Ca0900531 Atomic Energy of Canada Limited
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CA0900531 ATOMIC ENERGY OF CANADA LIMITED HOW MUCH OF THE ROCKS AND THE OCEANS FOR POWER? - EXPLOITING THE URANIUM-THORIUM FISSION CYCLE by W. B. Lewis DM-72 AECL-1916 Chalk River, Ontario April, 1964 BM-72 AEGL-1916 HOW MUCH OF THE ROCKS AND THE OCEANS FOR POWER? -. EXPLOITING THE URANIUM-THORIUM FISSION CYCLE by W* B. Lewis Abstract Even at quite low costs there appear to be many routes avail able to supply the world population of the future with its power for electricity, heat, energy storage, portable fuel, desalting water and local climate control. For example, sufficient power could come from nuclear fission in thermal neutron reactors. When rich uranium ores have become scarce, the price will rise from the current $13/kg U, but with improved techniques of extraction and the choice of an economical fuel cycle, abundant uranium for many centuries appears to be available in the rocks and the oceans. Even from reactors already developed to the stage of engineering design it is possible to choose a fuel cycle to which uranium at $250/kg U would contribute no more than 2 mill/kWh. Without suggesting when such a high cost might be reached, its implications are examined. The optimum fuel cycle would balance the financing charges on the fuel inventory and the costs of fuel make-up supply and reprocessing. By using uranium and thorium in combination at least 50,000 MWd can be derived per tonne of uranium. At a current low net conversion efficiency of 30$ and an overall rating of 6 thermal kW/kg, the natural uranium inventory would cost at the suggested high price $250/(6 x 0.3) » $139/ekW and for 7000 hr/yr at 7$ annual charges would contribute 1.4 mlll/ekWh. At 50 MWd/kg U the make-up supply contributes 250/(50 x 24 x 0.3) - 0.7 mill/ekWh. Probably higher efficiency and possibly higher specific power ratings would be used to lower such costs. The value of uranium is related to its content of the fissile U-235, and even though most power may be derived from thorium, its value will not rise comparably with that of uranium. In the course of time a ceiling will be set on the value of fissile material by the introduction of processes other than the thermal neutron fission chain reaction for producing power or neutrons. The total cost of nuclear power includes also contributions from the cost of equipment and plant operation as well as of fabricating and processing materials. Any other technique or fuel cycle cannot supplant the thermal fission reactors merely by showing a lower demand for fissile material, but if plant and operating costs are not excessive, may enter the competition or become a valuable supplement. Three such techniques have for many years been recognized as (1) Fast neutron breeders (2) Neutron production by the excitation of heavy nuclei to high energies (3) Controlled thermonuclear fusion. DM-72 HOW MUCH OF THE ROCKS AND THE OCEANS FOE POWER? - EXPLOITING THE URANIUM-THORIUM FISSION CYCLE by W. B. Lewis !• Introduction Since the first achievement of the controlled large scale release of nuclear energy in fission reactors, several different views have been expressed concerning the relative importance In the long term of the very abundant sources of energy, namely, the light elements, the heavy elements and the sun. So abundant are the light elements that one isotope deuterium would alone be sufficient. This point was expressed with poetic vision by H. J, Bhabha as President of the first United Nations Conference on the Peaceful Uses of Atomic Energy at Geneva in 1955 (!) when he said, "I venture to predict that a method will be found for liberating fusion energy in a controlled manner within the next two decades. When that happens, the energy problems of the world will truly have been solved forever for the fuel will be as plentiful as the heavy hydrogen In the oceans." Another widely held view looks to the already established technique of deriving fission energy from a large fraction of the abundant isotopes of uranium and thorium. Since most uranium and thorium is widely dispersed through the rocks of the earth's crust and relatively little is available in concentrated ores, A. M. Weinberg has described the process as "Burning the Rocks"(2) and the technique established, although not yet economically competitive, involves generating new fissile Isotopes in breeder reactors. A word of warning has been expressed by J. R. DietrichO), "Although the breeder industry could afford to burn the expensive uranium and thorium extracted from the low-grade ore deposits, it is by no . means clear that it could afford to use for inventory the high cost U-235 that may be extractable from such deposits." The writer has, however, pointed out that "Breeders are not necessary"^), and if their inventories become too costly, alternative means of producing fissile material,, and in particular U-233 from thorium and extra energy from lead and bismuth, are likely to compete. Somewhat earlier a sterner view was expressed by the late C. G. Darwin in his book "The Next Million Years".(5) He concluded that on that long time scale coal would soon be gone and uranium too. He recognizes, however, two sources of energy, the sun and deuterium, and would agree with Bhabha that this would be a plentiful fuel, but he backed the sun, and admitted that in doing so he foresaw man condemned to great labour to gather up the. energy in sufficient amount for the needs of the population. All these views agree in the implication that a considerable fraction of the future cost of power will lie in the human effort required to manipulate the energy resources. It is most difficult to forecast whether technical developments will make it easier to extract the lower grade ores or to construct and operate more _2- effective nuclear energy machines and processing plants that -produce either more energy directly or more of the artificial fissile isotopes U-233 or plutonium. To make any quantitative assessment, it becomes important to establish the contribution of the cost of extracted ore concentrate to the cost of power from the reactors that are now the targets of design. To establish a particular example, a basic cost of $250/kg U will be considered that is ten to twenty times the current market price. It is shown to contribute perhaps less than 2 mill/kWh to the cost of power, a cost that should not be crippling since it has already been exceeded in the large scale use of coal for power. This performance would be offered by thermal neutron reactors already developed to the stage of engineering design for large generating stations. The optimum fuel cycle would balance the financing charges on the fuel inventory and the costs of fuel make-up supply and reprocessing. At a current low net conversion efficiency of 3^ and an overall rating of 6 thermal kW/kg of natural uranium inventory, at $250/kg U the inventory would cost $250/6 x 0.3 *= $139/ekW and for 7000 hr/yr at 7^ annual charges would contribute 1.4 mill/ekWh. At 50 MWd/kg U (50,000 MVfd/tonne U) the make-up supply would contribute 250/50 x 24 x 0.3 = 0.7 mill/kWh. By the distant time when such high costs are encountered, it is to be expected that higher efficiencies and possibly also higher specific power ratings would be achieved to lower the contribution to the cost of power. Before substantiating the above claim in detail, the availability of natural uranium supplies at $250/kg U or less to meet the potential demand may be reviewed. 2. Availability of Uranium Any long term forecast can be challenged and disputed and so will be avoided; instead, some relations gleaned from a general knowledge of the world will be presented so that the reader can make his own deductions. The present population explosion of the world made possible by technical organization that is able to reduce premature death, is expected to encounter some limit. The world population may soon be about 3000 million, and it may be supposed that within the next century or two it is not likely to exceed 14000 million. Suppose the electrical generating capacity of the world rises to 14,000 MkW; this would provide 2 kW per capita for a population of 7000 million or 1 kW per capita for 14,000 million. At a modest energy yield of 50,000 thermal MWd/tonne natural uranium at 35$ efficiency for 7000 hr/yr the uranium consumption would be 240,000 tonnes/yr or 24 million tonnes/century. Using breeder reactors, the practical energy yield might be 15 times greater, i.e., 75^ of the theoretical maximum, and at 40$ efficiency the corresponding consumption would be only 1,4 million tonnes U/century. The use of additional power for desalting water and for local climatic control would perhaps be limited to a similar amount by economic considerations of the cost of the plant in relation to the population served. The corresponding total demand for uranium -3- DM-72 would not need to exceed a few tens of millions of tonnes per .century. The U.S.A.E.G. Report to the President of 1962(6) suggests that the United States alone may have 10 million tonnes reasonably assured and 20 million tonnes of estimated resources of uranium recoverable for less than $100/lb U3O8 or approximately $250/kg U.