In previous articles, we found out that neither solar energy will be able to meet the needs of mankind (due to the rapid failure of batteries and their cost), nor thermonuclear (because even after reaching positive energy output in experimental reactors, there remains a fantastic amount problems in the way of commercial use). What remains?

Not for the first hundred years, in spite of all the progress of mankind, the bulk of electricity comes from the banal burning of coal (which is still the source of energy for 40.7% of the world's generating capacity), gas (21.2%), oil products (5.5%) and hydropower (another 16.2%, in total, all this - 83.5% as of 2008 ).

It remains - nuclear power, with conventional thermal neutron reactors (requiring rare and expensive U-235) and fast-neutron reactors (which can process natural U-238 and thorium in a "closed fuel cycle").
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What is this mythical "closed fuel cycle", what are the differences between reactors on fast and thermal neutrons, what designs exist when we expect happiness from all this and of course - a safety issue - under the cut.

About neutrons and uranium

All of us at school were told that U-235 when a neutron hits it is divided with the release of energy, and another 2-3 neutrons fly out. In reality, of course, everything is somewhat more complicated, and this process strongly depends on the energy of this initial neutron. Let's look at the cross section plots (= probabilities) of the neutron capture reaction (U-238 + n -> U-239 and U-235 + n -> U-236), and the fission reaction for U-235 and U-238 depending on the energy (= velocity) of neutrons:



As we see, the probability of neutron capture with fission for U-235 increases with decreasing neutron energy, therefore in conventional nuclear reactors the neutrons “slow down” in graphite / water to such an extent that their speed becomes the same order as the speed of thermal oscillation of atoms in the crystal lattice (hence the name - thermal neutrons). And the probability of fission of U-238 by thermal neutrons is 10 million times less than U-235, therefore it is necessary to process natural uranium in tons in order to pick U-235.

Someone looking at the bottom chart can say: Oh, great idea! And let's fry 10-MeV neutrons with a cheap U-238 - you should get a chain reaction, because there just the cross section graph for the division goes up! But there is a problem - neutrons released as a result of the reaction have energy of only 2 MeV or less (~ 1.25 on average), and this is not enough to trigger a self-sustaining fast-neutron reaction in U-238 (either more energy is needed or more neutrons took off with each division). Oh, mankind was not lucky in this universe ...

However, if a self-sustaining fast-neutron reaction in U-238 were obtained so easily, there would be natural nuclear reactors, as was the case with U-235 in Oklo , and accordingly U-238 would not occur in nature in the form of large deposits.

Finally, if we abandon the “self-sustainability” of the reaction, it is still possible to divide U-238 directly with the production of energy. It is used, for example, in thermonuclear bombs — 14.1 MeV neutrons from the D + T reaction divide U-238 in the bomb shell — and thus the power of the explosion can be increased almost free of charge. Under controlled conditions, the theoretical possibility remains of combining a fusion reactor and a blanket (shell) from U-238 so that the energy of thermonuclear fusion can be increased by ~ 10-50 times due to fission.

But how to divide U-238 and thorium in a self-sustaining reaction?

Closed fuel cycle

The idea is as follows: let's look not at the fission cross section, but at the capture cross section: With suitable neutron energy (not too small and not too large), U-238 can capture a neutron, and after 2 decays - become plutonium-239:



From spent fuel - plutonium can be separated by chemical means, and MOX-fuel (a mixture of plutonium and uranium oxides) can be made which can be burned both in fast reactors and in conventional, thermal ones. The process of chemical reprocessing of spent fuel can be very difficult due to its high radioactivity, and so far it has not been completely solved and has not been practically worked out (but work is on).

For natural thorium, a similar process, thorium captures a neutron, and after spontaneous fission, it becomes uranium-233, which is divided in much the same way as uranium-235 and is separated from spent fuel by chemical means:



Of course, these reactions also take place in conventional thermal reactors - but because of the moderator (which greatly reduces the chance of neutron capture) and control rods (which absorb some neutrons), the amount of generated plutonium is less than uranium-235 burns. In order to generate more fissionable substances than it burns - you need to lose as few neutrons as you can on control rods (for example, using control rods from ordinary uranium), construction, coolant (see below) and completely get rid of the neutron moderator (graphite or water). ).

Due to the fact that the fission cross section with fast neutrons — less than thermal — has to increase the concentration of fissile material (U-235, U-233, Pu-239) in the core of the reactor from 2-4 to 20% and higher. And the operating time of the new fuel is being conducted in cassettes with thorium / natural uranium located around this core.

By happy coincidence, if fission is caused by a fast neutron, not a thermal one, the reaction produces ~ 1.5 times more neutrons than in the case of fission by thermal neutrons - which makes the reaction more realistic:



It is this increase in the number of generated neutrons that makes it possible to produce a larger amount of fuel than it was originally. Of course, the new fuel is not taken from the air, but is produced from the "useless" U-238 and thorium.

About heat carrier

As we found out above - water in a fast reactor cannot be used - it is extremely effective in slowing down neutrons. What can replace it?

Gases: It is possible to cool the reactor with helium. But because of the small heat capacity, it is difficult to cool powerful reactors in this way.

Liquid metals: Sodium, Potassium - are widely used in fast reactors around the world. The advantages are low melting point and work at about-atmospheric pressure, but these metals burn very well and react with water. The only operating energy reactor in the world, BN-600 , works precisely on a sodium coolant.

Lead, bismuth are used in the BREST and SVBR reactors currently being developed in Russia. Of the obvious drawbacks - if the reactor is cooled below the lead / bismuth freezing temperature - it is very difficult to warm it up for a long time (not obvious ones - you can read it on the link in the wiki). In general, there are many technological issues on the road to implementation.

Mercury - with a mercury coolant was the reactor BR-2, but as it turned out, mercury relatively quickly dissolves the structural materials of the reactor - so that more mercury reactors are not built.

Exotics: A separate category - molten salt reactors - LFTR - operate on different types of fluorides of fissile materials (uranium, thorium, plutonium). Two “laboratory” reactors were built in the United States at the Oak Ridge National Laboratory in the 1960s, and since then there have not been any other reactors, although there are many projects.

Existing reactors and interesting projects

The Russian BOR-60 , an experimental fast neutron reactor, has been operating since 1969. On it, in particular, they are testing the elements of the construction of fast-neutron reactors.

Russian BN-600, BN-800 : As mentioned above, the BN-600 is the only fast neutron power reactor in the world. It has been operating since 1980, while on uranium-235.

In 2014, a more powerful BN-800 is planned to be launched. It is already planned to begin using MOX fuel (with plutonium) on it, and to start working out the closed fuel cycle (with reprocessing and burning of the produced plutonium). Then there may be a serial BN-1200 , but the decision on its construction has not yet been made. According to the experience of construction and industrial operation of fast-neutron reactors, Russia has advanced much further than anyone else, and continues to actively develop.

Small operational research fast reactors exist in Japan ( Jōyō ), India ( FBTR ) and China ( China Experimental Fast Reactor ).

The Japanese Monju reactor is the most unfortunate reactor in the world. In 1995, it was built, and in the same year a leak of several hundred kilograms of sodium occurred, the company tried to hide the scale of the incident (hello Fukushima), the reactor was stopped for 15 years. In May of 2010, the reactor was finally launched at reduced power, however, in August, during a fuel transfer into the reactor, a 3.3-ton crane was dropped, which immediately sank in liquid sodium. They managed to get the crane only in June 2011. On May 29, 2013, it will be decided to shut down the reactor permanently.

Traveling wave reactor : Of the well-known unrealized projects - “traveling-wave reactor” - traveling wave reactor, by TerraPower. This project was promoted by Bill Gates - so about it wrote twice on Habré: 1 , 2 . The idea was that the “core” of the reactor consisted of enriched uranium, and around it - cassettes with U-238 / thorium, in which the future fuel would be accumulated. Then, the robot would move these cassettes closer to the center - and the reaction would continue. But in reality, without chemical processing, it is very difficult to make it all work, and the project never took off.

About nuclear power safety

How can I say that humanity can rely on nuclear power - and this is after Fukushima?

The fact is that any energy is dangerous. Let us recall the accident at the Banqiao dam in China, which was also built to generate electricity - then 26 thousand died. up to 171 thousand. person. The accident at the Sayano-Shushenskaya HPP - 75 people died. In China alone, 6,000 miners die every year in coal mining , and that is not counting the health effects of inhalation of CHP emissions.

The number of accidents at nuclear power plants does not depend on the number of power units, since Each accident can occur only once in a series. After each incident - the causes are analyzed and eliminated on all blocks. So, after the Chernobyl accident, all the blocks were refined, and after Fukushima, the Japanese took away nuclear power engineering in general (however, there are conspiracy motifs here - the United States and its allies expect a deficit of uranium-235 in the next 5-10 years).

The problem with spent fuel is directly solved by fast neutron reactors, since in addition to improving the technology of waste processing, less waste is formed: heavy (actinides), long-lived reaction products are also “burned” by fast neutrons.

Conclusion

Fast reactors - have the main advantage, which everyone expects from thermonuclear - the humankind will have enough fuel for them for thousands and tens of thousands of years. He does not even need to mine - it has already been mined, and lies in warehouses and dumps . Technical problems - although they remain, they seem to be solved, not epic - as in the case of fusion reactors.

Fuel in the “closed fuel cycle” does not appear from the air, but from previously useless uranium-238 and thorium after irradiation in a fast reactor, and further chemical processing to separate useful plutonium-239 and uranium-233 from spent fuel. Fast reactors compared to thermal neutron reactors - they give 1.5 times more neutrons per 1 division, and there is enough of them both for the chain reaction and for the production of new fuel.

From an economic point of view - with mass construction, fast reactors are more expensive than conventional thermal nuclear reactors, but not by orders of magnitude. Mass construction of fast reactors seems to simply not start ahead of time, because uranium-235 and conventional fuel are still enough for most countries in the short term (15-30 years), and there is time to work out the technology.

So, when cheap oil and uranium-235 are finally over, our grandchildren will not have to sit without light, will colonize Mars on what, and slowly finish thermonuclear fusion for the next 10,000 years.