Introduction

The various activities associated with the production of electricity from nuclear reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of used fuel as an option for nuclear energy, the stages form a true cycle.

Uranium

The nuclear fuel cycle.

Uranium is a slightly radioactive metal that occurs throughout the Earth's crust. It is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in many rivers and sea water. It is, for example, found in concentrations of about four parts per million (ppm) in granite, which makes up 60% of the Earth's crust. In fertilizers, uranium concentration can be as high as 400 ppm (0.04%), and some coal deposits contain uranium at concentrations greater than 100 ppm (0.01%). Most of the radioactivity associated with uranium in nature is in fact due to other minerals derived from it by radioactive decay processes, and which are left behind in mining and milling.

There are a number of areas around the world where the concentration of uranium in the ground is sufficiently high that extraction of it for use as nuclear fuel is economically feasible. Such concentrations are called ore.

Uranium Mining

Both excavation and in situ techniques are used to recover uranium ore. Excavation may be underground or open pit mining.

In general, open pit mining is used where deposits are close to the surface and underground mining is used for deep deposits, typically greater than 120 meters deep. Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. As a result, the quantity of material that must be removed in order to access the ore may be large. Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine.

An increasing proportion of the world's uranium now comes from in situ leaching (ISL), where oxygenated groundwater is circulated through a very porous orebody to dissolve the uranium and bring it to the surface. ISL may be with slightly acid or with alkaline solutions to keep the uranium in solution. The uranium is then recovered from the solution as in a conventional mill.

The decision as to which mining method to use for a particular deposit is governed by the nature of the orebody, and safety and economic considerations.

In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect against airborne radiation exposure.

Uranium Milling

In situ uranium leaching. (Source: WISE Uranium Project)

A uranium mill is a chemical plant that extracts uranium from mined ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. At conventional mills, the ore arrives via truck and is crushed and leached. In most cases, sulfuric acid is the leaching agent, but alkaline leaching can also be done. The leaching agent not only extracts uraniumfrom the ore but also several other constituents: vanadium, selenium, iron, lead, and arsenic. Conventional mills extract 90 to 95 percent of the uranium from the ore. Mills are typically in areas of low population density, and they process ores from mines within 50 kilometers (30 miles). Milling produces a uranium oxide concentrate that is then shipped from the mill. It is sometimes referred to as yellowcake and generally contains more than 80% uranium. The original ore may contains as little as 0.1% uranium

In situ leach (ISL) facilities are one means of extracting uranium from underground. ISL facilities recover uranium from low grade ores that may not be economically recoverable by other methods. In this process, a leaching agent, such as oxygen with sodium carbonate, is injected through wells into the ore body to dissolve the uranium. The leach solution is pumped from there to the processing plant and ion exchange separates the uranium from the solution. After additional purification and drying, the yellowcake is placed in 55-gallon drums.

Potential Industrial Hazards and Wastes from Milling

Because the uranium is not enriched, there is no criticality hazard and little fire or explosive hazard for it. The ISL process does present a fire hazard, however. The primary hazards associated with milling operations are occupational hazards found in any metal milling operation that uses chemical extraction plus the chemical toxicity of the uranium itself.

Radiological hazards are low at these facilities as uranium has little penetrating radiation and only moderate non-penetrating radiation. The primary radiological hazard is due to the presence of radium in the uranium decay chains and the production of radon gas from the decay of radium and radon progeny (short-lived radon decay products).

The solid (sandy) waste from the conventional uranium milling process is called mill tailings. Uranium mill tailings, which contain most of the progeny of uranium, are a significant source of radon and radon progeny releases to the environment. The hazards from radon involve inhalation of radon progeny that may be deposited in the respiratory tract. Alpha radiation could be emitted into those tissues and can pose a cancer risk to those workers.

Conversion

The product of a uranium mill is not directly usable as a fuel for a nuclear power reactor. After the yellowcake is produced at the mill, the next step is conversion into pure uranium hexafluoride (UF6) gas suitable for use in enrichment operations. During this conversion, impurities are removed and the uranium is combined with fluorine to create the UF6 gas. The UF6 is then pressurized and cooled to a liquid. In its liquid state it is drained into 14-ton cylinders where it solidifies after cooling for approximately five days. The UF6 cyclinder, in the solid form, is then shipped to an enrichment plant. UF6 is the only uranium compound that exists as a gas at a suitable temperature.

As with mining and milling, the primary risks associated with conversion are chemical and radiological. Strong acids and alkalis are used in the conversion process, which involves converting the yellowcake (uranium oxide) powder to very soluble forms, leading to possible inhalation of uranium. In addition, conversion produces extremely corrosive chemicals that could cause fire and explosion hazards.

Enrichment

Natural uranium consists, primarily, of a mixture of two isotopes (atomic forms) of uranium. Only 0.7% of natural uranium is "fissile", or capable of undergoing fission, the process by which energy is produced in a nuclear power reactor. The fissile isotope of uranium is uranium-235 (²³⁵U); the remainder is uranium-238 (²³⁸U).

In the most common types of nuclear reactors, a higher-than-natural concentration of ²³⁵U is required. The enrichment process produces this higher concentration, typically between 3.5% and 5% ²³⁵U, by removing over 85% of the ²³⁸U. This is done by separating gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium. The other stream is progressively depleted in ²³⁵U and is called 'tails'.

There are two enrichment processes in large-scale commercial use, each of which uses uranium hexafluoride as feed: gaseous diffusion and gas centrifuge. They both use the physical properties of molecules, specifically the 1% mass difference, to separate the isotopes. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide.

Gaseous diffusion

Source: U.S. Nuclear Regulatory Commission

In the gaseous diffusion enrichment plant, the solid uranium hexafluoride (UF6) from the conversion process is heated in its container until it becomes a liquid. The container becomes pressurized as the solid melts and UF6 gas fills the top of the container. The UF6 gas is slowly fed into the plant’s pipelines where it is pumped through special filters called barriers or porous membranes. The holes in the barriers are so small that there is barely enough room for the UF6 gas molecules to pass through. The isotope enrichment occurs when the lighter UF6 gas molecules (with the U-234 and U-235 atoms) tend to diffuse faster through the barriers than the heavier UF6 gas molecules containing U-238. One barrier isn’t enough, though. It takes many hundreds of barriers, one after the other, before the UF6 gas contains enough uranium-235 to be used in reactors. At the end of the process, the enriched UF6 gas is withdrawn from the pipelines and condensed back into a liquid that is poured into containers. The UF6 is then allowed to cool and solidify before it is transported to fuel fabrication facilities where it is turned into fuel assemblies for nuclear power reactors.

The primary hazard in gaseous diffusion plants include the chemical and radiological hazard of a UF6 release, and the potential for mishandling the enriched uranium, which could create a criticality accident (inadvertent nuclear chain reaction).

Gas centrifuge

The gas centrifuge uranium enrichment process uses a large number of rotating cylinders in series and parallel formations. Centrifuge machines are interconnected to form trains and cascades. In this process, UF6 gas is placed in a cylinder and rotated at a high speed. This rotation creates a strong centrifugal force so that the heavier gas molecules (containing U-238) move toward the outside of the cylinder and the lighter gas molecules (containing U-235) collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. Significantly more U-235 enrichment can be obtained from a single unit gas centrifuge than from a single unit gaseous diffusion stage.

Fuel fabrication

Nuclear fuel fabrication converts the enriched UF6 into fuel for nuclear power reactors. Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide that is sintered (baked) at a high temperature (over 1400ƒ C). The pellets are then encased in metal tubes to form fuel rods, which are then arranged into a fuel assembly and ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of fuel bundles.

In a fuel fabrication plant, great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel, criticality is highly unlikely, but in plants handling special fuels for research reactors, this is a vital consideration.

Power generation

Inside a nuclear reactor the nuclei of uranium-235 atoms split (fission) and, in the process, release energy. This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator that produces electricity. Some of the uranium-238 in the fuel is turned into plutonium in the reactor core. The main plutonium isotope is also fissile and it yields about one third of the energy in a typical nuclear reactor. The fissioning of uranium is used as a source of heat in a nuclear power plant in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant.

Used fuel

With time, the concentration of fission fragments and heavy elements formed in the same way as plutonium in a fuel bundle will increase to the point where it is no longer practical to continue to use the fuel. So, after 12-24 months the 'spent fuel' is removed from the reactor. The amount of energy produced from a fuel bundle varies with the type of reactor and the policy of the reactor operator.

Typically, more than 36 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic meters of gas.

Used fuel storage

When removed from a reactor, a fuel bundle will be emitting both radiation, principally from the fission fragments, and heat. Used fuel is unloaded into a storage pond immediately adjacent to the reactor to allow the radiation levels to decrease. In the ponds, the water shields the radiation and absorbs the heat. Used fuel is held in such pools for several months to several years.

Depending on policies in particular countries, some used fuel may be transferred to central storage facilities. Ultimately, used fuel must either be reprocessed or prepared for permanent disposal.

Reprocessing

Used fuel is about 95% uranium-238 but it also contains about 1% uranium-235 that has not fissioned, about 1% plutonium and 3% fission products, which are highly radioactive, with other transuranic elements formed in the reactor. In a reprocessing facility the used fuel is separated into its three components: uranium, plutonium and waste, containing fission products. Reprocessing enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste (compared with treating all used fuel as waste).

Uranium and Plutonium Recycling

The uranium from reprocessing, which typically contains a slightly higher concentration of ²³⁵U than occurs in nature, can be reused as fuel after conversion and enrichment, if necessary. The plutonium can be directly made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides are combined.

In reactors that use MOX fuel, plutonium substitutes for the ²³⁵U found in normal uranium oxide fuel.

Used fuel disposal

At present, there are no disposal facilities (as opposed to storage facilities) in operation in which used fuel, not destined for reprocessing, and the waste from reprocessing can be placed. Although many technical issues related to disposal have been addressed, there is currently no pressing technical need to establish such facilities, as the total volume of such wastes is relatively small. Further, the longer the wastes are stored, the easier they are to handle due to the progressive diminution of radioactivity. There is also a reluctance to dispose of used fuel because it represents a significant energy resource that could be reprocessed at a later date to allow recycling of the uranium and plutonium. Technical issues aside, opponents of nuclear power point to the lack of a long term disposal option as a significant problem for the industry.

A number of countries are carrying out studies to determine the optimum approach to the disposal of spent fuel and wastes from reprocessing. The general consensus favors its placement into deep geological repositories, initially recoverable.

Wastes

Nuclear waste management is a critical part of the fuel cycle. Wastes from the nuclear fuel cycle are categorized as high-, medium- or low-level wastes by the amount of radiation that they emit. These wastes come from a number of sources and include:

• low-level waste produced at all stages of the fuel cycle
• intermediate-level waste produced during reactor operation and by reprocessing
• high-level waste, which is waste containing fission products from reprocessing, and in many countries, the used fuel itself.

The enrichment process leads to the production of much 'depleted' uranium, in which the concentration of uranium-235 is significantly less than the 0.7% found in natural uranium. Small quantities of this material, which is primarily uranium-238, are used in applications where high-density material is required, including radiation shielding and in the production of MOX fuel. While ²³⁸U is not fissile, it is a low specific activity radioactive material and some precautions must, therefore, be taken in its storage or disposal.

Material balance in the nuclear fuel cycle: The following figures make various assumptions but may be regarded as typical for the operation of a 1000 MWe nuclear power reactor:

Uranium concentrations are sometimes expressed in terms of U₃O₈ content (U₃O₈ is a mixture of two uranium oxides approximately as they occur in nature). Pure U₃O₈ product contains about 85% uranium metal.

Further Reading

• Uranium Information Centre Ltd
• The Nuclear fuel cycle, The U.S. Nuclear Regulatory Commission