The fast neutron reactor BN-800 has reached the power level of 880 MW
The unique Russian fast-neutron reactor operating at the Beloyarsk NPP was brought to capacity of 880 megawatts, according to the press service of Rosatom.
The reactor is operating at power unit No. 4 of the Beloyarsk NPP and is now undergoing planned tests of generating equipment. In accordance with the test program, the power unit maintains electrical power at a level not lower than 880 megawatts for 8 hours.
The power of the reactor rises in stages, so that in the end, according to the test results, to obtain certification at the design power level of 885 megawatts. At the moment, the reactor is certified for a power of 874 megawatts.
Recall that the Beloyarsk NPP has two fast neutron reactors. Since 1980, the BN-600 reactor has been operating here - for a long time it was the only reactor of this type in the world. But in 2015, the phased launch of the second BN-800 reactor began.
Why is it so important and considered a historical event for the global nuclear industry?
Fast neutron reactors make it possible to realize a closed fuel cycle (currently not implemented in BN-600). Since only uranium-238 is “burned”, after processing (extraction of fission products and the addition of new portions of uranium-238), fuel can be reloaded into the reactor. And since in the uranium-plutonium cycle, plutonium is formed more than decayed, excess fuel can be used for new reactors.
Moreover, this method can be used to recycle surplus weapons-grade plutonium, as well as plutonium and minor actinides (neptunium, americium, curium) recovered from the spent fuel of conventional thermal reactors (minor actinides currently represent a very dangerous part of radioactive waste). At the same time, the amount of radioactive waste in comparison with thermal reactors decreases by more than twenty times.
Why, for all their merits, fast neutron reactors are not widespread? This is primarily due to the peculiarities of their design.As mentioned above, water cannot be used as a coolant, since it is a neutron moderator. Therefore, in fast reactors, mainly metals in the liquid state are used - from exotic lead-bismuth alloys to liquid sodium (the most common option for nuclear power plants).
“Thermal and radiation loads are much higher in fast-neutron reactors than in thermal reactors,” explains Mikhail Bakanov, chief engineer of Beloyarsk NPP. - This leads to the need to use special structural materials for the reactor vessel and in-reactor systems. The bodies of fuel elements and fuel assemblies are not made of zirconium alloys, as in thermal reactors, but of special alloyed chromium steels that are less prone to radiation “swelling”. On the other hand, for example, the reactor vessel is not subjected to loads associated with internal pressure - it is only slightly above atmospheric. ”
According to Mikhail Bakanov, in the first years of operation the main difficulties were associated with radiation swelling and cracking of fuel. These problems, however, were soon solved, new materials were developed - both for fuel and for TVELs.But even now, campaigns are limited not so much by fuel burnout (which at the BN-600 reaches 11%), as by the resource of the materials from which fuel, fuel rods and fuel cells are made. Further operational problems were mainly associated with leakage of secondary sodium, a chemically active and flammable metal, rapidly reacting to contact with air and water: “Only Russia and France have long experience in operating industrial fast neutron power reactors. Both we and French specialists from the very beginning faced the same problems. We successfully solved them, from the very beginning by providing special means to control the leaktightness of the circuits, localize and suppress sodium leaks. And the French project was less prepared for such troubles, and as a result, in 2009 the Phenix reactor was finally shut down. ”
“The problems were really the same,” adds the director of the Beloyarsk NPP Nikolay Oshkanov, “but we solved them here and in France in various ways. For example, when the head of one of the assemblies was bent on Phenix to capture and unload it, French specialists developed a complex and rather expensive “vision” system through a layer of sodium.And when the same problem arose with us, one of our engineers suggested using a video camera placed in a simple design like a diving bell — an open-bottom pipe with argon blowing from above. When the sodium melt was displaced, the operators were able to capture the mechanism via video link, and the bent assembly was successfully removed. ”
The fast-neutron reactor core is structured like a bulb, in layers
370 fuel assemblies form three zones with different enrichment in uranium-235 — 17, 21, and 26% (initially there were only two zones, but in order to equalize the energy release, three were made). They are surrounded by side screens (blankets), or reproduction zones, where assemblies containing depleted or natural uranium are located, consisting mainly of isotope 238. At the ends of the fuel elements above and below the core are also depleted uranium tablets, which form end screens ( reproduction).
Fuel assemblies (FAs) are a set of fuel elements assembled in a single package (fuel rods) - tubes of special steel filled with uranium oxide tablets with various enrichments.In order to prevent the fuel rods from touching each other, and the coolant could circulate between them, a thin wire is wound into the tubes. Sodium enters the fuel assembly through the lower throttling holes and exits through the windows in the upper part.
In the lower part of the fuel assembly there is a shank inserted into the manifold socket, in the upper part there is the head part, for which the assembly is seized upon overload. Fuel assemblies of various enrichment have different seats, so it is simply impossible to install the assembly in the wrong place.
To control the reactor, 19 compensating rods containing boron (neutron absorber) to compensate for fuel burnup, 2 automatic control rods (to maintain a given power), and 6 rods of active protection are used. Since uranium's own neutron background is small, for controlled reactor start-up (and control at low power levels), a “backlight” is used — a photoneutron source (a gamma emitter plus beryllium).
Power units with fast-neutron reactors can significantly expand the fuel base of nuclear energy and minimize radioactive waste through the organization of a closed nuclear fuel cycle.Only some countries possess such technologies, and the Russian Federation, according to experts, is the world leader in this field.
The BN-800 reactor (from “fast sodium”, with an electrical capacity of 880 megawatts) is an experimental industrial fast neutron reactor with liquid metal coolant, sodium. It should become a prototype of commercial, more powerful power units with BN-1200 reactors.