The fuel handling and storage system provides for the safe, efficient and economical handling and storage of the fuel from the arrival of new fuel to the despatch of fully irradiated fuel. This includes the receipt, inspection and storage of new fuel and core control
components, the disassembly of the reactor for the re­fuelling operation, the refuelling of the reactor core, the reactor reassembly after refuelling, the storage of irradiated fuel and control components, and the dis­patch of fully irradiated fuel from the power station. The fuel handling facilities. Fig 2.138, are sited in

two areas:

• The refuelling cavity within the reactor building.

• The fuel building situated adjacent to the reactor building.

The fuel building contains the fuel storage pond with­in which are racks for storing both new and irra­diated fuel assemblies. Adjacent to the fuel storage pond and normally separated from it by removable gates, are the fuel transfer canal and two bays for the preparation and filling of the irradiated fuel transport flask, All the bays are of reinforced concrete con­struction with stainless steel line plates. Also within the fuel building are facilities for the receipt and in­spection of new fuel and the loading bay for the road transport vehicles. In addition, the building houses the pumps and heat exchangers required for the re­moval of the decay heat given out by the stored irra­diated fuel, and the heating and ventilation equipment for the building.

The tw’o areas are connected by the fuel transfer tube which penetrates the reactor building contain­ment. This tube is isolated by a blind flange during reactor operation, to maintain the reactor building containment integrity.

Refuelling is carried out at approximately yearly intervals with the reactor off-load. To refuel, the re­actor is shutdown and depressurised and the closure head is lifted off to gain access to the vessel upper internals and, when they have been removed in turn, the fuel assemblies.

After cooldown and clean-up of the reactor coolant system, the electrical and instrumentation cables serv­ing the CRDMs are disconnected, the cooling air duct between the fan plenum and the reactor vessel head cooling shroud is removed, and insulation is removed from the reactor vessel head area. The vessel head assembly is then clear to permit the multistud ten­sioner to be lowered into position over the reactor vessel studs securing nuts. The studs are de-tensioned, the nuts are unscrewed and both they and the studs are removed by the multistud tensioner which is fully automatic and remotely controlled. After the studs have been removed, the reactor vessel head assembly is removed by the reactor building polar crane and stored on the operating floor.

As the head is removed, the refuelling cavity is flooded with water from the refuelling water storage

tank (RWST). The control rod drive shafts are de­coupled from the rod cluster control assemblies and the upper internals are removed, exposing the core to permit refuelling. The internals are stored under water on a stand at the far end of the refuelling cavity.

The normal refuelling operation involves discharg­ing one-third of the core of fully irradiated fuel to the fuel storage pond, shuffling the positions of the remaining two-thirds of the core of partially irradiated fuel and charging the core with one-third of a core of new fuel from the fuel storage pond. During the process, the core control components are transferred between the fuel assemblies by the refuelling machine to maintain their correct position in the core. Alter­natively, when it is required to carry out reactor pres­sure vessel in-service inspection, the complete core of 193 fuel assemblies together with the core control components, is discharged to the fuel storage pond where the new fuel assemblies are already stored. The core control components are shuffled between fuel assemblies in the storage pond, utilising the various components handling tools suspended from the mono- rail hoist of the pond fuel-handling machine. The new core, made up of new and particularly irradiated fuel assemblies, is then charged into the reactor, leaving the fully irradiated fuel in the fuel storage pond.

At all positions along the fuel handling route, until it is placed and sealed into the irradiated fuel trans­port flask, the fuel is handled under water; this pro­vides an effective, economical and optically transparent radiation shield as well as a reliable medium for the removal of decay heat generated by the irradiated fuel. The water used in all the fuel handling facilities contains boron, normally at a concentration of 2000 ppm, to ensure sub-critical conditions during all fuel handling operations.

Upon completion of the refuelling, the upper in­ternals are replaced over the core, the control rod drive shafts are recoupled and the refuelling cavity is drained. The reactor vessel flange shield is lowered into place around the control rod drive shafts to provide radiation protection for the reactor vessel flange and the sealing face cleaning operations.

The flange shield is then removed and stored on the storage floor and the reactor vessel head assembly is lowered, guided by three guide studs, onto the re­actor vessel. The reactor vessel studs and nuts are replaced and tensioned by the multi-stud tensioner and the tensioner removed. The two cable bridges are lowered into position and the cable connections are made, the cooling air duct is refitted and the insulation is installed.

These operations are followed by the reactor start­up tests, plant heat-up and power raising.

The components of the fuel handling system are designed to handle only one fuel assembly at a time. The main components are the refuelling machine, the fuel transfer system, the pond fuel handling ma­chine and the new — fuel elevator. Various tools are also provided for handling fuel assemblies and core control components.

The refuelling machine is essentially a rectilinear bridge and trolley crane system with a vertical mast extending down from the trolley into the refuelling cavity; the bridge spans the refuelling cavity and runs on rails set into the edge of the cavity at the operat­ing floor level. The bridge and trolley motions are used to position the vertical mast over the fuel as­sembly positions in the core and the up-ender position of the fuel transfer system. The bridge and trolley controls automatically position the mast over the op­erator selected position, with co-ordinates programmed into the control console computer. A television mo­nitor display indicates the position selected and the actual position of the mast.

Separate manual controls are provided for non­automatic operation. The vertical mast supports and guides the gripping devices and the main hoist for the handling of core components.

Only one core component, e. g., a rod control clus­ter assembly, a thimble plug or a fuel assembly, which may or may not have inserted a control component, can be handled at any one time within the mast.

The fuel transfer system comprises a fuel assembly container basket mounted on a conveyor carriage that runs on rails from the fuel transfer canal in the fuel building, through the fuel transfer tube, into the re­fuelling cavity in the reactor building. Located at each end of the transfer tube is a hydraulically operated up-ending device which turns the basket between the vertical and the horizontal. The basket is designed to hold one fuel assembly and the control components can only be transferred inserted in the fuel assembly. In the vertical position, the basket is loaded and un­loaded with a fuel assembly by the refuelling machine in the reactor building, or by an irradiated fuel han­dling tool suspended from the pond fuel handling machine in the fuel building. The fuel is passed hori­zontally through the transfer tube.

The pond fuel handling machine is a wheeled bridge structure with a steel deck walkway and two hoists that are carried on an overhead monorail structure. The bridge spans both the fuel storage pond and the fuel transfer canal, and runs on rails set into the edge of the pond and canal at operating floor level. The machine is used primarily to transfer single new and irradiated fuel assemblies between the fuel storage racks and the fuel transfer system, new fuel assemblies between the new fuel elevator and the storage racks, and irradiated fuel assemblies between the storage racks and the irradiated fuel transport flask in the flask fill bay. The machine, together with various tools, also performs any transferring of control com­ponents between the fuel assemblies in the storage racks. All the core components are handled vertically and underwater. Additionally, the machine handles the opening and closing of the fuel transfer gates, utilising the second hoist on the monorail structure.

The new fuel elevator, situated in the flask fill bay, comprises a fuel basket carriage assembly and a guid­ance system of rollers. The rollers are confined by rajls attached to the bay wall in order to maintain the ertical orientation of the carriage as it is raised and lowered by a winch, mounted above the operat­ing floor. The elevator is used exclusively to hold

“d lower single new fuel assemblies to the bottom of the bay; from there they are transferred to the fuel storage racks by the irradiated fuel handling tool suspended from the hoist of the pond fuel handling machine. For new core control components to be introduced into the storage pond, they must first be inserted into new fuel assemblies.

Fuel storage facilities are located in two areas of the fuel building; one is a transit storage area for new fuel assemblies upon arrival at the station, and the second is the storage pond which contains the permanent fuel storage racks. New fuel arrives at the power station in containers on a road transport vehicle. Each container can carry two fuel assemblies that are clamped to a strongback for support during transit. The containers are lifted off the road trans­port vehicles by a monorail hoist suspended from the underside of the fuel building crane, and are placed for temporary storage in the new fuel reception and preparation area on the operating floor. In this area the fuel assemblies are removed from their containers, detached from the strongback and inspected. The fuel is then transferred via the monorail hoist, the new fuel elevator and the pond fuel handling machine to the fuel storage racks.

The fuel storage racks rest on the fuel storage pond floor. The racks are not tied to the stainless steel line plate at the floor or walls. Each rack consists of a number of stainless steel vertical storage cells formed into a square lattice with full height neutron absorber material on all sides. The inclusion of this neutron absorber permits the storage of a fuel assem­bly in any empty position within the storage racks, without regard to the degree of burn-up, decay peri­od, or initial enrichment, thereby minimising the han­dling of irradiated fuel. Allowance is made in the rack design for the passage of cooling water through the stored fuel assemblies. The pitch of the storage cell lattice is 267 mm which results in an ultimate stor­age capacity for 1323 fuel assemblies. This capacity would suffice for sixteen annual discharges of irra­diated fuel, plus one-third of a core of new fuel, plus the contingent capacity for one full core discharge during in-service inspection of the reactor vessel.

The fuel storage pond cooling system consists of two 100% capacity cooling trains for the removal of decay heat generated by the irradiated fuel that is stored in the fuel storage pond. Each cooling train comprises two 50% capacity horizontal centrifugal pumps, one 100°7o shell and U-tube heat exchanger, a strainer, manually operated valves, and the instrumen — tation required for system operation.

The decay heat generated by the irradiated fuel is transferred from the fuel storage pond, through the heat exchangers of the cooling system, to the com­ponent cooling water system.

During normal system operation, both pumps of one train take suction from the fuel storage pond and transfer the water through a heat exchanger back to the storage pond. The pumps’ suction line penetrates the wall of the fuel storage pond near the normal water level; the return line terminates in a distribution header at the bottom of the pond. An ami-siphon hole in each return line is located near the surface of the water to prevent inadvertent draining of the pond. Normal make-up water for the fuel storage pond, to compensate for evaporative losses, is supplied from the reactor make-up water system. For emergency make­up, the borated water in the flask preparation bay water system is available for use. Two further systems are employed to clean-up the water in the fuel storage pond, fuel transfer canal and handling bays, and in the refuelling cavity of the reactor building.

The fuel storage pond clean-up system contains two centrifugal pumps in parallel, two filters in parallel, a mixed bed demineraliser, a strainer and four float — type skimmers. The pumps and filters are designed for 50% of the system capacity and the demineraliser and strainer are designed for 100% system capacity. The filters are provided to remove particulate matter. The demineraliser removes ionic corrosion impurities and fission products, and the strainer downstream of the demineraliser removes resin fines. The float-type skimmers are positioned so that there is one in the fuel transfer canal, one in the flask fill bay and two in the fuel storage pond; each has inlets positioned for the removal of surface debris.

The similar refuelling pool clean-up system provides a capability for purifying the water in the refuelling cavity during refuelling and, at other times, the con­tents of the refuelling water storage tank. It contains one 100% duty centrifugal pump, four filters in par­allel, two 50% mixed-bed demineralisers each with a strainer, and two float type skimmers stationed in the refuelling pool.

The irradiated fuel transport flask is conveyed to and from the station on a road vehicle, fitted with a transport and tilting frame onto which the flask is secured horizontally. Inside the fuel building reception bay, the fuel building crane is used with a special lifting attachment to elevate the flask to the vertical position. The flask is then lifted the minimum distance necessary to disengage it from the transport and tilting frame, and transferred to the flask fill bay where it is filled with irradiated fuel underwater. Finally the flask is returned lidded, decontaminated and in all ways prepared for loading onto the vehicle for trans­port off site.

The flask preparation bay provides the access, tools and equipment for checking and preparation of the flask, including lid bolt removal/replacement, and for flask decontamination. A position is provided at op­erating floor level for the inspection and refurbishing of the flask lid and its seals.

Three water retaining gates are provided between the fuel storage pond and the vehicle loading bay:

• A flask transfer gate between the vehicle loading bay and the flask preparation bay.

• A flask transfer gate between the flask preparation bay and the flask fill bay.

• A fuel transfer gate between the fuel storage pond and the flask fill bay.

During flask handling operations, interlocks prevent more than one of these three water retaining gates from being open at any one time.