Since the uranium in the magnox element is natural uranium and contains only 0.7% of the fissile U-235, criticality of the fuel cannot result from its trans­portation. Thus, the sole requirement is the protection of the dements from mechanical damage. This is achieved by packing the wrapped (brown paper and polythene) elements in preformed fibre packing such that the element is constrained from movement and provided with all-round protection. They are trans­ported in this packing in sealed metal boxes designed to hold 20 elements. The lack of criticality problems allows the storage of the boxed fuel to be relatively simple and only precautions against fire damage have to be considered. These are essentially the suitable stacking of the boxes to a maximum of six high with access to at least one side of the box. A fusible salt extinguisher as employed for metal fires is provided for fire control purposes.

New fuel is unpacked in clean conditions, the pur­pose of which is to protect the fuel from damage and contamination. The exclusion of certain materials from this area is essential. In particular, contact between the element canning and metals or alloys which could result in a low melting point magnesium alloy is rigorously avoided, e. g., lead. Personnel han­dling the fuel are provided with ‘clean condition cloth­ing’ (including linen gloves) to avoid contamination of the fuel by salts and grease during handling.

Unpacking usually occurs immediately before in­spection, the purpose of which is to eliminate elements which may give rise to problems whilst they are within the reactor. To date some 2 x 106 magnox elements have been manufactured, the number of elements giv­ing rise to reactor problems and associated with a manufacturing defect being only a few hundred and mostly from the early days of production.

Nevertheless, since the removal of failed elements from reactors at power can be costly and failure of a single element may involve the discharge of a complete channel of fuel, inspection of the fuel before charging is essential. The following typical features are checked: [37]

• Type identification (LTA, HTA, etc.).

• Freedom from grease and dirt.

• Freedom from damage of the can finning.

• Weld damage, cracks, or pin holes in end caps.

• Soundness of fittings such as splitters, braces and lugs.

A particularly vital area examined for damage is the end cap closure weld and the upstand of the can into which the-cap is screwed. If the defect is such that the fission products, which are produced when the re­actor is operating at power, can readily escape from the can, then the BCD equipment will effectively indicate the channel containing the defective fuel ele­ment. If, however, as a consequence of even appa­rently minor damage, the leak is very small or the leak path is long, it may give rise to the phenomenon of a ‘fast burst’. This results from the ingress of the coolant gas through the leak to the uranium and leads to local oxidation of the bar. The accumulation of oxide causes can-swelling and eventual rupture. A long path length allows fission products diffusing outwards to decay before leaving the can and thus evade detection. At a later stage, the fission products are suddenly released in larger quantities due to fail­ure of the can giving rise to the nomenclature of a ‘fast burst’. Normally it is only during unpacking and inspection that the fuel is handled, thereafter the fuel is remotely handled.

Optimisation studies of reactor systems covering core physics, heat transfer characteristics, gas circu­lator power and material constraints are carried out for each design concept.

The following characteristics of magnox elements contribute to the optimisation of the physical size of magnox elements and the steam cycle conditions of the associated reactor plant:

• Uranium experiences a metallurgical phase change at 663°C when the metal changes from an orthor­hombic to a tetragonal crystalline structure. This places an upper limit on the operating temperature of the centre of the bar.

• Magnox has a limiting ignition temperature of the order of 630°C and under operating conditions, including any transients resultant from postulated fault conditions, this temperature must not be exceeded.

These characteristics and other constraints result in an optimum uranium bar diameter of about 25 mm, a finned can of 60 mm diameter and associated with a channel gas outlet temperature of 400°C. The latter will determine the steam cycle conditions and efficien­cy. As a result, there is a limit to achievable steam conditions associated with magnox fuel beyond which progress is marginal.