Decommissioning of the Floating Maintenance Base Lepse
Sosny R&D Company is practically involved in preparations for decommissioning of the Floating Maintenance Base Lepse under the contract with JSC FCNRS. Funds are provided by the Northern Dimension Environmental Partnership set up by the European Bank of Reconstruction and Development as part of technical assistance to the Russian Federation. The work is done jointly with the National Research Center Kurchatov Institute with the participation of FSUE Atomflot and SC Zvezdochka, branch of the Nerpa Shipyard.
In 1961 a dry cargo ship was re-equipped to create a two-deck vessel with three platforms and a superstructure. For a long time, the Lepse carried out operations to recharge nuclear fuel on the nuclear-powered icebreakers Lenin, Arktika and Siberia. Also, it was used as a repository of spent nuclear fuel (SNF), radioactive waste, and equipment. In 1988 the Lepse was taken out of service and in 1990 it was re-classified as a ‘berth connected ship’. In 2014 the Lepse was placed in a dry dock at the Nerpa shipyard where it was separated into five parts: the stern section; the section with liquid radioactive waste tanks; the engine and boiler room, the section with spent nuclear fuel and and the bow section.
As the SFA storage tubes and caissons of the Lepse storage tanks are filled with damaged fuel assemblies of ОК-150 Type and ОК-900 Type, it is planned to retrieve all spent fuel from the Lepse storage tanks and load it into transport canisters, and then load full canisters into the cask. These activities will be carried out in the Lepse Shelter using the special SNF retrieval equipment and the SNF handling equipment. Then the cask with full transport canisters will be transported to the Serebryanka ship and the canisters will be reloaded into TK-18 casks for their further transportation to the site of FSUE Atomflot for subsequent reloading onto the rail conveyance for shipment to the reprocessing plant at Mayak PA.
According to the Technical Design documentation prepared by Afrikantov OKBM, in the period of 2015-2016 the specialists of Sosny R&D Company prepared the Detailed Design documentation on the equipment for spent fuel retrieval from the Lepse storage tanks, such as:
- SFA unload cask,
- Cask-to-storage tank installer,
- Cask-to-caisson installer,
- Manipulator for SFA retrieval from the caissons,
- Equipment for retrieval of the spent fuel fragments from the caissons,
- Thin-walled overpacks and special equipment for loading SFAs into thin-walled overpacks (loading station),
- Equipment for cutting off the SFA lifting features and a reamer,
- Equipment for cutting the SFA storage tube out of the Lepse storage tank tube plate,
- Lifting adaptors,
- Tool for installation of lifting adapters and tool for removal of lifting adapters,
- SNF retrieval equipment remote control system,
- TV system for SNF retrieval,
- Special-purpose lifting beam for handling the equipment, and
- Necessary tools and accessories.
All the SNF retrieval equipment is made in compliance with the requirements of NP‑022-2000 and can be operated remotely from the Control Room located far from the Lepse storage tanks. The manual override devices are intended to complete the current operations performed by the equipment in case of the power failure. The SNF retrieval equipment was fabricated at the manufacturing plant of Sosny R&D Company in 2016.
SNF unload cask is designed for safe transportation of the fuel assemblies, the SFA storage tubes which are cut-out of the Lepse storage tank tube plate and the thin-walled overpacks within the Lepse Shelter. The winch, the grapple position sensors and load sensors are on the cask top. The rotary gate is at the cask bottom. The cask has a biological shielding. The weight of the SNF unload cask is 10 tons. The scope of supply includes an associated lifting beam, a rotary frame, a telescopic tower and a drip pan which are intended to transport, tilt, handle and store the SNF unload cask. The cask is equipped with the removable grapples for handling ОК-150 fuel assemblies and ОК-900 fuel assemblies.
Cask-to-storage tank installer is intended to arrange the SNF unload cask above the SFA storage tubes located in the Lepse storage tank, and to release those fuel assemblies which are stuck inside their storage tubes. The installer has a guide funnel in its top for proper positioning of the SNF unload cask. The slide gate is located below the guide funnel and intended to close the opening providing the access to the SFA storage tube. The SFA release drive is mounted under the slide gate. All drives are remotely controlled and equipped with the manual override devices. The scope of supply includes an associated lifting beam and a support which are intended to transport and store the cask-to-storage tank installer. This installer weights 2.4 tons.
Cask-to-caisson installer is intended to arrange the SNF unload cask or the manipulator for SFA retrieval from the caissons above the caisson of the Lepse storage tank. This installer is equipped with the in-built manipulator for SFA retrieval from the caisson. The cask-to-caisson installer enables moving the manipulator for SFA retrieval from the caissons in the horizontal plane, while the manipulator grapple can be moved up and down, and rotated in both the horizontal and vertical planes.
Capacity of the manipulator is 200 kg. All drives of the cask-to-caisson installer and the manipulator for SFA retrieval from the caissons are equipped with the manual override devices. This installer weights 7.3 tons and the manipulator weights 0.7 tons. The scope of supply includes an associated lifting beam and a support which are intended to transport and store the cask-to-storage tank installer, and an associated lifting beam and a rotary frame which are intended to transport and store the manipulator for SFA retrieval from the caissons.
Equipment for retrieval of the spent fuel fragments from the caissons includes:
- Tools for collection of the spent fuel debris and loading them into accumulation space of the guiding device and into the special container for fuel debris,
- A guiding device to be mounted above the caisson and intended to install the tool for collection of the spent fuel debris and a TV camera in proper positions, to provide the radiation shielding and interim accumulation of the fuel debris,
- A container for fuel debris with a set of replaceable filtering cans,
- A dust extractor for collection of the spent fuel debris and loading them into the container,
- An adapter plate for unloading of the collected fuel debris and full filtering cans to the thin-walled cask.
A set of thin-walled overpacks includes 27 airtight cylindrical overpacks for transportation of the fuel assemblies and fuel debris. The special equipment for loading spent fuel into thin-walled overpacks (loading station) shall be arranged in an empty caisson in order to fill the thin-walled overpack with radioactive contents. An empty thin-walled overpack shall be placed into the loading station. The SNF unload cask or the guiding device, which is a part of the equipment for retrieval of the spent fuel fragments from the caissons, shall be placed on the top of the overpack.
Equipment for cutting off the SFA lifting features is intended to perform horizontal cut in confined spaces of the Lepse storage tanks using the abrasive cut-off wheel. It is based on the multi-speed machine with controlled rotation speeds of the spindle and a flexible shaft with shears. This cutter has three electric drives: the main motion drive, the horizontal motion drive and the rotation drive. The horizontal motion drive is equipped with the load cell for monitoring the loads applied to the fuel assembly.
Below is the equipment designed for recovering of the fuel assembly lifting part after its lifting feature has been cut off:
- A reamer which is intended to remove burrs,
- Lifting adapters,
- A tool for installation of lifting adapters on the fuel assemblies.
Equipment for cutting off the weld between the SFA storage tube and the SFA storage tube seating plate (or weld shears) is intended to cut the SFA storage tube out of the Lepse storage tank tube plate. It is a special purpose metal working machine which shall be aligned with the SFA storage tube and secured to the Lepse storage tank tube plate before each weld cut. According to the nuclear safety requirements, a weld between two corrosion-resistant steel surfaces shall be cut off without any cutting fluid using a special design cutter head. The inner diameter of the cutter head is limited with its inner collet which is intended to secure the cutter on the SFA storage tube while the outer diameter can not be larger than the diameter of the opening in the Lepse storage tank rotary plate. Because of these strict requirements for the cutter head and the weld shears itself, a series of design activities was carried out in order to develop and fabricate the cutter heads of various designs. After selection of the optimal cutting mode and the cutter head design, the operability of the weld shears was demonstrated.
Lifting adapters are intended to be attached to the top of the cut-off SFA storage tubes for their further handling using the SNF unload cask.
A set of auxiliary tools and accessories includes more than 150 items for installation, testing, transportation, maintenance and repair of the SNF retrieval equipment, as well as for training the operating and maintenance personnel.
TV system for spent fuel retrieval is based on the radiation-resistant TV cameras made by DIAKONT Company which can be operated at the radiation dose rate not more than 106 Rad/h. This system includes:
- A side view camera with a vertical travel device to be placed inside a free opening of the Lepse storage tank plate,
- A camera for proper positioning of the Lepse storage tank rotary plate,
- A camera mounted on the manipulator for SFA retrieval from the caissons, and
- A camera for monitoring the collection of spent fuel debris inside the caissons.
All cameras are equipped with build-in lighting modules and operated remotely from the control panel located in the Control Room of the Lepse Shelter. The monitors of the Shelter TV system are also located in the Control Room. They are intended to display images from the cameras of the TV system for spent fuel retrieval.
SNF Retrieval Equipment Remote Control System is based on the industrial computer, industrial controllers, operator panels and frequency converter. The control panel located in the Control Room of the Lepse Shelter is used to control the equipment. The special software has been developed for the purpose of the equipment control, the malfunction diagnostics, the equipment maintenance and adjustment. This system includes four electrical cabinets for communication equipment and control equipment, eight wiring cabinets and cables needed to operate the SNF retrieval equipment both on the Lepse storage tanks and on the mock‑up facility which is designed for the combined trials of the equipment and the personnel training at the Nerpa Shipyard.
In 2017, the combined trials of the fabricated equipment were carried out on the mock‑up facility on the site of Sosny R&D Company. On completion of the trials all the equipment and the mock-up facility were delivered to the Nerpa Shipyard and installed in the Lepse Shelter. Stage 2 of the combined trials and the personnel training were successfully completed in the Lepse Shelter in 2018.
To handle radioactive waste, three storage and transfer tanks for low-level liquid waste were designed and fabricated. Capacity of each tank is 3 m3. The tanks are equipped with automatic monitoring of the liquid waste temperature and automatic turn-on of the in-built electric heaters. Each tank has a drip pan, the level sensors, a pump and valves for filling and discharging the tank. After the Sanitary-Epidemiological Certificate verifying compliance of the equipment with the applicable standards and regulations had been obtained, the tanks were delivered to the Nerpa Shipyard.
The tilting device for Type KB‑650B cask and the empty transfer and storage container, which are intended to transport spent fuel within the territory of the Nerpa Shipyard, were designed, fabricated and tested. The tilting device is used to transport the Type KB‑650B cask loaded with spent fuel on the vehicle within the territory of the Nerpa Shipyard from the Lepse Shelter to the Serybranka ship and to transport the empty cask back to the Shelter.
Under the contract with ROSATOM State Corporation, 234 one-cell canisters and three-cell canisters, which are intended to transport the spent fuel retrieved from the Lepse storage tank caissons and the SFA storage tubes in TK-18 cask, were fabricated at the manufacturing plant of Sosny R&D Company.
In 2018 the specialists of Sosny R&D Company participated in the works on installation, adjustment and testing the equipment and systems of the Lepse Shelter, as well as the combined trials of the SNF retrieval equipment. In addition, the specialists of Sosny R&D Company jointly with the National Research Center Kurchatov Institute trained the personnel of FSUE Atomflot and the personnel of the Nerpa Shipyard to perform activities related to the Lepse spent fuel retrieval.
The retrieval of the spent fuel from the Lepse storage tanks were launched in 2019.
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