The drier plant is provided for the continuous re­moval of moisture from the coolant gas. The incoming gas passes first through a recuperative heat exchanger where it is cooled by gas returning to the reactor. A water cooler further reduces the gas temperature before it enters one of the two absorption towers filled with desiccant. The desiccant is a bead form of silica gel which does not deteriorate significantly due to the trace organics which are in the coolant gas. The dried gas returns to the bypass circuit via the recuperative heat exchanger. After a period, the de­siccant becomes saturated and the gas flow is auto­matically switched to the other absorption tower. The desiccant is reactivated by part of the cooled gas from the recuperative heat exchanger which is mixed with some of the inlet gas to give a temperature of about 200°C. The gas from the absorption tower during the reactivation cycle passes through a water cooled intercooler before entering the separator vessel where the water is removed. The gas is returned to the gas flow through the absorbing tower in service. The extracted water contains tritium and care has to be taken in its handling and storage.

Recombination unit

The function of the recombination unit is to control the carbon monoxide level in the coolant gas. Part of the bypass flow passes through a platinum catalyst in the unit where the carbon monoxide is combined with oxygen. The catalyst is supported on a perforated plate surmounted by stainless steel mesh to prevent the carry-over of dust from the catalyst bed. Any hydrogen which is present in the gas is converted to water. The rise in temperature caused by the reactions is approximately 35°C.

The catalyst and the desiccant in the driers have a finite life and provision is made for their replacement

and storage.

Filters

The filters in the bypass circuit are designed to re­move particulate matter down to one micron with a high efficiency. In each filter there are a number of elements which are made from either sintered stain­less steel mesh or particles. An alternative design of element is made from packs of etched stainless steel discs. The filters can be back-blasted with clean car­bon dioxide to remove particulate matter and provi­sion is made for its removal and collection.

Electrolysis plant

The electrolysis plant provides oxygen for the recom­bination unit, hydrogen for generator cooling and for the methanation plant if necessary. Oxygen and hy­drogen gases from the electrolytic cells are collected and passed to their associated low pressure gas hol­ders. Compressors take the gases from the holders and their pressures are raised to values suitable for their safe and economic storage and for injection into the reactor system. Driers and sometimes water separators are installed after the compressors to ensure that the moisture content of the gases is within the specified limit. Gas cylinders are used for high pressure storage and may be supplemented with off-site produced gas. The storage capacity for these gases is relatively small, being equivalent to a few days’ usage.