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Spent fuel pool at a nuclear power plant.
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant) to the point where it is no longer useful in sustaining a nuclear reaction.

Nuclear reprocessing can separate spent fuel into various combinations of reprocessed uranium, plutonium, minor actinides, fission products, remnants of zirconium or steel cladding, activation products, and the reagents or solidifiers introduced in the reprocessing itself.

Alternatively, the intact Spent Nuclear Fuel (SNF) can be disposed as radioactive waste. The United States planned disposal in deep geological formations, such as the Yucca Mountain nuclear waste repositorymarker, where it has to be shielded and packaged to prevent its migration to mankind's immediate environment for thousands of years. On March 5, 2009, Energy Secretary Steven Chu told a Senate hearing "the Yucca Mountain site no longer was viewed as an option for storing reactor waste."

Nature of spent fuel

Large John H Radioactive Decay Characteristics of Irradiated Nuclear Fuels, January 2006.

Nanomaterial properties

Spent low enriched uranium nuclear fuel is an example of a nanomaterial which existed before the term nano became fashionable. In the oxide fuel intense temperature gradients exist which cause fission products to migrate. The zirconium tends to move to the centre of the fuel pellet where the temperature is highest while the lower boiling fission products move to the edge of the pellet. The pellet is likely to contain lots of small bubble like pores which form during use, the fission xenon migrates to these voids. Some of this xenon will then decay to form caesium, hence many of these bubbles contain a large concentration of 137Cs.





In the case of the MOX the xenon tended to diffuse out of the plutonium rich areas of the fuel, and it was then trapped in the surrounding uranium dioxide. The neodymium tended to not be mobile.

Also metallic particles of an alloy of Mo-Tc-Ru-Pd tends to form in the fuel. Other solids form at the boundary between the uranium dioxide grains, but the majority of the fission products remain in the uranium dioxide as solid solutions. A paper describing a method of making a nonradioactive uranium active) simulation of spent oxide fuel exists.Microstructural features of SIMFUEL - Simulated high-burnup UO2-based nuclear fuel, P.G. Lucuta, R.A. Verrall, Hj. Matzke and B.J. Palmer, Journal of Nuclear Materials, 1991, 178, 48–60.

Fission products

  • 3% of the mass consists of fission products of 235U and 239Pu (also indirect products in the decay chain); these are considered radioactive waste or may be separated further for various industrial and medical uses. The fission products include every element from zinc through to the lanthanides, much of the fission yield is concentrated in two peaks, one in the second transition row (Zr, Mo, Tc, Ru, Rh, Pd, Ag) while the other is later in the periodic table (I, Xe, Cs, Ba, La, Ce, Nd). Many of the fission products are either non radioactive or only shortly lived radioisotopes. But a considerable number are medium to long lived radioisotopes such as 90Sr, 137Cs, 99Tc and 129I. Research has been conducted by several different countries into partitioning the rare isotopes in fission waste including the fission platinoids (Ru, Rh, Pd) and silver (Ag) as a way of offsetting the cost of reprocessing, however this is not currently being done commercially.


The fission products can modify the thermal properties of the uranium dioxide, the lanthanide oxides tend to lower the thermal conductivity of the fuel while the metallic nanoparticles slightly increases the thermal conductivity of the fuel.


Table of chemical data

The chemical forms of fission products in uranium dioxide
Element Gas Metal Oxide Solid solution
Br Kr Yes - - -
Rb Yes - Yes -
Sr - - Yes Yes
Y - - - Yes
Zr - - Yes Yes
Nb - - Yes -
Mo - Yes Yes -
Tc Ru Rh Pd Ag Cd In Sb - Yes - -
Te Yes Yes Yes Yes
I Xe Yes - - -
Cs Yes - Yes -
Ba - - Yes Yes
La Ce Pr Nd Pm Sm Eu - - - Yes


Plutonium

  • About 1% of the mass is 239Pu and 240Pu resulting from conversion of 238U, which may either be considered a useful byproduct, or as dangerous and inconvenient waste. One of the main concerns regarding nuclear proliferation is to prevent this plutonium from being used by states, other than those already established as nuclear weapons states, to produce nuclear weapons. If the reactor has been used normally, the plutonium is reactor-grade, not weapons-grade: it contains much 240Pu and less than 80% 239Pu, which makes it less suitable, but not impossible, to use in a weapon. If the irradiation period has been short then the plutonium is weapon-grade (more than 80%, up to 93%).


Uranium

  • 96% of the mass is the remaining uranium: most of the original 238U and a little 235U. Usually 235U would be less than 0.83% of the mass along with 0.4% 236U.


Reprocessed uranium will contain 236U which is not found in nature; this is one isotope which can be used as a fingerprint for spent reactor fuel.

If using a thorium fuel to produce fissile U-233, the SNF will have U-233, with a half-life of 1.592E5 years. This will have an impact in the long term radioactive decay of the spent fuel. If compared with MOX fuel, the activity around 10E5 in the cycles with thorium will be higher due to the presence of the not fully decayed U-233.

Minor actinides

  • Traces of the minor actinides are present in spent reactor fuel. These are actinides other than uranium and plutonium. These include neptunium, americium and curium. The amount formed depends greatly upon the nature of the fuel used and the conditions under which it was used. For instance, the use of MOX fuel (239Pu in a 238U matrix) is likely to lead to the production of more 241Am and heavier nuclides than a uranium/thorium based fuel (233U in a 232Th matrix).


For natural uranium fuel:Fissile component starts at 0.71% 235U concentration in natural uranium). At discharge, total fissile component is still 0.50% (0.23% 235U, 0.27% fissile 239Pu, 241Pu) Fuel is discharged not because fissile material is fully used-up, but because the neutron-absorbing fission products have built up and the fuel becomes significantly less able to sustain a nuclear reaction.

Some natural uranium fuels use chemically active cladding, such as Magnox, and need to be reprocessed because long-term storage and disposal is difficult.

For highly enriched fuels used in marine reactors and research reactors the isotope inventory will vary based on in-core fuel management and reactor operating conditions.

Fuel composition and long term radioactivity

Activity of U-233 for three fuel types
Total activity for three fuel types


Long-lived radioactive waste form the back end of the fuel cycle is especially relevant when designing a complete waste management plan for SNF. When looking at long term radioactive decay, the actinides in the SNF have a significant influence due to their characteristically long half-lives. Depending on what a nuclear reactor is fueled with, the actinide composition in the SNF will be different.

An example of this effect is the use of nuclear fuels with thorium. Th-232 is a fertile material that can undergo a neutron capture reaction and two beta minus decays, resulting in the production of fissile U-233. The SNF of a cycle with thorium will contain U-233, an isotope with a half-life of 1.59E5 years. Its radioactive decay will strongly influence the long-term activity curve of the SNF around 10E5 years. A comparison of the activity associated to U-233 for three different SNF types can be seen in the figure on the top right.

The burnt fuels are Thorium with Reactor-Grade Plutonium (RGPu), Thorium with Weapons-Grade Plutonium (WGPu) and Mixed Oxide fuel (MOX). For RGPu and WGPu, the initial amount of U-233 and its decay around 10E5 years can be seen. This has an effect in the total activity curve of the three fuel types. The absence of U-233 and its daughter products in the MOX fuel results in a lower activity in region 3 of the figure on the bottom right, whereas for RGPu and WGPu the curve is maintained higher due to the presence of U-233 that has not fully decayed.

The use of different fuels in nuclear reactors results in different SNF composition, with varying activity curves.

Spent fuel corrosion

Noble metal nanoparticles and hydrogen

According to the work of the corrosion electrochemist Shoesmith the nanoparticles of Mo-Tc-Ru-Pd have a strong effect on the corrosion of uranium dioxide fuel. For instance his work suggests that when the hydrogen (H2) concentration is high (due to the anaerobic corrosion of the steel waste can) the oxidation of hydrogen at the nanoparticles will exert a protective effect on the uranium dioxide. This effect can be thought of as an example of protection by a sacrificial anode where instead of a metal anode reacting and dissolving it is the hydrogen gas which is consumed.

See also



References

  1. [1] Testimony of Robert Meyers Principal deputy Assistant Administrator for the Office of Air and Radiation U.S. Environmental Protection Agency before the subcommitee on Energy and Air Quality Committee on Energy and Commerce U. S. House of Representatives, July 15, 2008
  2. [2]
  3. Dong-Joo Kim, Jae-Ho Yang, Jong-Hun Kim, Young-Woo Rhee, Ki-Won Kang, Keon-Sik Kim and Kun-Woo Song, Thermochimica Acta, 2007, 455, 123–128.



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