A BN-600 fast reactor has been in commercial operation at the Beloyarsk nuclear power station in Russia’s Sverdlovsk Region since 1980. With a clear government policy to construct new fast reactors, a joint-stock company has been set up by the power station along with regional energy organisations to raise the money to construct an 800 MWe version, the BN-800. The estimated cost of the project is $1.1 billion. How soon the new plant can be built at Beloyarsk will depend on finance.
The new company includes the Beloyarsk station, the Sverdlovsk regional authority, Sverdlovenergo (which operates the local grid), Sverdlovenergostroy (a construction company) and Rosenergoatom (Russia’s national nuclear generation company). Some initial investment has been made, although little in the form of money. Rosenergoatom is providing the site and buildings, while the federal government will take a number of shares in the project and provide various needed documentation, explains Alexander Shestekov, head of Power Production and Engineering at Beloyarsk. Electricity charges will also be increased to raise money.
Beloyarsk has some financial difficulties – it is owed a large amount of money for electricity supplied to the national and local grids – but is under less pressure than many other Russian nuclear plants. Some 7% of its power is supplied directly to the nearby uranium enrichment facility which pays promptly from its export earnings. As a result, salaries are only two months in arrears and workers receive their pay regularly every two weeks. However, this leaves nothing for investment in the new reactor, and the new company hopes to finance much of the construction using offset and barter arrangements. Construction will take seven years if there is no interruption in financing.
EARLY DEVELOPMENT
Fast reactors had long played an important role in the nuclear development plans of the Soviet Union, an option which Russia still prefers particularly considering the need to find ways of disposing of Pu from dismantled nuclear weapons. Today, the BN-600 at Beloyarsk and the BN-350 at Shevchenko are the world’s only commercially operating fast reactors. While both are cooled with liquid sodium, Russia has also developed small lead-bismuth-cooled fast reactors as power units for high speed submarines.
The first fast reactor designs were drawn up by the Soviets in 1949, and in 1955 the BR-1 zero power reactor was commissioned at the Institute of Physics and Power Engineering (IPPE) in Obninsk. The larger BR-5 commissioned in 1959 acted as a testbed for the first sodium-cooled reactors. It has since been upgraded to 8 MW and is now known as the BR-10. Fast reactor research is also under way at the Research Institute of Atomic Reactors (RIAR) in the city of Dimitrovgrad, near Ulyanovsk. In 1969 the BOR-60, with a power capacity of 60 MWe, was commissioned at RIAR. It is used mainly to test different types of fuel assembly. On the basis of the research done at these institutes, the Kazakhstan and Beloyarsk reactors were commissioned and new designs developed for larger sodium-cooled fast reactors, including the BN-800.
Site preparations for four BN-800s were already under way before the Chernobyl accident, one at Beloyarsk (the station’s fourth unit) and three at the South Urals plant. But in 1988, after Chernobyl, public protests brought activities to a standstill. Then, in June 1992, President Yeltsin ordered work to continue but, while local approval was obtained, the lack of finance has stalled progress. However, Beloyarsk plant director, Oleg Saraev, is now confident that the first BN-800 will be built at his plant which already has the necessary infrastructure and operating experience.
BN-600 EXPERIENCE
The BN-600 has been in commercial operation for over 17 years during which time valuable experience has been gained. “We have collected data on failures and problems, including leaks and sodium fires, changes in core material and structural flaws,” says Lev Kochetkov of IPPE, which acts as scientific supervisor. As a result many improvements have been made and the BN-600 is now among the most efficient nuclear plants in Russia. The plant has an availability of 77% with scheduled refuelling and maintenance outages accounting for 21% of unavailability and unplanned outages only 2%. Other performance figures are given in the table above.
The BN-600 is a three-loop design with the reactor and primary pumps submerged in a large pool of liquid sodium. The secondary circuits comprise three loops each with a steam generator and a secondary sodium pump. The steam, exiting at 510°C, supplies three 200 MW turbines. Each steam generator consists of eight sections, and each section comprises three modules – an evaporator, superheater and reheater. The sections are connected by manifold and can be isolated on both sodium and steam sides. With this design it is possible to isolate a failed section and the steam generators repaired while the plant is on-line. This has proved so successful that the BN-800 design, which originally had a single large turbine, is to be suitably changed, explains Shestekov.
However, the use of sodium as a coolant raises the possibility of sodium fires in the event of a leak coming in contact with water. The BN-600 has experienced a total of 27 leaks but none of them serious (see table below). Of these five have occurred on radioactive sodium systems, of which 14 leaks have resulted in active sodium burning; five of these were caused by incorrect action during maintenance or during return to or removal from service. A total of 12 water-to-sodium leaks have accounted for only 0.3% loss of power generation.
DESIGN IMPROVEMENTS
As an experimental plant, it was expected that modifications would have to be made to the BN-600 and various procedures have been upgraded, including those for on-load loop re-connection, decay heat removal and on-load steam generator section re-connection.
The unit was designed so that, in case of one loop being disconnected, reactor power would automatically fall by 1/3 and the reactor would continue on two loops. On-load re-connection of the loop was not envisaged in the design. However, the system has been modified so that loop connection is now possible at 35% of nominal power.
A major problem arose in the design of the reactor decay heat removal system due to overcooling of water downstream of the steam generators. This was solved by closing the decay heat removal route. And as indicated above, problems also arose because the design did not allow for re-connection of any steam generator section which had been isolated without disconnecting a loop or shutting the unit down. New procedures had to be developed to overcome this.
FUEL UPGRADES
As to the core, the BN-600 has mainly used enriched uranium as fuel, although some experimental mixed uranium-plutonium (MOX) fuel assemblies have been tried. The core was designed with two enrichment zones of 21% and 33% uranium 235 enrichment and a peak burn-up of 9.7% of heavy atoms (ha). However data available just before commissioning led to burn-up being limited to 7.3% ha with reloads every 100 days. But even then there was considerable subassembly distortion and too many fuel failures. Subassembly bending was subsequently avoided by replacing the austenitic wrappers with ferritic ones.
To reduce fuel failure the core was modified in 1987. This included introducing a three zone layout (17%, 21% and 26% enrichment with 8.3% ha peak burn-up), preventing any fuel rotation or mispositioning, and increasing the fissile section height from 75 cm to 100 cm (leading to a linear rating decrease of from 51 to 47 kW/m) with reloads every 165 days. In 1993 the introduction of new structural materials made possible a rise in the peak burn-up limit to 10% ha.
Further modifications are planned with the aim of achieving an 11% ha burn-up and changing the refuelling scheme to avoid outages in winter. This is seen as an urgent task as the unit is used for co-generation of heat for the central heating systems of the town of Zarechny.
The control rod guide tubes have also caused some concern. They were designed to last for 30 years but had to be replaced after 10 due to radiation damage. The use of new materials is expected to have solved this problem.
A FAST REACTOR FUTURE
Overall, the BN-600 has performed very well, once the teething troubles were over. It is due to be decommissioned in 2010, but plans are now in hand to extend the design life by up to 10 years, explains Shestekov. And by then the new BN-800 should be operating.
Construction costs for fast reactors remain somewhat higher than for a VVER. A comparison between the BN-600 and a VVER at Novovoronezh shows that the BN-600 is 1.5 times more expensive to build. However, it is cheaper to run and reduces the overall cost difference to a factor of only 1.2. And this would be reduced further if BN reactors were put into serial production. Fuel costs are only 30% those for a VVER when the BN-600 uses uranium oxide fuel, but this increases by 20% if MOX fuel is used. Simultaneously with power production, the BN reactors can also be used to produce isotopes for medical and industrial use, and with slight adaptation they can also be used to burn wastes. But whatever their advantages, it is finance that will dictate their future and whether they come to be viewed as a serious solution to the plutonium problem.
Pavel Nikolayevich Alexeyev of the Kurchatov Institute points out that the nuclear community is under pressure to find a quick solution to the plutonium issue. Nuclear power plants need to be adapted to use MOX and in Russia those most suitable for this at present are the BOR-60 and the BN-600. However, to meet all the conditions set by the regulatory agencies, only 12% of the BN-600 core can at present use MOX fuel. Considerable adaptation would be required to use MOX fully, including reconstruction of the core and removal of the blanket, Alexeyev points out. If plutonium is to be used long-term, however, new reactor designs, such as the BN-800, will be needed. However, it will be necessary to improve control rod efficiency, moderator performance and corrosion prevention, before licensing is possible. “I am sure that our nuclear energy complex can rise to the challenge of efficient weapons grade plutonium management,” he says, “but we will need international funding for this.”
