Nuclear waste management

Published: July 25, 2010, 3:42 am

Topics:

Nuclear Energy (main)

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).

Introduction

All parts of the nuclear fuel cycle produce some radioactive waste (radwaste). The cost of managing and disposing of these wastes is part of the electricity cost, i.e., it is internalized and paid for by the electricity consumer. At each stage of the fuel cycle there are proven technologies to dispose of the radioactive wastes safely. For low- and intermediate-level wastes these are mostly being implemented. For high-level wastes some countries await the accumulation of enough of it to warrant building geological repositories, others, such as the USA, have encountered political delays. The radioactivity of all nuclear waste decays with time. Each radionuclide contained in the waste has a half-life - the time taken for half of its atoms to decay and thus for it to lose half of its radioactivity. Radionuclides with long half-lives tend to be alpha and beta emitters - making their handling easier - while those with short half-lives tend to emit the more penetrating gamma rays. Eventually, all radioactive wastes decay into non-radioactive elements. Eventually all radioactive wastes decay into non-radioactive elements. The more radioactive an isotope is, the faster it decays. The main objective in managing and disposing of radioactive (or other) waste is to protect people and the environment. This means isolating or diluting the waste so that the rate or concentration of any radionuclides returned to the biosphere (Nuclear waste management) is harmless. To achieve this, practically all wastes are contained and managed - some need deep and permanent burial - so that harmful pollution is avoided. From nuclear power generation, none is allowed to cause harmful pollution.

In OECD nations, some 300 million tonnes of toxic wastes are produced each year, but conditioned radioactive wastes amount to only 81,000 cubic metres per year. In countries with nuclear power, radioactive wastes comprise less than 1% of total industrial toxic wastes (the balance of which remains hazardous indefinitely). All toxic wastes need to be dealt with safely, not just radioactive wastes.

Types of radioactive wastes

Mine tailings: Traditional uranium mining generates fine sandy tailings, which contain virtually all the naturally occurring radioactive elements found in uranium ore. These are collected in engineered tailings dams and finally covered with a layer of clay and rock to inhibit the leakage of radon gas and ensure long-term stability. In the short term, the tailings material is often covered with water. After a few months, the tailings material contains about 75% of the radioactivity of the original ore. Strictly speaking these are not classified as radioactive wastes.

Exempt Waste & Very Low Level Wastes (VLLW) is radioactive waste which contains radioactive materials at a level which is not considered harmful to people or the surrounding environment. It consists mainly of demolished material (such as concrete, plaster, bricks, metal, valves, piping etc) produced during rehabilitation or dismantling operations on nuclear industrial sites. Other industries, such as food processing, chemical, steel etc also produce VLLW as a result of the concentration of natural radioactivity present in certain minerals used in their manufacturing processes (see also paper on NORM). The waste is therefore disposed of with domestic refuse, although countries such as France are currently developing facilities to store VLLW in specifically designed VLLW disposal facilities.

Low-level Wastes (LLW) are generated from hospitals and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters, etc. that contain small amounts of mostly short-lived radioactivity. These wastes do not require shielding during handling and transport and are suitable for shallow land burial. To reduce the waste's volume, it is often compacted or incinerated before disposal. LLW comprises some 90% of the volume but only 1% of the radioactivity of all radwaste.

Intermediate-level Wastes (ILW) contains higher amounts of radioactivity and some requires shielding, usually of lead, concrete or water. It typically comprises resins, chemical sludges, and metal fuel cladding, as well as contaminated materials from reactor decommissioning. Smaller items and any non-solids may be solidified in concrete or bitumen for disposal. ILW makes up some 7% of the volume and has 4% of the radioactivity of all radwaste. The maintenance of a 1000 MWe nuclear reactor produces less than 0.5m³ of long-lived ILW each year. If fuel is reprocessed this is increased to 3m³.

High-level Wastes (HLW) arise from the "burning" of uranium fuel in nuclear fission reactors. HLW contains the fission products and transuranic elements generated in the reactor core. It is highly radioactive and hot, so requires cooling and shielding. It can be considered as the "ash" from "burning" uranium. These wastes contain the fission products and transuranic elements generated in the reactor core. It is highly radioactive and hot and thus requires cooling and shielding. HLW accounts for over 95% of the total radioactivity produced in the process of electricity generation. There are two distinct kinds of HLW: Note that there are virtually no radioactive wastes in nuclear fusion technology.

• used fuel itself in fuel rods, or
• separated waste from reprocessing the used fuel as described below.
HLW has both long-lived and short-lived components, depending on the length of time it will take for the radioactivity of particular radionuclides to decrease to levels that are considered no longer hazardous for people and the surrounding environment. If generally short-lived fission products can be separated from long-lived actinides, this distinction becomes important in management and disposal of HLW.

With new reactor technology, the amount of HLW reduces. The following is a comparison published by Areva. Apparently considering HLW separated from reprocessing: first-generation gas-graphite reactors produced about 50 cubic metres per billion kilowatt-hours (TWh). Second-generation reactors initially produced about 11 m3/TWh, which reduced to half that by 1995. Third generation reactors produce about 3 m3/TWh and the goal with fourth generation plants is one cubic metre.

Fuel Cycle stages

Conversion, enrichment, making fuel

Uranium oxide concentrate from mining, essentially "yellowcake" (U₃O₈), is not significantly radioactive - barely more so than the granite used in buildings. It is refined and then converted to uranium hexafluoride gas (UF₆). As a gas, it undergoes enrichment to increase the U-235 content from 0.7% to about 3.5%. It is then turned into a hard ceramic oxide (UO₂) for assembly as reactor fuel elements.

The primary by-product of enrichment is depleted uranium (DU), principally the U-238 isotope, which is stored either as UF₆ or U₃O₈. About 1.2 million tonnes of DU is now in storage. Its extremely high density makes it valuable for use in the keels of yachts and military projectiles. It is also used (with recycled plutonium) for making mixed oxide fuel and to dilute highly-enriched uranium from dismantled weapons, now being used for reactor fuel.

Electricity Generation

In terms of radioactivity, High-level Waste (HLW) is the major issue arising from the use of nuclear reactors to generate electricity. Highly radioactive fission products and also transuranic elements are produced from uranium and plutonium during reactor operations and are contained within the used fuel. Where countries have adopted a closed cycle and utilised reprocessing to recycle material from used fuel, the fission products and transuranic elements are separated from uranium and plutonium and treated as HLW (uranium and plutonium is then re-used as fuel in reactors). In countries where used fuel is not reprocessed, the used fuel itself is considered a waste and therefore classified as HLW.

Low and intermediate level waste is produced as a result of operations, such as the cleaning of reactor cooling systems and fuel storage ponds, the decontamination of equipment, filters and metal components that have become radioactive as a result of their use in or near the reactor.

A typical large (1000 MWe) light water reactor will generate 200 - 350 m³ low and intermediate level waste per year. It will also produce about 20m³ (27 tonnes) of used fuel per year, which corresponds to a 75m³ disposal volume following encapsulation if it is treated as waste. Where that used fuel is reprocessed, only 3m³ of vitrified waste (glass) is produced, which is equivalent to a 28m³ disposal volume following placement in a disposal canister.

Managing HLW from used fuel

Figure 1. Storage pond for spent fuel at UK reprocessing plant

Used fuel gives rise to HLW that may be either the used fuel itself in fuel rods, or the principal waste arising from reprocessing the used fuel, a process discussed in the following section. In either case, the amount is modest - about 27 tonnes of used fuel or three cubic metres per year of vitrified waste for a typical large nuclear reactor. Both can be effectively and economically isolated, and have been handled and stored safely since the production of nuclear power began.

Figure 2. Decay in radioactivity of fission products

Storage is mostly in ponds at reactor sites, or occasionally at a central site. Some 90% of the world's used fuel is stored thus and some of it has been there for decades. The ponds are usually about seven metres deep, to allow three metres of water over the used fuel to fully shield it. The water also cools it. Some storage is in dry casks or vaults with air circulation and the fuel is surrounded by concrete.

If the used fuel is reprocessed, as it is in UK, French, Japanese, and German reactors, HLW comprises highly-radioactive fission products and some transuranic elements with long-lived radioactivity. These are separated from the used fuel, enabling the uranium and plutonium to be recycled. The remaining HLW generates a considerable amount of heat and requires cooling. It is vitrified into borosilicate (Pyrex) glass, encapsulated into heavy stainless steel cylinders about 1.3 metres high, and stored for eventual disposal deep underground. This material has no conceivable future use and is unequivocally waste. The hulls and end-fittings of the reprocessed fuel assemblies are compacted, to reduce volume, and usually incorporated into cement prior to disposal as ILW.

But if used reactor fuel is not reprocessed, it will still contain all the highly radioactive isotopes, and the entire fuel assembly is treated as HLW for direct disposal. As HLW, it generates considerable heat and requires cooling. However, since it largely consists of uranium (with a little plutonium), it represents a potentially valuable resource. Hence there is an increasing reluctance to dispose of it irretrievably.

Either way, after 40-50 years, the heat and radioactivity have fallen to one-thousandth of the level at removal. This provides a technical incentive to delay further action with HLW until the radioactivity has reduced to about 0.1% of its original level.

After being stored for approximately 40 years, the used fuel assemblies are ready for encapsulation or loading into casks ready for indefinite storage or permanent disposal underground.

Direct disposal of used fuel is the procedure followed in the US, Switzerland, and Sweden, among others, although evolving concepts are tending towards making it recoverable if future generations see it as a resource. This means allowing for a period of management and oversight before a repository is closed.

Increasingly, reactors are using fuel enriched to over 4% U-235 and burning it longer, to end up with less than 0.5% U-235 in the spent fuel. This provides less incentive to reprocess.

Used fuel from light water reactors contains approximately:
95.6% uranium (less than 1% of which is U-235)
2.9% stable fission products
0.9% plutonium (about two thirds fissile Pu-239 & Pu-241)
0.3% cesium & strontium (fission products)
0.1% iodine and technetium (fission products)
0.1% other long-lived fission products
0.1% minor actinides (americium, curium, neptunium)

Recycling used fuel

Any used fuel will still contain some of the original U-235 as well as U-238 and various plutonium isotopes that have been formed inside the reactor core. In total, these account for some 96% of the original uranium and over half of the original energy content (ignoring U-238). Reprocessing, undertaken in Europe and Russia, separates this uranium and plutonium from the wastes so that they can be recycled for re-use in a nuclear reactor as a mixed oxide (MOX) fuel. This is the "closed fuel cycle".

Plutonium (Pu) arising from reprocessing comprises only about 1% of commercial spent fuel. It is recycled through a MOX fuel fabrication plant where it is mixed with depleted uranium oxide to make fresh fuel. European reactors currently use over 5 tonnes of plutonium a year in fresh MOX fuel, although all reactors routinely burn much of the plutonium which is continually formed in the core by neutron capture. The use of MOX simply means that some plutonium is incorporated into fresh fuel. Plutonium arising from the civil nuclear fuel cycle is not suitable for bombs. It contains far too much of the Pu-240 isotope due to the length of time the fuel has spent in the reactor.

Major commercial reprocessing plants operate in France, the UK, and Russia with a capacity of some 5,000 tonnes per year and cumulative civilian experience of 80,000 tonnes over 50 years. France and the UK also undertake reprocessing for utilities in other countries, notably Japan, which has made over 140 shipments of used fuel to Europe since 1979. Presently, most Japanese used fuel is reprocessed in Europe, and the vitrified waste and recovered U and Pu (as MOX) are returned to Japan to be used in fresh fuel. Russia also reprocesses some of its spent fuel from Soviet-designed reactors in other countries.

A proposed development of this reprocessing and recycling procedure is to separate plutonium with the minor actinides as one product. However, this product is not simply put into MOX fuel and recycled in conventional reactors; it requires fast neutron reactors that are not yet common in the commercial market. However, were this procedure to become common, it would make disposal of high-level wastes much easier.

Costs of radioactive waste management

Financial provisions are made for managing all kinds of civilian radioactive waste. The costs of managing and disposing of wastes from nuclear power plants represents about 5% of the total cost of electricity generation.

Most nuclear utilities are required by their governments to put aside a levy (e.g., 0.1 cents per kilowatt hour in the USA, 0.14 c/kWh in France) to provide for management and disposal of their wastes. So far, some US$ 28 billion had been committed to the US waste fund by electricity consumers.

The actual arrangements for paying for waste management and decommissioning also vary. The key objective is however always the same: to ensure that sufficient funds are available when they are needed.

Disposing of spent fuel and other HLW

There is about 270,000 tonnes of spent fuel in storage, much of it at reactor sites. About 90% of this is in ponds, the balance in dry storage. Approximately 12,000 tonnes of used fuel are created annually, with 3,000 tonnes being reprocessed. Final disposal is therefore not urgent in any logistics sense.

To ensure that no significant environmental releases occur over tens of thousands of years, 'multiple barrier' disposal is used. This immobilizes the radioactive elements in HLW and some ILW, preventing their release into the biosphere.

Steps in multiple barrier disposal:

• Immobilize waste in an insoluble matrix such as borosilicate glass or synthetic rock (fuel pellets are already in the form of a very stable ceramic: UO₂);
• Seal it inside a corrosion-resistant container, such as stainless steel;
• Locate it deep underground in a stable rock structure; and
• Surround containers with an impermeable backfill such as bentonite clay if the repository is wet.

HLW from reprocessing must be solidified. France has two commercial plants to vitrify HLW left over from reprocessing oxide fuel, and there are plants in the UK and Belgium as well. The capacity of these western European plants is 2,500 canisters (1,000 t) per year, and some have been operating for three decades.

Figure 3. Loading silos at a UK site

Figure 3 shows loading silos with canisters containing vitrified high-level waste in UK. Each disc on the floor covers a silo holding ten canisters

The Australian Synroc (synthetic rock) system is a more sophisticated way to immobilize wastes, and this process may eventually come into commercial use for civil wastes.

To date there has been no practical need for final HLW repositories, as surface storage for 40-50 years is first required so that heat and radioactivity can decay and dissipate to levels at which handling and storage are easier.

The process of selecting appropriate deep geological repositories is now under way in several countries, with the first expected to be commissioned some time after 2010. Finland and Sweden are in the advanced stages, with plans and site selection for direct disposal of used fuel, since their Parliaments decided to proceed on the basis that the procedure was safe using existing technology. The US has opted for a final repository in Nevada, at Yucca Mountain, although this proposal has generated significant controversy and is now stalled by political decision. There have also been proposals for international HLW repositories at sites with optimum geology - Australia or Russia are possible locations.

Figure 4. Decay in radioactivity of HLW

A question remaining is whether wastes should be emplaced so that they are readily retrievable from repositories. While there are sound reasons for keeping such options open, long-term security is also vital. After being buried for approximately 1,000 years, most of the radioactivity will have decayed. The amount of radioactivity then remaining would be similar to that of the naturally-occurring uranium ore from which it originated, though it would be more concentrated.

Disposing of other radioactive wastes

Generally, short-lived intermediate-level wastes (mainly from the decommissioning of nuclear reactors) are buried, while long-lived intermediate-level wastes (from fuel reprocessing) will be disposed of deep underground.

Low-level wastes are disposed of in shallow burial sites. Some low-level liquid wastes from reprocessing plants are discharged to the sea. These include radionuclides that are distinctive, notably technetium-99 (sometimes used as a tracer in environmental studies), which can be discerned many hundred kilometers away from the site of discharge. However, such discharges are regulated and controlled, and the maximum radiation dose potential is only a small fraction of natural background levels.

Nuclear power stations and reprocessing plants release small quantities of radioactive gases (e.g., krypton-85 and xenon-133) and trace amounts of iodine-131 to the atmosphere. However, they have short half-lives, and the radioactivity in the emissions is diminished by delaying their release. Additionally, krypton-85 and xenon-133 are chemically inert; the net effect is too small to warrant consideration in any life-cycle analysis.

It is important to note that coal combustion produces some 280 million tonnes of ash per year, most of it containing low levels of natural radionuclides. Some of this is classified as LLW and it is simply buried.

Wastes from decommissioning nuclear power plants

In the case of nuclear reactors, about 99% of the radioactivity is associated with the fuel that is removed before moving to any of the three options. Apart from any surface contamination of the plant, the remaining radioactivity comes from "activation products" such as steel components that have been exposed to long-term neutron irradiation. Their atoms change into different isotopes such as iron-55, cobalt-60, nickel-63, and carbon-14. Iron-55 and cobalt-60 are highly radioactive, emitting gamma rays, but have short half-lives so that 50 years after closedown their hazard is much diminished. Some caesium-137 may also be in decommissioning wastes.

Some scrap material from decommissioning may be recycled. However, for uses outside the industry very low clearance levels are applied, so most is buried.

Natural precedents for geological disposal

Nature has already proven that geological isolation is possible through several natural examples (or "analogues"). The most significant case occurred almost 2 billion years ago at Oklo, in what is now Gabon in West Africa, where six spontaneous nuclear reactors operated within a rich vein of uranium ore. At the time, the concentration of U-235 in all natural uranium was about 3%. These natural nuclear reactors continued for about 500,000 years before dying away. They produced all of the radionuclides found in HLW, including over 5 tonnes of fission products and 1.5 tonnes of plutonium, all of which remained at the site and eventually decayed into non-radioactive elements.

The study of such natural phenomena is important for any assessment of geologic repositories, and is the subject of several international research projects. However, it must be noted that the Oklo reactions proceeded because groundwater was present as a moderator in the "enriched" and permeable uranium ore.

Legacy Wastes

This article mainly addresses the routine wastes arising from current nuclear power generation and its supporting activities.

In several pioneer-nations of nuclear power, and especially where power programs arose out of military programs, there are other radioactive wastes that require management and disposal. These are sometimes voluminous and difficult, and are referred to as 'legacy wastes'. They arose in the course of countries getting to a position where nuclear technology was a commercial proposition for power generation, and they represent a liability not covered by current funding arrangements. In the UK, some 70 billion GBP is estimated to be involved in addressing these wastes - principally from Magnox and some early AGR developments - and about 30% of the total wastes can be attributed to military programs. In the US, Russia, and France the liabilities are also considerable.

Further Reading

• WNA paper on Waste management in the Nuclear Fuel cycle

Citation

Association, W. (2010). Nuclear waste management. Retrieved from http://editors.eol.org/eoearth/wiki/Nuclear_waste_management