main content:
- 1 Uranium-235-Take the lead
- 2 Fast neutrons are ready to go
- 3 The artificial sun has shown the light
1 Uranium-235-Take the lead
Uranium-235 is the only nuclide that exists in nature and is prone to nuclear fission, and it can form a chain reaction. This kind of chain reaction is relatively easy to control, so it was the first to be used to make nuclear weapons, and it was also the first to be used as nuclear fuel for the peaceful use of nuclear energy. So far, the vast majority of nuclear power plant reactors in the world still rely on uranium-235 as nuclear fuel.
However, the content of uranium-235 in natural uranium is very low (approximately 0.7%), and it is difficult to use natural uranium as a nuclear fuel to meet the high-power design requirements of nuclear power plants.
In order to increase the concentration of uranium-235 in uranium, the production of enriched uranium must be carried out. The production of enriched uranium, that is, the separation of uranium isotope, the process is very complicated and the cost is very high. At present, only a few countries in the world can produce nuclear fuel. The method of producing enriched uranium in various countries is basically the traditional gas diffusion method. This method has many processes and requires thousands of diffusers to be used in series. Beginning in the late 1970s, a new enrichment method called "gas centrifugation" appeared. The uranium vapor was passed through a high-speed centrifuge to separate uranium-235 and uranium-238. The cost has been greatly reduced. The "laser separation method under development" is a promising method for uranium enrichment. Two lasers with different wavelengths are used to irradiate uranium vapor to allow uranium-235 to be preferentially ionized and separated from natural uranium. The production cost and cost are possible. Further decrease.
The reserves of uranium on the earth are very limited, only about 5 million tons have been proven, and only half of them have economic mining value, namely 2.5 million tons, of which uranium-235 is only about 20,000 tons, equivalent to 40 billion. The calorific value of a ton of standard coal is obviously very small. At present, the world's natural uranium consumption (mainly used to extract uranium-235) reaches 60,000 tons per year. Even if the current proportion of nuclear power in total electricity remains unchanged, the existing recoverable reserves can only be maintained for decades. Therefore, trying to find uranium resources and improve the utilization rate of uranium resources has become the direction of people's efforts.
2 Fast neutrons are ready to go
In the process of searching for uranium resources and producing nuclear fuel, people discovered that there are also uranium resources in seawater. There are 3 grams of uranium in every 100 tons of seawater, and the global seawater has 15×1014 tons. Theoretically, the uranium content is as high as 45×106 tons, which is a thousand times more than the uranium reserves on land. However, the extraction of uranium from seawater is after all a "needle in a haystack" project. Although there are many methods, there are still some technical difficulties and the cost is too high. It is more realistic if the uranium resources mined on land are fully utilized. The current development of nuclear energy only uses 0.7% uranium-235 of uranium element. If the remaining 99.3% of uranium-238 can also be used as nuclear fuel, it is equivalent to expanding nuclear energy resources by hundreds of times. The 2.5 million tons of uranium mined is equivalent to the global geological reserves of coal in terms of calorific value. In addition, thorium can also be used to make nuclear fuel, which is enough to support all the energy consumption in the world for hundreds of years. How tempting is this Prospects!
The solution is already there! That is the fast breeder reactor.
It turns out that after uranium-238 absorbs a neutron in the reactor, it undergoes two "beta decays" (in a very short time) and becomes plutonium-239, which is another nuclear fuel that can be used as a fissile material. This kind of nuclear fuel can only fission and release energy by the "fast neutrons" with greater energy. In a slow neutron reactor, even if uranium-238 absorbs neutrons and becomes plutonium-239, it will not fission, but exists. In spent fuel. Therefore, people designed fast neutron reactors, the core fuel is natural uranium, and neutron moderators are not used. In the chain reaction, each plutonium nucleus will be fissioned by fast neutrons and will produce an average of 2.6 neutrons. In addition to maintaining the chain reaction using one neutron, there may be more than one neutron remaining for regenerating materials. The conversion of uranium-238 is absorbed by uranium-238 to generate plutonium-239. As a result, the ratio of newly generated plutonium to consumed plutonium (called the breeder ratio) can reach about 1.2, thus realizing the proliferation of fission fuel, so this kind of reactor is called "fast breeder reactor", or "fast reactor" for short. ".
Since the fast reactor does not have a moderator, the core structure is compact, small in size, and the power density is 4 to 9 times higher than that of the general light water reactor. However, the heat transfer problem is particularly prominent and needs to be strengthened. Generally, liquid metallic sodium must be used as the coolant, which is very technically difficult, and the structure of the reactor and heat transfer system is much more complicated. At present, only a few countries have installed fast reactors in a few nuclear power plants. More than 20 fast reactors have been built, including one in China. The fast reactor has a good momentum of development and has broad prospects.
3 The artificial sun has shown the light
We have already understood before that light nuclei with lower average binding energy can be fused into nuclei with larger binding energy, that is, nuclear fusion reaction. This is another way to obtain nuclear energy, and the energy obtained is much larger than the energy released by nuclear fission. The nuclear fusion of light nuclear matter is usually the process of the polymerization of hydrogen into helium, which is the same as the nuclear fusion reaction inside the sun. Therefore, devices that achieve controllable nuclear fusion and continuously release energy are regarded as miniature "artificial suns." Among the hydrogen elements, heavy hydrogen (deuterium) and superheavy hydrogen (tritium) are the most important nuclear fuels for nuclear fusion. Therefore, to obtain energy from nuclear fusion, the first step is to find these nuclear fuels.
Deuterium, one of the fuels for nuclear fusion, is particularly abundant on the earth. Each liter of seawater contains 0.034 grams of deuterium (existing in heavy water). This should be said to be a lot. The global seawater contains 45 billion tons of deuterium, which is almost inexhaustible and inexhaustible. Another nuclear fuel, tritium, is not so much. It has very little content in sea water, and it cannot be expected to be extracted from sea water. But tritium exists in the earth’s rich lithium mines. It can be separated out, and tritium can also be produced by artificial synthesis, which is to put deuterium, lithium, boron or nitrogen and other substances into powerful neutrons. In the current nuclear reactor, or use fast deuterium nuclei to bombard compounds containing a large amount of deuterium (such as heavy water), so that tritium can be obtained. Therefore, it can be said that tritium, as one of the fusion nuclear fuels, is also rich in sources.
With nuclear fuels, how to make them realize a controllable nuclear fusion reaction and make the nuclear energy released by people to be used by people is more complicated and difficult. The nuclear fusion reaction can only occur at extremely high temperatures. At extremely high temperatures, the extranuclear electrons of the atoms participating in the reaction are stripped off and become exposed nuclei. The substances participating in the reaction become completely composed of positively charged nuclei and nuclei. A "plasma" of negatively charged electrons. If nuclear fusion occurs at this time, since the radiation heat transfer method is proportional to the 4th power of the temperature, the ultra-high temperature plasma loses a huge amount of heat in the form of radiation. If the energy released by the fusion is less than the radiation loss, the thermonuclear reaction will stop. There is a critical temperature. When this temperature is exceeded, the fusion reaction can continue. This critical temperature is called the "critical ignition temperature". For the deuterium-tritium reaction, this temperature is 44 million ℃; pure deuterium can also be fused, but the critical ignition temperature is higher, about 200 million ℃. To maintain the continuous progress of the fusion reaction, a temperature much higher than the critical ignition temperature is also required. For example, the minimum operating temperature of the deuterium-tritium reactor is as high as 100 million ℃, and the deuterium-deuterium is 500 million ℃. In addition to the requirement of ultra-high temperature, there are also severe constraints, namely plasma density and confinement time. The greater the plasma density, that is, the greater the number of nuclei per unit volume, the easier it is for nuclear fusion reactions to continue; when the density increases by 10 times, the possibility of fusion reactions increases by 100 times. The longer the plasma is confined, the more conducive to the fusion reaction. To enable the controllable nuclear fusion reaction to proceed, the product of the plasma density (in units/cubic centimeter) and the confinement time (in seconds) must satisfy a certain Numerical value, called "Lawson condition". It is 1014 for the deuterium-tritium reaction and 1016 for the deuterium-deuterium reaction. The temperature of the plasma is so high, where is it installed for the reaction? It won’t work anywhere! A container made of any material cannot withstand it, so the plasma must be reduced to isolate it from the surrounding environment. For example, a magnetic confinement system can be used. The most promising magnetic confinement device is the "circulator" (circular current device), also known as the "tokamak".
Picture: Tokamak test device
At present, controllable nuclear fusion has made very encouraging and significant progress. In 1991, controlled nuclear fusion was achieved for the first time in a laboratory in the UK. Good news came out in December 2005, the thermonuclear test reactor of international cooperation.
(TER) finally settled in France. China has gone through 40 years of research on controllable nuclear fusion, and has achieved significant results and has entered the world's advanced ranks. Although there is still a long way to go to obtain nuclear energy from nuclear fusion, people have already seen the dawn of the success of the artificial sun. Because nuclear fusion fuel resources are abundant, the energy released is large, the raw material hydrogen and the product helium are harmless to the environment, and there is no worry of radioactive pollution, so the realization of controllable nuclear fusion to provide clean energy is the common ideal of mankind. Once successful, it will bring a great revolution in the development of new strategic energy for mankind.