arrangements with industry, including demonstrations of particular applications. At such time as Plowshare technology advances to the stage where it is economically and technically practicable and there is an active industrial demand for explosives, Congressional action would be required to make nuclear explosives available for commercial purposes. Under any forseeable circumstances, the Federal Government is expected to retain certain responsibilities for transportation and detonation to assure the safety of the general public.

In order to assist industry in evaluating possible future uses, the AEC has published projected charges for nuclear explosives for use as a guide in evaluating Plowshare excavation applications. Table I shows charges for specific yields picked from the published chart. The AEC believes that the projected charges are sufficiently representative of the future situation to warrant their use in feasibility studies. While these charges are for explosives designed for excavation purposes, they are presented here for illustrative purposes.

World consumption and production of copper has risen progressively from about 18,000 tons in 1800, to more than 6 million tons in 1965. The U.S. Bureau of Mines reports that the United States refined copper consumption increased from 1.35 million tons in 1960 to 2.35 million tons in 1966. This represents an increase of 74 percent in the last six years. The annual rate of increase has more than doubled, from 8 percent in 1961 to 17 percent in 1966. This rapid increase has been much greater than either consumers or producers had anticipated, and is a combination of population growth, business upsurge and exceptional military need. Although the recent consumption rate may have been inflated by the present extraordinary defense requirements and may tend to level off, a more moderate but sustained annual increase of 4 to 5 percent appears inevitable.

Mine productive capacity in the United States has remained substantially constant at 1.7 million tons from World War I to the present with a 10 percent increase effected in the last two years. Prior to World War II, the United States possessed a high degree of self-sufficiency in copper and exported substantial quantities to other users. Today this basic metal is in short supply in the United States with the result that the country, to satisfy its industrial and defense demands, is in the unfavorable position of being a net importer of copper. This dependency on imports is expected to continue since the higher

grade domestic reserves have become depleted while richer reserves still remain to be developed in other parts of the world. Rising standards of living and increasing populations in these developing nations indicate a growth in consumption for the world approxi mately twice the United States level. The availability of, and competition for, metal to import will be more difficult in the future.

The U.S. Government's current minimum copper stockpile objective, for strategic defense purposes, is 775,000 tons. The size of this objective has created some confusion since it has been changed frequently and presently, because of the extrordinary demand, the reserve has been depleted to about 260,000 tons. To meet its needs, the Government has been required to adopt a "set aside" policy for defense which now amounts to 29 percent of the domestic producers' copper production.

To insure adequate supplies of this strategic commodity at reasonable prices for industrial and defense needs, the copper industry has intensified its efforts to discover and develop deposits in the country. Some additional copper production will come from expansion of existing operations and from new deposits with ore grades equal to those now being mined. How ever, the major portion, in the long run, must come from development and utilization of deposits with ore grades not presently considered economic.

Exploration for new deposits is born of the necessity to replace the depleting deposits being mined. Its targets and successes are rigidly controlled by the economics that determine whether a mineral deposit can be developed into a successful mine. These economic criteria are not fixed, and the industry has been able in the past to gradually alter them. Copper represents a triumph for the technology that now permits profitable extraction from a grade of material that not so long ago could only be classified at best as a potential resource. Development of a low cost nuclear fracturing - in-situ leaching method of recovery may still further increase the domestic ore reserves and provide the needed copper from vast quantities of material never before considered economic for mining.

How large are the United States' reserves of copper? The answer depends upon which authority one consults. In a 1960 survey the U.S. Bureau of Mines estimated the United States to have 32.5 of the world's 212 million tons of copper in ores averaging 0.9 percent copper. In 1965 these domestic reserves were indicated to be 75 million tons in ores averaging 0.86 percent. Reported reserves have always been only a fraction of what the earth will ultimately yield. Large sums of money are required to outline a deposit sufficiently for it to be classed as a mining reserve. Producers can only justify investigating that portion of the total that offers a reasonable promise of profitable operation. If many years of potential production exisit with presently commercial grade material, there is often little impetus for diverting funds to prove the existence of sub-ore material. As producers approach their known ore reserve limits, they attempt to develop additional reserves. These

additions can be obtained by new discoveries, by lowering recovery costs through advances in technology, or raising the price. Thus, the border between mineral reserves and mineral resources is constantly shifting.

It would be very difficult to establish reliable figures for the amount of sub-ore material that could be reclassified as ore reserves by the development of a mining method that would allow a substantial lowering of the present economic grade limits. The closest approximation is available in what is often referred to as "potential" ore, namely, material known by its location and quality and considered likely to be profitably minable in the future.

The amount of such potential reserves is known to be enormous. Vast tonnages of sub-ore material exists as halos around the economic limits of operating properties in this country. Exploration activities, presently on a 100 million dollar a year quest for additional ore, often observe, partially define, and are forced to abandon great quantities of this type of material in the search for today's commercial ores. The U.S. Bureau of Mines estimates that an additional 58 million tons of copper probably exist in potential ores averaging about 0.47 percent (9.4 pounds of copper in 1 ton of ore). Some definite information is available for eighteen such deposits containing about 16 million tons of copper.

A complete answer to the question of how much copper is in the United States in a sub-ore resource category will be determined only when a new method of mining technology is tried and proven, giving industry an impetus to fully explore the extent of these

resources.

Leaching is the process of dissolving metal values from an ore by means of a solvent, removing the resulting solution from the undissolved materials, and extracting the valuable constituents from the solutions.

Leaching of copper ores is not a new process. It was used as early as 2,500 B.C. in Cyprus, and perhaps in other historical copper producing areas of the Middle East. With the development of open pit copper mining operations on low-grade ores in the 1930's, the production of leach copper in the U.S. was greatly increased. To extract additional copper, the mining companies began to leach the dumps of sub-ore material that had been excavated in the mine stripping operations. By 1963, dump or "heap" leaching accounted for 9 percent of the 1,200,000 tons of copper produced in the U.S. By 1965, copper from leaching

had grown to 12 percent of the domestic production.

Successful practice of in-situ leaching methods has been previously restricted to the abandoned workings of old higher grade underground mines. Mines in Butte, Montana; Ray and Miami, Arizona; and Bingham Canyon, Utah, have practiced in-situ leaching to a limited extent in the mined out areas of old block caving operations. The zones treated by leaching were well fractured and had been made permeable by the previous mining operations. Solution recovery in these operations was generally accomplished by using the existing underground openings.

The investigations of a nuclear blasting technique are principally concerned with the development of deposits that are not currently economic. These deposits currently cannot be economically mined by conventional methods but none the less contain mil

lions of tons of recoverable copper. Types of presently uneconomic deposits to which in-situ leaching techniques may be applied include:

1. Large low-grade deposits in which the copper content of the ore is insufficient to justify mining the ores and concentrating the copper minerals; and

2. Small high-grade deposits in which the copper content is insufficient to permit direct smelting and for which the available ore reserves are too small to justify the large expenditure for treatment plant facilities.

Other considerations including physical and geographic location, characteristic metallurgy, depth and ground water, can also significantly influence the applicability of the techniques to a prospective deposit.

Present leaching technology is generally concerned with two types of copper mineralization, the oxide state and the non-oxide or sulfide state. Bornite and chalcopyrite are considered to be "primary" sulfide minerals deposited from igneous sources. Covellite and chalocite are largely "secondary" sulfide formations naturally leached from sulfides near the surface and precipitated near the water level. The oxide minerals such as chrysocolla, malachite and brochantite were

formed through oxidation of surface sulfides and are also secondary.

Copper is extractable from both types of mineralization by a number of common acid and alkali leaching agents. However, aqueous leaching is far more rapid and efficient when applied to the oxide mineral types. Several operating mines with oxide ore reserves utilize in-plant leaching as the primary copper extraction process. The sulfide minerals must be oxidized for effective leaching recovery. This may be by weathering or by employing oxidizing leach sc'utions. The "primary" sulfides are especially resistant and conventional practice has been to employ the flotation process for recovery of copper from these ores.

In-situ leaching eliminates the high costs of excavating and transporting the material to a plant for further treatment. To be economically effective, this method of leaching requires preparation of the deposit so that the dissolution process may proceed at an economic rate. The deposit must be shattered and broken to develop the permeability required to allow air and leaching solutions adequate contact with the minerals. A suitable means for collecting the copper leaching solution must also be provided so that the dissolved copper is not lost in the ground.

A. Rock Breaking

There are two general types of explosive emplacement configurations that could be used to fracture an orebody for leaching purposes. They are distinguished by the resulting fracturing effect which is dependent on the depth of explosive burial. If the explosive is deeply buried, the forces would not be sufficient to break through to the surface. The force would remain contained within the earth and create a completely buried cylinder of broken rock. At a shallower depth, determined by the explosive size and rock type, the blast can break through to the surface and heave the ground upward leaving a cone of broken rock. Instead of a buried cylinder, an inverted cone of broken rock extending from the shot point to the surface would be formed. Although the shallower emplacement yields about seven times more broken rock than a fully contained blast, it would permit some venting of radioactive gases to the atmosphere. All discussions in this report refer only to the contained emplacement configuration.

Upon detonation, the energy of an underground nuclear explosive is released in a fraction of a microsecond and vaporizes, melts and crushes the surrounding rock (Figure 1). A cavity forms and expands

spherically around the blast center following the outward moving shock wave until the cavity gas pressure approaches equilibrium with the weight of the overlying rock. The molten rock that initially lines the cavity walls will flow and form a pool on the cavity bottom. As this material cools and solidifies into a relatively inert glass, it traps and retains up to 90 percent of the radioactive fission products generated by the explosion. The roof over the cavity, having been fractured by the shockwave and effectively undercut, will start to collapse and a cylindrical chimney of caved and very permeable broken rock will develop upward. The chimney would have a radius that approximates that of the cavity and would normally extend to a height of four or five cavity radii. Chimney material formed by nuclear explosions in granitic rock is extremely permeable, and has been observed to have about 25 percent void space with 75 percent of the particles smaller than 12 inches in size. The force of the explosion would also fracture rock out beyond the chimney boundary. This outside fracturing would increase the rock's original permeability for a distance approaching three cavity radii. However, without the physical displacement caused by cavity collapse, this additional permeability would be very much lower than that within the chimney zone.

B. Safety Considerations

The major safety considerations for a deeply buried and fully contained nuclear explosion for leaching experiments are the direct effects of the blast and the indirect effects of the leaching program. The principal effects at the time of the blast are ground motion, possible accidental venting of radioactive gases to the atmosphere and the possibility of radioactivity from the explosion entering the ground water system. These "operational" safety considerations are discussed on page 32 and in Appendix A.

During the leaching portion of the experiment, the primary concern would be for potential industrial radiological safety problems that might be encountered as a result of solution treatment of rock broken by the nuclear explosive. These would be primarily due to tritium and to acid soluble fission products entering the circulating leach solutions. Investigations indicate that radiation from the leaching process solutions would be at such low levels that no shielding would be required for personnel protection.* Very little additional operating cost would be incurred by the housekeeping type precautions required to assure complete operational safety in handling these solutions. Tritiated water vapor from the leach solutions could constitute a hazard in the underground workings or in the precipitation plant, if allowed to collect and concentrate where it could be inhaled or absorbed through the skin. Process plant design specifying enclosed pipeline handling of the solutions and adequate ventilation would minimize this potential hazard.

The tritium content of the process solutions could be greatly reduced by initially flushing the chimney with water prior to the start of leaching. The flushing fluids would be chemically controlled in order to dissolve a minimum amount of copper. If contaminated, the flushing fluids would be disposed of in compliance with established AEC and State regulations.

C. Contamination of Copper*

Extensive laboratory scale experiments have been performed to investigate the possibility of radioactive contamination of the finished copper causing a poten

See Appendix B

tial health hazard or marketing difficulty. The investigations have indicated that the normal industry sequence of leaching and precipitation followed by smelting and refining, would result in a finished copper that is essentially free of any radioactivity.

The copper itself is not rendered radioactive by the nuclear explosive for any significant period of time. The radionuclides of copper formed by the explosion are very short lived and decay rapidly. Most of the fission products of the explosion are trapped at the bottom of the chimney in the relatively insoluble slag formed from vaporized and melted rock created by the detonation. Some fission products would be dispersed in the chimney in a more leachable form, but many of these would be strongly held on the ore by adsorption mechanisms and would not build up to significant concentrations in the circulating solutions.

Metallic copper is extracted from the leach solutions by precipitation with metallic iron. The only important long-lived radioisotope that would precipitate with the copper is ruthenium 106. Most of the ruthenium impurity would remain with the copper through the semi-finished smelting step; however, nearly all of the ruthenium would be removed during the electrolytic refining process. The refined copper product should contain less than 1 to 2 percent of the original small quantity of ruthenium that enters the leach solutions.

Substantial quantities of cement copper would be produced during the leaching tests conducted as part of the Sloop experiment. Decisions regarding the handling, smelting and refining of this copper and the resulting by-products and waste material would have to be based upon safety investigations and analysis of samples of the actual materials. After the experimental requirements of Sloop are satisfied, commercial usage of copper from Sloop could be permissible under suitable regulatory arrangements. Marketing of the copper could be permitted after a determination has been made that the sale and use of the copper produced from the experiment would not result in a significant increase in the radiation exposure normally received by the general public.

See Appendix B for detailed discussion.

The Safford deposit of Kennecott Copper Corporation is located in the Lone Star mining district of southeastern Arizona approximately nine miles northeast of the town of Safford (Figure 2). The test site is located on the northern flank of this large disseminated deposit, which is situated within the Gila Mountains at an elevation of 5,000 feet. Safford, the

Graham County Seat with a population of 4,700, is located adjacent to the Gila River at an elevation of 2,920 feet. The climate is typical of the southwestern desert, with little rainfall, hot summers and mild winters.

The recorded metal production for the Lone Star mining district amounted to 194,270 pounds of copper