Plutonium processing Radioactive decay

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

Elements slightly heavier than uranium, produced by radioactive decay (see later), are found in tiny amounts in natural uranium ores. Plutonium (element 94) has also been found in nature, a product of the element-forming processes that happen in dying stars. So it is a tricky matter to put a precise number on the natural elements. 

Plutonium is formed and transmuted through radioactive decay. Three common types of radioactive processes involve the release of alpha or beta particles or gamma rays. Alpha decay results in the release of an alpha particle, which is a charged particle emitted from the nucleus of an atom having a mass and charge equal in magnitude to a helium nucleus (i.e., two protons and two neutrons). In alpha decay, the atomic mass of the nuclide is reduced by four and the atomic number by two. For example, plutonium-239 undergoes alpha decay to form uranium- 235. 

Plutonium does not undergo transformation processes in the air beyond those related to radioactive decay. Radioactive decay will be important for the short-lived isotopes with half-lives less than the average residence time in the troposphere of approximately 60 days. For example, plutonium-237 has a half-life of 46 days and undergoes electron capture to form neptunium-237 which has a half-life of 2.1x10 years (Nero 1979). Therefore, neptunium-237 may form in the stratosphere prior to deposition of plutonium-237 on the earth s surface as fallout. 

The other more short-lived actinides must be made synthetically by using high-energy collisions in a particle accelerator. These machines collide a particle such as a gamma ray with an atom of the naturally formed actinide elements. They split after collision the other elements are formed in the process of radioactive decay. The first of these new elements were named after the planets in a similar fashion to uranium (planet Uranus) — neptunium (Neptune) and plutonium (Pluto). The rest have been named for historical themes or places in which they were first created. 

Because PUREX does not need process salting to work efficiently and because solvent and aqueous reagent streams can be reclaimed for recycle, the volumes of waste fluids are greatly reduced. The FP fractions can be discharged in the concentrated form to holding tanks for the radioactive decay prior to final vitrification and disposal. The process system works very reliably and produces a plutonium product with a decontamination factor of FPs >10. The uranium fraction has a similar decontamination factor. 

Fig. 123.1 The mean radius of earth is about 3,960 mi (6,373 km). By contrast the radius of the georeactor is only about 6 mi (10 km), although there is much uncertainty in that estimate. The georeactor, thought to reside within the inner core at the center of earth, is comparably simple in stmcture. The georeactor sub-core consists of the actinide fuel, the uranium and heaver fissionable elements, such as plutonium, formed by the fission process. The surrounding sub-sheU, which is thought to be liquid or a slurry, consists of radioactive decay products and fission products. Heat produced by nuclear fission in the sub-core, causes convection in the sub-shell which will interact with the CotioUs forces produced by planetary rotation and act like a dynamo, a magnetic amplifier, generating the geomagnetic field (Herndon, 2007,2009). (Source Maverick s Earth and Universe, Herndon 2008, with permission)...

The half-Ufe for radioactive decay (a first-order process) of plutonium-239 is 24,000 years. How many years does it take for one mole of this radioactive material to decay until just one atom remains ... 

Plutonium exists in trace amounts in nature. Most of it isotopes are radioactive and manmade or produced by the natural decay of uranium. Plutonium-239 is produced in nuclear reactors by bombarding uranium-238 with deuterons (nuclei of deuterium, or heavy hydrogen). The transmutation process is as follows + deuterons—> 2 nuclei + Np + p— ... 

Americium does not exist in nature. All of its isotopes are man-made and radioactive. Americium-241 is produced by bombarding plutonium-239 with high-energy neutrons, resulting in the isotope plutonium-240 that again is bombarded with neutrons and results in the formation of plutonium-241, which in turn finally decays into americium-241 by the process of beta decay. Both americium-241 and americium-243 are produced within nuclear reactors. The reaction is as follows Pu + (neutron and X gamma rays) —> " Pu + (neutron and X gamma rays) —> Pu—> Am + beta minus ([ -) followed by " Am—> jNp-237 + Hej (helium nuclei). 

Einsteinium does not exist in nature and is not found in the Earth s crust. It is produced in small amounts by artificial nuclear transmutations of other radioactive elements rather than by additional explosions of thermonuclear weapons. The formation of einsteinium from decay processes of other radioactive elements starts with plutonium and proceeds in five steps as follows ... 

Now imagine the problem with nuclear waste. We cannot alter the rate at which it decays. This is defined by the half-life. We can t heat it, stir it, or add a catalyst to speed up the process as we can with chemical reactions. Furthermore, the half-lives of many nuclear waste products are very long plutonium, for example, has a half-life in excess of 24,000 years. Ten half-hves are required for the radioactivity of a substance to reach background levels. So we are talking about a very long storage time. 

In Section 2.5, we discussed the production of energy by nuclear fission, and the reprocessing of nuclear fuels. We described how short-lived radioactive products decay during pond storage, and how uranium is converted into [U02][N03]2 and, finally, UFg. One of the complicating factors in this process is that the fuel to be reprocessed contains plutonium and fission products in addition to uranium. Two dilferent solvent extraction processes are needed to elfect separation. 

Plutonium is a radioactive element. Radioactive elements are those that undergo spontaneous transformation (decay) in which energy is released (emitted) either in the form of particles, such as alpha or beta particles, or waves, such as gamma or X-ray. This transformation or decay results in the formation of new elements, some of which may themselves be radioactive, in which case they will also decay. The process continues until a stable (nonradioactive) state is reached (see Appendix B for more information). 

This process happens slowly since the half-lives of plutonium isotopes tend to be relatively long Pu-238 has a half-life of 87.7 years Pu-239 has a half-life is 24,100 years, and Pu-240 has a half-life of 6,560 years. The decay process continues until a stable, non-radioactive element is... 

The SNF (after a cooling period to allow for decay of short-lived radionuclides) is chopped up and dissolved in nitric acid. The gasses emitted in the process are treated to avoid their release to the environment. The solution is filtered to separate the insoluble residues and sent to the solvent extraction stage in which the uranium and plutonium are extracted into the organic phase (usually TBP in a hydrocarbon solvent) and the fission products and minor actinides remain in the aqueous phase. The radioactive fission products may then be treated as high-level-waste while the uranium and plutonium are then separated from each other by selective back-extraction. 

A radioactive element, like everything else in life, decays (ages). When uranium or plutonium decays over billions of years, they go through a transformation process of degrading into lower energy element forms until they settle into one that is stable. 

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