The purpose of the coolant is to remove heat from the fuel elements and to transfer it to the boilers. With most designs of gas cooled reactor some leakage of coolant occurs and availability of supply has to be considered. If it were not for this factor, the gas helium would be ideal: as it is carbon dioxide is chosen for magnox and AGR stations.

Carbon dioxide is a minor constituent of the atmos­phere: it is colourless, odourless and heavier than air. Chemically speaking it is an acid anhydride, i. e., it will react with water to form an acid-carbonic acid. Although carbonic acid is a weak acid, moist carbon dioxide is corrosive to some metals, particularly mild steel. In addition other minor constituents found in the reactor gas, e. g., ammonium chloride, reduce the relative humidity required to produce condensation and it is therefore essential to control the moisture concentration to prevent dewing out of water.

The individual elemental constituents of carbon di­oxide, carbon and oxygen possess the main nuclear property of low neutron absorption cross-section. Car­bon dioxide is also chemically compatible with the reactor environment, since it decomposes only slowly under irradiation. However, it can react with the core graphite, which can be expressed by the simple reaction C + CO; ~ 2CO.

This reaction can proceed by either of two processes — radiolytic or thermal reaction. During normal operation the former is important for both reactor designs but the latter has also to be considered for the highest temperature graphite components in an AGR, i. e., the upper inner graphite sleeves and in some reactor designs — graphite boiler support pads. The equation is inadequate to describe the complicated sequence of reactions w’hich take place in the core
but it serves to illustrate the net reaction by which the carbon dioxide can attack the graphite. The re­action can proceed to an equilibrium with the reverse reaction (hence the double arrow’ symbol), termed in the thermal case as the Boudouard Reaction. This is subject to the Law of Mass Action which means that as the concentration of carbon monoxide is increased the forward reaction is progressively reduced.

Since the graphite is a structural member, and graphite strength is reduced by loss of carbon, the reaction can only be permitted to proceed at an ac­ceptable rate. In addition the graphite is the reactor moderator and hence significant loss of carbon would affect the nuclear properties of the reactor. In magnox reactors where the rate of the reaction is low, due to the low dose rate and low total gas pressure, it is necessary merely to maintain a small proportion of carbon monoxide in the coolant, in order to limit the rate of the forward reaction, and so to reduce the loss of graphite to acceptable levels. In AGR reactors where the dose rate and total gas pressure are higher, the reaction would proceed to an unacceptable loss of core integrity if limited by carbon monoxide alone and further protection has to be provided.

The rate of the reaction is dependent on the type of graphite and the material used in AGR (giiso car­bon graphite), which was primarily developed to reduce the fast-neutron-induced dimensional change, exhibits a rate approximately one-half of that of the graphite used in magnox reactors (pitch coke graphite). Even this is not sufficient to reduce carbon loss to accept­able values. It was demonstrated that low concentra­tions of methane can further significantly reduce the rate of reaction. However, the methane itself under­goes radiolytic oxidation according to the equation CH4 + 3C02 — 4CO + 2H20.

The consequent rate of formation of carbon mono­xide and water is significantly higher than in magnox reactors and hence in the AGR it is necessary to install large recombination units (2CO + 02 2C02)

and driers (H20 removal) to maintain the coolant composition within specification.

A further important consideration is that, if the concentration of methane or carbon monoxide are too high, carbonaceous deposit may begin to form on fuel pin and boiler surfaces, inhibiting heat transfer from the fuel to the coolant. The detailed choice of coolant requires optimisation between core corrosion, steel corrosion, fuel pin deposition, boiler deposition, and coolant composition control.

Helium is also added to the coolant in both mag­nox and AGR reactors being maintained at a concen­tration between 50 and 500 vpm. The reasons for the addition are:

• Asa measure of the reactor carbon dioxide leak rate.

• As a measure of the reactor carbon monoxide leak

rate which is used as a measure of the core corrosion

rate.