In the Russian Federation, a series of integral reactor designs are being actively pursued by the Experimental Machine Building Design Bureau (OKBM).

The reactor called Atom Thermal Electric Plant (ATEC 200) comes in sizes from 80 to 250 MWe. They have natural circulation systems for residual heat removal, positioned in the upper head. They are intended for use in remote locations and are designed to be sited below ground level.

The "AST 500" is a natural circulation district heating reactor which is ready for operation but due to public acceptance problems, the project has been suspended for the present. The safety of its design and operation have been reviewed by an IAEA OSART team and found to be adequate.

The Passive Safety Integral Reactor Plant (VPBER 600) is a 640 MWe reactor designed earlier A number of significant changes have been made to its design, including moving the pumps from the bottom of the vessel to a position above the core, and addition of a core catcher. A comprehensive in-service test and inspection programme has been set up and equipment design carried out.

Integral Reactor for a Floating NPP (NIKA 120) is designed by the Research and Development Institute for Power Engineering (RDIPE) as a floating power unit for use in northern Russia and is under development. The plant consists of two reactors, each of 42 MWt. The reactor vessels are enclosed in a safeguard vessel which is immersed in a bubbling tank. The fuel is 21% U-235 as UO2 in a zirconium matrix.

Control rods are designed to stay inserted even if the floating unit is inverted in the sea.

Development work is also being carried out on a small unit called the Autonomous Co-generation NPP (UNITHERM), with a power of 30 MWt or 6 MWe for use in difficult-to-reach regions. Emphasis is on maintenance-free operation between the annual visits of the maintenance team. There is an intermediate heat exchanger to isolate the final steam supply including that to the turbine, from any risk of radioactive contamination. Such an intermediate circuit is necessary for a district heating system to act as a pressure barrier Maintenance of the steam generator and turbine can be carried out using conventional methods as the steam is free from radioactive contamination. The penalty is an increased generation cost due to loss of thermal efficiency and the cost of an extra plant system.

Development work is in progress with regard to emergency heat removal systems on the "ABV-6" reactor system, planned for installation in the floating nuclear power plant, "Volonolom". This is a 38 MWt plant using a uranium-aluminum(U-Al) alloy fuel in a natural circulation reactor with nitrogen pressurization There are five

emergency heat removal paths some of which are dedicated systems and others are shared systems(e. g use of the coolant purification system heat exchanger for emergency heat removal). There is a passive system using the flow of stored water to the normal steam generators and discharging the resulting steam to the atmosphere The ABV-6 containment system embodies two rupture discs The first releases steam from the reactor containment shell to the pressure suppression pool, and the second releases pressure in the suppression pool compartment to the auxiliary building The reactor and containment systems were modeled using the computer code RELAP5 MOD-3, with modification of some code modules to allow for release of dissolved nitrogen A severe accident scenario with a double ended break of the pressurizer surge line, failure of the emergency core cooling system (ECCS), failure of the shielding tank coolers and no operator action, was modeled. The results of the analysis have conclusively shown the very high level of safety achievable by this design.

For many years, OKBM has been conducting design and development work on highly efficient cassette steam generators. These cassettes are produced on an automated assembly line and have been successfully used in many reactors They are also specified for VPBER 600 design, where 216 independent sub-sections are assembled into two different shapes of boxes to fill the annular space available The design is based on straight tube steam generators with secondary flow inside the tubes to ensure that they are always in compression There is considerable practical experience with these cassette steam generators in WER plants (500,000 hr) and they are backed by a large programme of developmental tests

OKBM is also working on radiological aspects of decommissioning integral reactors The main advantage of integral reactors results from the large water filled space between the core and the pressure vessel This results m a reduction in activation

level of the RPV by a factor of up to 10^ compared to WER reactors, with a corresponding reduction in activity in the adjacent concrete structures The overall effect on occupational dose during decommissioning, involving breaking up and removing all active plant components, is a reduction by a factor of ten

Design work on a new concept called Integral Reactor with Inherent Safety (IRIS) based on the "PIUS" reactor was started under the leadership of IPPE It is a 600 MWe reactor with natural circulation The steam generator is within the main vessel and the upper density lock is replaced by a by-pass tube which descends below the water level in the outer tank, which in turn, is below the water level in the reactor vessel during normal operation An increase in power leads to vapour formation and flow of primary water/steam into the outer tank Borated water enters the primary through the lower density lock and shuts the reactor down The design is still in the conceptual stage

Design work at the Kurchatov Institute is continuing on the use of super critical water as the primary fluid in the "V-500 SKDI" reactor With subcritical water, power increase is limited by the possibility of departure from nucleate boiling (DNB) This possibility is eliminated in super critical conditions, since the fluid remains in a single phase through all the temperature range Furthermore, the enthalpy of super critical water approaches that of steam, giving improved heat transfer in the steam generator Density variation with temperature provides an excellent negative reactivity

feedback for reactor stability The water density effect is strong enough to allow compensation for reactivity changes with bum up, without control rod movements and only a small change in primary temperature Some modem fossil-fueled stations operate with supercritical water in the boiler at 26-28 MPa and 560-580 K, giving a basis of practical experience for the use of these conditions Safety analyses have confirmed the high level of safety achievable by this concept, even in a very severe transient caused by multiple failures