The Hinkley Point В fuelling machine weighs 600 tonnes. It comprises a vessel some 26 m high with a design pressure of 65.5 bar. The hoist is at the top and the grab is suspended from two roller chains which pass over a double hoist sprocket, the chain slack passing into an external chain locker tube (Fig 2.107).

The multicore electrical cables from the grab run between and with the support chains. The hoist system embodies a soft suspension system of weights and springs so that the grab chain tension does not sud­denly change if a fuel assembly snags during movement.

Below the hoist head shaft is a turret comprising three storage tubes. When a fuel assembly has been lifted into a storage tube, latches are engaged under the closure to support it. The grab can then be dis­engaged and the turret rotated to bring another stor­age tube below the grab. The bottom of the pressure vessel is closed when required by a slide valve. Below the valve is a telescopic snout, which can be lowered to connect with either an access hole to one of the fuel handling facilities in the central block or the make-up shield pressure vessel.

The lower half of the pressure vessel is surrounded by external iron shot/concrete/steel shielding. The vessel and its shielding is carried on a crab running on a gantry bridge which traverses the length of the charge hall.

At Hinkley Point В, the reactor shield floor is 1.5 m below the charge hall floor level. A make-up shield is therefore coupled to the fuelling machine when it is at the reactor. The elastomer seals in the machine, the make-up shield telescopic snouts and the grab are cooled by a cold CO2 supply, the flow totalling 0.5 kg/s under normal circumstances.

The grab uses spring-assisted gravity to engage and lock the grab hooks onto the closure-pin cage lifting ring. The grab locking system is electrically energised to unlock and disengage (Fig 2.108). The grab has duplicated load cells so that the load on the grab is recorded and monitored by the machine interlock/ control system, which also monitors the outputs from the microswitches which register the positions of es­sential components in the grab.

At the reactor, the fuelling machine is aligned to the standpipe, prior to engaging the snouts, by op­erator control using TV cameras in the base of the make-up shield. TV cameras are also fitted in the

successfully completed, all relevant components are jn their correct starting positions and conditions, and other movements are locked out.

When refuelling at the reactor, the standpipe is pre-

ared by removing a number of slabs, disconnecting the closure cables, removing the gag motor/gearbox and placing an extension sleeve over the standpipe. The machine, containing a new fuel assembly and fitted with the make-up shield, is pressurised to re­actor pressure and connected to the reactor by lower­ing the make-up shield snout to engage with the sleeve. The machine valve is opened after the inter­space between the valve and the closure has been pressurised and all pressures have been equalised.

The fuelling machine grab then engages with the dosure/Iifting head of the spent fuel assembly and lifts it into the storage tube. After latching and grab disengagement, the turret is rotated and the new fuel assembly is lowered and locked into the standpipe. The machine valve is closed and, after interspace de — pressurisation, the machine is disconnected from the standpipe which is then reinstated. After disconnection of the make-up shield, the machine moves to the fuel handling facilities in the central block to dis­charge the spent fuel assembly into either a buffer storage tube, where it remains for a period of time to allow the fuel decay heat to reduce, or directly to the irradiated fuel disposal facility (IFDF).

The Dungeness В fuelling machine, which weighs some 1000 tonnes, is similar to that at Hinkley Point В in that it has a З-tube turret, but each storage tube has its own grab, chain system and hoist sprocket driven by an external hoist motor when a tube is in the operating position. The charge hall floor is all at one level so there is no make-up shield. The lowest part of the machine shielding has interlocked doors which are opened to give access for manual coupling of the telescopic adaptor (attached to the grab), to the closure and operation of its interlock key system. The Dungeness refuelling system permits the passage of hot reactor gas up into the machine which has a recirculating gas cooling system operating from start of hoisting until the fuel assembly is in the irradiated fuel disposal facility (IFDF). (At Hinkley Point B, the cold CO; injection prevents the reactor hot gas from rising into the bulk volume of the fuelling ma­chine pressure vessel, and when the assembly is stored within the machine its decay heat is dissipated by natural convection cooling.) The refuelling sequence at the Dungeness В reactor is similar to that at Hinkley Point B.

The Hartlepool/Heysham I fuelling machine is very different {Fig 2.109). It is a single-chamber ma­chine weighing some 300 tonnes. The pressure vessel is lowered on jacks through the shielding to seal the snout on a standpipe. There are two similar grabs, the fuelling grab and the plug grab. The fuel­ling grab (Fig 2.110) is suspended on hoist chains, the chain passing over hoist sprockets at the top of the vessel and the chain slack passing into a sepa­rate chain storage tube. When at the bottom of its travel the grab seals the bottom of the pressure vessel, The portion of the grab below the seal is telescopic and is extended and rotated by external drives engaging with the grab at this level to en­gage with the lifting features on the fuel assembly closure. The plug grab (Fig 2.111) has similar features and is stored in the lower portion of the machine pressure vessel. This grab carries a standpipe clo-.ure seal plug which is swung over and inserted into the standpipe.

When refuelling at the reactor, the empty machine is jacked dowm onto the standpipe and the hoist grab seals the bottom of the pressurised machine vessel. The interspace is pressurised and the fuelling grab is extended and engaged with the spent fuel assembly closure which is rotated by the grab to unlock it. The grab and fuel assembly are hoisted up into the machine. The plug grab is now swung over, lowered and operated to seal both the machine vessel and standpipe, the fuel channel being left empty. This grab is disengaged from the plug, the interspace is depressurised and the machine vessel is jacked off the standpipe.

The fuelling machine travels to a buffer storage tube, where the plug grab picks up the plug which seals the pressurised tube. The spent fuel assembly is lowered and locked into the buffer tube, the hoist grab sealing the machine. The fuelling machine now travels to another buffer storage tube, or to the new fuel facility, and picks up a new’ fuel assembly which is subsequently inserted into the empty fuel channel in the reactor.