Fast reactions

BN-800, the newest power reactor - the fourth - at Beloyarskaya is the most powerful example so far of a fast-neutron reactor. Construction is complete: all power unit tests specified in the power start-up programme were completed in February this year, and the test results confirmed all the project criteria required for power unit operation. The power generation plan anticipates production of 3,500GWh in 2016.

Russian nuclear engineers view the new plant as an important step in moving to a closed nuclear fuel cycle. It will be used for testing different fuel types and variations.

Considering the existing fuel resource base, experience from operating BN-800 can help make it possible to generate electricity for several thousand years even without controlled thermonuclear fusion. The technology used in the new plant makes it possible to increase the utilisation factor of mined natural uranium, as well as increasing in the level of safety to meet all the requirements set for prospective nuclear power units.

Passive safety

The safety of BN-800 is assured by the consistent implementation of the principle of defence in depth, and by the use of passive systems.

In all the BN reactor series at Beloyarsk the design is "inherently safe". That means in the event of any deviation from normal operating mode the nuclear chain reaction is stopped by the laws of nature, even though no command is given by a person or automatic device.

Following the recent trend, the reactor accommodates a number of additional passive safety systems that function by laws of nature
and cannot be "switched off". For example, the additional reactor emergency cool-down system uses natural air circulation through the heat exchangers. That is why there are four tubes over the power unit instead of one, as is usually the case. Physics law dictates that heated air rises automatically, while forcing inside another portion of cooled air.

As in all power units with fast-neutron reactors in Russia, in the BN- 800 sodium is used as a heating medium. Over 70 years of development, Soviet and foreign nuclear scientists have tried more than 50 kinds of heating methods, sodium has emerged as one of the most convenient options. It is relatively cheap and is characterised by good heat capacity and a relatively low melting temperature - only 95C. What is remarkable about the BN-800 is that sodium is preferable to water as a heat transfer medium: water is dangerous during phase change because a small pipeline leakage can lead to the water boiling (since it is under pressure and at high temperature) and overpressure. But in the case of an emergency in the BN-800, sodium leaves the reactor at its working temperature of 550C, and only starts boiling at 850C.

The reactor vessel is not at high pressure, just slightly higher than atmospheric pressure. The reactor vessel consists of two components - the main vessel and the guard vessel - inserted into one another like the elements of a Matryoshka doll. There is no water in the reactor, and the primary (in-vessel sodium) circuit is separated from the tertiary (water/steam) circuit by an intermediate circuit containing pure non-radioactive sodium.

The reactor has in integrated layout: all the primary circuit equipment that is exposed to radiation is enclosed in the vessel. The high heat capacity and temperature tolerance of the liquid sodium coolant prevents the reactor from overheating even if there is no cooling at all.

There is an additional trip system based on rods suspended (floating) in the sodium flux. They will fall into the core under the action of gravity in any situation in which the liquid metal stops circulating. Inside the reactor vessel there is a tray that is intended to confine the corium if it becomes necessary.

During stress testing the reactor's earthquake resistance was calculated. Based on the results all safety equipment, buildings and other plant facilities were upgraded to improve their earthquake- resistance. Based on the completed testing, they found the systems and schemes of heat removal from the reactor would allow it to continue in normal operation, even during an earthquake. Design estimates confirm that the power unit equipment is resistant to earthquakes up to level 7. Systems tested included the circulating water supply system - a dam and water reservoir hydroelectric facility, an onshore pump station, pressure and drainage water circulation pipes. This system can resist earthquakes up to level 6.

An additional estimate was made of the robustness of buildings and facilities in the case of a severe wind load and as a result the
wall panels, turbine building load-bearing structures, and some of the metal framework elements of the diesel generator plant building were reinforced in the course of project implementation.

BN-800 reactor development

The BN-800 power unit is the latest evolution of the 'BN' design and technology. Its predecessors were: BN-600 (at the Beloyarsk plant); BN-350 (at Mangyshlack Atomic Energy Combine; and a series of research, pilot and demonstration fast reactors known as BR-1, BR-5 and BOR-60.

Projects associated with the BN-350 and BN-600 reactors used well- understood enriched uranium oxide fuel in order to move to sodium reactor technology as soon as possible. The new project - BN-800 - takes this on to use mixed uranium and plutonium oxide fuel (MOX) and aim for a closed fuel cycle.

Project development on the BN-800 began soon after work on the BN-600 was completed. The BN-800 project had the following basic development and licensing stages:

• 1984 - the engineering design of the power unit was developed;
• 1985 - the power unit design was approved by Gosatomnadzor of the USSR, and construction of two power units started;
• 1989-1993 - an environmental assessment was made and there were expert appraisals by the State Planning Committee, State Sanitary Surveillance bodies, the Fire Safety Authority and the Ministry of Economy;
• 1990 - expert appraisal was carried out by the USSR Academy of Sciences;
• 1993 - the project was updated in accordance with new regulations (OPB-88 and PBY RP AS-89) and taking account of the notes made by the USSR Academy of Sciences expert group;
• 1994-1997 - expert appraisal by Gosatomnadzor of the Russian Federation;
• 1997 - a licence was issued by Gosatomnadzor of the Russian Federation for the renewal of construction work at Beloyarskaya 4; and
• 1998 - a licence was issued by Gosatomnadzor of the Russian Federation for the restart of construction at the Yuzhno-Uralskaya power plant.
  

The BN-800 project development continued despite the Chernobyl accident. Engineering solutions related to the project were finally approved in the 1990s. They were required to comply with new safety regulations for nuclear plant, and with more ambitious performance indicators. The works performed in this direction in the 1990s were considered successful. In 1997, the licence was awarded that allowed construction of BN-800 to restart at Beloyarskaya. The following year the licence was issued for Yuzhno-Uralskaya nuclear power plant. They were the first licences for nuclear plant construction in Russia after the Chernobyl accident.

The project was revived nearly ten years later after the breakup of the USSR. In 2006, the government of the Russian Federation approved "Development of the Nuclear Generation Complex in Russia in 2007-2010 and up to 2015," a Federal Target Programme aimed at fast-reactor development. The same year a government decree was published that restarted construction of the BN-800 plant at Beloyarskaya 4.

As specified in the Federal Target Programme, the main purpose of building BN-800 is similar to the task set back in the early 1980s when project development first started, "execution of closed nuclear fuel- cycle technology."

BN-800 specifications

The structure and specifications of the BN-800 have no significant differences from those of BN-600. However, using newly discovered capabilities of the design and with some other improvements it was possible to increase the reactor power by about 40%. That improved the technical and economic indicators of the power plant. The number of turbine sets was reduced from three to one for the same purpose. With one turbine set the installed capacity-utilisation factor of the BN- 800 is expected to increase to 0.85.

The number of modules in the BN-800 project was reduced when it was decided not to use sodium reheating. This improved reliability while preserving steam generator sectionality. This also made the steam-heat pipeline layout simpler and allowed a standard design of steam-heater to be used. Although this solution reduced efficiency slightly, overall it was thought to be the most efficient.

Sodium temperature in the circuits of the BN-800 is lower than in the BN-600 because chromic steel is used in the steam heating units instead of stainless steel, which is vulnerable to intercrystalline corrosion under voltage when the heat-exchange surface is wet.

New solutions implemented in the BN-800 project are primarily aimed at safety enhancement as well as at improved performance indicators (see Table 1).

The improvements made it possible to enhance BN-800 project safety so it complies with the requirements set for advanced nuclear power units. That includes, in particular, no requirement for human evacuation in any of the accident levels considered in the project.

The BN-800 project is intended to use MOX fuel, and the design affords the opportunity of a transition to high-density nitride fuel in future. Over the period of the BN-800 project development, extensive research and development was performed to develop technology for the manufacture of fuel elements, new materials and fuel blends.

For the first operating period of the reactor, fuel elements coated with ChS-68 cold-deformed austenitic steel will be used; such steel is used in the BN-600 and makes it possible to reach a maximum burnup of 10% heavy atoms (the damaging dose when using MOX fuel is about 90 dpa). In future, improved cold-deformed austenitic steel (EK-164) will be used to increase the maximum burnup to 13% h.a. after which ferrite-martensite steels will be used. For the fuel assembly cladding ferrite-martensite steel (EP-450) will be used - as in the BN-600 reactor. Corresponding studies are performed in the BN-600 reactor to confirm that the new steels can be used successfully for fuel element cladding.

This suggests that the BN-800 reactor is capable of supporting closed fuel-cycle technology and tests the efficiency of new engineering solutions. The steps required to take this forward are well understood.

A methodology for calculating the strength of the main components of a fast-neutron reactor unit, using sodium as the heat-transfer medium (RD EO 1.1.2.09.0714-2007) was developed during work on life extension of BN-600. Regulations for future projects will be improved, as nuclear and radiation safety requirements become more stringent.

BN-800 project development and licensing, included software development as follows:

• new codes for neutronic and thermo physical characteristics of the reactor core were developed;
• radiation safety and safety assessment calculations were enhanced; and
• three-D codes were integrated (ANSIS, Flow Vision, CFD) to carry out the calculation and distribution of temperature and voltage in the reactor structures, along with temperature fields and sodium flow rates.
  

The reactor is equipped with the following control and measuring devices:

• body element position sensors to manage temperature change in the equipment and pipelines;
• fuel element status monitoring systems;
• metal control system during operation;
• spectrometry system and sampling devices for quality control of the sodium heat-transfer medium; and
• gas sampling devices for gas control above the sodium coolant.

Supply of the automated control system, venting, electro technical and other equipment is arranged similarly to the same equipment for other nuclear power plants that have recently been commissioned (Kalininskaya, Rostovskaya, etc.), since they use the same type of equipment. BN-800 construction achieves the most important practical task: development of BN reactor technologies, which is essential for making them a commercial product.

Future experimental capability

BN-800's experimental capability was developed in tandem with the reactor technology. This capability was based on data derived from BN-350 and BN-600 project development.

BN-800 is the most important stage in fast-neutron reactor technology: one that shows that fast-neutron reactor technology is ready for commercial development:

• engineering solutions for major components and for reactor plant assembly have been developed and tested;
• the most important construction materials have been created and qualified and the direction of future development has been defined;
• applicable regulations and methodology are understood and there are up to date calculation codes for project characteristics and for fast sodium reactor safety feasibility;
• there is a wide test base, created for the BN series feasibility study and will be used for new project development;
• manufacturing technologies for equipment and instrumentation have been restored or set up;
• stable cooperation between technology Russian manufacturers of such technologies has been created; and
• human resource policy issues have been resolved.

The BN-800 reactor was created primarily for the use of mixed uranium and plutonium fuel. It will also be used for closed fuel cycle system design and justification of the technical safety solutions to be used in the next project: BN-1200. The aim of the BN-1200 project is to create a power unit by 2020 that has the technical, financial and safety indicators to comply with the requirements for generation IV units, and for the commercial construction of this type of power unit in the future.

The accumulated experience of BN development and operation in Russia demonstrates that sodium technology can achieve the following targets: search for a solution to the fuel supply problem in nuclear energy over the long term; design the closed fuel cycle on an industrial scale; reduce radioactive waste by allowing the processing of spent nuclear fuel from pressurised water reactors; and the use of plutonium and minor actinides separated from spent fuel, solving the issues of non-proliferation.