Irradiated magnox fuel

Approximately 500 consignments or irradiated fuel per year are made from the CEGB magnox power stations to BNFL for reprocessing. In general, the fuel loads and the flask and transport conditions are con­trolled to within limits which are specified either di­rectly or by reference in the approval certificates. They are based on a Package Design Safety Report in which an analysis is made to show that under normal transport and accident conditions, the regula­tory requirements will be satisfied.

The bulk of the fuel is consigned in Mk 2 magnox flasks (Fig 4.6), although AGR flasks are also used at WFylfa power station.

There are two design variants of the Mk 2 magnox flask, one having a body made from a single-piece forging and the other being made using full penetra­tion welds. Basically the flask is a massive steel box

Л lid-to-body seat is achieved using two compressed viion rubber rings recessed into grooves in the lid. The! id and seal area are protected by a bolted-on aluminium impact absorber. The base incorporates a heat shield comprising 25 mm of Marinite 36 and a 13 mm steel cover plate.

There are two valves, one in the flask wall to allow the water level to be controlled and one in the lid. These valves also allow the ullage space to be purged or vented in a similar fashion to the magnox flask. The water not only acts as a heat transfer medium but also as a radiation shield. The water used is pond water, i. e., demineralised water dosed with hydroxide and boric acid to maintain a specified boron concentration and pH. Boron is a thermal neutron absorber and enhances the criticality safety margins, although its presence is not essential. The standard type of fuel skip is sub-divided by boron-loaded steel plate to produce 20 compartments which allows unbottled ele­ments to be held in vertical positions during storage and transport. Twelve compartment skips are used
for bottled fuel. Holes are provided in the skip lead liners to facilitate cooling water circulation and give enhanced heat dissipation through the finning on the outer flask surface.

The minimum allowable cooling periods before des­patch are 35 days for bottled fuel and 60 days for un­bottled. However, to meet restrictions on the overall thermal load, 12 and 14 kW respectively, and to meet BNFL requirements, it is usually necessary to extend the period beyond 90 days to optimise the number of elements despatched. The use of covers on flatrols has a very small effect on the maximum allowed heat toad.

The despatch procedures are similar to those em­ployed for magnox flasks.