On January 18, 2018, the Prizes of the Government of the Russian Federation in the Field of Science and Technology were awarded. Among the prize-winners was the work entitled Enhancing Operation Efficiency and Radiation Safety of the BN-600 Reactor Plant; Using the Gained Experience for Advanced Designs of Sodium-Cooled Fast-Neutron Reactor Plants. Boris Vasiliev, who headed the work and is the Chief Designer of SFR cores at JSC “Afrikantov OKBM”, disclosed the details of the large project.
– Mr. Vasiliev, please tell us about the studies that were prize-winning for the team headed by you.
– Over the last years, the mass media, which cover events in the nuclear industry, have been speaking a lot about the leadership of Russia in the area of fast neutron reactors. Largely, this assertion is associated with the fact that only Russia has power reactors of this type, BN-600 and BN-800, operating at the Beloyarsk NPP.
The BN-600 reactor was commissioned in 1980 and has been successfully operating for 37 years. The BN-800 reactor started its commercial operation in 2016. The experience with BN-600 is unique in terms of both duration and reactor plant operational efficiency. This experience is a basis for further development of the SFR technology.
The successful operation of BN-600 is determined, first of all, by the reliability of the reactor design that was developed as far back as in the 1970s. At the same time, the efficiency and safety of the reactor were ensured by design studies and improvements done in the course of reactor operation. Our work is dedicated exactly to these studies and improvements. They were performed in a number of areas — the reactor core and the nuclear fuel, radiation safety, sodium-water steam generators and extending the reactor service life.
– Tell us about the co-authors of the work. Which companies, institutes do they represent?
– Specialists from six organizations took part in the work.
Three specialists represented the lead organization, JSC “Afrikantov OKBM”. Oleg Vilensky was responsible for the reactor structural strength and operability analyses for extending the reactor operation life. Sergey Osipov was responsible for the radiation safety studies.
The major creative area in my work is the improvement of the reactor core, increasing the burnup of the nuclear fuel. Along with this, as the Chief Designer of BN Reactor Plants, I was taking part in managing and performing of other studies from 2000 to 2016.
Alexander Tsikunov and Vladimir Poplavsky from JSC SSC RF IPPE took part in the work. Tsikunov studied radiation safety. Poplavsky performed the research work on steam generators and in a number of other areas. Mr. Poplavsky was being the IPPE Deputy Director for Research in Fast Reactors for 20 years and made a big contribution to the development of these reactors.
It is very sad that the laureate title was given to him posthumously.
Vladimir Denisov from OKB Gidropress conducted studies and was involved in developments to substantiate reliable, long-time operation of the steam generators.
Andrey Tselishchev from VNIINM developed structural materials for the nuclear fuel.
Boris Margolin from CRISM “Prometey” took part in the work. He was involved in the materials science studies and developed the strength analysis methodology for reactor structures in long-time operation.
Oleg Potapov and Vladimir Chuev represented the Beloyarsk NPP. Potapov managed the work in all research and improvement area. The Chuev’s work area was studies to substantiate the increased burnup of the nuclear fuel.
– What are the major results of the studies?
– I will try to describe every area in brief. A significant volume of studies was done to improve the reactor core in order to enhance the nuclear fuel utilization efficiency. This efficiency is determined by two interconnected factors — burnup level and fuel operation time (duration of the fuel life). It should be noted that under the operating conditions in the fast reactor, the fuel is under intensive irradiation — the neutron fluxes are more than 10 times higher than the neutron fluxes in the PWR cores. At the time BN-600 was commissioned, the fuel behavior under these conditions had not been studied sufficiently. Nevertheless, it was already at the initial stage of BN-600 operation when the average fuel burnup of ~ 42 MW·day/kg was reached.
And thanks to the work done in this area, it was possible to increase the average (uranium dioxide) fuel burnup in BN-600 to 74 MW·day/kg. In a VVER reactor, this parameter is at the level of 50 MW·day/kg. The activities to optimize the nuclear fuel operating conditions were determinative as were the activities to improve the radiation resistance of structural materials for the fuel sub-assembly wrapper tubes and for the fuel cladding.
Another part of the work was associated with the radiation safety. Studies in this area were also largely associated with the reactor core. An efficient system was developed to detect fuel sub-assemblies with leaking fuel rods and to remove them from the reactor. For this, brand new engineering solutions were proposed. Additionally, a series of studies was conducted on various radiation characteristics of the BN-600 reactor, which has the integral layout type. It is this layout that the advanced fast reactor designs are aiming at. Eventually, it was shown that the BN-600 reactor surpasses reactors of other types in terms of radiation safety. The personnel exposure is negligible, radiation releases are minimal and iodine is absent from them. The last important circumstance is associated with the fact that this isotope — one of the most dangerous isotopes — chemically binds with sodium and remains in the reactor all the time.
In terms of technology, the operability of the BN-type fast reactors is largely determined by a possibility of developing a reliable steam generator design where water is heated and steam with the required parameters is generated by heat transferred from the sodium coolant. To prevent steam generator failures caused by inter-circuit leaks during operation, the steam generator design was improved, and optimal steam generator operating modes were determined and introduced.
As a result, no inter-circuit leaks have been in the steam generators of BN-600 over the last 25 years. Additionally, materials science studies and computational studies resulted in that the specified service life of steam generator modules was extended by 20–50%.
And finally, in order to substantiate the BN-600 operation life extension, a series of materials science studies was conducted (including the studies on specimens taken from withdrawn elements of BN-600). At the same time, a new strength analysis methodology was developed for reactor structures, and in so doing possible flaws were taken into account that had been overlooked during fabrication.
Eventually, the operability of the BN-600 reactor was substantiated for 45 years in operation, which is 15 years longer than the design service life. In April 2010, Rostechnadzor issued a service life extension license for the BN-600 reactor for ten years with a perspective on further extension for another five years.
– What is the significance of the accomplished work for the advanced BN-type reactor plant designs?
– The obtained operating experience with the BN-600 reactor, the results of the studies have a great significance for the recently developed BN-800 reactor and for BN-1200 under development. The BN-600 reactor has been successfully operating for 37 years with the minimal unplanned losses in the power factor, which was stably close to 80%. The unplanned losses were 1–2% on average per year. This is why we can speak with confidence that major engineering solutions implemented in the BN-600 reactor design may be used for advanced designs. And this has been implemented in the BN-800 and BN-1200 designs.
Also all the results of the work done have a great significance for the advanced designs. Based upon the studies accomplished during the operation of BN-600, it is planned for the BN-800 and BN-1200 reactors to get the average burnup of MOX fuel at around 90 MW·day/kg. The feasibility of reaching high burnup in the MOX fuel, which is used in the BN-800 design and is considered for application in BN-1200 along with the nitride fuel of higher density, has been demonstrated during the testing of test fuel sub-assemblies in the BN-600 reactor. The studies on the mixed uranium-plutonium nitride fuel (MNIT) are also being conducted in BN-600 for a few years.
The studies on steam generators of BN-600 made it possible to substantiate reliability and, along with that, to simplify and make cheaper the steam generator design for the BN-800 reactor plant and especially for the BN-1200 reactor plant.
And the most important thing in terms of economics is that the feasibility of long-time operation has been substantiated for the BN-type reactors. It is associated with the fact that all nuclear power stations are characterized by high capital intensity, and in the electricity generation cost structure, the depreciation portion makes the highest contribution. Therefore, based upon the accomplished studies, for water-cooled reactors widespread in the world, the operation life is set at 60 years, and a possibility of extending it to 80 years is considered. Of course, this is done based on the obtained experience with successful long-time operation of this type of reactors. For the fast neutron reactors, a quite broad experience with long-time operation has been gathered too. However, it is the experience with the studies on BN-600 that allows one — with a sufficient degree of confidence — to set the design service life at 60 years for the BN-1200 reactor with account of the steel with higher heat resistance widely used in the design. Possibilities are also considered for extending the operation life of BN-600 and BN-800 in excess of 45 years. This kind of studies has already been started for BN-600.
All in all, the results of the studies presented in our work testify to the fact that Russia has created the scientific and technological background for the development of the fast neutron reactors on the industrial scale — for serial construction of fast reactors based on the BN-1200 project.
– Under the modern conditions, it is not enough to have the scientific and technological background — the reactors must be competitive.
– Of course, it is necessary to strive to the minimum cost of electricity generated by nuclear power stations. The objective for the designers is to ensure that the specific capital costs per BN-1200 power unit do not exceed the analogous characteristic for the best projects of nuclear power stations with PWRs. According to our estimates and according to the calculations made by the General Designer — considering the improvements are made to the BN-1200 design — this is ensured. As for the fuel component in the cost of generated electricity, the competitive ability may be ensured through a high fuel burnup. As of today, sufficiently high burnup of the MOX fuel has been substantiated. There are plans to further increase the reached burnup both for the MOX fuel and for the MNIT fuel.
Aside from that, the competitive ability of the nuclear energy as a whole, without being specific about reactor types, will also be determined in the future by its environmental advantages — absence of any carbon emissions and other large-scale environmental contaminations. Along with that, consideration should be also given to environmental problems of the nuclear energy per se — it is a buildup of radioactive wastes and a need to deal with them later on. And in this respect, the fast reactors have an advantage — as it is well known, they can burn minor actinides, which, accordingly, are not going to get into the category of long-lived radioactive wastes. Included here may be plutonium burning — the possibility of plutonium recycling in the fuel of thermal-neutron reactors is quite limited.
– Can we say that you see the future of the BN-type reactors in an optimistic way?
– The future of the nuclear energy largely depends upon when and how the transition to the closed nuclear fuel cycle will be made with the use of the fast neutron reactors, which is basically required in connection with the limitations in the raw materials for the fuel if thermal neutron reactors are used. And it is only in Russia that such transition could be made quite fast and efficiently, because Russia has the fast neutron technology developed on the industrial scale. And this is the BN-reactor technology.