Partitioning and transmutation
Radioactivity of spent nuclear fuel
Spent nuclear fuel when removed from a nuclear reactor is a billionfold more radioactive than fresh fuel (enriched uranium). During the first year after removal the radioactivity of the spent fuel falls with a factor 100 by natural decay of short-lived radionuclides. During the next four centuries the radioactivity decreases another factor 100 by natural decay. From then on the radioactivity decreases very slowly: another factor 100 takes ten million years. After ten million years of cooling the specific radioactivity of spent fuel, measured in radioactivity units per kilogram (Bq/kg), is still nearly ten million times higher than the natural level of the human body [more i10].
The idea behind the partitioning & transmutation concept is to reduce the hazards posed by the radioactive content of spent fuel. Theoretically there are three conceivable ways to achieve this aim:
• Conversion of long-lived radionuclides into short-lived radionuclides, Then the waste would remain dangerous for a shorter time, so less demanding permanent storage facilities would be appropiate.
• Conversion of radionuclides into stable nuclides. Then the amount of radioactivity in the nuclear waste would be reduced.
• Conversion of the most dangerous radionuclides into less dangerous ones. Then the radiotoxicity of the nuclear waste would be reduced.
If the long-lived radionuclides could be converted into short-lived or stable nuclides, the concentrations of the long-lived dangerous nuclides in the remaining wastes could be reduced to below an official standard, so it would be safe for release the waste into the environment after a storage time of only a few centuries. At least, the amount of high level waste to be stored permanently in a geologic repository would be reduced to a small fraction of the spent fuel.
The conversion of atoms by neutron capture into other atoms is called transmutation.In an operating nuclear reactor, where free neutrons are present, radionuclides can be converted into other radionuclides or into stable nuclides. Likewise the reverse process works out: stable nuclides are converted into radioactive nuclides. The net result is always an increase of the radioactivity of the reactor and its contents. In fact the generation of plutonium from uranium-238 in a common nuclear reactor is a transmutation process, but also activation, the unintended process by which non-radioactive construction materials become radioactive. Neutron capture is a random process: it occurs by chance, because the randomly moving neutrons in a reactor cannot be directed at a specific nuclide.
The nuclear industry focuses its research on the transmutation of the so-called 'minor actinides', that are the heavy radionuclides which are formed by neutron capture of plutonium nuclides. The designation 'minor' regards the relatively small amounts of these radionuclides, compared to the amount of plutonium [more i11]. The minor actinides have long half-lifes (thousands to millions of years) and are extremely dangerous in the human body.
The nuclear industry asserts to be able to fission the minor actinides, so they will contribute significantly to the nuclear energy production. However, the amounts of fissile minor actinides are negligible compared to the amounts of fissile uranium and plutonium, even in closed-cyclereactor systems, such as the breeder [more i33].
Apart from the minor actinides a number of other long-lived radionuclides are present in nuclear waste, which seem to be ignored by the nuclear industry.
For reason of the fact that neutron capture is a random, non-selective process, the spent fuel has to separated into a number of partitions: uranium, plutonium, actinides other than plutonium, lanthanides plus one or two other partitions. Partitioning is required to remove as much as possible those radionuclides or stable nuclides which would generate unwanted radionuclides in the transmutation cycle, or would interfere otherwise.
Radioactive isotopes cannot be separated from non-radioactive isotopes of the same chemical element, because partitioning is based on chemical separation processes. In fact partitioning is a more complicated version of reprocessing of spent fuel [more i31]. Another word does not mean another process with less problems.
A partitioning & transmutation system consists of a cycle of three components: a transmuter reactor, a partitioning plant and a plant for fabrication of fuel elements and target elements. The fuel elements are needed for a sustained fission and neutron generation in the reactor, the target elements contain the radionuclides te bo irradiated by neutrons for transmutation. After a certain period in the transmuter reactor, the target elements have to be removed and to treated in the partioning plant again, to separate fission products from it. From the remaining amount of transmutable radionuclides new target elements are to be fabricated, which can be placed in the transmuter reactor.
The partitioning & transmutation cycle is much alike the breeder cycle, albeit more demanding. All three components have to work nearly flawlessly and finely tuned to each other, during many decades, if the system were to work as planned.
The target elements, containing the radionuclides to be transmuted, have to be recycled repeatedly. Each cycle increasing the radioactivity of the material will increase and the concentration of radionuclides exhibiting spontaneous fission will rise. This will render the material unmanageable and will increase the chance of criticality incidents in the separation processes, igniting small nuclear explosions.
Even if the transmutation system would work as advertised, it would have its limitations:
• On nuclear-physical grounds only a limited number of the many kinds of long-lived radionuclides in nuclear waste are candidate for transmutation, so the waste reduction would be marginal at best.
• It would take centuries (!) to reduce a given quantity of a specific radionuclide with a factor 100, Only a small fraction of it can be transmuted in one cycle, so the amount has to pass through many cycles to achieve a significant reduction.
The concept of partitioning & transmutation is implicitely based on the assumed availability of 100% perfect materials and 100% complete separation processes. None of these two conditions could be met, as follows from the Second Law of thermodynamics [more i42, i43]. By virtue of this law can be argued beforehand that the transmutation cycle will not work as advertised. Worse still, it would be counterproductive:
• A transmutation system would produce more radioactivity than it starts with and would generate more long-lived radionuclides, of other kinds, than it would reduce.
• The energy consumption of a transmutation system would be prohibitive. The partitioning and target fabrication have to be performed robotically, if it would work at all, due to the extremely high radioactivity of the materials. The energy production by fission of the actinides would be negligible.
• As a result of the increase of radioactivity in the transmutation cycle, the required long operation periods and the increased number of facilities containing highly radioactive materials, intended and unintended discharges of radioactivity into the human environment would sharply increase by the implementation of the transmutation cycle.
The partition & transmutation concept is unfeasible due to fundamental technical limitations. The question rises for what reasons the nuclear industry promotes this concept.
Figure 34-1. Partitioning and transmutation system outline.
A partitioning and transmutation system is in concept similar to the breeder system concept. The input of the transmutation cycle would comprise uranium, chemicals and spent nuclear fuel, containing fission products, activation products and actinides. The output of the system would consist of more fission products and activation products than went into the system and less actinides than the input, if the cycle would work. In addition a appreciable part of the radioactive content of the transmuter cycle would be discharged into the human environment. What to do with the highly radioactive output of the system remains an unanswered question in the proposals of the nuclear industry.
Figure 34-2. Partitioning of spent fuel.
Actually the partitioning of spent fuel and irradiated targets would be a more complicated version of reprocessing. More demanding because the spent fuel would have to be separated into more pure fractions than in a conventional reprocessing procedure and because of the extremely high radioactivity of the irradiated targets. Partitioning would produce large volumes of radioactive waste, a part of which would released into the human environment.