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Prepared by Jill Tatko, CNS Graduate Research Assistant Early Soviet research in the 1950s into nuclear propulsion reactors pursued a dual track: one oriented towards a water-moderated design and the other towards the use of heavy-metal coolant. Despite certain start-up and operational advantages in using heavy-metal as a coolant, Soviet designers eventually abandoned this option due to the greater safety hazards involved and the difficulty of keeping the reactors hot enough while the submarine was in port. Running the reactors at less than full capacity caused the heavy-metal coolant to congeal, causing the reactors to seize up and essentially "freeze" themselves, causing irreparable damage. For these reasons, besides a few test designs, the Soviet Navy opted to produce only water-cooled reactors for use in active-duty submarines.[1] CHARACTERISTICS OF COMMON DESIGNS USED IN NUCLEAR SUBMARINES Most Soviet-designed nuclear submarines are powered by one or two water-cooled, pressurized water reactors with a total thermal capacity of 50 to 200 MWt. Depending upon the type of reactor, each reactor core contains approximately 248-252 fuel assemblies. One fuel assembly holds tens of fuel rods, which can be round or flat. Flat fuel rods enlarge the surface area for improved thermal efficiency and are more common in later generation reactors.[2] Out of the estimated 468 naval reactors that have been installed on 258 submarines and service ships, 24 use fuel enriched to 90% U-235. Most of the reactors were fueled with U-235 enriched to 21-45%. A typical reactor core contains 315 kg of uranium.[3] The level of enrichment of uranium fuel varies with reactor design. First- and second-generation reactors are fueled with 21 percent U-235. Third-generation reactors have cores with different enrichment zones.[4] Fuel assemblies in the middle section of the reactor are enriched to 21 percent U-235, while the outermost fuel assemblies are enriched up to 45 percent U-235. A second-generation submarine reactor contains about 250 kg of uranium, of which 50 kg are U-235. Third-generation nuclear submarines contain approximately 115 kg of U-235.[2] The Russian naval nuclear fuel cycle significantly overlaps the fuel cycles of the military's fissile material production and commercial nuclear power reactors. In the 1960s and 1970s, the nuclear power industry, the shipbuilding industry, and the Russian Navy established a naval fuel cycle infrastructure. During this period, the fresh fuel was fabricated in Elektrostal, near Moscow. Until the 1970s, the fuel was also fabricated in Kazakhstan at the Ulba Metallurgical Plant. From these facilities, the fuel was then transported to naval facilities for refueling the nuclear fleet.[4] Production of weapons-grade uranium fuel required about 1.5 tons of HEU annually from national stockpiles for use in both naval and research reactors.[5] In the post-Soviet era, the overall demand for HEU has decreased due to reduced naval activities and the subsequent reduced demand for naval reactor fuel.[6] Until the early 1990s, the uranium component of naval fuel was recovered from tritium production reactors at Mayak in Ozersk (Chelyabinsk-65) and from HEU rods from plutonium production reactors in Krasnoyarsk-26 and Tomsk-7. The RT-1 plant at Mayak reprocessed irradiated HEU fuel. Uranium enriched approximately to 50 percent and recovered from irradiated fuel was sent to the Machine Building Plant for production of submarine fuel rods and assemblies. After the fuel was irradiated through use in active-duty submarines, it was stored for several years before being sent to Mayak for reprocessing. Spent naval fuel underwent reprocessing together with the spent fuel from research reactors. Separated plutonium was stored at Mayak. Recovered uranium from naval reactors was shipped to Ulba Metallurgical Plant in Ust-Kamenogorsk, Kazakhstan to produce RBMK fuel pellets.[4] In the 1990s, the fabrication and reprocessing of naval reactor fuel has occurred at Mayak.[7] Standard naval reactor fuel is stainless steel or zirconium-clad Cermet material (dispersed fuel) in which uranium particles are embedded in a non-fissile aluminum matrix. Fresh fuel is sent to the Sevmorput and Shkotovo waste sites for temporary storage. From there, it is shipped to central storage facilities. Naval fuel is later transferred to service ships for distribution to operating nuclear submarines. Physical protection of naval fuel is a matter of serious concern today due to inadequate levels of safety at many Russian storage facilities. At least two documented cases of the diversion of fresh submarine fuel have occurred since 1991, one from Andreyeva Guba and another from Sevmorput, both in the Northern Fleet. Many unsuccessful attempts have also been noted.[8] In 1996, the US Department of Energy began a cooperative program in the naval fuel sector to assist in the design and construction of safe and secure fuel storage facilities for the Northern Fleet and the civilian icebreaker fleet (Atomflot). A similar effort to address problems at Pacific Fleet facilities began in 1998.[9] (For more information, please see the Foreign Assistance Overview.) Since 1995, little spent fuel storage has been available on land for additional reactor cores from decommissioned submarines. This problem has caused decommissioned submarines and service ships to become de facto long-term spent fuel storage facilities.[10] Reactors containing fuel (both low-irradiated and spent fuel) remain in operation on decommissioned submarines. Low-irradiated fuel in submarines decommissioned before the end of their service lives retains a large quantity of highly-enriched uranium (HEU). Separation of HEU from low-irradiated fuel is much easier than chemical reprocessing required for plutonium separation and can be done at smaller facilities. Naval reactor fuel assemblies are smaller and easier to handle than power reactor assemblies. It is important to note that irradiated fuel in naval nuclear reactors requires a significant cooling time. A primitive nuclear explosive device manufactured from U-235 is more likely to be successful than a primitive device made of plutonium.[15] Spent fuel is also kept in service ships, which receive spent fuel assemblies from active-duty submarines during refueling operations and from decommissioned vessels as a result of defueling and reactor shutdown operations.[4] Once the service ships are filled, the spent fuel is sent to on-shore central storage facilities where it is stored for three years. There is a shortage of these service ships in both fleets, and to compound the problem, the existing service ships need repairs and are often inoperational. The spent fuel is then placed into shipping containers to be transported via rail to Mayak. Newly constructed containers, in some cases, exceed the weight capacity of local rail terminals. In addition, the design of new containers requires repackaging of spent fuel. Special rail cars are needed to carry the containers safely, yet only one such train is currently available to service both fleets. Service is therefore intermittent. When the fuel rods eventually arrive at Mayak, past processes involved storage and eventual reprocessing. It is unclear how much is still being reprocessed. Damaged or non-standard fuel, however, cannot be reprocessed.[10] Older classes of Russian nuclear-powered submarines were refueled every seven to ten years of service, whereas newer generations of Russian nuclear submarines are being refueled after three to five years of active duty.[11] Nuclear submarines were previously refueled at dry docks in shipyards. In recent years, submarine reactors are refueled in the water between a pier and a service ship, called a plavuchaya masterskaya (PM) or floating workshop.[12] Fuel is removed using PM-124-class nuclear submarine support barges (capacity of 560 fuel assemblies each) and PM-2020 Malina-class submarine support ships (with a storage capacity of 1,400 fuel assemblies or six reactor cores with spent fuel). The PM-124 service ships are remodeled Finnish cargo barges. According to a US Government Office of Technology Assessment report, the frequency of refueling has decreased in the last several years due to the lack of fuel transfer and storage equipment, saturation of the spent fuel storage capacity, and difficulties in removing fuel from submarines with damaged reactor cores.[13] The removal of spent nuclear fuel is initiated at least 90 days after shutdown, during which time the reactors are allowed to cool. The process of removing fuel takes about one month. Refueling and preparing the reactor takes two to three months. The steps involved in changing fuel in the reactor include:
The refueling process generates radioactive waste in addition to the spent nuclear fuel. The refueling process produces about 10 cubic meters of high-level liquid radioactive waste. Solid waste is generated in the form of control rods, reactor tank tailings and contaminated equipment. The installation of new reactor filters generates about one cubic meter of highly radioactive ion exchange sorbent and two to three cubic meters of liquid radioactive waste. A regular refueling process produces from 155 to 200 cubic meters of waste.[14] (For more information, please see the Decommissioning and Dismantlement Overview.) REFUELING FACILITIES
Northern Fleet: Pacific Fleet:
Page last updated 7 April 2000 Comments or questions? Contact Cristina Chuen at MIIS
CNS: Cristina.Chuen@miis.edu
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Naval Reactor Technology
cutting away the segment of the hull that covers the reactor;
