The procedures currently adopted in the UK to pro­tect magnox fuel during pond storage have evolved over many years, the main objective being to avoid exposure of the uranium fuel to pond water with con­sequent corrosion of the fuel and release of activity.

Pond water chemistry is accordingly specified to suppress magnox corrosion and hence penetration of the cladding [38]. Chloride and sulphate ions, and to a lesser extent silicate, have been shown to be aggres­sive towards magnox, but for pHs greater than about 11, the higher the pH, the higher the levels of chlo­ride and sulphate which can be tolerated. Water treat­ment is needed to maintain such alkaline conditions in an open pond where carbon dioxide can be absorbed from the air, and the demands for treatment can ef­fectively limit the sensibly achievable pH. Accordingly, magnox pond water is specified to be not less than pH 11.5 with a target value of 11.7, and the combined chloride and sulphate limit is specified at I g/m3 with a target value of 0.5 g/m3. Magnox fuel is most vulnerable to chloride excursions in the early stages of pond residence during the time in which a protec­tive corrosion film is forming on the surface [39]. Recently discharged fuel has been affected by transient increases in bulk pond water chloride levels of < 5 g/m3. In view of the widespread use of sand pressure filters and the less aggressive effect of silicate ion, only a target level for silicate of 1 g/m3 is set.

CVCS — CHEMICAL 4 VOLUME CONTROL SYSTEM ^-4 — NORMALLY CLOSED HX — HEAT EXCHANGER

Fig. 1.65 Boron thermal regeneration system

Several stations have installed cooling plant to re­duce water temperatures to about 15°C and hence reduce magnox corrosion rates further.

Magnox corrosion can be increased by galvanic coupling with the mild steel from which the storage skips are made. The skips are painted but the paint
deteriorates in use and therefore it is recommended that only skips with paint in good condition are used for storage.

Occasionally, corrosion product sludge has accumu­lated in station ponds and has had a deleterious effect when in contact with magnox. The sludge is known
to concentrate chloride and this may be responsible for the increased magnox corrosion, but it is also possible that the blanketing effect of the sludge allows local departures from the bulk pond water chemistry to develop. It is therefore recommended that ponds should contain minimum corrosion product.

Exposure of uranium to pond water has occur­red through mechanical damage to element1;. In one instance the cause was traced to a discharge route, which was subsequently modified to reduce element impact, but the desplittering and delugging of ele­ments has been found to be a more common source of damage. Efforts have been made to reduce the amount of desplittering and delugging damage and the operation is often delayed until shortly before element despatch.

As uranium corrodes in pond water, the fission product caesium it contains is released essentially com­pletely, strontium to a lesser extent [40]. The con­sequences of having failed fuel in a pond depend essentially on the area of uranium exposed. This can change substantially as corrosion progresses, particu­larly if a swollen element is involved and the corrosion penetrates the porous annulus. In this case, release rates from a single element can be of the order of a hundred mCi Cs-137/day, whereas the release from an element with a damaged end fitting exposing un­swollen uranium can be only a few /rCi Cs-137/day. More rapidly releasing elements can be identified and given priority for despatch. Caesium removal plant is common on magnox stations. The uranium dioxide corrosion produced is not adherent but forms a sludge, which if disturbed and redistributed can become an airborne radiological hazard. This is another reason why pond sludge should be kept to a minimum level, preferably by preventive measures but alternatively by mechanical removal [41].

When magnox fuel is identified as failed in reactor, it may be bottled before discharge to the station pond and then later sent for PIE to characterise the failure. Occasionally bottle seals leak, admitting pond water. The corrosion of uranium in moist atmospheres, as opposed to immersion conditions can lead to uranium hydride, accompanying uranium dioxide, as a corro­sion product in significant amounts, such that when the bottle is eventually opened, the uranium hydride may oxidise rapidly and exothermically, causing an occasional ignition of the uranium bar. Good sealing of fuel bottles is therefore important.