This is an excerpt from a paper prepared for the Cabinet by John Wilmot, Minister of Supply, dated 7 May, 1947. (National Archives PRO CAB 134/21.) It sets out the case for using air cooled piles rather than water cooled, as initially planned, and would lead to the construction of the two piles at Windscale. After an introduction, the paper carries on with:
II. Alternative plan for air-cooled piles.
4. While the work of designing the water-cooled pile has been proceeding, some thought was devoted at Harwell and Risley to the possibility of designing gas-cooled piles to give the same output of plutonium as the water-cooled pile. The results are such that it was found necessary to review again the original decision in favour of building a water-cooled pile.
5. The U.S. project considered in 1942-43 the construction of high pressure helium-cooled piles as an alternative to the Hanford water-cooled piles and the Smyth report states that all their preliminary studies, including their initial plans for choice of site, were based on the construction of a helium-cooled pile. As a result of engineering studies by Du Pont the plan was changed for the following reasons: –
(a) hazard from leakage of a high pressure gas coolant carrying radio-active impurities.
(b) the difficulty of getting large blowers quickly.
(c) the large amount of helium required.
(d) the difficulty of loading and unloading uranium from the pile.
(e) the relatively low power output per kilogram of uranium metal.
7. The design now proposed differs from that studied by the U.S. project and is in general very similar to the air-cooled pile which the US project built at Oak Ridge and to the air-cooled pile which is under construction at Harwell.
8. In the Harwell pile, cooling air is drawn through the pile and ejected through a 200 ft. stack, the total power dissipation being 6,000 kilowatts, this being 150 kilowatts per ton of uranium metal used.
9. The design now being proposed for the production piles is broadly similar to the Harwell pile but the cartridges containing the uranium slugs are finned to increase the area available for removal of heat. Calculation shows that this should enable the power output to be raised to the order of 1,000 kilowatts per ton of metal, without using excessive pumping power.
10. The difficulties of the U.S. plan referred to in para. 6 now largely disappear for the following reasons:-
(a) since high pressure gas is not used, the danger of leakage of the gas coolant is largely reduced.
(b) pumping power is much reduced due to the use of finning and the blower supply difficulty is eased.
(c) air is used in place of helium.
(d) since a pressure envelope is not used the difficulty of building and unloading metal disappears.
(e) the output per ton of metal can be raised to the Hanford level by use of finning.
11. With the disappearance of these difficulties and in the light of the advantages of the air-cooled pile referred to in Section IV below, it is now recommended that the plans previously approved be changed and that we should build two or three air-cooled piles instead of one water-cooled pile.