Nuclear fuel cycle

Like nuclear weapons, which also use unmoderated or "fast" neutrons, these fast-neutron reactors require much higher concentrations of fissile isotopes in order to sustain a chain reaction.

The natural concentration (0.71%) of the fissile isotope U-235 is less than that required to sustain a nuclear chain reaction in light water reactor cores.

Accordingly, UF6 produced from natural uranium sources must be enriched to a higher concentration of the fissionable isotope before being used as nuclear fuel in such reactors.

The bulk (96%) of the byproduct from enrichment is depleted uranium (DU), which can be used for armor, kinetic energy penetrators, radiation shielding and ballast.

While transport casks vary in design, material, size, and purpose, they are typically long tubes made of stainless steel or concrete with the ends sealed shut to prevent leaks.

SIMFUEL is the name given to the simulated spent fuel which is made by mixing finely ground metal oxides, grinding as a slurry, spray drying it before heating in hydrogen/argon to 1700 °C.

Because of the thermal gradient which exists in the fuel during use, the volatile fission products tend to be driven from the centre of the pellet to the rim area.

In addition the isotope signature of a hypothetical accident may be very different from that of a planned normal operational discharge of radioactivity to the environment.

For example, caesium (Cs) binds tightly to clay minerals such as illite and montmorillonite, hence it remains in the upper layers of soil where it can be accessed by plants with shallow roots (such as grass).

Note that a source of data on the subject of caesium in Chernobyl fallout exists at [1] (Ukrainian Research Institute for Agricultural Radiology).

The IAEA states that under a series of different conditions different amounts of the core inventory can be released from the fuel, the four conditions the IAEA consider are normal operation, a spike in coolant activity due to a sudden shutdown/loss of pressure (core remains covered with water), a cladding failure resulting in the release of the activity in the fuel/cladding gap (this could be due to the fuel being uncovered by the loss of water for 15–30 minutes where the cladding reached a temperature of 650–1250 °C) or a melting of the core (the fuel will have to be uncovered for at least 30 minutes, and the cladding would reach a temperature in excess of 1650 °C).

Each tube can be individually isolated and refueled by an operator-controlled fueling machine, typically at a rate of up to 8 channels per day out of roughly 400 in CANDU reactors.

This increase in efficiency is partially offset by the added complexity of having hundreds of pressure tubes and the fueling machines to service them.

The spent fuel rods are usually stored in water or boric acid, which provides both cooling (the spent fuel continues to generate decay heat as a result of residual radioactive decay) and shielding to protect the environment from residual ionizing radiation, although after at least a year of cooling they may be moved to dry cask storage.

As an alternative to the disposal of the PUREX raffinate in glass or Synroc matrix, the most radiotoxic elements could be removed through advanced reprocessing.

After separation, the minor actinides and some long-lived fission products could be converted to short-lived or stable isotopes by either neutron or photon irradiation.

Strong and long-term international cooperation, and many decades of research and huge investments remain necessary before to reach a mature industrial scale where the safety and the economical feasibility of partitioning and transmutation (P&T) could be demonstrated.

The highly radioactive medium-lived fission products Cs-137 and Sr-90 diminish by a factor of 10 each century; while the long-lived fission products have relatively low radioactivity, often compared favorably to that of the original uranium ore. Horizontal drillhole disposal describes proposals to drill over one kilometer vertically, and two kilometers horizontally in the Earth's crust, for the purpose of disposing of high-level waste forms such as spent nuclear fuel, Caesium-137, or Strontium-90.

Spent fuel includes fission products, which generally must be treated as waste, as well as uranium, plutonium, and other transuranic elements.

Due to current particle accelerators not being optimized for long continuous operation at least the first generation of accelerator-driven sub-critical reactor is unlikely to be able to maintain a constant operation period for equally long times as a critical reactor, and each time the accelerator stops then the fuel will cool down.

As a consequence, it is important that the chosen matrix allows the reactor to keep the ratio of fissile to non-fissile nuclei high, as this enables it to destroy the long-lived actinides safely.

In contrast, the power output of a sub-critical reactor is limited by the intensity of the driving particle accelerator, and thus it need not contain any uranium or plutonium at all.

The promising strategy that consists of utilising plutonium and minor actinides using a once-through fuel approach within existing commercial nuclear power reactors e.g.

[44][45] This option has the advantage of reducing the plutonium amounts and potentially minor actinide contents prior to geological disposal.

An advantage of mixing the actinides with uranium and plutonium is that the large fission cross sections of 235U and 239Pu for the less energetic delayed neutrons could make the reaction stable enough to be carried out in a critical fast reactor, which is likely to be both cheaper and simpler than an accelerator driven system.

[57] An extraction rate of kilotons of U per year over centuries would not modify significantly the equilibrium concentration of uranium in the oceans (3.3 ppb).

[citation needed] Under these optimal conditions the consumption of natural uranium would be 7 tons per year and per gigawatt (GW) of produced electricity.

The coupling of uranium extraction from the sea and its optimal utilisation in a molten salt fast reactor should allow nuclear energy to gain the label renewable.

[61] The main thorium-bearing mineral, monazite is currently mostly of interest due to its content of rare earth elements and most of the thorium is simply dumped on spoils tips similar to uranium mine tailings.

The term nuclear fuel is not normally used in respect to fusion power, which fuses isotopes of hydrogen into helium to release energy.

The nuclear fuel cycles describes how nuclear fuel is extracted, processed, used, and disposed of
The lifecycle of fuel in the present US system. If put in one place the total inventory of spent nuclear fuel generated by the commercial fleet of power stations in the United States, would stand 7.6 metres (25 ft) tall and be 91 metres (300 ft) on a side, approximately the footprint of one American football field . [ 1 ] [ 2 ]
Nuclear fuel cycle begins when uranium is mined, enriched and manufactured to nuclear fuel (1) which is delivered to a nuclear power plant. After usage in the power plant the spent fuel is delivered to a reprocessing plant (if fuel is recycled) (2) or to a final repository (if no recycling is done) (3) for geological disposition. In reprocessing 95% of spent fuel can be recycled to be returned to usage in a nuclear power plant (4).
The solid state structure of uranium dioxide, the oxygen atoms are in green and the uranium atoms in red
Temperature profile for a 20 mm diameter fuel pellet with a power density of 1 kW per cubic meter. The fuels other than uranium dioxide are not compromised.
A once through (or open) fuel cycle
A fuel cycle in which plutonium is used for fuel
The integral fast reactor concept (color), with the reactor above and integrated pyroprocessing fuel cycle below. A more detailed animation and demonstration is available. [ 36 ]
IFR concept (Black and White with clearer text)
A pair of fuel cycles in which uranium and plutonium are kept separate from the minor actinides. The minor actinide cycle is kept within the green box.