, lower cost salt (NaCl) thermal energy storage than other reactors can use. The small modular reactor and large grid-scale versions can use conventional UNF as the much of their fuel. In all versions, at the end of the reactor life the fuel has a higher fissile content than conventional UNF, so it can continue to be used as most of the fuel in other reactors of the MUBR type. The design is new and involves higher temperatures than other designs. While the MCNP simulations show that the theory is sound, there remains much engineering work and development before such a reactor can be built.
page last modified 09/26/2023
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