Description of Recovery Process

Briefly and roughly stated, the Plutonium recovery process is understood to proceed as follows: As beforesaid, the treated uranium discharged from a pile after about three months' operation at full capacity, should contain on the average about twenty-five hundred-thousandths (0.00025) parts by weight of Plutonium, together with about an equal amount of radioactive chemical by-products. On account of its radioactivity, the treated uranium cannot be touched or closely approached, or even be looked at with safety except when under some 16 feet of water. It must be handled mechanically behind heavy shielding (5 feet of concrete or 1 foot of lead) and by remote control, and must be moved to isolated pits and stored under water for two months in order that its radioactivity may decrease to the point where it is reasonably safe to begin recovery of Plutonium from it.

The treated uranium spontaneously produces heat and must be cooled continuously during handling and during the early part of its two months' storage in order that it may not melt and burn to radioactive smoke. After storage it must be transported, properly shielded, to isolated recovery units. In these units the Plutonium will be recovered from the uranium and purified by chemical means, using dissolving, precipitating and filtering equipment which must be operated unseen and by remote control from behind shields (7 feet of concrete) calculated to be adequate to protect operating personnel against the extreme radioactivity.

When the treated uranium is dissolved in acid to start recovery operations, radioactive gas will be evolved. This must be vented to the atmosphere above the recovery units with sufficient dilution so that radioactivity from it may not injure operating personnel by radiating down to the ground from the sky with harmful intensity or by being carried back to the ground in harmful concentration by air currents. Radioactive xenon, the principal known radioactive gas which will be encountered in the recovery process, is so toxic that one cubic foot of it must be diluted in a cube of air two miles on each edge, before it can be breathed safely. Recovery operations must be limited to a small amount of Plutonium per recovery-batch, in order to avoid heat and explosion hazards from the Plutonium itself.

The chemical solutions involved in the recovery process are corrosive, and it has not been possible to determine conclusively whether corrosion will be increased to a prohibitive extent by the presence of the degree of radioactivity which will be encountered in the Plant as compared with the lesser degree available for laboratory experiments. The treated uranium will contain so small an amount of Plutonium that, when the Plutonium is precipitated from solution in the course of recovery operations, it will not form a visible precipitate in the solution. Consequently, by addition of suitable chemical reagents, bulky "carrier" precipitates must be formed along with the Plutonium precipitate throughout the recovery process. The chemical recovery process is an ionic process, and it has not been possible to test the process with materials and under conditions adequate to determine conclusively that the process will "work" in the presence of the intensity of radioactivity which will be encountered in the Plant.

Experimental development of the recovery process has dealt, all told, with about four millionths (0.000004) of a pound of Plutonium; and this small quantity has been made, not by the proposed transmutation process, but by cyclotron bombardment of uranium or chemical compounds of uranium. Only by conducting experiments in glassware on a micro-chemical scale has it been possible to work with anticipated Plant concentrations of Plutonium and anticipated Plant concentrations of radioactive by-products, and it is doubtful that the radioactive by-products so tested have been altogether representative of those to be met in large-scale operation. Extremely dilute solutions of Plutonium and radioactive by-products have been all that have been available for experiments in metal apparatus on a large laboratory scale. Therefore, in the development of the recovery process, non-typical raw material has had to be used, in micro-chemical quantity or at great dilution, and it has not been possible to carry out plant-type recovery operations in the presence of radioactivity even distantly approaching in intensity that which will be faced in the recovery units in the Plant.

Purification of Plutonium must be carried at least to the point where radioactivity is reduced to proportions requiring only normal precautions in handling and shipping. Purification to this degree requires removal of radioactive impurities so that the purified product possesses only one ten-millionth (0.0000001) as much radioactivity as the treated uranium entering the recovery units after two months' storage. Plutonium is itself moderately radioactive.

No purification process can be established as adequate until treated uranium from large-scale pile operation at rated output has been purified to the desired degree. The impurities to be removed may turn out to be related qualitatively as well as quantitatively to the scale and rate of the transmutation operation; consequently new impurities may be encountered in uranium treated on the large scale. And until purification has been carried to the desired degree, it cannot be determined that the purification process is not selectively removing certain impurities and leaving other impurities whose presence cannot finally be tolerated.

For the sake of simplicity, the transmutation process has been described as the direct transmutation of uranium to Plutonium. Actually another element, "Neptunium", is first formed. Upon relatively rapid radioactive decay, Neptunium spontaneously transforms itself into Plutonium. Since Neptunium is dangerously radioactive and furthermore may have a deleterious effect on the final use of the Plutonium, it cannot be allowed to contaminate the recovered and purified Plutonium. No practical process for separating Neptunium from Plutonium has been developed. Therefore, two months storage between the pile and recovery operations must be provided to allow the Neptunium content of the pile product to be transformed almost completely into Plutonium, as well as to allow the radioactivity of the pile product to be reduced by decay to a level reasonably safe for recovery operations. At the high intensity of large-scale pile operation, various isotopic modifications of Plutonium may be formed of which little is known. These, unlike Neptunium, may persist in the treated uranium after storage and interfere with the Plutonium recovery and purification processes or too greatly increase the radioactivity of the purified Plutonium.

There may be as great difficulty in modifying or repairing recovery units in which operation has been attempted, as in modifying or repairing piles; on account of the absorption of radioactive materials by the recovery equipment and the difficulty, if not impossibility, of washing these absorbed radioactive materials out of the recovery equipment so that it can be approached and handled safely. However, individual recovery units can be small and can be made of less critical materials than pure graphite and uranium. Therefore, individual recovery units can be provided in sufficient number so that forced abandonment of a recovery unit is not so serious a matter as abandonment of a pile.

All waste solutions resulting from dissolving the treated uranium and from the series of the precipitations and re-solutions employed in the recovery and purification of the Plutonium, will contain uranium or radioactive by-products or both. These wastes will be corrosive, will evolve heat spontaneously for long periods, and because of their radioactivity may not be discharged either into the river or into the ground and so must be stored indefinitely. Accordingly, facilities must be provided for isolated, cooled and shielded waste storage in the amount of 25,000,000 gallons for each year's operation of the Plant at full capacity.

The value of the uranium contained in these waste solutions will be very great, and under normal conditions attempts would be made to reclaim this uranium. However, no process has been developed for reclamation nor could such a process be developed in sufficient time to be of use in the present undertaking without seriously affecting the effort available to more nearly assure the production and recovery of Plutonium. Accordingly, no provision looking to reclamation will be made beyond safe and permanent storage. The radioactive by-products in the waste solutions may be of value for various scientific purposes or for military use; but for the same reasons as apply to uranium reclamation, the salvage of radioactive by-products is not a part of the work contemplated under the Contract.