Technologies and Equipment for Decommissioning of the Lepse Service Ship
The Sosny R&D Company was involved in preparations for decommissioning of the Lepse service ship under the contract with JSC FCNRS. The Northern Dimension Environmental Partnership set up by the European Bank of Reconstruction and Development as part of technical assistance to the Russian Federation allocated funding. The work was done jointly with the National Research Center Kurchatov Institute with the participation of FSUE Atomflot and SC Zvezdochka, a branch of the Nerpa Shipyard.
In 1961, the dry goods freighter was retrofitted to build up a two-deck vessel with three platforms and a superstructure. For a long time, the Lepse participated in refueling operations aboard the Lenin, Arktika, and Siberia nuclear-powered icebreakers. 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 put in a dry dock at the Nerpa shipyard where it was cut into five sections: the stern section, the section with liquid radioactive waste tanks, the engine and boiler room, the section with spent nuclear fuel, and the bow section.
As the SFA storage tubes and caissons of the Lepse storage tanks contained damaged fuel assemblies of the ОК-150 and ОК-900 types, a decision was taken to load the SNF into transport canisters and place them into casks in a dedicated shelter using specifically designed retrieval and handling equipment. Then, the casks were transported to the Serebryanka ship where the SNF-containing canisters were reloaded into the TK-18 casks for further transportation to the Atomflot site and subsequent rail shipment to the reprocessing plant at Mayak PA.
In 2015-2016, according to the Technical Design elaborated by Afrikantov OKBM, Sosny developed detailed design documentation for the SNF retrieval and handling equipment including:
- an SFA transfer cask,
- a cask-to-storage tank installer,
- a cask-to-caisson installer,
- a manipulator for SFA retrieval from the caissons,
- equipment for retrieval of spent fuel fragments from caissons,
- thin-walled canisters and SFA loading equipment (a loading station),
- equipment for cutting off SFA upper fittings and a reamer,
- equipment for cutting SFA storage tubes out of the tube plate in the storage tank,
- lifting adapters,
- tools for installation/removal of lifting adapters,
- a remote control system for the SNF retrieval equipment,
- a video surveillance system,
- a special-purpose lifting beam for handling the equipment,
- a set of auxiliary tools and accessories.
All the SNF retrieval equipment was made in compliance with the NP 022‑2000 requirements and can be operated from the control room remotely from the Lepse’s storage tanks. Manual override devices were provided to complete an operation in case of a power loss. The SNF retrieval equipment was fabricated at Sosny’s manufacturing facility in 2016.
The SNF transfer cask was designed for the safe transfer of the fuel assemblies, the SFA storage tubes cut out of the tube plate in the Lepse’s storage tank, and the thin-walled canisters in the Lepse shelter. The cask has a winch, grapple position sensors, and load sensors on the top, and the rotary gate on the bottom. It is biologically shielded. The weight of the SNF transfer cask is 10 tons. The scope of supply includes a lifting beam, a rotary frame, a telescopic tower, and a drip pan to transport, tilt, handle, and store the SNF transfer cask. The cask is equipped with replaceable grapples for handling the ОК-150 and ОК-900 fuel assemblies.
The cask-to-storage tank installer is designed to place the SNF transfer cask above the SFA storage tubes in the Lepse’s storage tank and release the fuel assemblies stuck in the storage tubes. The installer has a guide funnel on the top for precise positioning of the SNF transfer cask. The slide gate is designed below the guide funnel to close the access to the SFA storage tube. An SFA release drive is mounted under the slide gate. All drives are remotely controlled and have manual override devices. The scope of supply included an associated lifting beam and a support ring to transport and store the cask-to-storage tank installer. The installer weighs 2.4 tons.
The cask-to-caisson installer is intended to place the SNF transfer cask or the manipulator above a caisson in the Lepse's storage tank. The installer has an in-built manipulator for SFA retrieval from the caisson.
The cask-to-caisson installer is designed to install and align the manipulator for SFA retrieval from the caissons; the manipulator moves the grapple rod up and down, revolves it, and turns the grapple horizontally and vertically.
The manipulator capacity is 200 kg. All drives of the cask-to-caisson installer and the manipulator are equipped with manual override devices. The weight of the installer is 7.3 tons and that of the manipulator is 0.7 tons. The scope of supply included a lifting beam and a support ring for handling the cask-to-storage tank installer, as well as a lifting beam and a rotary frame for handling the manipulator.
The equipment for retrieval of spent fuel fragments from caissons includes:
- tools for spent fuel debris collecting and loading into the accumulation buffer of the guiding device and a fuel debris container,
- a guiding device mounted above a caisson to position the fuel debris tool and a TV camera and to ensure radiation shielding and interim accumulation of the fuel debris,
- a fuel debris container with a set of replaceable filtering cans,
- a dust extractor for fuel debris collecting and loading into the container,
- an adapter plate for unloading the collected fuel debris and filtering cans into a thin-walled canister.
A set of thin-walled canisters includes 27 airtight cylindrical canisters for transportation of the fuel assemblies and the fuel debris. The thin-walled canisters are filled up in a specially designed loading station placed in an empty caisson. An empty thin-walled canister is put in the loading station with the SNF transfer cask or the guiding device atop.
The equipment for cutting off SFA upper fittings is designed for horizontal cutting with an abrasive cut-off wheel in small spaces of the Lepse's storage tanks. It is based on a multi-speed machine with a controlled rotation velocity of the shears' spindle and flexible shaft. The cutter has three electric drives for primary motion, horizontal transfer, and rotation. The horizontal transfer drive is equipped with a force sensor for monitoring the forces applied to the fuel assembly.
The following equipment was designed to rebuild the SFA pintle after cutting off the upper fitting:
- a debarring reamer,
- lifting adapters,
- a tool for the installation of lifting adapters on the fuel assemblies.
The weld shear is designed for cutting an SFA storage tube out of the seating plate in the storage tank. It is a special-purpose metalworking machine that, before each cutting, should be aligned with the SFA storage tube and secured on the tube plate. Once secured, the weld shear cuts the tube-plate junction. The nuclear safety rules require that corrosion-resistant steel welds be cut off using a specially designed cutter head without any lubricating fluid. The inner diameter of the cutter head is limited to the size of the inner collet securing the cutter on the SFA storage tube. The outer diameter cannot be larger than that of the cell in the storage tank rotary plate. Strict requirements for the cutter head and the weld shear necessitated design work to develop and fabricate various cutter heads. Once the most appropriate cutting mode and the cutter head design were selected, the performance of the weld shear was demonstrated.
The lifting adapters are installed on the top of the cut-off SFA storage tubes to handle them with the SNF transfer cask.
The set of auxiliary tools and accessories included more than 150 items for mounting, testing, transportation, maintenance, and repair of the SNF retrieval equipment, as well as training of the operating and maintenance personnel.
The TV system for spent fuel retrieval is based on DIAKONT radiation-resistant TV cameras that operate at a radiation dose rate of up to 106 Rad/h. The system includes:
- a periscope side view TV camera installed in a vacant opening of the storage tank plate,
- a TV camera for the rotary plate positioning surveillance,
- a manipulator-mounted TV camera,
- a TV camera for the fuel debris collecting surveillance.
All cameras are fitted with built-in lighting units and can be operated remotely from the control room in the shelter. In addition, there are TV monitors in the control room to display images from the process monitoring TV cameras.
The remote SNF retrieval control system is based on an industrial computer, industrial controllers, operator's panels, and a frequency converter. The equipment is operated remotely from the control room in the shelter. Specially developed software not only controls the equipment, but also allows implementing diagnostics, maintenance, and adjustment of the equipment. The system includes four switchboard and control cabinets, eight wiring cabinets, and associated cables to operate the SNF retrieval equipment both in the storage tanks and at the mock-up facility designed for combined trials of the equipment and personnel training at the Nerpa shipyard.
In 2017, the Sosny R&D Company conducted combined trials of the fabricated equipment at the mock-up facility on its site. On completion of the trials, all the equipment and mock-up facility were delivered to the Nerpa shipyard and installed in the shelter followed by another round of the combined trials and personnel training in 2018.
To handle the resulting radioactive waste, three storage and transfer tanks for low-level liquid waste were designed and fabricated, the capacity of each making up 3 m3. The tanks have automatic temperature controls for the liquid waste and automatic switches for the in-built electric heaters. Each tank has a drip pan, level sensors, a pump, and valves for tank filling/discharging. Once the sanitary-epidemiological certificate of equipment compliance with the applicable standards and regulations was obtained, the tanks were delivered to the SRY Nerpa.
Handling the spent fuel at the SRY Nerpa required designing, fabricating, and testing an upender for the KB‑650B cask and a container for storage and transfer of empty tubes. The upender is used for on-site transfers of the KB‑650B cask containing the spent fuel from the shelter at the Nerpa shipyard to the Serybranka ship and for the transport of the empty cask back to the shelter.
Under a contract with ROSATOM State Corporation, the Sosny R&D Company manufactured 234 one- and three-cell canisters for transportation of the spent fuel retrieved from the Lepse's storage tank caissons and the SFA storage tubes in the TK-18 cask.
In 2018, Sosny specialists participated in installation, debugging, and testing of the equipment and systems for the shelter, as well as in the acceptance tests of the spent fuel retrieval equipment. Jointly with the National Research Center Kurchatov Institute, the Sosny R&D Company conducted Atomflot and Nerpa personnel training in removing the Lepse spent fuel.
Hands-on operations on retrieval of the spent fuel from the storage tanks started in 2019. In September 2019, the Serebryanka ship completed the first SNF shipment from the Nerpa shipyard to the Atomflot site in Murmansk.
In July 2020, the last of six batches of the spent fuel was removed from the storage tanks. In total, 620 spent fuel assemblies were retrieved; the plans are to remove 19 damaged SFAs remaining in the caissons in 2021.
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