Limitations of separation processes
Separation processes play a vital role in the process industry, especially in the nuclear energy system. The nuclear process chain starts with the extraction of uranium from its ore, a sequence of physical and chemical separation processes.
Separation processes are based on chemical and physical distribution equilibria. These dynamic equilibria are governed by the laws of thermodynamics and never go to completion, as a consequence of the Second Law. For that reason it is impossible to separate a mixture of different chemical species into separate fractions without losses.
Separation becomes more demanding, requiring more useful energy and specialistic materials and equipment, and goes less completely as:
• more different kinds of species are present in the mixture,
• the concentration of the desirable species in the mixture is/are lower,
• the constituting species of the mixture are chemically and/or physically more alike,
• the purity specifications of one or more of the fractions are more stringent.
Purification of a substance is based on separation processes, aimed at removal of contaminants from the substance. A higher purity means a lower concentration of contaminants. Extracting a species at a lower concentration requires more useful energy and is coupled to greater material losses. Higher purity means better predictable properties of a material. As pointed out above 100% pure materials are impossible. Purity specifications depend on the application of a material. Actually purity in the process industry is an economic notion.
One of the many purifications performed in nuclear technology is the fabrication of Zircalloy, the clading material of nuclear fuel. Zircalloy is made of exceedingly pure zirconium, with a few percent of another very pure metal added. Zirconium as found in nature is always contaminated with hafnium, a highly undesirable element in nuclear cladding. So natural zirconium has to be purified, an intricate process requiring a high input of useful energy and auxiliary chemicals, because hafnium and zirconium are chemically much alike.
Extraction of uranium
Above observation means that the extraction of uranium from uranium-bearing rock, usually named mining and milling, consumes more energy per kilogram recovered uranium and goes less completely with decreasing ore grade. That implies that the recovery yield (the fraction of uranium present in the rock which is actually extracted) declines with declining ore grade. From rock containing 1 gram per kilogram rock some 95 % can be recovered. At a content of 0.1 g/kg less than 50% of the uranium present in the rock can be recovered, the other 50% are lost in the waste stream (mill tailings). This phenomenon greatly contributes to the phenomenon of the energy cliff [more i38].
The specific energy consumption of the extraction of uranium, measured in energy units per kilogram recovered uranium, is determined by two variables: the dilution factor and the extraction yield. The dilution factor is proportional to the uranium content of the ore: to get hold of 1 kg uranium from ore at a grade of 1 kg U/tonne at least 1 tonne of rock has to be processed, from ore at a grade of 100 g U/tonne at least ten times as much rock has to be processed and at least ten times as much energy is consumed per kg recovered uranium.
On top of the dilution factor comes the declining recovery yield of the extraction process with declining ore grade, and consequently the specific energy consumption per kg recovered uranium rises steeply at low uranium ore grades. As the energy production per kg recovered uranium has a fixed value, the energy invetment of the uranium recovery surpasses the energy content at a given ore grade. This is called the energy cliff. The critical ore grade lies in the range of 0.1-0.2 gram uranium per kg rock, depending on the ore properties.
Of course the phenomenon of steeply rising energy investment per kg metal with declining ore grade is not typical for the recovery of uranium from the earth's crust.
Enrichment of uranium
Natural uranium, at the isotopic composition as found in nature, contains 0.7% of the fissile uranium-235 atoms and 99.3% of the non-fissile uranium-238. For use as nuclear fuel the uranium has to be enriched in U-235 atoms. This involves a physical separation process, based on the slightly different masses of the U-238 and U-235 atoms, which is done by means of diffusion or ultracentrifuge plants. Due to the scantness of the physical differences between the two isotopes, a large number of separation steps are needed to enrich the uranium to the desirable isotopic composition of 2-5% U-238. As a result enrichment is a very energy-intensive process. It is not possible to extract all U-235 atoms from natural uranium, a consequence of the Second Law as pointed out above, Unavoidaby the enrichment process generates a large waste stream of depleted uranium with a lower content of U-235 atoms than natural uranium of 0.2-0.3%.
Reprocessing of spent nuclear fuel is an intricate sequence of separation processes, aimed at the recovery of plutonium and unused uranium from spent fuel. Reprocessing is a pivotal process in several advanced nuclear concepts, such as closed-cycle reactors, breeders, partitioning + transmutation of long-lived radionuclides and nuclear fusion [more i30].
Separation of the involved highly radioactive mixtures, containing dozens of kinds of radionuclides, into pure fractions is impossible, as follows from the Second Law. The separation losses increase with higher radiation levels, due to deterioration of the separation chemicals and equipment, and with a higher number of chemical constituents.
One consequence of the inherently incomplete separation is that all nuclear concepts relying on 100% separation efficiency are doomed to fail.
Another consequence is that a reprocessing plant generates large waste streams, which are larger and more hazardous as the radioactivity of the spent fuel is higher. The separation losses increase with higher radiation levels, due to deterioration of the separation chemicals and equipment.
Figure 42-1. Extraction of uranium from a solution.
To recover uranium from its ore, the rock is ground to a fine powder. By means of chemicals the uranium atoms are dissolved as ions in a watery solution, together with some other elements from the host rock. Then a special mixture of kerosene and chemicals is added in which the uranium ions better dissolve than in the water phase. After some time a distribution equilibrium is established, when the number of uranium ions diffusing from the water phase to the kerosene phase equals the number diffusing in the opposite direction. When the two liquid phases are decanted, inevitably a part of the original amount of uranium ions are left in the water phase and are lost in the waste stream.
Figure 42-2. Extraction yield of uranium from ore.
Maximum attainable extraction yield (also named recovery yield) of uranium from its ore, as function of the uranium content of the ore. The yield is defined as the actually recovered fraction of the uranium as present in the original rock. In practice the yields are generally lower than in this diagram. The world averaged uranium content of the currently exploited ores is 0.5-1 gram uranium per kg rock.
Figure 42-3. Specific energy consumption of the recovery of uranium from ore.
The dilution factor is the relationship between ore grade and energy investment per kg recovered uranium (green dotted line). On top of this comes the effect of the recovery yield, which declines with declining ore grade. A lower yield means that relatively more ore has to be processed to obtain the same amount of uranium and consequently more energy is needed.
Figure 42-4. Isotopic enrichment of uranium, a physical separation.
In an enrichment plant natural uranium is split up into two fractions: enriched uranium containing 2-5% U-238 and depleted uranium containing 0.2-0.3% U-235; in this symbolic diagram 3.3% respectively 0.2%. In practice the U-235 atoms are randomly dispersed between the U-238 atoms. To produce nearly pure uranium-235 (weapons-grade) a much larger amount of natural uranium has to be processed, generating a massive waste stream of depleted uranium.
Figure 42-5. Outline of reprocessing of spent nuclear fuel.
Reprocessing is an intricate sequence of separation processes, aimed at the recovery of newly formed plutonium and remaining uranium from spent nuclear fuel. Due to the inherently incomplete separation, a part of the uranium and plutonium end up in the waste streams, and the recovered uranium and plutonium are contaminated with other nuclides. Purification of the uranium and plutonium generates large waste streams, as a consequence of the high purity specifications of the two metals.
The other radioactive (and non-radioactive) consituents of spent fuel are distributed over large volumes of solid and liquid waste. Gaseous and volatile radionuclides are set free from the spent fuel during the first steps of the reprocessing sequence, are not retained and are released into the human environment. In addition the waste streams originating in the last steps of the separation and purification processes are released into the human environment. Inevitably these waste streams contain all kinds of radionuclides, those with high water-solubility in high concentrations, those with low water-solubility in low concentrations. For above reasons reprocessing is an extremely polluting process.
Virtually nothing has been published on the discharges of non-radioactive pollutants, especially greenhouse gases, by reprocessing plants. From a chemical point of view it seems unlikely that reprocessing would not emit greenhouse gases other than CO2. So the contribution of reprocessing to the nuclear emission of CO2-equivalents remains a well-kept secret.
The volumes of the decommissioning and dismantling waste of a reprocessing plant may amount to hundreds of thousands cubic meters. This waste stream is never mentioned by the nuclear industry. Unavoidably a significant part of this future radioactive waste stream will end up uncontrollably in the human environment. The cost of decommissioning and dismantling of the reprocessing plant at Sellafield (UK) is estimated at some Û100bn, more than the total cost of the American Apollo program, in Û(2010), which resulted in the landing of six crews on the Moon (1969-1972). These preliminary cost estimates, which most likely will turn out to be too low, are an ominous indication of the unheard scale of decommissioning and dismantling a eprocessing plant