After breakdown of irradiated AGR stringers in the irradiated fuel dismantling cell, individual elements are discharged to a cooling pond, where they are stored vertically in compartmented skips to allow cooling and radioactive decay before transport off site.

To provide criticality control, the pond water con­sists of demineralised water dosed with boron in the form of boric acid at a level of 1250 ppm boron, the pH being adjusted to 7 with caustic soda. Chillers are provided to maintain pond water temperatures of approximately 25°C.

In anticipation of release of activation products from fuel cladding, especially from the clad oxide, ion exchange plant is provided to remove active cations such as Co-60, and also to provide some measure of chloride-ion control to meet the specification of a target value of 0.5 g/m3 (upper limit 10/m3) to sup­press corrosion. For sulphate ion a target level of <0.5 g/m3 is set. In practice to date (1988), the graphite sleeves rather than fuel cladding have been found to be the major source of activation products. The activation products form in the graphite itself, and also in originally inactive material deposited from the reactor coolant on the large graphite surface area. Caesium 134 is among the activation products found

and this element is not removed by the cation ex­change resin to any extent. However, caesium selective resins are available if required. Fuel pin failure is not expected in station ponds where storage periods are of limited duration. However, activity release from deliberately defected pins has been examined and is expected to be slow, and limited in amount to the cap inventory (i. e., the amount released from the fuel pellets to the pin interior during service). The fuel does not react significantly with water.

Slight visual signs of corrosion are observed on elements where the steel components have been irra­diated in a temperature range conducive to sensitisa­tion. For example, parts of the brace which include welded strip sections, are affected and occasionally pin end caps. Metallography has shown slight inter­granular corrosion after typical pond residence times of about 250 days but pin and brace integrity has not been significantly impaired.

At this early stage of the AGR programme, ex­perience of fuel behaviour in pond storage [45] is necessarily limited and inspection of fuel behaviour both in pond by underwater techniques and by de­structive examination of a limited number of pond — stored elements is planned to continue.

MAGNOX REACTOR