5 Nuclear fuel management

Table of Contents
1 Introduction
2 Electricity in Alberta
3 Options for meeting Alberta’s needs
4 An overview of nuclear power
5 Nuclear fuel management
6 Nuclear safety
7 Nuclear electricity in Alberta
8 Nuclear regulation in Canada
9 Conclusion

5.1 Overview
This chapter looks at the fuel used for nuclear reactors: how it produces energy; how it is mined and milled; how it operates in a reactor; and how it is disposed of. These stages make up the complete ‘nuclear fuel cycle.’

As discussed in Chapter 4, the majority of power reactors operating today are cooled either by light water (PWR, BWR) or heavy water (CANDU, PHWR).  The fuel for these reactors is made up of two principal components: ceramic pellets of uranium dioxide (UO2), and zirconium alloy tubes that encase the pellets and are referred to as either the fuel sheath or cladding.

For purposes of this report, a hypothetical nuclear unit of 800 MW has been used, as being most comparable to a base-load coal plant in the Alberta system. This does not correspond to any particular nuclear reactor design currently on the market and is not meant to suggest a specific type of plant. Rather this hypothetical nuclear unit is used to compare it to standard supply-side solutions.

5.2 The nuclear fuel cycle
The nuclear fuel cycle consists of two main parts:

Front-end processes:

• mining;
• processing the ore into a form suitable for manufacturing fuel;
• enriching the concentration of uranium-235 (for reactors requiring enriched fuel);
• fabricating the nuclear fuel pellets, fuel bundles and assemblies that are inserted into the reactors.

Back-end processes:

• storing the used fuel discharged from reactors;
• ultimately disposing of the waste products.

Figures 7 and 8 show the difference between natural uranium and enriched-uranium fuel cycles, which takes place primarily during the front-end processes. For the enrichment process, an intermediate conversion step produces uranium hexafl uoride (UF6), which facilitates the concentration of uranium-235. Then a second conversion process produces uranium dioxide (U02) powder, the product required for manufacturing the uranium fuel pellets.

The figures also show the typical mass of materials involved at different stages of the fuel cycle.

Nuclear fuel cycles can also be classified as ‘open’ or ‘closed.’ In an open cycle, the fuel is placed in a reactor only once. After discharge it is stored prior to ultimate disposal.
A closed cycle, on the other hand, involves recycling the significant energy content that still remains in the fuel so that fissionable material can be incorporated into newly fabricated fuel. This recycling requires the use of reprocessing technology to separate the true waste material – the very small amounts of fission products – from the material that can be further fissioned to yield energy. Reprocessing and fuel recycling will be discussed later in this chapter.

5.2.1 Mining and milling uranium
Uranium occurs widely in the earth’s crust, at an average of four parts per million. Like other metals, it forms mineral compounds rather than being found as a pure metal. The distribution of uranium in the earth’s crust is not uniform. In certain localities, higher concentrations in ore bodies can be economically mined.

Countries with the largest reserves of uranium ore are Australia, Kazakhstan and Canada. Other countries with significant uranium reserves include South Africa, Namibia, Niger and the USA. World-wide the average grade of uranium ore is 0.2% (Uranium ore grade is defined as the ratio of the mass of uranium metal produced to the mass of ore mined. Therefore, 10 kg of uranium metal can be produced by mining 1 tonne (1000 kg) of ore with a grade of 1%).

Canada is the only country in the world to possess high-grade ore bodies, defined as ore with more than 2% uranium by mass. The McArthur River mine in Saskatchewan has the highest-grade ore found anywhere on earth, at 20.5% on average (This is 100 times the world-wide average ore grade.) The Cigar Lake Mine, currently being brought into service, will have the second highest-grade ore in the world. By comparison, ore bodies in other parts of the world have grades in the region of 0.01% to 1% (1/20 to 5 times the world average).

In some locations, uranium is extracted through in situ leaching, a process in which a solution is injected into the ore body to dissolve the uranium-bearing compounds and then pumped back to the surface for further processing. This process is currently employed, for example, in mining operations in the United States where the ore grade is low.

Typically, mining and milling involves extracting the uranium-bearing ore, crushing and grinding it to coarse particle form and leaching it with an acid to extract the uranium as a solution. After further refining to remove impurities, the uranium is precipitated as U3O8 powder, referred to as ‘yellowcake’ because of its colour.

5.2.2 Fabricating reactor fuel
The yellowcake powder is refined and converted either directly to uranium dioxide (UO2) (for use in natural uranium fuel CANDU reactors) or to uranium hexafl uoride (UF6) for subsequent enrichment. The U-235 isotope naturally makes up 0.711% of the uranium found in nature.  The enrichment process increases this concentration to between 3% and 5%, as required by light-water reactors.

The early enrichment process developed in the United States was based upon gaseous diffusion. Uranium-235 atoms are lighter than uranium-238 atoms, and so diffuse through a membrane barrier slightly more often. However, the separation efficiency of gaseous diffusion is relatively low and the process requires large amounts of energy.  In Europe an alternative process based upon centrifuge technology was developed, which has significantly higher separation efficiency and much lower power requirements. The throughput capacity of individual centrifuges is low, and so a large number of centrifuge machines must operate in parallel to yield the required mass of enriched product. Nevertheless, centrifuge separation plants require approximately 25 times less energy to produce the same amount of enriched product as a gaseous diffusion plant. As a result modern enrichment plants employ the centrifuge process (A new process based on laser separation technology being developed in the USA offers even higher separation efficiencies with the potential to extract uranium-235 from the depleted uranium in current enrichment tails (typically containing between 0.2% and 0.3% uranium-235)).

Fuel pellet fabrication involves a number of steps:

• Powder granulation involves increasing the effective particle size of the powder so that it will flow more freely. This is necessary in order to produce consistent quality and density of the pressed pellets.
• Pressing compacts the UO2 powder to produce uniformly sized cylindrical pellets of relatively low density.
• Sintering passes the pellets slowly through a high-temperature hydrogen sintering furnace which increases their density. The process produces hour-glass-shaped pellets, which must be ground with water lubrication to the cylindrical shape needed for insertion into the fuel sheath.
• Stacking lines up the pellets end-to-end to the desired length for insertion into the zirconium alloy fuel sheaths. The sheath tube is filled with helium gas and hermetically sealed by welding end caps onto the ends. This forms a fuel element.
• Fuel bundle assembly is the final step where fuel elements are arranged in a regular array (cylindrical in the case of CANDU fuel bundles or a square array for light-water reactor fuel assemblies). Structural supports along the length of the fuel elements keep them in a desired spacing and structures at either end hold them together.

In Canada, uranium fuel processing facilities are located in Ontario. Yellowcake produced from mining and milling of uranium ore in Saskatchewan is shipped to Cameco’s refinery in Blind River, Ontario. Here it is refined to remove impurities and produce high quality uranium trioxide (UO3). The uranium trioxide is shipped to Cameco’s conversion facility in Port Hope, Ontario.  Here, it is converted either to uranium dioxide (UO2) for CANDU fuel or to uranium hexafl uoride (UF6) which is sent to uranium enrichment facilities around the world.

The uranium dioxide destined for use in CANDU reactors is then sent to Canadian General Electric in Peterborough, Ontario or to Zircatec Precision Industries in Port Hope, Ontario where it is further processed into fuel bundles.

5.3 Fuel utilization in a reactor

Inside the reactor, once it is operating, uranium-235 in the fuel pellets undergoes fission as described in Chapter 4, releasing energy. In addition to the uranium-235 fission, some of the uranium-238 (by far the predominant uranium isotope in the fuel) undergoes ‘transmutation’ – in other words, it captures a neutron to form a new element called plutonium-239. Plutonium-239 undergoes fission just like uranium-235.

This combination of fission and transmutation processes occurs in all operating reactors. In a CANDU fuel bundle, for example, about equal amounts of energy are released from fissioning uranium and plutonium atoms.  The amount of energy produced by nuclear fuel before it is discharged from the reactor is termed the fuel burnup.(Typical units of measurement for fuel burnup are gigawatt-days per tonne of uranium metal (GWd/tU) or the equivalent unit megawatt-days per kilogram of uranium) metal (MWd/kgU).  As fuel burnup increases, more of the original uranium-235 is consumed by fission, more plutonium isotopes are produced by transmutation, and more plutonium atoms also undergo fission. 

Each fission event, whether it involves uranium or plutonium, produces two fragments from the original atom. Each is one of a number of possible isotopes of lighter elements. These fragments are short-lived, highly ionized (“Ionized” means the atom does not have an equal balance between its protons and electrons, and so is positively or negatively charged)and unstable, and they deposit energy in the fuel pellet through interaction with other atoms and by emission of radiation. These unstable isotopes are referred to as fission products.

Fission products are the true waste from the fission process, since the uranium and plutonium that have not undergone fission still represent a significant energy source. For example, a new CANDU fuel bundle has approximately 18,800g of uranium metal. On discharge (typically after 8 months), it contains approximately 18,660g of heavy metal, mostly uranium-238, and only about 140g of fission products. In other words, fission or waste products represent only about 0.74% of the original mass of the uranium in the fuel bundle.

Table 4 shows typical fuel utilization and the fission products generated in CANDU and light-water reactors of the same size. For perspective, the table indicates that generating an amount of electricity equal to about 12% of Alberta’s 2007 energy consumption could result in less than one tonne of fission or waste products, leaving the heavy-metal component of the fuel available for recycling and reuse.

5.4 Managing spent fuel
Once fuel is discharged from the reactor, it is highly radioactive and continues to produce heat through decay of the fission products. The heat energy is only a small fraction of the heat generated in the bundle at full power, but it is sufficient to require continued cooling.  This is provided by storage in ‘spent fuel bays’ – large water-filled pools. (Water provides a shield against all three forms of radiation.(The three types of radiation are alpha, beta and gamma. They are discussed in more detail in section 6.2).) About 10 years after discharge, the heat has decayed to a sufficiently low level that the fuel can be transferred to concrete dry-storage structures in which the fuel is air-cooled.

Used fuel can be recycled to separate the waste fission products from the heavy actinide metals (i.e., uranium, plutonium and other heavy metals). This is an attractive option for maximizing the fission energy from mined uranium. Recycling fuel also has the benefit of significantly reducing the time frame over which final waste products have to be stored. This is because heavy metals have a very long half-life before they decay by emitting alpha particles from the nucleus. The lighter fission or waste products decay more quickly, mainly by emitting beta particles (electrons). Most fission products decay away to the natural background levels of radioactive material found in the earth’s crust within approximately 500 to 1000 years.(The exception is two very long-lived fission products, the isotopes Iodine -129 (I-129) and Technetium-99 (Tc-99). Because they decay very slowly this means that they emit radioactivity at a slow rate and, hence are very mild sources of radiation)

So recycling and fissioning the heavy metals can accelerate the process of breaking down their radioactivity, leaving a much smaller volume of shorter-term waste products to deal with. Fuel recycling in the form of Mixed Oxide (MOX) fuel is currently being performed in France and Japan. Reprocessing facilities have been established in France and the United Kingdom, and a facility is about to be brought into service in Japan. The nuclear fuel cycle based upon MOX fuel recycling is shown in Figure 9.

Figure 9 demonstrates that fuel reprocessing and reuse significantly reduces the amount of waste for which final disposal will be required, to 0.115 cubic meters of waste fuel  from the original 3600 tonnes of uranium ore.

5.4.1 Fuel disposal
In Canada the Nuclear Waste Management Organization (NWMO) was tasked with recommending to the Federal Government an approach to managing Canada’s used nuclear fuel. Their recommended approach, which has been accepted, is Phased Adaptive Management.

The Phased Adaptive Management approach was developed following an extensive public consultation process. Its key elements are to provide safe, monitored storage of used fuel and the flexibility for future generations to make their own decisions regarding fuel management as technological advances are made.  The approach involves three phases, during which options will be continuously evaluated:

1. In phase one, dry storage of used fuel at generating station sites will continue as currently practiced, while the option is assessed of a centralized shallow underground facility where used fuel could be stored on an interim basis and from which it could be retrieved. During this first phase, which will extend over approximately 30 years, work will be carried out on site selection for the centralized interim storage, as well as an environmental assessment, licensing and construction.
2. In the second phase, to be conducted over an additional 30 years, used fuel may be transferred to the centralized repository. Meanwhile, research and design will be carried out on a deep repository for permanent storage.
3. In the third phase, (after approximately 60 years) used fuel would be transferred to the deep geological repository for permanent storage.  Depending on technology developments, in particular for fuel recycling, used fuel could be retrieved for reprocessing and recycling and only the waste fission products buried. Alternatively, if fuel recycling is not chosen, the used fuel could be prepared for burial in the deep geological repository while still retaining the option to retrieve it later.

The amount of waste material to be disposed will likely be significantly reduced through deployment of recycling technologies which are currently under research and development. As mentioned previously, the true waste fission products decay much more rapidly than the heavy metal actinides that are potentially recyclable as fuel.

5.4.2 Security
The nuclear proliferation issue concerns the possibility that nations will surreptitiously develop technology and facilities that allow the development of material for nuclear weapons. This can involve the enrichment of uranium to very high levels of purity – material referred to as Highly Enriched Uranium – or reprocessing spent fuel to remove plutonium-239.  However, reprocessing/recycling reactor fuel does not produce weapons-grade plutonium, since power reactor fuel contains different isotopes of plutonium that reduce its effectiveness for explosions. 

Currently, the main means of limiting the proliferation of weapons-grade material are the international safeguarding of nuclear materials by the International Atomic Energy Agency (IAEA) and development of new technologies. Used fuel is stored either in water pools or in dry storage structures made of high-strength reinforced concrete. These structures provide high levels of protection against possible hostile actions aimed at disrupting safe storage of the used fuel.  Modern safety analysis evaluates the capability of these structures to withstand hostile attacks from a wide range of threats. In addition special seals are used by the IAEA to establish safeguarded facilities in conjunction with random inspections to verify that there has been no tampering with stored used fuel.

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