The Secret of Nuclear Fuel Diffusion
To understand the secret of nuclear fuel appreciation, we must first understand the secret of nuclear fuel fission. Generally speaking, according to the abundance of nuclear fuel in nature, the extraction cost and the difficulty of fission, we choose uranium isotope U-235 (element symbol is U) as the fuel of nuclear reactor, and its fission process is as follows:
Chain reaction of uranium -235
Because the total number of protons and neutrons in the nucleus of heavy elements is very high, uranium is element 92, and it has 92 protons. According to different isotopes, the number of neutrons will be different, so that the huge nucleus is unstable under the action of strong force and electromagnetic repulsion, so it may decay or fission. When a uranium atom is hit by a thermal neutron, it begins the fission process:
Uranium-235 fission process
First of all, the U-235 atom will become U-236 after obtaining a neutron, but the base of U-236 is unstable and will split into two smaller nuclei, namely KR-92 (element 36 of KR-92) and Ba-141(Ba-Ba-/Kloc-).
Nuclear fission process of U-235 atom
The key to chain reaction
The key of chain reaction lies in this redundant 3n, that is to say, three redundant neutrons will run out in the process of fission, but the fast neutron U-235 will not be absorbed, or the absorption rate is extremely poor, and it will run out of the reactor before being captured by the next nucleus, so it is impossible to continue fission, so the chain fission will hang up and the fission reactor will stall! Therefore, it is necessary to decelerate neutrons. Because U-235 prefers low-speed neutrons, the light water reactor and heavy water reactor of nuclear reactors are named according to the properties of moderator:
Light water reactor moderator: water H2O
Heavy water reactor moderator: heavy water D2O
Most modern commercial nuclear reactors are of these two types, others are only different in safety structure.
The secret of breeder reactor
The chain reaction depends on the number of 3n extra neutrons, which is also the key to increase the value of nuclear fuel. Because U-238 can be converted into plutonium -239 under the bombardment of fast neutrons, each fission can produce 2-3 neutrons to convert U-238 into plutonium -239, so each uranium nucleus burned in such a reactor will produce more than one plutonium nucleus, so there are more and more fuels, so it is called this.
What happens when the nucleus of uranium -238 absorbs a neutron and becomes uranium -239? Decay to neptunium -239, and then pass? Decay to plutonium -239, which is a perfect proliferation process!
The number of neutrons released by different atoms is different during nuclear fission. The number of neutrons that an atom releases and induces fission is called? Value, so? If the value is greater than 2, it may burn more and more. Different nuclei? These values are as follows:
Uranium -235: 2. 10
Plutonium -239: 2.45
Uranium -233: 2.3 1
The value of plutonium -239 is the highest, so most of the fast reactors in operation or operation use plutonium -239 as nuclear fuel everywhere (the former Soviet Union used uranium -235 with relatively high concentration), and at its periphery, U-238 (uranium isotope which is not easy to fission) can absorb neutrons and then convert them into combustible nuclear fuel. The real breeder reactor was born, using plutonium -239 as fuel, but plutonium -239 burned more and more, which is simply a perfect reactor!
The installed capacity of fast breeder reactor is very poor.
But in fact, there are only a few breeder reactors operating in the world! Why is this? Is it resisted by the traditional energy industry?
Why is this fast breeder reactor not built in large quantities?
In the last article, we learned that there are two kinds of moderator in modern nuclear reactors, one is light water reactor, the moderator is water, the other is heavy water reactor, and the moderator is heavy water. But fast neutron breeder reactor needs fast neutrons, not deceleration neutrons, so water and heavy water can't appear around the reactor core!
However, water and heavy water are not only moderators, but also important media for heat exchange. For example, light water reactors include pressurized water reactors and boiling water reactors. The former transfers high temperature to the medium in the secondary loop through high-pressure primary supercritical water, usually water! Boiling water reactor directly heats water into steam (radioactive), and then drives steam turbine to generate electricity. The process of heavy water reactor is the same as that of pressurized water reactor!
What does a fast breeder reactor use to transfer heat?
Heat transfer must have a medium, so it is not an exaggeration to say that the history of human civilization is a history of boiling water. Even if water is burned in the future nuclear fusion reactor, it is only a renovation, and it is impossible to burn water directly in the fast breeder reactor, so it must be changed. The media has the following requirements:
First of all, this medium can't slow down neutrons.
Secondly, the neutron absorption surface is small.
Finally, this medium must flow easily.
But to everyone's surprise, scientists chose sodium metal as the heat transfer medium, because it is the perfect heat transfer medium for fast reactors if the shortcomings of sodium solid and easy to burn and explode are ignored, so most countries that built early fast reactors chose sodium as the coolant. In fact, the structure of fast breeder reactor is similar to that of pressurized water reactor. The water or heavy water in the primary circuit is replaced by sodium metal, which is a flowing liquid at high temperature, and is exchanged by turbine outside to make water boil, and then returns to the reactor to absorb heat after cooling.
fast breeder reactor
The boiling point of sodium metal is 883℃, so the temperature of the reactor must be controlled below this temperature. But there is no high pressure in sodium-cooled reactor, so the pressure risk is not great. However, sodium is a substance that burns and explodes when it comes into contact with air or water. Once the high-temperature metal sodium leaks, the risk is great, so the biggest problem encountered by cold fast neutron reactors is the fire caused by sodium leakage, which seems to be an unsolved problem!
Sodium cooled fast neutron value-added reactor
The famous sodium-cooled fast reactor accident
The well-known accident of sodium-cooled fast reactor is Manjusri reactor in Japan. 1February 8, 995, the thermometer (thermocouple) sleeve of the secondary cooling system of Manjusri Reactor was damaged, and 640 kilograms of sodium steam spewed out of the pipeline, causing a fire. The reactor was shut down urgently, and the person in charge of the investigation committed suicide in the Tokyo hotel for one month.
Manjusri breeder reactor in Japan
Since then, the safety design of Manjusri reactor and the measures taken during leakage have made major mistakes. So the tug-of-war about whether to restart has been going on for nearly 15. May 6, 20 10 However, on August 26 of the same year, the mechanical arm for fuel replacement in the reactor failed, which made it impossible to replace the concentrated fuel (Japan will separate plutonium -239 from it), leading to the fall of the person in charge.
Japan officially scrapped manjusri reactor
20 1 6 65438+February 2 1, the Japanese government decided to start construction on 1970, and started operation on 1995. The total cost was1trillion yen, but the Manjusri reactor was retired after only 250 days of normal operation!
What about other coolants?
Metal lead as coolant can also meet the demand. Lead has smaller neutron absorption cross section and higher power density than sodium, and can be used to build larger nuclear power plants. However, its problems are as obvious as its advantages. Because the melting point of lead is 327℃, it is difficult to melt in the pipeline, and once it is started, it must keep flowing. Lead vapor is highly toxic and the construction cost is too high!
Structure diagram of Soviet lead-cooled fast reactor
Gas-cooled fast breeder reactor is actually the earliest reactor type in the history of gas-cooled nuclear reactor (graphite as moderator and Chernobyl as pressure tube graphite boiling water reactor), but it is replaced by boiling water reactor and pressurized water reactor because of its low power density and low fuel consumption. However, compared with sodium and lead in fast neutron breeder reactor, the disadvantages of air cooling are not so obvious.
Gas cooled fast breeder reactor
The gas-cooled fast breeder reactor is still under study. It belongs to the fourth generation nuclear reactor and uses helium or carbon dioxide as cooling gas. I hope its appearance can solve the problem, give full play to its advantages and make use of the spent fuel that is difficult to handle in modern times, so that even if nuclear fusion has not been realized, human beings can persist for thousands of years.
Besides the fast breeder reactor, are there any other reactors that burn more and more?
Generally speaking, in addition to fast breeder reactors, heavy water reactors can also be used to make nuclear fuel, but the efficiency and utilization rate are not as high as fast reactors. The best cycle of modern nuclear fuel is to obtain the original plutonium -239 accumulation through heavy water reactor, and then burn plutonium -239 with fast reactor to obtain 70% nuclear fuel utilization rate, which will increase human nuclear fuel by hundreds of times. But unfortunately, due to various restrictions, it cannot be achieved perfectly for the time being.
Reactor core
Similar to fission, the way of neutron proliferation will also appear in the nuclear fusion reactor, which comes from the deuterium-tritium fusion reactor that humans are trying to break through, because these are the two materials that are the easiest to meet the fusion conditions. The content of deuterium in seawater is 0.02%, and tritium basically does not exist, but it can be produced by neutron bombardment of lithium -6, so it also provides a way to realize tritium self-sustaining production by bombarding lithium -6 with redundant and intractable neutrons in deuterium-tritium fusion.
Deuterium-tritium fusion reaction
Because deuterium-tritium fusion does not need neutrons, neutrons are a very annoying thing, which can't be handled, but only contribute little to the production of fusion fuel tritium. Theoretically, a tritium nucleus and deuterium fusion can produce a neutron, but because the neutron is not well controlled, this efficiency is still debatable, but getting it back a little bit is much better than the first wall.
For this technology mastered by modern humans, nuclear fusion is still far away for the time being, while fast reactor technology is limited by various safety technologies, but in general, fast reactor is still relatively close to us. After all, all countries have failed experiences and mature experiences, such as China's fast reactor. China experimental fast reactor? 20 12 was put into production and is now running normally. In the future, with the maturity of the fourth generation gas-cooled fast reactor technology, there will be a nuclear revolution in global energy technology, which is undoubtedly the best news before nuclear fusion is realized.