The nuclear power plants now widely constructed can only utilize 1% to 2% of uranium resources. It buys us decades of precious time to develop new energy sources. Our ultimate goal, however, is to find lasting solutions to energy problems. Many countries are currently actively researching a new reactor type - fast breeder reactor. When they can be promoted like pressurized water reactors, the world's energy tensions will change.
What is a breeder reactor? When any reactor is running, it can convert part of the uranium-238 or thorium-232 in the core into fissile material. The ratio of newly produced fissile material to consumed fission material in the reactor is called the conversion ratio. The conversion ratio of a pressurized water reactor is only about 0.6, while the conversion ratio of a fast neutron reactor is greater than 1, that is, every time a fission atom is "burned", more than one new fission atom will be formed. Therefore, the nuclear fuel in the reactor is not "burned" less, but more "burned". After a few years, the fuel accumulated in one reactor will be used by two reactors. This is called the "proliferation" of nuclear fuel.
There are two ways to multiply nuclear fuel. One is to use uranium-238 to produce plutonium-239. The plutonium-239 is made into fuel elements and then fission in the reactor, which can also convert uranium-238. This approach is called the uranium-plutonium cycle. Another approach is to use thorium-232 to produce uranium-233. When uranium-233 is used as fuel, it converts thorium. This approach is called the thorium-uranium cycle. When these two fuel cycles actually come into operation one day, the energy reserves that people have will be expanded dozens of times.
To achieve fuel proliferation, the use of fast neutron reactors is an effective way. What is the reason for this? When the speed of neutron movement increases, generally speaking, the chance of nuclear reactions with various nuclides decreases. But there is one exception: its chance of being captured by uranium-238 increases. Because uranium-238 has the ability to "absorb vibrations." Relying on this ability, it can capture many neutrons and turn itself into plutonium-239 to achieve the purpose of proliferation.
In order to ensure that neutrons are not moderated, the design of fast neutron reactors has many features. First, it has no moderator and the fuel elements in the core are arranged very compactly. Secondly, since fast neutrons are not as capable of fissioning uranium-235 as thermal neutrons, the concentration of uranium-235 in the core fuel must be increased or a portion of plutonium-239 must be added. Third, depleted uranium (a substance containing less uranium-235 than natural uranium is called depleted uranium) or thorium-232 forms a breeding zone around the core to capture fast neutrons escaping from the core. Compared with thermal neutron reactors, fast neutron reactors generate a large amount of heat energy in a smaller core, so the coolant is required to have better thermal conductivity. Liquid metal is generally used to remove heat from the reactor core.
Breeding was first achieved in 1946 on a small test reactor in the United States. This is a reactor that uses plutonium as fuel and mercury as coolant. On this basis, the United States built the experimental breeder reactor EBR-i. It uses enriched uranium as fuel and uranium-potassium alloy as coolant. This reactor occupies a memorable page in the history of nuclear power development. Because it was the first to convert nuclear energy into electrical energy. In February 1951, it used the energy of fission atoms to drive a small turbine generator for the first time, illuminating the dark Idaho desert with the light from four light bulbs.
In the 1940s, fast breeder reactors have experienced the development process of experimental reactors, prototype reactors and commercial demonstration reactors in many countries. At present, the most mature fast breeder reactor uses liquid sodium as coolant. Relying on the extremely excellent heat transfer properties of this metal, the reactor core can reach a very high power density, thereby shortening the time required for nuclear fuel doubling.
Countries around the world have built dozens of fast neutron reactors and experimental devices, several of which were scrapped after a short period of time. The United States has built seven fast neutron reactors. The last one was a demonstration fast neutron reactor with a power of 300 megawatts. The final construction plan was shelved because it failed to pass safety approval.
France has developed better fast neutron reactors. Its "Phoenix" fast neutron reactor and "Super Phoenix" fast neutron reactor both adopt integrated pool structures. The reactor vessel is a large stainless steel pool with a diameter of 22 meters, a height of 10 meters, a wall thickness of 35 to 50 millimeters, and a 3-meter-thick steel and concrete cover on the top of the reactor. In this steel tank, in addition to the reactor core, a primary loop sodium pump and a sodium-sodium heat exchanger are also placed, which ensures that radioactive sodium will not leave the reactor vessel.
The sodium in the primary loop passes through the nuclear fuel from bottom to top, is heated to 545°C, and then enters the sodium-sodium heat exchanger. Outside the reactor vessel, there is a steel vessel of the same thickness. The entire device is installed in a 1-meter-thick concrete containment shell, which can be said to be heavily fortified and safe. In the 1991 world nuclear power plant statistics table, nine fast neutron reactor nuclear power plants can be found, but only four are actually in operation. France's "Super Phoenix" reactor is one of them.
Fast neutron reactors are very expensive due to complex technology and high safety requirements. Its investment is about five times that of a pressurized water reactor nuclear power plant.