The Russian technical specialists of Krasnaya Zvezda (Red Star) and Kurchatov Institute of Atomic Energy (KIAE) of Moscow were tasked in the early 1970's to develop a compact, transportable power system that would work reliably and autonomously at remote locations. The original system specifications required that it be able to operate for several years without maintenance in sea water at a depth of 6 km. To meet these requirements the Russia specialists designed a compact nuclear system with water coolant coupled to a thermoelectric power conversion subsystem. The goal was to design a system that was scalable from 300 kWth to 300 MWth. A proof-of-principle system, the GAMMA reactor system, was designed, constructed and tested at KIAE. The system construction was completed in 1981 and testing began the same year. The GAMMA reactor system is still operational today. The follow-on system for remote sites is the ELENA reactor system. It has not been built, but the final assembly drawings have been completed and are ready to be used. The ELENA was designed to provide both direct electric power and local space heating. It can also be used for desalination. The design of the ELENA meets post-Chernobyl safety requirements and includes features to assure its safety under autonomous operation. This paper provides an overview of the GAMMA and ELENA power systems including their design criteria, testing, and transportation to remote sites. Also included is a discussion on an evaluation and technology transfer program that could make this system available for use throughout the world. The Russian technical specialists at Krasnaya Zvezda and Kurchatov Institute of Atomic Energy (KIAE) have designed and tested a prototype of a compact, transportable power system that works reliably and autonomously at remote locations. The GAMMA reactor system is the prototype that has been built and tested at KIAE. The GAMMA was designed as a proof-of-principle unit for an undersea power plant. The follow-on system, the ELENA, has been designed for operation at remote sites to provide power and local heating. The assembly drawings of the ELENA have been completed, and are ready for fabrication and testing of the system. The ELENA system was designed with inherent safety features to ensure it remains in a safe configuration under any condition. This paper provides an overview of the GAMMA reactor system and its operation, the design requirements for the ELENA reactor, and a discussion of the technology transfer program that could make this system available for use throughout the world. GAMMA Reactor S vstem In the early 1970's the Russian specialists were t<?sked to develop a highly reliable power source that could operate within the ocean to a depth of 6 km for several years without field service. The primary design principles of the GAMMA reactor system were: 1. natural circulation and self-regulation of the reactor and secondary coolant loops, 2. an inherent negative reactor temperature coefficient, 3. steady-state reactor operation without active control at nominal power, 4. static power conversion, 5. 10,000 hours of autonomous operation, 6. a reduction of primary coolant temperature of less than 8-10°C for 5 years of operation, and 7. a thermal power range scalable from 300 kWth to 300 MWth. The stated power output for the GAMMA at 220 kWth is below the lower bound of 300 kWth for the pwer range. The GAMMA reactor system is designed to provide electricity for either remote villages or underwater operations. The output characteristics can be optimized for heating, as well as the production of electricity, depending on the requirements. A proof-of-principle reactor system, the GAMMA, completed manufacturing and began testing at the KIAE in 1981. It produces 6.6 kWe at a thermal power of 220 kWth. The system is designed to be able to deliver electric power at 28, 110 or 220 volts of electric power. Thermoelectric (TE) power conversion was selected due to its inherent long-term stability and high reliability. The TE power conversion units were based upon the design and test data obtained at KIAE as part of the Romashka space reactor system development during the mid1960s. The GAMMA system is 6 m diameter by 10 m high. A schematic of the GAMMA reactor system is shown Figure 1. Preliminary tests on the GAMMA reactor system were completed to ensure the reactor was stable during power transients or from external disturbances, such as load changes. Overall a large number of experiments were completed on the unit, including investigating the effect of the load on the performance of the TE power conversion and the associated feedback on the reactor. The Russian specialists checked their experimental data against computer calculations to develop an analytical basis. Up to the present, the GAMMA has operated for more than 10,000 hours at nominal power. It has provided the proof-of-principle for the followon ELENA reactor system. GAMMA Reactor Svstem Descrintion The GAMMA reactor system and power conversion system operate in a pool of water which removes the waste heat from the system by conduction. There is roughly twenty feet of water from the top of the reactor vessel to the top of the water in the pool. The reactor system uses water as both the primary and secondary coolant. The coolant from the reactor outlet goes directly to the TE power conversion and heat exchanger units. There are a total of twenty-four TE power conversion modules located above the core and arranged in a circular pattern around the outside of the reactor vessel. The coolant heats the hot-side of the TE power conversion on the inside of the coolant flow path and the TE cold-side is cooled by the pool of water. The primary coolant is pressurized and does not boil. There are no pumps in the primary coolant loop, rather the heat is transferred by natural circulation. Initially there was some problem with air stagnation at the top of the TE power conversion-to-heat exchanger units, but the Russian technical specialists believe they have solved this problem. The reactor is fueled by uranium-dioxide (U02) enriched to -20% U-235. The enrichment level was chosen to ensure it was below the level of concern for nuclear proliferation. The power density in the reactor core is very low. The system is designed for 10 years of operation without refueling. ELENA Reactor System The reactor system was named ELENA after the first remote site that the system was to be operated. The ELENA system has not been built or tested, but the technology is based on the principles and the operation of the GAMMA reactor. It was designed specifically to provide electric power and district heating for remote villages or locations. The baseline design of the ELENA reactor system is to produce 90 kWe at 3 MWth, although the design can be scaled to produce higher pwer, depending on the intended location and use. Of the 90 kWe produced, 20 kWe is used to operate the system and 70 kWe is available to the user. The TE power conversion system has low electrical conversion efficiency, and the waste heat is used for district heating. The electrical power output versus the excess heat for district heating can be optimized for the specific location and use. Table 1 provides a listing of the key parameters for the ELENA reactor. The reactor thermal power is kept constant and does not vary with the electrical load. There are two options for controlling the electrical power output: 1. use shunt resisters, or 2. to short circuit the TEs. Both of these have been analyzed by the Russians, but no final decision has been made as to which option should be used. They believe the decision should be dependent upon the specific electric loads. Two other options are to attach the third loop to a natural draft cooling tower, or to use it to heat greenhouses.