Patent Application
20230245791
As a continuous effort to battle global warming, the nuclear energy industry has been looking into various options for the next generation technologies towards more sustainable, reliable, economical, and safe solutions. Meanwhile, there is an urgent need globally to replace the aging fleet of nuclear reactors, especially after the Fukushima Daiichi accident in 2011.
Prior Art #1
Among the proposed solutions so far, Sodium Fast Reactor (SFR) has become one of the options drawing close attention from the industry as well as financial support from the US Congress. In a nutshell, molten sodium is applied as the coolant in an SFR to replace other traditional coolant options, such as light water, heavy water, etc. When combined with molten salt as a secondary coolant and the heat source for steam based turbine generators, it has the potential to revolutionize the nuclear energy industry through improved power efficiency and flexibility.
As in
Although there are other similar options using various coolants, e.g. molten lead, high pressure water, or high pressure gas, the molten sodium is representative enough and hence it is referred to as the primary prior art in this invention.
Prior Art #2
Although not directly related to conventional nuclear reactors, plasma-based Fusion technologies under development in the past decades are relevant in terms of fundamental physics. Therefore, it is listed here as context and another prior art. No matter what the plasma confinement options are, e.g. magnetic or laser, the basic thermal nuclear reactions are all about combining smaller elements into ones with higher binding energies.
Since the invention is based on the Lithium based nuclear reactions, the following is called out as a typical fusion option related to Lithium.
Lithium-6+Deuterium→2×Helium-4+22.37 MeV (1)
While hoping the industry's effort on the fusion technologies will eventually be able to benefit humanity in the long run, the method in this invention is however not based on the concept in (1), and will not use Deuterium as nuclear fuel in the process.
This invention introduces the nuclear fusion process into the conventionally fission-based reactors. It's an evolution of the current nuclear reactor designs rather than a total revolution based on fusion-only based plasma technologies.
Particularly, new coolant is introduced in the reactor to serve as the secondary fuel that absorbs neutrons from the fission core, and releases secondary power through fusion reactions. Molten Lithium is considered the preferred coolant in the invention, as it produces Helium gas through the neutron-Lithium fusion without leaving any radioactive or chemical impact to the environment.
A Helium pressure controller is also introduced in the system to manage the Helium gas produced by nuclear fusion reactions using the secondary fuel. The pressure controller keeps gas pressure balance between the reactor chamber and the external environment by either releasing Helium gas from the reactor chamber or injecting Helium gas into the reactor chamber. It also manages the pressure in the outer prediction shell space.
To leverage the higher operating temperature of the new coolant, Lithium Chloride (LiCl) is proposed as the secondary coolant in lieu of the commonly used molten salt in order to achieve higher operating temperature of the secondary coolant and hence higher power production efficiency. It is expected the proposed system could bring reactor power efficiency beyond 50%.
A reactor based on the proposed system requires less space than a conventional reactor of the same power capacity. It is a better choice when space is a key constraint, for example, on a container ship.
1.
2.
Naturally occurring Lithium is composed of two stable isotopes, Lithium-6 and Lithium-7, with Lithium-7 being more abundant (92.5% natural abundance).
The following is a comparison of some basic physical properties between Lithium and sodium, which is the primary coolant used in Prior Art #1.
Based on the physical properties above, there are several immediate benefits of using molten Lithium as coolant in a reactor.
Next, looking into deeper aspects of the physics, neutrons created by the fission reactions in the Fission Core can trigger the following reactions.
Lithium-7+neutron→Lithium-8+2.03 Mev (2)
Lithium-8→Beryllium-8+electron+electron-antineutrino+15.49 Mev (3)
Beryllium-8→2×Helium-4+0.09 Mev (4)
Overall, it takes one neutron to turn one Lithium-7 into 2 Helium-4 plus an electron and an antineutrino, with a total energy of 17.61 Mev released.
There is no radioactive element (or nuclear waste) as the final outcome, since Helium-4 is a very stable nucleus. And, there is no chemically active element either, with Helium-4 gas being a noble gas.
What about the other isotope of Lithium, since Lithium-6 is still 7.5% of the element in nature if not separated purposely. The following is the fusion reaction between Lithium-6 and neutrons.
Lithium-6+neutron→Lithium-7+7.25 MeV (5)
It basically turns Lithium-6 into Lithium-7 with a significant energy release, and the Lithium-7 will then join the same reaction chain as described in (2)-(4).
In general, the Lithium coolant can slowly clean up neutrons from the fission reactions and produce extra energies as the secondary fuel, and release Helium-4 gas.
The proposed system in this invention is largely the same as illustrated in
The molten Lithium in the reactor chamber will get most heat from the fossil core, and then produce further energy when absorbing neutrons produced by the fission core.
The reactor can operate in a temperature range between 650 to 1000 Celsius with enough safety margin. It allows the secondary coolant to operate at up to 1000 Celsius, which is way beyond what a sodium based reactor could achieve. The high temperature of the secondary coolant can lead to a power production efficiency beyond 50%, so will be a huge jump in the energy industry.
To allow the secondary coolant to benefit from the higher temperature range of the reactor, there is a need for a new material that is stable and has a higher boiling point. In the proposed system, Lithium Chloride (LiCl) is a replacement of the typically molten salt consisting of sodium nitrate and potassium nitrate. Lithium Chloride has a higher boiling point of 1383 Celsius, a melting point of 610 Celsius, a lower density (1.02 g/cm{circumflex over ( )}3), and a similar heat capacity per mass (1132 J/Kg*K).
As illustrated in
Based on laws of thermodynamics, the efficiency of an ideal heat engine is defined as
Efficiency=(T_high−T_low)/T_high (6)
where T_high is the temperature of the heat source (e.g. the hot tank in an SFR) measured in Kelvin, T_low is the temperature of the cold sink in Kelvin. Given a fixed T_low, the higher the T_high, the higher the efficiency. For example, when T_low is 200 Celsius (471 Kelvin), if T_high changes from 500 Celsius (771 Kelvin) to 800 Celsius (1071 Kelvin), the heat engine efficiency would change from 39% to 56%, which is a significant improvement.
Since Helium gas will be released as the residuals of the secondary/fusion reactions, the Helium Gas Pressure Controller is responsible for balancing the relative gas pressure between the main chamber and the outside environment.
The space between the reactor chamber and the outer protection shell (230) should be filled with noble gas as well, e.g. Helium-4. It should always have a pressure slightly higher than the gas pressure in the reactor chamber, therefore also higher than the external environment as well. The same Helium Gas Pressure Controller can be used to balance among all three pressures: external environment, reactor chamber, and outer protection shell space. The outer protection shell space should always have the highest pressure, followed by the reactor chamber, then the external environment.
The Lithium fusion action is a slower process than the reactions in the fission core. When the fission fuel burns out and needs to be replaced, e.g. in every ˜24 months, additional Lithium coolant can also be added based on its consumption. There is no need to replace the remaining Lithium.
Due to the additional power produced by the Lithium fusion process, and the higher heat capacity of Lithium, a reactor with specific power capacity can have smaller size than conventional reactors or the prior art #1. Therefore, a Lithium based reactor can potentially be a better option whenever space is a constraint, for example, on a vessel, with the expectation that future container ship fleets should not burn fossil fuels.
