Spent nuclear fuel
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). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.[1]
Nuclear fuel rods become progressively more radioactive (and less thermally useful) due to neutron activation as they are fissioned, or "burnt", in the reactor. A fresh rod of low enriched uranium pellets (which can be safely handled with gloved hands) will become a highly lethal gamma emitter after 1–2 years of core irradiation, unsafe to approach unless under many feet of water shielding. This makes their invariable accumulation and safe temporary storage in spent fuel pools a prime source of high level radioactive waste and a major ongoing issue for future permanent disposal.
Nature of spent fuel[edit]
Nanomaterial properties[edit]
In the oxide fuel, intense temperature gradients exist that 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 many small bubble-like pores that form during use; the fission product 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 135
Cs.
In the case of mixed oxide (MOX) fuel, the xenon tends to diffuse out of the plutonium-rich areas of the fuel, and it is then trapped in the surrounding uranium dioxide. The neodymium tends to not be mobile.
Also metallic particles of an alloy of Mo-Tc-Ru-Pd tend 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 non-radioactive "uranium active" simulation of spent oxide fuel exists.[2]
Fission products[edit]
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) and the other later in the periodic table (I, Xe, Cs, Ba, La, Ce, Nd). Many of the fission products are either non-radioactive or only short-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 segregating 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; 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 increase the thermal conductivity of the fuel.[3]
Spent fuel corrosion[edit]
Noble metal nanoparticles and hydrogen[edit]
According to the work of corrosion electrochemist David W. Shoesmith,[7][8] 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 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 that is consumed.