The safety considerations in respect of ponds at PWR stations are similar to those at magnox and AGR stations.

In the Sizewell В design, the main fuel storage pond is linked to three bays which are respectively for the purpose of fuel transfer into the pond, loading fuel into the transport flask and preparation of the flask before and after fuel loading. Each bay can be sealed off from the pond and/or adjacent bay by means of stop gates. The flask preparation bay is routinely drained and refilled for flask transfer purposes, and the fuel canal can be drained for maintenance, if necessary.

The fuel, which consists of assemblies of rods of uranium dioxide pellets in zircalloy cladding, is stored in racks which stand on the pond floor.

A range of protective measures is incorporated into the design to control radiation and contamina­tion exposure in normal operation, and to prevent accidents including criticality and damage to the pond containment.

There are interlocks on fuel handling machinery to prevent dropping or collisions during fuel transfer operations. The height to which skips may be raised during transfer is also limited.

Measures to prevent inadvertent pond drainage include the extraction of water from near the pond surface and fitting of anti-syphon devices on the pond water return line. Installed instrumentation in­cludes radiation monitors and high and low water level alarms.

The fuel storage racks incorporate neutron absorb­ing plates, and they are so arranged as to be safe even in the absence of boron from the pond water.

A pond water treatment plant is provided, incor­porating water/coolers, filters and ion exchange resins, together with a boric acid dosing facility.

Skimmers are included for the removal of debris and surface microlayers and an active ventilation sys­tem with filtered extract is provided.

The pond water is dosed to 2000 ppm with boron for criticality control purposes. The water would be maintained at pH near neutral by dosing with lithium hydroxide.

The pond construction is of reinforced concrete with an internal stainless steel liner which is keyed to the concrete. A leakage monitoring system associated with the liner welds is incorporated.

There is now considerable experience in the op­eration of cooling ponds at PWR and other water reactor stations, both in the United States and in other countries. This experience [38] supports the view that good radiological protection standards can be achi­eved and radiation exposure maintained at acceptably low levels.