The earth's crust holds huge amounts of uranium, dispersed over a range of rock types at widely different uranium contents, varying from a few grams uranium per tonne rock to more than 100 kilograms per tonne rock. The lower the uranium content of a rock type, the more of that rock type is present in the crust and the larger the total amount of the metal in that geologic compartment. This is a common geological phenomenon which applies to all metals in the earth's crust. Obviously it is not possible to extract all uranium from the erthÕs crust, not even from the accessible part of it.
Industrial view on resources
The nuclear industry bases its prospects of the future uranium supply mainly on an economic relationship between market price and uranium resources The economic point of view on mineral scarcity can be summarized as follows:
• The market price is the criterion of the mining of a metal or mineral.
• Higher prices will lead to more intensive exploration.
• More exploration will lead to more discoveries of new mineral deposits.
• The newly discovered deposits will contain more of the desired mineral than the already known deposits.
• At higher price more and larger resources are economically recoverable.
Ergo: the world mineral resources, in casu uranium, are practically inexhaustible.
Uranium ores and resources are defined In the mining industry as those deposits that can be exploited economically at the prevailing market price of uranium. Consequently the size of the uranium resources, that are the economically recoverable resources, can increase within a short time when the uranium market price rises.
However, above industrial paradigm is founded on a fallacy, because it ignores the essential difference between the quantity and the thermodynamic quality of uranium occurencies as present in the earth's crust or oceans. Below we will explain briefly this observation.
Thermodynamic quality of uranium resources
Nuclear fuel, consisting of enriched uranium, is produced from natural uranium which in turn has been extracted from uranium-bearing rock in the earthÕs crust. The amount of useful energy required to extract one mass unit of pure uranium from the resources as found in nature varies widely, depending on the geological and mineralogical proprties of the uranium deposits. Usually the ore grade is the most important variable.
From the laws of thermodynamics [more i43] follows that the energy investment per mass unit of extracted uranium increases exponentially with declining ore grade of the deposits from which the uranium is extracted. Below a certain grade the useful energy investment per mass unit of extracted uranium may be as large as or larger than the amount of useful energy which can be generated from that mass unit. Uranium deposits at grades below the critical threshold, called the energy cliff, are in effect energy sinks, not energy sources. It should be stressed that the threshold is not at a fixed value of the ore grade but is also determined by other variables, such as the mineralogy of the uranium-bearing rock and the depth of the deposit below surface.
The thermodynamic quality of a given uranium resource is here defined as the degree of usefulness of that resource for net useful energy production. The thermodynamic quality becomes zero if the net energy balance of the nuclear system reaches zero and no net useful energy can be generated from the resource in question. The higher the thermodynamic quality of a given uranium deposit (above the critical threshold), the more useful energy can be delivered to the consumer per mass unit of uranium extracted from that deposit.
Depletion of uranium resources
The notion of the thermodynamic quality points to the conclusion that depletion of uranium resources is actually not a matter of quantity, but a matter of quality. From a quantitative point of view the uranium resources may be considered inexhaustible as pointed out above, in accordance with the economic point of view.
However, the use of uranium for energy generation puts a lower limit to the uranium content of rocks to be considered an energy source. Below a certain grade of a uranium-bearing rockformation (about 100-200 grams of uranium per tonne rock) the energy balance of the nuclear systems turns negative and no useful energy can be delivered to the consumer, as pointed out above. This observation implies that only a fraction of the uranium occurencies in the earthÕs crust qualify for exploitation as energy source.
Depletion of uranium resources is a matter of quality, not of quantity.
In practice the richest and easiest accessible ores, those at the highest available thermodynamic quality, are always mined first, for these offer the highest return on investments. Consequently the world average energy quality of the available uranium resources are declining over time. When the thermodynamic quality of the yet to be exploited uranium occurencies will near zero, the uranium resources for the nuclear energy supply will get depleted. The gradual decline of the recoverable amount of net useful energy with decreasing thermodynamic quality of the natural uranium resources is here called the energy cliff.
Energy cliff over time
During the next decades the net energy from nuclear power will gradually decline, due to the depletion of high-quality uranium resources. If no new large high-quality resources will be discovered the net energy will decline to about zero when the lowest-grade known uranium resources are to be mined. The nuclear system then falls off the Ôenergy cliffÕ. This could happen during the next 5-7 decades, within the lifetime of new nuclear build.
It should be noted that the energy cliff as phenomenon is not typically reserved for uranium as energy source. Similar developments are observable in the fossil fuel recovery from the crust: deeper wells at more remote and harsh locations are needed, the recovery of oil from tar sands consumes at least half of its energy content, recovery of gas from shales (fracking) consumes a substantial part of its energy content. Coal mining meets similar problems.
The easy oil, gas and coal, having a high thermodynamic quality, are getting depleted. Exploitation of increasingly lower-quality resources is the trend.
New discoveries of uranium resources
New uranium deposits will likely be found in the future, when exploration continues. What matters is the thermodynamic energy quality of the yet-to-be discovered deposits.
The easiest discoverable and easiest accessible deposits are already known for decades. Based on currently available evidence the majority of the yet-to-be discovered uranium deposits may be of lower thermodynamic quality than similar deposits currently known, and consequently may lay closer to the energy cliff. A lower thermodynamic quality results not only from a lower grade, but also from other factors, such as: smaller ore body, greater depth below surface, less favourable geologic conditions and more refractory mineralogy.
Unconventional uranium resources
Vast amounts of uranium are known to be present in black shales, phosphate rock, lignite and coal. Due to the low grades, typically less than 0.1 gram per kilogram rock, extremely large amounts of the uraniferous deposits would have to be mined and processed. The energy consumed in the uranium extraction would push the nuclear energy system off the energy cliff.
In addition to the unfavourable effect of lower ore grades, another phenomenon greatly increases the energy input per kilogram recovered uranium. Below a certain uranium content the uranium atoms in a uraniferous rock do not form separate uranium mineral grains, but are dispersed in the matrix of the host rock. This implies that the whole mass of uraniferous rock has to be brought into solution to make possible the extraction of the uranium atoms from it. The consumption of chemicals and energy in this case is a factor 10-100 higher than the amounts needed for extraction from conventional ores, which contain separate uranium mineral grains which can be separated from the other minerals in the host rock by physical means, such as flotation. This phenomenon is called the mineralogical barrier.
Uranium from seawater
The optimistic view of the nuclear industry with regard to the possibilities of extraction of uranium from seawater, is based on laboratory-scale experiments, untried technology, unproven assumptions and a billion-fold extrapolation of a few experiments.
The size of the installations required for recovery of uranium from seawater at a substantive scale would be measured in tens of kilometers. Such installations would have to be positioned in warm ocean currents, such as the Gulfstream, because recovery of uranium from seawater is only practically feasible at water temperatures exceeding 20 °C. The consumption of materials and energy would be prohibitive.
Figure 38-1. Known recoverable uranium resources of the world.
Distribution of the known recoverable conventional uranium resources as function of decreasing ore grade. The ore grade distribution shows that the resources are larger at lower grade, a common geologic phenomenon of all metals in the earth's crust. Below grades of 200-100 grams of uranium per tonne rock, no economically recoverable resources are reported. Poorer ores tend to be harder to exploit, because of more refractory mineralogy. Recovery of uranium from hard ores requires more useful energy than from soft ores at the same ore grade. The known ore grade distribution of uranium exhibits a so-called bimodal character, a not very common phenomenon in geology. At ore grades between 5-50 gram uranium per kg rock only a few, small deposits are known.
Figure 38-2. Energy cliff.
The net energy represented by 1 kg uranium as present in the earth's crust declines with declining ore grades, due to exponentially rising energy consumption per kilogram uranium with falling ore grades, while the gross energy generated per kg recovered uranium has a fixed value. The green colored area represents the ore grade domain within which nuclear power has a positive energy balance. Below a grade of 1 gram U per kg rock the net energy from nuclear power steeply falls to zero (red curve): the energy cliff. The critical ore grade of the energy cliff, around 0.1 gram uranium per kg rock, turns out to be independent of the assumed energy investments of the nuclear power plant. In the background of this graph the ore grade distribution of the known uranium resources is given.
Figure 38-3. Known uranium resources over time.
Usually the easiest recoverable uranium resources are mined first, because these deliver the highest return on investment. The easy uranium resources are easily discoverable, are at shallow depth, have high ore grades and are mechanically and chemically easy for mining and milling. As a result of this common practice the remaining uranium resources are less easy, meaning that the energy investments per kilogram recovered uranium rises, even if the ore grade would remain flat. This graph is exclusively based on the declining ore grade over time; other factors, such as depth and hardness of the ores are left aside.
Figure 38-4 Net energy from nuclear power: energy cliff over time.
The energy return on energy investment (EROEI) is here defined as the ratio of the energy delivered to grid over the energy investments, both measured over the full cradle-to-grave (c2g) period. At EROEI = 1 the nuclear energy system consumes as much useful energy as it produces. Below a value EROEI = 1 the nuclear energy system becomes an energy sink, instead of an energy producer. This graph can be seen as the energy cliff over time and is the synthesis of the energy cliff and ore grade, presented by Figure 38-2, and the declining ore quality over time as presented by Figure 38-3. It turns out that the year of the plunge into the energy sink does not depend on the assumed parameters of the nuclear reactor, but mainly on the quality of the available uranium resources.