Ail the magnox stations except Wvlfa use cooling t‘■akh about 6 m deep for spent fuel storage in Tip’- tor a minimum ot some three months before Tspateh, this allows the post-irradiation heat and radiation to decay to an acceptable level. Water is a cheap and elfective medium for cooling and shield­ing whilst allowing the handling operations to be observed.

A typical sequence of events is shown diagramma — tically in Fig 2.29, which shows the irradiated fuel handling and storage arrangements at Oldbury. There are essentially three basic routes depicted on the dia­gram; the incoming flask and skip route (stages /-6), the outgoing irradiated fuel route (stages 7-17) and the splitter flask route (stages 18-26).

A brief description of the facilities required and the 26 stages shown on Fig 2.29 is given as follows:

Incoming flask and skip

1 An empty skip arrives inside a road transport flask.

2 The flask is transferred to the storage bay by the flask crane.

3 When required, the flask is transferred to the washdown bay where it is washed, the lid bolts removed and the lid jacked open.

4 The prepared flask is transferred into the dispatch bay of the cooling pond: the lid is removed and returned to the decontamination bay, and the empty skip is removed from the flask and trans­ferred to the pond storage bay by the skip crane.

5 Flask lid seals are inspected, if defective, the lid is decontaminated and transferred to the leak testing bay where seals are renewed.

6 When required, the empty skip is transferred to a pond handling bay.

Outgoing irradiated fuel route

7 Fuel elements and bottles discharged from the reactor arrive in the unloading tray in the cooling pond. From here they are transferred to a storage skip by manipulators.

8 When full, the skip is moved to the pond storage bay for a cooling period.

9 After cooling (for at least 90 days), the skip is returned to the pond handling bay w’here elements are removed to check that the correct cooling period has elapsed.

10 Polyzonal fuel elements are desplittered and placed in a second skip. Herringbone fuel elements are not desplittered and are placed in a separate skip,

11 When a skip is ready to leave the pond, it is transferred to the caesium sampling position in the storage bay.

Fig. 2.29

Irradiated fuel handling and storage arrangements at Oldbury

A Road Transport Flask containing an empty Fuel Element Skip arrives at Oldbury on a Low Loader and is transferred to the Storage Say by the Flask Crane.

When the Flask is to be processed it is transferred to the Washdown Bay for a Pre-Ponding Wash. The Lid Bolts are removed and the Flask Lid is jacked open.

After cleaning and preparation the Flask is transferred to the Despatch Bay in the Cooling Pond by the Flask Crane.

Once the Flask is in the Cooling Pond the Lid is removed and transferred to the Washdown Bay and the empty Skip inside the Flask is removed and transferred to a Storage Bay by the Skip Crane.

The Lid Seals are visually inspected ready for replacement on the Flask. If the Seals are seen to be defective the Lid is decontaminated and transferred to the Leak Testing Bay where the Seals are renewed.

When the empty Skip is required it is transferred to a Handling Bay.

Fuel Elements and Bottles discharged from the Reactor pass through the Unloading Well equipment and arrive in the Unloading Tray in the Cooling Pond. From here they are removed by manipulators and placed in a Storage Skip.

When the Skip is full it is moved to the Storage Bay by the Skip Crane and left to cool.

After the necessary cooling period has elapsed (at least 90 days) the Skip is brought back into the Handling Bay.

Polyzonal Fuel Elements are removed from the Skip, monitored to check that the correct Cooling Period has elapsed, Desplittered and placed in a second Skip. Herringbone Fuel Elements are monitored but not Desplittered and are put in a separate Skip.

Once a Skip is ready to leave the Pond it is transferred to the Caesium Sampling Position in the R1 Storage Bay.

When the Skip is ready to leave the Pond it is checked under the Caesium Sampling Hood and then placed inside an empty Flask in the Despatch Bay.

Once the Skip has been placed in the Flask by the Skip Crane the Flask Crane collects the Flask Lid trom the Washdown Bay. lowers it onto the Flask and then transfers the Flask to the Washdown Bay. The Flask is Decontaminated and the Lid is bolted down. Health Physics check that decontamination is satisfactory then the Flask is transferred to the Leak Testing Bay. The Flask is Leak Tested and Nitrogen purged, if required by the Transport Approval, it is Fluoride Dosed then transferred to the Storage Bay. When a Low Loader is available the Flask is checked tor contamination and then transferred to the Low Loader.
14
15
16
Once clearance has been obtained, the Low Loader leaves Site for either the British Rail Railhead, Berkeley Power Station or Berkeley Nuclear Labs.
17
When a Splitter Flask is full of Magnox debris it is transferred to the Despatch Bay by the Skip Crane and deposited in a Cradle. The Flask Crane now transfers the Splitter Flask and Cradle to the Washdown Bay for Decontamination. After decontamination the Splitter Flask and Cradle are transferred to the Splitter Flask Transporter on the Magnox Debris Corridor. The splitter Flask Transporter transfers the Sputter Flask and Cradle along the Corridor to the Active Waste Oump Loading Bay. At the Active Waste Dump the Splitter Flask is removed from the Transporter and Cradle by the Dump Crane and lowered onto a Magnox Debris Vault Door. The Splitter Flask contents are emptied into the Vault and the Flask is then transferred back onto the Transporter by the Dump Crane. The Splitter Flask Transporter transfers the Splitter Flask and Cradle along the Magnox Debris Corridor to the Flask Crane. The Splitter Flask and Cradle are transferred to the Despatch Bay by the Flask Crane. When required, the Splitter Flask is removed from the Cradle by the Skip Crane and is transferred to a Handling Bay adjacent to a Desplittering Machine.
19 20
21
22
23
24
25
storage arrangements at Oldbury

12 Caesium sampling complete, the skip is placed inside an empty flask in the pond dispatch bay.

Note: At Sizewell A and Hinkley Point A, the skip

is removed from the pond up a shielded ramp before being placed in a flask, but the arrange­ment at Oldbury is more representative.

13 The flask crane collects a flask lid from the decon­tamination bay, lowers it onto the flask and returns the flask and its lid to the decontamination bay (see Fig 2.30).

14 The flask is decontaminated and the lid is bolted down. Health physics check for satisfactory de­contamination and the flask is transferred to the leak testing bay.

15 The flask is leak tested, nitrogen purged and, if necessary, fluoride dosed.

16 The flask is transferred to the storage bay to await road transport and a final contamination check.

17 Transfer to road transport for delivery to re­processing plant.

Splitter flask route

18 Splitter flask full of magnox debris.

19 Full splitter flask deposited on a cradle in the dispatch bay.

20 Flask and cradle transferred to the washdown bay for decontamination.

21 Decontamination flask and cradle transferred to the splitter flask transporter for delivery to the active waste dump loading bay.

22 Splitter flask removed from transporter and cradle and lowered onto a magnox debris vault door by the dump crane.

23 Magnox debris emptied into the vault and the empty flask returned to its cradle and transporter.

24 Splitter flask and cradle transported to flask crane.

25 Splitter flask and cradle transferred to the pond dispatch bay.

26 When required, splitter flask is removed from cradle and transferred to the pond handling bay, next to the desplittering machine.

Whilst ponds provide convenient shielding, careful

and rigorous water treatment and pond management

is necessary if magnox fuel can corrosion and re­

lease of contamination are to be acceptable. The decay heat also has to be removed. Ponds there­fore are provided with water circulating systems em­bodying filters, coolers and water treatment equipment. Contamination is removed by the filter and water treat­ment media and hence there is supplementary shield­ing provision for the exchange and storage of these arisings. Early experience with storage ponds was un­satisfactory, particularly when storage periods were excessive. Can-corrosion occurred and contamination spread from the water surface and from equipment as it was moved in and out of the water. Considerable ingenuity by station and other staff was necessary to cope with the difficulties.

It was these difficulties that led to the adoption of dry spent fuel storage at Wylfa (Fig 2.31). Each dry store cell consists of a bundle of vertical thimble tubes with a carbon dioxide atmosphere in w’hich the spent fuel is placed by the charge machine. The outside of the tubes is cooled by natural convection air. After the decay period the elements are discharged using a separate hoist to a shielded vault where they are de- lugged, placed in skips and dispatched in the trans­port flask. Subsequently, further dry storage has been provided at Wylfa in low head air-cooled vaults.

2.3 Reactor charge/discharge In principle, refuelling consists of coupling the pres­surised FM to the reactor, removal of the spent fuel elements one at a time from the selected channel and subsequent charging of the new elements.