In compliance with ventilated cask storage, casks with spent nuclear fuel are located on a special concrete storage site (base plate). Each cask in its turn consists of two components: multi-sealed basket (MSB) and external ventilated concrete cask (VCC).
The proposed SFDS project uses technology of ventilated concrete cask for storage of spent fuel assemblies in vertical position. The casks ensure dry, tight and safe fuel storage.
SFDS is calculated for safe storage of 380 casks containing 9000 spent fuel assemblies from WWER-1000 reactors.
The system is passive; after installation of casks on the site it does not require any significant maintenance.
VCC-WWER system for dry storage of ZNPP spent nuclear fuel consists of the following components:
Multi sealed basket is a tight vessel calculated for 24 spent fuel assemblies from WWER-1000 reactor type, previously cooled down in spent fuel ponds of the power units during not less than 5 years after their unloading from the reactor vessel.
Reloading container - transporter - is a vessel serving for temporary placement and transporting of the loaded multi sealed basket from the SFP to the ventilated concrete cask, and for installation of the MSB in the VCC. The basic purpose of the reloading container is to ensure NPP personnel protection from radiation impact during transporting and technological operations with the multi sealed basket.
Ventilated storage cask serves for long-time intermediate storage of the MSB with spent fuel assemblies at SFDS site, ensuring their cooling down and the required biological protection. SF cooling down is done by means of natural air convection around steel walls of the baskets, circulating in the cylindrical gap between the external MSB surface and internal VCC surface. Ventilated concrete casks are transported by a specially designed transporter to the SFDS concrete site located at ZNPP site.
- Temperature detector
- Air inlet
- Concrete site
- Air outlet
- Cask lid
- Load bearing and protection lids of the basket
- Bank of 24 guiding tubes for spent fuel assemblies
- Guiding tubeа
- Multi sealed basket body
- Ventilated concrete cask
Multi sealed basket - is a tight vessel serving for dry safe storage of spent fuel assemblies in vertical position and their cooling by means of natural air convection.
The basket comprises the body – external liner – housing the guiding tube bank, bottom, protection and load-bearing lids.
Basket body is manufactured from boiler steel of improved strength used for pressurized vessels. The body and internals have protective coating excluding corrosion under impact of water from SFP and impact during the basket loading.
Guiding tubes bank is manufactured from steel plates bent into hexahedron shape with wall thickness equal 5 mm. The structure rigidness in horizontal directions is ensured by bent supporting horizontal belts located in five points along the cask height. These belts are connected by guiding tubes with the help of welding and represent a kind of a rack for 24 spent fuel assemblies installed during the basket loading.
Basket protection lead is manufactured from three steel sheets; a RX-277 material liner is located between the steel sheets 2 and 3 for protection from neutron radiation. For water extraction, vacuum drying and MSB filling with helium, the protective lid has an opening. The protective lid is welded to the basket body.
After the protection lid, the load bearing lid is welded to the basket body. The load bearing lid is a steel disk with openings to grant access to MSB draining and drying valves. Then, the protective and bearing lids are welded together via the opening of access to the valves. After completion of operations on MSB drying and filling with helium, openings in the bearing lid are welded by two disk plugs, using multi-pass welding.
The basket tightness is reached by welding of two parts: the upper one consisting of the power and bearing lids, and the lower one – the basket bottom. Quality control of the weld seams is by means of hydraulic tests and using advanced diagnostic techniques. After water removal from the basket and its final drying, inert gas – helium is fed into the basked. The inert medium, complete absence of moisture and basket tightness reliable ensures possibility of long-time storage of spent fuel. The rated pressure inside the gas medium of the basked during the storage period is 0,6 - 1 atmospheres.
The basket structure together with organizational and technical activates ensure nuclear safety both in the normal loading conditions and during design-basis accidents. The basket integrity, as the second circuit of radioactive material isolation (primary localization circuit is the tight wall of the fuel elements) is monitored in the operation process by indirect means, based on the results of prescribed operation of air and smear from the internal surface of the ventilation channel outlets, as well as by means of visual examination of the external surface of the basket liner.
The main structural parameters of the basket:
|Hexagonal tube bank||Unit height||4320|
|External diameter of load-bearing belt||1625,6|
|Bearing belt thickness||19|
|Hexagonal guiding tubes||Length||4320|
|Fuel area length||3530|
|Internal distance between plates||249,6|
|Number of tubes||24|
|MSB liner||External diameter||1715|
|Material||Steel, ASME SA516|
|Protection lid||Diameter||Total: 1657.4|
|Material||1 - steel, ASME SA516
2 - RX-277
3 - steel, ASME SA516
Reloading container serves for temporary storage and transportation of loaded MSB from the SFP to the VCC, and installation of the MSB in the VCC. The main purpose of the reloading container is to ensure NPP personnel protection from radiation impact during transporting – loading operations with the multi-sealed basket and to ensure the MSB protection during loading operations in the reactor halls.
Reloading container is a cylinder with walls made as a “sandwich” from the following layers of protective materials: steel + lead + RX-277 material + steel. RX-277 material has a special composition and ensures effective neutron protection. The reloading container in its upper part has a lid (limitation ring), the internal diameter of which is smaller than the basket diameter. The limitation ring is installed after MSB is installed in the reloading container. Its purpose is prevention of accidental extraction of the loaded MSB via the top of the reloading container. The reloading container bottom is formed by two mobile gates used during lowering of the basket to the ventilated concrete cask. The gates move in steel guiding grooves located in the lower container part. Four steel pins prevent accidental opening of the gates. Gate opening is dune using a hydraulic drive.
All transport and technological operations with the reloading container are adapted and identical to those performed on the transporting container TK-13 (serving for spent fuel assembly transportation) which allows for using the existing standard equipment of the reactor compartment: the polar crane and TK-13 cross-arm.
The main structural parameters of the reloading container:
|Overall dimensions||External diameter||2223|
|Lid (limiting ring)||Diameter||2057|
|Material||Steel, ASME SA516|
|Radial screen||Thickness||Total: 247.6|
1 - steel, ASTE A588
2 - lead
3 - RX-277
4 - steel, ASTM A588
|Material||Steel, ASTM A36|
Ventilated concrete cask serves as a support structure ensuring protection from radiation and MSB cooling during its storage period by means of natural air convection.
The internal part of the VCC is formed by thick steel cylinder (internal lining). The external reinforced structure is formed by vertical reinforcement bars with hooks and horizontal bars circularly bent. The concrete mixture selected in the design is Portland cement ensuring rigidness and durability of the object.
The air flow duct is formed by air intake channels, air inlets, cylindrical gap between MSB shell and internal liner of the VCC, and air output channels. The air inlets and outlets are covered by plate steel and has complicated profile (channels with labyrinth sections) – in order to minimize the dose rates of the radiation generated by these channels.
VCC lid fixed by bolts is used for MSB protection from the environment: mechanical impact and precipitation, and for ensuring additional protection on the upper part of the container. A special IAEA seal is placed on the VCC-WWR lid bolts that prevents not-authorized opening of the VCC. The VCC bottom is a steel plate 9 mm thick. The Ventilated storage cask has reinforced rounded corners at the upper and lower parts in order to decrease possible damage of concrete during its hypothetical drop.
The container is manufactures from welded metallic parts and concrete. The external side of the internal VCC liner has reinforcement bars. Removable steel lining on bolts is fixed then around the internal container lining. The remaining gap between the internal metallic VCC liner and the removable lining is filled by concrete of normal density.
The main structural parameters of the VSC:
|Concrete shell||External diameter||3378|
|Material||Portland cement concrete|
|Material||Carbon steel A36|
The existing facilities, systems and equipment of the nuclear power plant are used to ensure spent fuel storage including the following: maintenance equipment, storage and transportation area equipment, decontamination system, fire fighting system, ventilation and conditioning systems.