USING FERTILE FUEL | SMRs & ADVANCED REACTORS
Left, figure 1: Various reactor concepts grouped by fuel type and refuelling requirements. The number of sides of the geometric figures (circle, square, etc) denotes level of difficulty
happens in the fuel salt, neutrons are absorbed by the blanket where Th232 is converted into U233 predominantly. Trace amounts of U232 and U234 are also generated in the blanket. A proprietary system is used to transfer all species of uranium from the blanket to the fuel salt periodically several times per operational hour. Each transfer is a few grams of uranium. The reactor is designed to output 100 MW of heat and the outer diameter of the Onion Core®
is
2.4 meters. There are two barriers between the hot salt and the
unpressurised heavy water and graphite insulation foam is placed in the cavity. The transfer of heat from the hot salts to the water has been measured to be ~35 kW. The majority of the temperature increase in the heavy water originates from neutrons and gammas. Between 5 - 7% of the total reactor energy is injected into the heavy water and can be used for water desalination or district heating. In order to be able to breed more new fissile fuel than is being consumed, we also need to remove the majority of fission products outside the lanthanides group. Figure 3 shows how the Onion Core®
is connected to pipes, pumps,
heat exchanger and tanks. The use of a heat exchanger creates three barriers between the radioactive salt and the salt which is sent outside the cocoon to the customer, eg, for electricity generation. The Cocoon wall and the Insulation wall are several meters thick to provide shielding from radiation. When power to the pumps is cut they stop and all liquids (salt & water) drain into their respective dump tanks in 10-300 seconds depending on the volume of each circuit. Cutting the power to the pumps is the primary safety feature. Not all pumps are shown in Figure 3. There is one pump for each of the four channels in the Onion Core®
. This molten salt reactor is configured such that it can
only load follow. This means that the reactor core can only produce the same amount of energy as is removed from the salt circuit leaving the left side of Figure 3. Control rods are not needed and the volume of heavy water in the inner water region can be adjusted to criticality. This type of reactor cannot have a loss of coolant
accident with subsequent core meltdown and it cannot have a rapid steam explosion of the type that can be envisaged for PWRs. It also does not have a spent nuclear fuel pool.
www.neimagazine.com | June 2025 | 25
Figure 4 shows a 25 year burnup simulation of the reactor
starting on a 5% enriched uranium fuel salt. It can be seen that the reactor consumes the majority U235 within the first 5 years, while it builds up U233 and plutonium in the fuel salt. It can be seen that ~70% of the power originates from uranium and ~30% originates from plutonium after 5 years. Gradually shifting towards ~85/15 % at 25 years. Every 5 years we siphon off fissile inventory (200 - 500 kg), which can be used to start other reactors. The simulation assumes lithium 7 (Li7) enriched to 99.999% purity and 99.9% deuterium content in the water. Neutron leakage from the reactor core is ~2%.
Planned tests Thermal expansion, thermal cycling and thermal heat transfer have been tested in a full scale prototype non- nuclear rig in Copenhagen. The first nuclear test of
Below, figure 2: Cross section view of the Copenhagen Atomics Onion Core® showing the different layers and their temperature
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