What Is a Thermal Reactor?

Thermal neutron reactor ^[1] is a device that uses a moderator to reduce the speed of fast neutrons, making them into thermal neutrons (or slow neutrons), and then using thermal neutrons to perform chain reactions. Since thermal neutrons are more likely to cause fission such as uranium-235, a chain fission reaction can be obtained with a small amount of fissionable material. Moderators are substances that contain light elements and absorb less neutrons, such as heavy water, beryllium, graphite, water, etc. Thermal neutron reactors generally arrange fuel elements in moderators regularly to form the core. The chain reaction is carried out in the core.

Thermal neutron reactor is a kind of nuclear fission reactor ^[2]

Moderator ^[3]

Heavy water reactors can be divided into pressure shell type and pressure tube type according to their structural types ^[6]

The so-called graphite gas-cooled reactor is a reactor using a gas (carbon dioxide or helium) as a coolant. This reactor has gone through three stages of development, producing three types of reactors: natural uranium graphite gas-cooled reactors, improved gas-cooled reactors, and high-temperature gas-cooled reactors.
(1) Natural uranium-graphite gas-cooled reactor Nuclear power plant Natural uranium-graphite gas-cooled reactor is actually a reactor with natural uranium as fuel, graphite as moderator, and carbon dioxide as coolant. This reactor is one of the types of reactors built for commercial power generation in Britain and France. It was developed on the basis of military plutonium production reactors. As early as 1956, Britain had built a nuclear power plant with a net power of 45 MW. Because it uses magnesium alloy as the fuel cladding, the British also called it the magnesium Knox reactor.
The core of this pile is roughly cylindrical, and is made of many graphite blocks with hexagonal prisms. There are many holes in the graphite masonry that contain fuel elements. This allows the coolant to flow through to take heat away. The hot gas from the core transfers heat to the water in the secondary circuit in the steam generator, thereby generating steam. These cooling gases are returned to the core by means of a circulating circuit. The steam generated by the steam generator is sent to the steam turbine, which drives the steam turbine generator set to generate electricity. This is the simple working principle of a natural uranium-graphite gas-cooled reactor nuclear power plant.
The main advantage of this kind of reactor is that it uses natural uranium as fuel. Its disadvantages are small power density, large volume, large charge, high cost, and natural uranium consumption is much larger than other reactors. Both Britain and France have stopped building nuclear reactors of this type.
(2) Improved gas-cooled reactor nuclear power plant The improved gas-cooled reactor was developed on the basis of natural uranium graphite gas-cooled reactor. The purpose of the design is to improve the steam conditions and the maximum allowable temperature of the gas coolant. In this reactor, graphite is still a moderator, carbon dioxide is a coolant, and low-concentration uranium is used for nuclear fuel (the concentration of uranium-235 is 2-3%). The outlet temperature can reach 670 ° C. Its steam conditions have reached the standards of new thermal power stations, and its thermal efficiency can be compared with it.
This kind of reactor is called the second-generation gas-cooled reactor. It was built in Britain. Because of many engineering and technical problems, its economics have been debated for many years, and the conclusion is uncertain.
(3) High-temperature gas-cooled reactors High-temperature gas-cooled reactors are called third-generation gas-cooled reactors. They are a reactor with graphite as a moderator and helium as a coolant.
The high temperature mentioned here means that the temperature of the gas has reached a relatively high level. Because in this kind of reactor, ceramic fuel and high temperature resistant graphite structural material are used, and inert helium gas is used as a coolant, so the temperature of the gas is raised to above 750 ° C. At the same time, as the structural material graphite absorbs less neutrons, the fuel consumption is deepened. In addition, due to the large surface area of the particulate fuel, the good heat transfer of helium, and the high temperature resistance of the core material, the heat transfer performance was improved and the power density was improved. In this way, the high-temperature gas-cooled reactor has become a type of reactor with high temperature, deep burnup and high power density.
Its simple working process is that helium coolant flows between the fuel bodies and becomes a high-temperature gas; the high-temperature gas generates steam through a steam generator, and the steam drives the turbine generator to generate electricity.
High-temperature gas-cooled reactors have special advantages: because helium is an inert gas, it cannot be activated, and it does not corrode equipment and pipes at high temperatures; due to the large thermal capacity of graphite, it does not cause a rapid increase in temperature in the event of an accident; Because the pressure shell is made of concrete, this way, the reactor does not have the risk of sudden rupture, which greatly increases safety; as the thermal efficiency reaches more than 40%, such high thermal efficiency reduces thermal pollution.
High-temperature gas-cooled reactors have the potential to provide high-temperature thermal energy to industrial sectors such as steel, fuel, and chemical industries, to realize new processes such as hydrogen reduction ironmaking, cracking of petroleum and natural gas, and coal gasification, and open up new ways to comprehensively utilize nuclear energy. But high-temperature gas-cooled reactor technology is more complicated.