Fusion Machine Thermal Blankets – The Basics
The role of thermal blankets in creating a sustainable power source.
Power Generation with Nuclear Fission
With some notable exceptions, every approach to generating electricity from fusion heats water into high-pressure steam. The steam then turns a turbine and a generator, producing electricity. In this respect, most fusion machines are no different from nuclear fission reactors or fossil fuel power plants. In fossil fuel power plants, heat from burning natural gas or coal superheats water in a boiler, turning it into steam.
In a nuclear fission reactor, pressurized water circulates through the reactor core (the “primary” coolant loop), which is superheated by fission. The super-heated water is radioactive, and it is run through a heat exchanger, where it hates “clean” (non-radioactive) pressurized water (the “secondary” coolant loop) into superheated steam, which turns a turbine and a generator. The pressurized water in the primary coolant loop also cools the reactor core, preventing overheating and fuel meltdown. Finally, the water in a nuclear fission reactor (specifically the hydrogen atoms in the water) moderates and slows down fast neutrons produced by fission, reducing neutron damage to the nuclear reactor.
Power Generation with Fusion Machines
The basic process of using a fusion machine to heat water, which drives a turbine generator, is similar to coal, natural gas, or fission, where they are all used to generate heat to create steam. The Fusion machine generates heat by releasing neutrons, which must be transferred from the inner containment vessel to some form of thermal transfer system to heat the water to drive the turbines. The heat transfer system in fusion machines is typically called a thermal blanket. These blankets typically utilize a liquid metal to conduct heat. As you can see in both graphs, the heat source changes, but the basic process is the same.
What is a Thermal Blanket?
A thermal blanket, also known as a fusion blanket, lithium blanket, or breeding blanket, is a key component of a fusion reactor that shields the reactor vessel and superconducting magnets from neutrons and heat. It also produces tritium fuel for the fusion reaction. Here are some of the functions of a thermal blanket:
Shielding - The blanket protects the reactor vessel and magnets from the high-energy neutrons produced by the fusion reaction.
Cooling - The blanket absorbs the energy from the neutrons and transforms it into heat energy, which is then collected by coolants. This heat can boil water and drive a steam turbine to produce electricity.
Tritium Breeding: The blanket reacts neutrons with lithium to produce tritium, which fuels the fusion reaction. Tritium is difficult and expensive to obtain, so it's more practical to make it on-site.
The Fusion Process Creates Heat and Neutrons
Fusion energy involves combining two light atomic nuclei to form a heavier nucleus, releasing energy. This is the same reaction that powers stars, including our Sun. Earth's most common fusion reactions involve isotopes of hydrogen, such as deuterium and tritium. The process combines deuterium and tritium, and the outcome is helium, a safe, inert gas, and neutrons that can be captured via a new technology known as a thermal blanket.
The “First Wall” of a Thermal Blanket and the Shield Block
As the thermal blanket will be on the inside of the fusion chamber and will directly face the fusion plasma, the thermal blanket’s “outside” fusion-facing wall panels (also known as the “first wall panels”) must be built with materials that survive the high temperatures and neutron radiation.
Beryllium was originally going to be used for first wall panels, but given its toxicity and the potential for panel failure from melting during “off-normal” plasma events, tungsten is now the material of choice for the thermal blanket first walls. The final component of the thermal blanket is the shield blocks, which provide mechanical support to the first wall and provide protection for the outer components, such as the vacuum vessel, and for magnetic confinement of the magnetic coils.
Thermal Blankets as Tritium Creator to Re-Fuel Fusion
The primary reaction targeted for commercial fusion energy is the deuterium-tritium (D-T) reaction, as shown on the right. While deuterium is prevalent (it makes up 0.0312% of the hydrogen in seawater), tritium is rare due to its radioactive half-life of roughly 12 years. The only man-made source of tritium today is “fast breeder” nuclear fission reactors, which are only a few worldwide and are expensive to operate. Because of this, a fusion machine should be able to “breed” (create) tritium as part of the fusion reaction process. While pressurized water could be used for steam generation in a fusion machine (outside the fusion chamber), water will not produce tritium.
One of the best materials for breeding tritium is lithium – fast neutrons hitting lithium-6 atoms produce helium and tritium. Lithium is also excellent at capturing the 14.1 MeV of energy from the free neutrons produced by D-T fusion and turning it into thermal energy.
Turning Neutron Energy into Thermal Energy
Most of the approaches to thermal blankets for production fusion machines utilize lithium “pebbles” (usually lithium-based ceramics) cooled by continuously pumped highly pressurized water; the pebbles (and, to an extent, the water) absorb the energy from the fast ions and turn it into heat. The thermal blanket also cools the fusion chamber and moderates the fast neutrons. As the water exits the thermal blanket, the tritium is separated from it and stored for future use. The water then goes through a heat exchanger, where it heats the pressurized water in the secondary loop, which drives the turbine and generator to produce power.
Your Grandma Cannot Knit A Fusion Thermal Blankets for You
The thermal blanket is widely seen as one of the most challenging components to develop in a fusion machine, perhaps the most demanding in terms of precision in material sciences, machining, and production. For a project like ITER, various versions of thermal blankets are being created by multiple nations to determine the best approach to capturing high-energy neutrons and producing electricity and tritium. Private companies working towards commercial fusion reactors all have their own thermal blanket models, but most are utilizing similar mechanisms to those being developed by ITER.