The proposed Molten Uranium Thermal Breeder Reactor (MUTBR) is based on five design features:
1. The fuel is in large fuel tubes instead of thin fuel rods to increase the rate of fast fission of U-238. This increases the number of neutrons produced and reduces the amount of fissile material used, both of which increase the conversion ratio. (this is not new, no fast reactor has thin fuel rods)
2. The fuel is molten uranium metal. The surface to volume ratio of the large fuel tubes is too low to allow for cooling at the tube surface. The fuel is cooled by circulating it through a heat exchanger. The uranium metal fuel has higher density and lower neutron loss than other fuel forms. (Not new but hotter, some molten salt reactors do this)
3. The reactor is moderated with heavy water to reduce neutron loss and increase the conversion ratio. (not new, google CANDU)
4. The reactor is controlled by negative feedback using moderator steam to displace liquid moderator between the core and the reflector. This method under-moderates excess thermal neutrons, causing them to be absorbed by resonance capture in U-238 and increasing the conversion ratio rather than absorbing them in control material and wasting them. (New, US patent 8416908)
5. Some fission products are continuously removed from the circulating molten metal fuel by simple physical processes (mostly evaporation), not chemical processes. (not new idea, new method)
Even though the MUTBR depends on some fast fission of U-238, it is important to emphasize that the MUTBR is a thermal nuclear reactor. It is a thermal nuclear reactor not only because more than 60 % of fission events are caused by thermal neutrons, but also because the reactor is controlled entirely by controlling the supply of thermal neutrons. Even though around 25% of fission events are caused by fast neutrons, most of those fast neutrons come from fission events caused by thermal neutrons. When the supply of thermal neutrons is decreased, this not only reduces the rate of thermal fission but also reduces the rate of fast fission. Neutron physics considerations limit the size range of the fuel tubes so higher power reactors have more fuel tubes. The thermal power per fuel tube is around 20 MW.
Uranium melts at 1135 degrees Centigrade, so the reactor is proposed to operate at 1200 C. on the cool side of the fuel loop and 1400 C. on the hot side. This is far hotter than most other proposed reactor designs, but extended use of silicon carbide composites is considered, together with magnetohydrodynamic (MHD) pumps to circulate the melted uranium. Silicon carbide composites are now being used as LWR fuel cladding and have been tested at temperatures up to 1600 C. in a nuclear environment. The high temperature allows for the use of high efficiency air-Brayton cycle turbine generators for electricity generation and allows for efficient, low cost storage of thermal energy in cheap ordinary salt (NaCl).
The MUTBR is controlled by confining the moderator in control cavities which surround each vertical fuel tube. The control cavities are closed at the top and open at the bottom. There is a bubble of moderator steam at the top of each cavity. Each cavity is cooled by a pumped flow of cool moderator into the top of the cavity and heated by the flux of fast neutrons. The size of the steam bubble is the size which makes keff = 1.0000 and is controlled entirely by negative feedback. The reactor power is changed by changing the flow rate of the moderator cooling pump. The control method reduces the reactivity by under-moderating neutrons so they undergo resonance capture in U-238 rather than causing thermal fission; this increases the conversion ratio rather than absorbing and wasting excess thermal neutrons. The diagram below shows the relationship of the fuel to the moderator, the reflector, the steam bubbles, and the liquid moderator in the core of the reactor. (See Control_Method for more details.)
While other heavy water reactors (CANDU) can use natural uranium (0.72% U-235) as fuel, the large diameter fuel tubes preclude this in the MUTBR. However, the required fissile content of the fuel is only around 1.3%, so the fuel can be either very low enriched uranium or Light Water Reactor Used Nuclear Fuel (LWR UNF)(which has a fissile content of around 1.4%) or a combination thereof.
The large diameter of the fuel tubes and the fuel circulating through the heat exchanger outside of the reactor means that the reactor has a large fuel mass. This combined with a breed and burn fuel cycle means that the fuel may last as long as the reactor (50+ years). At the end of the reactor life the fuel will have a higher fissile content than standard LWR UNF so it can be used again in a new reactor of this type. It is proposed that such a reactor be built on the site of a decommissioned or operating Light Water Reactor to take advantage of the existing site license, power lines, and stock of UNF. The UNF would be reduced from uranium and plutonium oxides to metal and would be safely stored in the new MUTBR reactor in molten form and producing power for 50+ years with no shutdowns for refueling or fuel manipulation.
The concept has been validated with MCNP simulations and various aspects have been presented at American Nuclear Society meetings since 2014.
page last modified 10/11/2021