Interim Results and Prospects for Spent Fuel Management at Bilibino NPP
M. Baryshnikov, E. Suvorova, A. Khaperskaya, N. Shorokhov (Rosatom Corporation), V. Fokin, K. Ozerov, E. Olenin, S. Petrov, F. Tukhvetov (Bilibino NPP), O. Barinkov, A. Dorofeyev, D. Ivanov, A. Leschenko (Sosny Research and Development Company)
Nuclear & Enviromental Safety, №3, 2012
The policy of Rosatom corporation with respect to spent nuclear fuel management, as stated in the industry's Concept of Spent Fuel Management (2008), is based upon the underlying principle that spent fuel is to be reprocessed in order to ensure environmentally acceptable management of fission products and recycling of all recovered nuclear materials. At the same time, the Concept envisages for all types of spent NPP fuel to first undergo interim storage and then reprocessing. The same approach lies in the core of the "Programme for Spent Fuel Management Activities and Infrastructure for 2012-2020 and Until 2030", which was approved in November 2011.
The only kind of spent fuel whose fate currently remains unidentified is of Bilibino NPP. The design of that station does not even have technological provisions for spent fuel shipment from the site, and lacks the hardware and transport vehicles to support spent fuel movements within or outside the site. The main spent fuel management option currently remains long-term storage in the now-dry existing cooling pools.
EGP-6 spent fuel is enclosed in long assemblies, and its fuel composition is close to that of one of AMB fuel modifications, making this type of spent fuel potentially reprocessable at Mayak once its fragmentation and canisterisation shops are commis¬sioned, i.e. after 2016. However, remoteness of the Bilibino site, absence of on-site infrastructure to support spent fuel recovery and removal, or adequate transport infrastructure in the region make potential de-fuelling an extremely expensive project to undertake. As part of the Federal Nuclear and Radiation Safety Programme, various options have been evaluated, including spent fuel removal from Bilibino for reprocessing using sea or air transport. At the same time, permafrost conditions in the region around Bilibino are favourable for final isolation of radwaste and spent fuel (in mined wells or galleries).
Spent fuel removal from reactor room
The existing NPP process provides for post-core fuel assem¬blies to be put into canisters, which are then placed into cooling pools. The originally designed SFA storage and handling shop was never built; instead, the cooling pools were re-racked to increase storage capacity. During the initial period of NPP operation, the canisters were made of carbon steel and coated with coal-based enamel to slow down corrosion, but it was impossible to guarantee canister in¬tegrity during indefinitely long storage in hot water conditions in the cooling pools. There was also the risk that pool walls would corrode. Hence, two pools full of SFAs were drained, transferring spent fuel to dry mode of storage. Therefore, Bilibino accommodates two drained cooling pools that hold densely-spaced SFAs tied to each other by wire and cotton bands (fig. 1). Some SFA canisters are located in cooling pool bays.
The status of canisters and walls in the drained cooling pools is unknown, they are impossible to enter and it would be unde¬sirable to re-flood them. In order to determine retrievability of all SFAs held in the drained cooling pools, a comprehensive engineering and radiological survey had to be performed using unique purpose-designed equipment. Then it became possible to develop the process and equipment to remove spent fuel from the reactor building.
In order to support retrieval of SFA canisters from the drained cooling pools, a special manipulator-gripper was designed, to be installed in the existing holes in the floor slabs, with its operating arm then transferred to the working position (fig. 2). The main dif¬ficulty experienced during manipulator design was achieving sufficient lifting capacity with strict limitations on structure size and significant arm reach required to recover fuel from the outlying pool bays. Other equipment was also designed, for example, for canister cutting and SFA encapsulation (fig. 3).
By the time the plant goes into shut-down, two more cooling pools (currently flooded) would be filled with SFAs; for them, the fuel retrieval technology is already provided. Therefore, there is technology available to de-fuel the entire reactor room, safety of the designs has been verified on an experimental rig, and safety provisions to be put in place during the performance of opera¬tions are compliant with the regulatory requirements taking into account the seismic properties of the region (a preliminary safety analysis report has been issued and endorsed by the Leipunsky Institute of Physics and Power Engineering - scientific supervisor of the site). This technology will be in demand whichever option is subsequently chosen for Bilibino spent fuel management.
Spent fuel management outside the reactor
As until a final decision regarding management of Bilibino spent fuel has been made, detailed design of the equipment re¬mains non-practical, therefore, technical and economic studies have been carried out to estimate the cost of future works, taking manufacturing of equipment into account. Continued SFA storage in cooling pools was considered, along with disposal in a nearby repository (in mined wells or galleries) for removal for reprocessing or storage. Let us consider the options in more detail.
Continued storage in cooling pools
Using the results of technical and economic studies carried out by Siberian Group of Chemical Enterprises in 2009-2010, it is possible to justify safety of SFAs storage in the cooling pools (in a gas environment or poured in concrete) for the next 50 years. The costs to be borne to prepare and maintain the fuel in safe storage during this period are estimated to be 70 billion roubles in 2009 prices. One major drawback of this option, however, is that SFAs storage in cooling pools is essentially a delayed solution, as it would keep the source of potential hazard in the region and fail to address Bilibino decommissioning. Therefore the problem would need to be revisited within just a few decades.
Disposal in a local repository
In order to review feasibility of constructing a pilot under¬ground repository for spent fuel and radwaste nearby Bilibino NPP, VNIPIpromtekhnologii conducted an investment study and environmental impact assessment of various options involving spent fuel disposal in a locally-mined well or gallery repository. Permafrost conditions in the Bilibino region are favourable for a final radwaste and spent fuel repository, due to the following:
– the host geological rock does not include free water, which hampers potential transfer of radionuclides from the repository into the environment;
– slowed-down oxidation-reduction reactions in permafrost rock, extending the time during which engineered barriers will remain effective;
– presence of cold as a natural thermophysical barrier;
– characteristic time of thawing zone persistence is a few decades;
– subsequent freezing of the entire repository effective volume with establishment of average temperatures characteristic of permafrost rock;
– engineered barriers effectively ensure containment of radio-nuclides and prevent their transfer to outside the repository during thaw periods;
– geological and geocryological conditions in the Bilibino region correspond to the concept of spent fuel and radwaste disposal in permafrost rock.
During environmental impact assessment, various aspects of safety assurance were considered (nuclear, radiation, transfer of radionuclides). The risk of beyond-design accident was estimated to be 10-7, i.e. below the level of negligible risk. At the same time, the most vulnerable aspect of the permafrost rock repository concept is poor demonstrability of permafrost stability over a million years.
It is essentially possible to move spent fuel assemblies out of Chukotka. Heavy loads (reactor vessels, cooling pool steel cover slabs, etc.) were delivered to the Bilibino site by the Northern Sea Route, so the same route could be used for shipment of fuel-loaded overpacks. It is also possible to export spent fuel using heavy-duty airplanes, such as the AN-124-100, justification of which was made by Sosny as part of technical and economic studies of 2010-2011.
Reviewed below are the various routes and methods for trans¬port of spent fuel overpacks.
Modes of transport
All loads are delivered to Bilibino using sea or air transport, and in either case, automobile transport must also be used. The same modes of transport are available for spent fuel export (fig. 4).
Sea route: by automobile ice road from Bilibino to a tempo¬rary storage location near a port, then, as summer navigation be¬gins, by sea transport to a mainland port, and therefrom by rail to the final destination. The overpack would be returned following the same sequence in reverse. Each part of the route is only avail¬able four months a year, so the full transport cycle, including re¬turn of empty overpacks, would last about two years. Regularity of heavy transport movement by the ice road depends upon the weather. Extremely volatile weather conditions in the region also influence passability of the Northern Sea Route. Nonetheless, automobile traffic by the ice road is organised every year, and three sea voyages to European Russia and back are possible if supported by icebreakers. The full cost of Bilibino de-fuelling using sea transport is estimated to be approximately 50 billion roubles. This includes fabrication, delivery and installation of equipment, construction, loading/offloading operations, setting up the necessary transport infrastructure and actual shipping, as well as reprocessing of spent fuel at Mayak.
Air route: transport by airplane from a nearby airfield to an airport next to a railway station, therefrom by train to the final destination. Various options may be possible: either use the local airfield as it is, capable of year-round service of up to 20 tonnes ca¬pacity airplanes, or upgrade the existing runway to make it suitable for airplanes with over 100 tonnes capacity, thus significantly re¬ducing the number of flights to be made. Amounts saved by fewer flights should about off-set the cost of runway upgrade.
The cost of the "airborne" option will depend on the type of overpack used:
– spent fuel in a type B(U) overpack using air transport may be possible if the contents meet the requirements of "special radioactive material" (which Bilibino spent fuel canisters meet). The cost of Bilibino de-fuelling using type B(U) overpacks by air transport is estimated to be approximately 70 billion roubles, of which 10 billion roubles would be spent on upgrading the local airfield runway;
– the main requirement to type C overpack is resistance to airplane crash accident, so type C overpacks are safer than B(U) for spent fuel transport by air. A type C overpack may also hold significantly more fuel. The possibility of Bilibino spent fuel transport in a type C overpack is currently being analysed by VNIITF under a contract sponsored by the Federal Nuclear and Radiation Safety Programme. Costs associated with this option are estimated to be approximately 40 billion roubles.
Russia's currently only reprocessing plant - Mayak - divides all spent fuel into two categories: reprocessable and non-reprocessable. So far, the EGP-6 spent fuel falls into the non-reprocessable, but as its fuel composition is close to that of one of AMB modifications, this type of spent fuel may potentially be reprocessable after commissioning of the new fragmentation and canisterisation shop and upgrade to the site transport infrastructure. The cost of repro¬cessing has been estimated and is already factored in the amounts quoted above. The plan is to complete reprocessing of AMB spent fuel (at the current rate) by 2023, which fits well with the schedule to start the work at Bilibino in 2022.
Currently, Rosatom has adopted a decision that envisages the following dates for decommissioning of Bilibino units 1-4:
– unit 1: 2018;
– units 2-4: 2019.
After shut-down of the Bilibino reactors, management of its spent fuel stockpiled over its operating life since 1974 will need to be urgently addressed. It is too expensive to keep the reactor in shut-down mode for a long time. Plant preservation in the Far North would be even more costly, as an enormous infrastruc¬ture would require conservation; staff familiar with the plant design and operational history would leave the site, and by the time decommissioning begins, a new workforce would have to be hired and trained, residential and production infrastructure re-built.
Thanks to the large on-site pool capacity and small power of the plant itself, at the current energy output the cooling pools are expected to be filled within ten years, which is also the natural limit for the NPP active life, as the operating lives of the reactor installations will expire in 2019-2021. But in order for de-fuelling operations to start in the reactor room immediately after the shutdown of the final reactor, the necessary equipment will have to be delivered and installed by 2022. The expected duration of prepara¬tory activities (pre-design, design, design reviews, equipment de¬velopment and adjustment in the mainland, equipment transport, personnel training) is more than ten years.
Since 2009, the activities to prepare the Bilibino cooling pools for de-fuelling are covered by the Federal Nuclear and Radiation Safety Programme. So far, technical and economic studies in a number of areas have been completed, along with some of design and engineering. Much still remains to be done.
Risk reduction in various options of Bilibino spent fuel management
In order to finalise the choice of Bilibino spent fuel manage¬ment option, it is essential to carry out an assessment of risks as¬sociated with normal conditions and hypothetical accidents, taking into account their probability and severity. In 2012, under contract within the framework of the Federal Nuclear and Radiation Safety Programme Sosny is to assess radiological risks for personnel and the public associated with various options of Bilibino EGP-6 spent fuel transport to Mayak for reprocessing.
Justified choice of EGP spent fuel management option is ex¬pected to be made by Rosatom in 2012, taking into account the recommendations by the working group that includes members of Rosatom Corporation, Rosenergoatom Concern, Chukotka regional authorities, developers of EGP-6 spent fuel transport and handling processes, as well as Rostekhnadzor experts (Science and Technology Centre for Nuclear and Radiation Safety).
Количество просмотров: 301