Magnox ponds It can be shown by calculation that a critical configuration of natural uranium fuel could not be attained in the cooling pond and therefore no criticality controls are required at the magnox stations.

AGR ponds Because it has been postulated that in certain fault/accident situations a critical mass of fuel

Fig. 4.9 Simplified flow diagram of the pond water treatment system at Hinkley Point В (AGR) power station

could be formed, criticality controls are required in respect of storage and handling of AGR fuel, the uranium of which is enriched in U-235 to 3.5 w/o.

For this reason the fuel elements are stored ver­tically in specially compartmented stainless steel skips, the walls of which contain boron as a neutron ab­sorber. In addition, all fuel is stored in pond water containing a normal concentration of 1250 ppm boron ions derived from the 0,7% boric acid dissolved in the pond water.

Precautions against criticality can therefore be summarised as:

• Boronation of pond water to 1250 ppm boron.

• Control of the location and distribution of fuel in the cooling pond.

In normal conditions both these factors are operative. The risk of breakdown of the controls through error
is minimised. All water added to the pond must be boronated and the boron concentration is continuously monitored by installed instruments which alarm if the boron level falls below a prescribed limit. In ad­dition there is manual sampling and analysis of the pond water.

Plant interlocks and standard procedures are de­signed to minimise the risk of occurrences such as the spurious introduction of elements into the ponds, the dropping of elements during handling, or collisions during skip transfer operations. Nevertheless, assess­ments have been made of the full range of accident situations which could conceivably arise in order to ensure that none of these would lead to unacceptable levels of reactivity. These include:

• Loss of pond boron.

• Presence of fuel elements or pins outside their skips, e. g., as a result of a skip being dropped or knocked over during transfer.

• Non-standard water distributions.

• Dropping of individual fuel elements during transfer

from pond receipt to in-skip storage.