2. Control of Chemical Processes Involving Uranium Materials by John E. Vance Dr. Vance is associate professor of chemistry at Yale Uni- Introduction As pointed out in the report of the Scientific and Technical Committee of the United Nations Atomic Energy Commission, the materials involved in the production of atomic energy are highly concentrated when the stage of chemical processing is reached, so that relatively small quantities may be significant. In the words of that report, . . only the application of very close and careful safeguards would provide an adequate assurance against the diversion from those plants of purified chemical compounds of natural uranium . . . for the surreptitious production of explosive material for atomic weapons." 8 In discussing the possibilities of control of the chemical processes used to convert black oxide, UsOs, to the feed materials for an isotope separation plant or a nuclear reactor, it is not necessary to examine the details of the preparation of these materials, but it is only required to consider the problem of controlling any large scale chemical procedure. In other words, the equipment used to convert UsOs to feed materials does not differ in any major way from the equipment employed in any combination of unit processes in chemical engineering. For this reason, such chemical plants do not have any outstanding characteristics which would make it easy to differentiate them from other normal chemical operations. There will be tanks for dissolving, for precipitations, for solvent extractions, and for the storage of "A First Report on the Scientific and Technical Aspects of the Problems of Control," September 27, 1946. liquors in process; there will be continuous filters and filter presses for removing precipitates and for the clarification of liquors; there will be mills for the grinding of solid materials; there will be furnaces for drying the products and for casting metal. In addition, there will be the auxiliary equipment characteristic of any chemical plant, such as sumps for collecting solid or liquid materials spilled on the floor, acid recovery systems, dust collectors, ventilators, etc. Material Balance The problem of controlling any chemical process to prevent diversion of materials must involve the problem of determining or checking the material balance of the process; therefore, an examination into the factors entering into a material balance must be made. Obviously, the balance is made up of the following items: (a) the material received by the plant; (b) the material produced by the plant; (c) the material held in process in the tanks, filter presses, griders, and furnaces and the material in residues and scrap of all kinds at the time the balance is struck; (d) known losses in effluents, in filter cloths, throughout dust collectors, ventilators, etc. The respective sources of error in the above items would include the following: (a) At the time of the balance, analyses must have been performed on all material received. This is rarely the case, so it is customary to estimate the assay of unanalyzed material with a consequent uncertainty. (b) The same requirement for analysis and the same uncertainty in estimation applies to material produced during the period covered by the balance. (c) The amount of solids and liquids in each piece of equipment must be estimated as well as the quantity of scrap materials produced during the period. Also, each item must be sampled and analyzed or an estimate of the content must be made. (d) Figures must be available on the quantity and content of all effluents discarded during the period of the balance, the amount of material lost through failure of dust collectors, etc. The difference between (a) and the sum of (b) through (d) is a measure of the unknown or unaccountable losses. Unaccountable Losses A competent management devotes great effort and thought to the problem of minimizing unaccountable losses, since that figure indi cates the efficiency of the process and directs attention to a possible flaw in the operation of the equipment. Even with careful attention on the part of the management and an earnest effort on the part of plant operators, a material balance is a rather uncertain thing, at least over short periods of time. Some months a positive balance may be obtained, i. e., apparently more material can be accounted for than was received; other months a large negative balance is registered. In the past years, careful records have been kept on all chemical processes used in the production of uranium compounds in the United States. It is fair to say that all those concerned went to extreme lengths in an attempt to achieve accuracy, using all the various analytical facilities made available to the operating plants and exerting the most careful supervision over those who prepared the material balances. The aim, which was to prevent losses, coincides with the aim of preventing diversion. A glance at a few of the actual balances indicates the accuracy which might be expected under a control agency, were enough competent personnel available to perform the task. In the following table, Conversion A and Conversion B are examples of individual steps in the chemical processes which are used for the preparation of the required products. The second column shows the average unaccountable losses over a year's time, while the third column gives the range of values of unaccountable losses recorded on a monthly basis in a twelve-month period; these figures are typical of the processes under discussion. Incidentally, the control of chemical processes used to convert UO; to feed materials is essentially identical with that of controlling the refining of uranium ores to produce U3O. In a typical case of the refining process, the corresponding figures show an average unaccountable loss for one year of 1.1% whereas the monthly figures range from an unaccountable gain of 2% to a loss of 4%. Conclusions It seems apparent that, over an extended period of time, a relatively smaller percentage of material is unaccounted for than over a shorter period; however, the yearly unaccountable losses apply to twelve months' production so the absolute uncertainty may well involve a considerable quantity of material. Large fluctuations seem inevitable and it is rarely possible to determine their significance. before the passage of an appreciable period of time, even though extreme precautions are observed in the operations. Where several 735654-47-3 successive processes are necessary to prepare a given material, the overall uncertainty is increased correspondingly. In any case, the problem of control would include that of verifying the material ba!ance; the only alternative would be the unsatisfactory one of accepting figures prepared by the management. The conclusion seems inescapable that a material balance can be checked only if the effort directed toward its verification is comparable to that devoted by the operators to the preparation of the balance. The services of several competent observers would be required and facilities, as well as staff. for duplicating or checking each weight, estimate, and analysis in each plant. In addition, the chemical plants should be suitably enclosed, and a system of protection with guards should be established against direct theft of material by employees or others. It is generally possible to improve the accuracy of a material bal ance to a marked degree by closing a plant, emptying all tanks. presses, sumps, and other equipment and completing analyses in incoming and outgoing material; if this is done, unaccountable losses might be reduced to less than 1%. This would be a time-consuming undertaking and would interfere seriously with normal plant operation; however, if any major question arose on possible diversion, a complete shut-down for the purpose of a more accurate balance might resolve the difficulty. Since the chemical efficiency of a plant depends not only on the process but also on the proficiency of the individual operators in the plant, it seems unlikely that any norm could be established for a particular process. Even within a single plant, fluctuations may occur which are without apparent explanation. Past experience has shown on more than one occasion that occasional large losses are recorded which are explained only after months of study. For example, on two occasions in recent years large unexplained losses were recorded which were eventually found to have been due to leaking sumps. In the event that sudden losses occurred, as they certainly would occur from time to time in a plant being controlled, virtually nothing could be done to assure oneself that the losses were real and were not the result of diversion. Many such possibilities of loss, or diversion masquerading as loss, exist in any plant; while these might be small losses individually, in sum they could be of consequence. In conclusion, it appears that control of any chemical process involves certification of a material balance and that this can be accomplished only by a staff comparable in size and skill with the plant staff devoted to the same task. 3. Control of Metal Production in Industry by James H. Critchett Mr. Critchett is vice president of the Electro Metallurgical Introduction The subject of controls covers one of the most important elements in industry, where they are employed to check the usage of material and consequently have an important bearing on economics. Even more important is the fact that through these controls the technology involved in the operation is continually being reassessed and its efficiency increased. These controls may be likened to the electric meters in a public utility company. It is possible to measure the fuel burned and the amount of energy generated and put on the wires at the central station. From here on, as the power passes through transformers and over transmission lines, there are steady losses. By placing meters at suitable locations, the amount of these losses can be continuously known, and the troubles that develop can be quickly found and eliminated. Likewise, with material going into the manufacturing plant, there may be losses in transporting the ore to the plant and in storage, and there are always unavoidable losses in the process itself. Finally, of course, there is always the possibility of theft or diversion. Material Accounting First, consider the methods which are used in a sound system of material accounting. They involve records based on weighing, sampling, and analyzing the material at various stages as it progresses through the cycle of manufacture. Fortunately, the question of weighing is on a pretty sound basis. In the United States, the Bureau |