Basic Overview Of Nuclear Reactor Waste

Waste from Nuclear Power

What is Nuclear Reactor Waste?

The generation of electricity in a Nuclear Power Plant is made by splitting uranium atoms. The uranium used as fuel in a nuclear plant is formed into ceramic pellets about the size of the tip of your little finger. The uranium atoms in these pellets are bombarded by atomic particles, they split (or fission) to release particles of their own. These particles, called neutrons, strike other uranium atoms, splitting them. When the atoms split, they also release heat. This heat is known as nuclear energy and is essentially responsible for creating electricity.

Other reactions also take place in the nuclear reactor such as neutron capture. Neutron capture is a term used for the scenario where a neutron comes close to a nucleus (in this case uranium) and the nucleus captures it and becomes a different nucleus. In this case when uranium-238 captures a neutron it becomes uranium-239. After uranium-239 emits a beta particle (electron) it becomes neptunium-239. Then neptunium-239 also emits a beta particle and becomes plutonium-239. The plutonium can also be used as nuclear fuel.

Certain changes take place in the ceramic fuel pellets during their time in the reactor of the nuclear power plant. The particles left over after the atom has split are radioactive. During the life of the fuel, these radioactive particles collect within the fuel pellets. The fuel remains in the reactor until trapped fission fragments begin to reduce the efficiency of the chain reaction. Some of the fission products are various isotopes of barium, strontium, cesium, and iodine. The spent fuel also contains plutonium and uranium that was not used up. The fission products and the left over plutonium and uranium remain within the spent fuel when it is removed from the reactor and are called high level waste as they are extremely hot and very radioactive.

A Short Review of Radioactive Isotopes and Half-lives

In this subsection a review on radioactivity is provided so that the reader will have a better understanding of the nuclear waste issue.

An atom that is radioactive will decrease its radioactivity in time and eventually decay. How fast the radioactivity decreases depends on the half-life. The half-life is defined as the time it takes for half of the radioactivity to decay. Hence an isotope with a short half-life will decay quickly. The half-life is also inversely proportional to the intensity of radioactivity. Therefore the higher the intensity of radioactivity the shorter the half-life.

The approximate half-lives of some of the isotopes in the spent nuclear fuel are listed below:

Isotope	Half-life
Strontium-90	28 years
Caesium-137	30 years
Plutonium-239	24,000 years
Caesium-135	2.3 million years
Iodine-129	15.7 million years

It is clear from the table above that the left over nuclear isotopes from the generation of electricity through nuclear power are extremely long lived and therefore must be shielded from humans and the environment for a long time.

Storage and Disposal of High Level Nuclear Reactor Waste

Since the spent nuclear reactor (SNF) fuel is highly radioactive initially it is too dangerous to handle and thus it is very important to shield the radioactivity from humans and the environment. The radioactive material in the SNF generally falls into three categories: (1) un-reacted fuel, usually uranium, (2) fission products, and (3) activation products, most notably plutonium. Because of the nature of radioactivity, on a per-atom or by-weight basis the fission products are by far the most radioactive, and have the shortest half-life. The un-reacted uranium and the plutonium have vastly longer half-lives, but are correspondingly less radioactive. Once the SNF has been removed from the nuclear reactor it is placed in interim storage at the reactor site. Usually this consists of putting the nuclear waste into large pools of water. The water cools the radioactive isotopes and shields the environment from the radiation. Nuclear waste is typically stored in these supervised pools between 20-40 years. As the SNF ages the radioactivity decreases, reaching the point where it does not need to be water cooled and can be placed in dry storage facilities. Throughout this time there is a great reduction in heat and radioactivity and this makes handling of nuclear waste safer and easier.

After this “cooling off” period the high level waste can be handled in different ways. It can be reprocessed then disposed of permanently or directly disposed permanently in a geological repository.

1. Reprocessing

Recall that the spent nuclear fuel contains uranium and plutonium which are used as fuel in a nuclear reactor. It is possible to isolate much of the uranium and plutonium from the other fission products in spent nuclear fuel so that it can be recycled as fresh fuel to power the nuclear reactor. This is called reprocessing. After reprocessing the left over waste is largely liquid. It is then embedded into borosilicate glass and put into interim storage. Eventually it will be disposed of permanently deep underground.

2. Final Disposal

Eventually the nuclear waste will have to be stored indefinitely because of the long time it takes for some of the waste isotopes to decay to a safe level. The consensus of most waste management specialist for final disposal is to bury the waste deep underground. In doing so we must ensure that the radioactive waste does not move from its burial site or that it does not escape into the environment. If it does it could have dire consequences for future generations such as contamination of drinking water. To ensure that the radioactive waste is contained the current consensus is to use a multi-barrier system to store the waste. The geological disposal system consists of firstly surrounding the conditioned and packaged solid waste by several human made barriers then placing this at a depth of several hundred meters in a stable geological environment. The geological formation is the most important of the isolation barriers. The barriers act in concert to initially completely isolate the radioactive particles so they can decay and then limit their release to the environment. The combination of man made and natural geological barriers is called a multibarrier system.

The solidification of nuclear waste (which is necessary for final disposal) usually consists of dispersal in a glass matrix. However, alternative techniques are being researched. One such technique consists of embedding the waste into a ceramic matrix such as Synroc.Synroc is a “synthetic rock” invented in 1978 by Professor Ted Ringwood of the Australian National University. Synroc can incorporate nearly all the elements contained in high level waste.

The barriers surrounding the solid waste vary from country to country. However most countries believe that the barriers should be made of materials that occur naturally in the earth’s crust. In Sweden, the barriers consist of

1. A copper canister with a cast iron insert. This barrier is closest to the waste and its function is to isolate the fuel from the environment.

2. The second layer consists of bentonite clay called a buffer. Its function is to protect the the canister against small movements in the rock and keep it in its place. The clay also acts as a filter in case any radioactive particles escape form the canister.

3. The geological rock. The rock also stops leaking of radioactive particles into the environment but its main function is to protect the canister and buffer from mechanical damage and to offer a stable environment for the isolation of the waste.

Current Programs for final disposal of nuclear waste

Currently, no country has a complete system for storing high level waste permanently but many have plans to do so in the next 10 years. There are a number of well-developed proposals from the USA, Sweden, Finland and France for the disposal of long-lived radio-active waste.

All the proposed disposal techniques employ multiple barriers, as discussed above, to isolate the waste from the biosphere for at least 100,000 years. Nevertheless every one of the proposed disposal methods faces strong opposition from environmental groups and it is true that humans have never attempted to do anything on this sort of timescale. However nature has plenty of examples of systems that are stable for much longer periods. The most spectacular being the trans-uranic products of the Oklo natural nuclear reactors, which are discussed below, which have not appreciably moved in over 1.7 billion years.

The World Nuclear Industry appears to have reached a consensus to pursue Geologic disposal as final phase of Nuclear waste management. The US National Academies of Science, Engineering and Medicine also conclude that deep Geologic Disposal can provide a safe means of disposing high level waste.

There are a number of programs that have seriously mishandled the issue of waste from Nuclear Power. The British decision to reprocess spent-fuel appears to have been both an environmental and financial mistake. The Nuclear Weapons program at the Hanford site in Eastern Washington State, U.S.A, created an enormous environmental impact that has so far cost 5.7 billion dollars to clean up.

Currently waste from Nuclear Power plants is being held in temporary storage facilities until such time as long-term disposal is decided. This is a feasible option because of the relatively small amount of material used to generate Nuclear Power.

New Technologies for Waste Disposal

Another option for disposal of long-lived (trans-Uranic) waste is to burn it via either Accelerator Driven Systems or within Fourth Generation reactors. However these technologies are not yet mature. Since the waste is stored in large tanks of water for 20-40 years first, it may be that by this time these new technologies will be sufficiently developed so that waste can be destroyed using these new methods.

Read an article on transmutation from The Economist.

There are also proposals to use a Fusion-Fission Hybrid for waste-disposal. These devices use the powerful neutrons from a Deuterium-Tritium plasma to drive nuclear transmutation. In some respects this technology is similar to Accelerator driven waste transmutation and the proponents believe that a fleet of 6 Fusion-Fission hybrids, when used in conjunction with a reprocessing waste cycle, would be sufficient to destroy the remaining long-lived nuclear waste from a fleet of 100 commercial power-reactors.

Finally there are experiments with Deep-Burn where fuels originating from reprocessed nuclear-waste would be used to power Very High Temperature Reactors (VHTR). The result would be that a single fuel loading derived from 4 years of operation of a light-water reactor could be used to deliver all the energy needed over the 60-year life of a VHTR. This technology would not only destroy most of the long-lived waste, it would make the existing stockpiles a very valuable source of energy, since it could be used to deliver ten times the energy of the original fuel.

How much high level nuclear waste is produced in a nuclear reactor?

According to the International Atomic Energy Agency a nuclear reactor which would supply the needs of a city the size of Amsterdam – a 1000MW(e) nuclear power station – produces approximately 30 tonnes of high level solid packed waster per year if the spent fuel is not reprocessed. In comparison, a 1000MW(e) coal plant produces 300,000 tonnes of ash per year.

Currently, worldwide, nuclear power generation produces 10,000m3 of high-level waste per year.

Who is responsible for the nuclear waste?

In most countries it is the responsibility of the power companies to take care of the waste. They are responsible for all the cost of nuclear waste management. In fact most countries are obliged to set aside a certain amount of money each year for waste management. Furthermore there are strict guidelines imposed by governments and other bodies for safe and responsible nuclear waste management. For example the International Atomic Energy Agency runs frequent conferences on the science of nuclear waste disposal, setting and enforcing safety standards, monitoring safety and security of nuclear waste. Recent conferences can be found on the IAEA website.

A natural nuclear reactor

Eventual final disposal of nuclear waste means that the radioactive isotopes will be buried (with appropriate barriers - see "Final disposal" above) in an appropriate disposal site for a very long time. One of the serious questions we should consider is are there going to be any problems far in the future associated with the burial of the waste. The system for final waste disposal is usually based on naturally occurring phenomena. One such natural analogue is naturally occurring nuclear reactors in Gabon, West Africa in the Oklo mine called the Oklo nuclear reactors.

About 1.7 billion years ago, deep underground in Africa, favorable natural conditions prompted nuclear reactions to take place. These natural conditions were sufficient amounts of Uranium-235 and the evolution of plants which subsequently caused rainwater to filter down through cracks in rocks. The water was necessary to slow down the neutrons emitted via uranium decay so that they could interact with other particles and produce nuclear chain reactions. The reactors operated for about 1 million years. The reactions stopped because the uranium depleted to amounts that were too small to keep the reactions going.

It has been shown that the Oklo reactors fissioned Uranium-238, Uranium-235 and Plutonium-239. This is exactly the elements that are fissioned in todays man-made nuclear reactors. Note also that there was no Plutonium-239 on earth when the Oklo reactors formed. This means that the reactors themselves must have produced this isotope. This is also the case for man-made nuclear reactors.

Once the natural reactors burned out they left radioactive nuclear waste. This waste is very similar to the waste generated by nuclear power stations. The nuclear waste was held in place deep underground by granite, sandstone and clays surrounding the reactors' site. The important point is that the waste has not moved much over approximately 2 billion years (see the fact sheet from Office of Civilian Radioactive Waste Management, USA and the article by Walton and Cowan from the "Proceedings of a Symposium on the Oklo Phenomenon", 1975 listed in the bibliography at the end of this section).

The Oklo reactors give us an opportunity to observe the effects of storing waste deep underground for billions of years. By analyzing the remains of these ancient natural nuclear reactors and gaining an understanding of the conditions needed to secure and contain the nuclear waste, we can apply the same techniques to the final disposal of man-made nuclear reactor high level waste. The study of the Oklo nuclear reactors means that we can have reasonable confidence in final disposal.

Bibliography for Waste from Nuclear Power

The sources on man-made high level nuclear waste have been,

International Atomic Energy Agency Factsheet on Nuclear Waste

Nuclear Energy Agency

Swedish Nuclear Waste Management

Finland Nuclear Waste Management

French Nuclear Waste Management

USA Office of Civilian Radioactive Waste Mangagement

Synroc

Technical Report series no. 43, International Atomic Energy Agency, 2003, "Scientific and Technical Basis for the Geological Disposal of Radioactive Wastes"

The sources for the Oklo natural nuclear reactors have been,

R.D Walton Jr., G.A. Cowan, "Relevance of Nuclide Migration at Oklo to the Problem of Geologic Storage of Radioactive Waste", from the International Atomic Energy Agency, Vienna, 1975, "Proceeding of a Symposium on the Oklo Phenomenon".

Curtin University Information on Oklo

Office of Civilian Radioactive Waste Management, USA - Oklo Fact Sheet

Other information on natural nuclear waste storage can be found through the following link,

Natural Analogues of Nuclear Waste Storage

• FFH_FED_Final08.pdf: FusionFissionHybrids

INFO #3 : Nuclear Fuel Cycle [IAEA]

The Nuclear Fuel Cycle

• The nuclear fuel cycle is the series of industrial processes which involve the production of electricity from uranium in nuclear power reactors. 
• Uranium is a relatively common element that is found throughout the world. It is mined in a number of countries and must be processed before it can be used as fuel for a nuclear reactor. 
• Fuel removed from a reactor, after it has reached the end of its useful life, can be reprocessed to produce new fuel. 

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

Uranium is a slightly radioactive metal that occurs throughout the Earth's crust (see page on Uranium and Depleted Uranium). 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 in 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 fertilisers, 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 and 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 m 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. Special precautions, consisting primarily of increased ventilation, are required in underground mines to protect against airborne radiation exposure.

An increasing proportion of the world's uranium now comes from in situ leach (ISL) mining, where oxygenated groundwater is circulated through a very porous orebody to dissolve the uranium oxide and bring it to the surface. ISL may be with slightly acid or with alkaline solutions to keep the uranium in solution. The uranium oxide 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, safety and economic considerations.

For more detailed information see the information pages on:

• Supply of Uranium 
• World Uranium Mining 
• In Situ Leach (ISL) Mining of Uranium 

Uranium milling

Milling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate which is shipped from the mill. It is sometimes referred to as 'yellowcake' and generally contains more than 80% uranium. The original ore may contain as little as 0.1% uranium, or even less.

In a mill, uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium oxide. The uranium oxide is then precipitated and removed from the solution. After drying and usually heating it is packed in 200-litre drums as a concentrate, sometimes referred to as 'yellowcake'.

The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in mined out pit). Tailings need to be isolated from the environment because they contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived.

Conversion and enrichment

The uranium oxide product of a uranium mill is not directly usable as a fuel for a nuclear reactor and additional processing is required. Only 0.7% of natural uranium is 'fissile', or capable of undergoing fission, the process by which energy is produced in a nuclear reactor. The form, or isotope, of uranium which is fissile is the uranium-235 (U-235) isotope. The remainder is uranium-238 (U-238). For most kinds of reactor, the concentration of the fissile uranium-235 isotope needs to be increased – typically to between 3.5% and 5% U-235. This is done by a process known as enrichment, which requires the uranium to be in a gaseous form. The uranium oxide concentrate is therefore first converted to uranium hexafluoride, which is a gas at relatively low temperatures.

At a conversion facility, the uranium oxide is first refined to uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium. Most is then converted into uranium hexafluoride, ready for the enrichment plant. The main hazard of this stage of the fuel cycle is the use of hydrogen fluoride. The uranium hexafluoride is then drained into 14-tonne cylinders where it solidifies. These strong metal containers are shipped to the enrichment plant.

The enrichment process separates 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 U-235 and is called 'tails', or simply depleted uranium.

There are two enrichment processes in large-scale commercial use, each of which uses uranium hexafluoride gas as feed: diffusion and centrifuge. These processes both use the physical properties of molecules, specifically the 1% mass difference between the two uranium isotopes, to separate them. The last diffusion enrichment plants are likely to be phased out by 2013.

The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide. Up to this point the fuel material can be considered fungible (though enrichment levels vary), but fuel fabrication involves very specific design.

Enrichment is covered in detail in the page on Uranium Enrichment.

Fuel fabrication

Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide (UO2) which is sintered (baked) at a high temperature (over 1400°C)^a. The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly 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 the fuel.

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 most unlikely, but in plants handling special fuels for research reactors this is a vital consideration.

Power generation and burn-up

Inside a nuclear reactor the nuclei of U-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 which produces electricity. Some of the U-238 in the fuel is turned into plutonium in the reactor core. The main plutonium isotope is also fissile and this yields about one third of the energy in a typical nuclear reactor. The fissioning of uranium (and the plutonium generated in situ) is used as a source of heat in a nuclear power station 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.

Typically, some 44 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 metres of gas.

An issue in operating reactors and hence specifying the fuel for them is fuel burn-up. This is measured in gigawatt-days per tonne and its potential is proportional to the level of enrichment. Hitherto a limiting factor has been the physical robustness of fuel assemblies, and hence burn-up levels of about 40 GWd/t have required only around 4% enrichment. But with better equipment and fuel assemblies, 55 GWd/t is possible (with 5% enrichment), and 70 GWd/t is in sight, though this would require 6% enrichment. The benefit of this is that operation cycles can be longer – around 24 months – and the number of fuel assemblies discharged as used fuel can be reduced by one third. Associated fuel cycle cost is expected to be reduced by about 20%.

As with as a coal-fired power station about two thirds of the heat is dumped, either to a large volume of water (from the sea or large river, heating it a few degrees) or to a relatively smaller volume of water in cooling towers, using evaporative cooling (latent heat of vapourisation).

Used fuel

With time, the concentration of fission fragments and heavy elements formed in the same way as plutonium in the fuel 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 that is produced from a fuel bundle varies with the type of reactor and the policy of the reactor operator.

When removed from a reactor, the fuel 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. It may be transferred to ventilated dry storage on site. 

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 94% U-238 but it also contains almost 1% U-235 that has not fissioned, almost 1% plutonium and 4.5% 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, which contains 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). See page on Processing of Used Nuclear Fuel.

Uranium and plutonium recycling

The uranium from reprocessing, which typically contains a slightly higher concentration of U-235 than occurs in nature, can be reused as fuel after conversion and enrichment.

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 U-235 in normal uranium oxide fuel (see page on Mixed Oxide (MOX) Fuel).

Used fuel disposal

At the present time, 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 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 it is stored the easier it is 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 which could be reprocessed at a later date to allow recycling of the uranium and plutonium. There is also a proposal to use it in Candu reactors directly as fuel. This proposal, known as DUPIC (direct use of used PWR fuel in Candu reactors) is covered at the end of the page on Processing of Used Nuclear Fuel.

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

Wastes

Wastes from the nuclear fuel cycle are categorised 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 U-235 is significantly less than the 0.7% found in nature. Small quantities of this material, which is primarily U-238, are used in applications where high density material is required, including radiation shielding and some is used in the production of MOX fuel. While U-238 is not fissile it is a low specific activity radioactive material and some precautions must, therefore, be taken in its storage or disposal.

See page on Waste Management in the Nuclear Fuel Cycle.

Material balance in the nuclear fuel cycle

The following figures may be regarded as typical for the annual operation of a 1000 MWe nuclear power reactor:^b 

Mining	20,000 tonnes of 1% uranium ore
Milling	230 tonnes of uranium oxide concentrate (which contains 195 tonnes of uranium)
Conversion	288 tonnes uranium hexafluoride, UF6 (with 195 tU)
Enrichment	35 tonnes enriched UF6 (containing 24 t enriched U) – balance is 'tails'
Fuel fabrication	27 tonnes UO2 (with 24 t enriched U)
Reactor operation	8760 million kWh (8.76 TWh) of electricity at full output, hence 22.3 tonnes of natural U per TWh
Used fuel	27 tonnes containing 240 kg transuranics (mainly plutonium), 23 t uranium (0.8% U-235), 1100kg fission products.

---

Further information

Notes

a. UO2 has a very high melting point – 2865°C (compared with uranium metal – 1132°C). [Back]

b. Figures are based on the following assumptions: enrichment to 4% U-235 with 0.25% tails assay – hence 140,000 SWU (separative work units) of enrichment needed (one SWU requires about 50kWh of electricity at a gas centrifuge enrichment plant); core load 72 tU, refuelling so that 24 tU/yr is replaced; operation – 45,000 MWday/t (45 GWd/t) burn-up, 33% thermal efficiency. In fact, a 1000 MWe reactor cannot be expected to run at 100% load factor – 90% is more typical, so an output of around 7.75 TWh/yr is more realistic, but this simply means scaling back the inputs accordingly, e.g. to 175.5 tU.

Canadian figures for tU/GWe/yr suggest slightly lower uranium requirements and utilization for PHWRs than for light water reactors. An International Atomic Energy Agency technical report¹ gives 157 tU at typical 7.5 GWd/t burn-up and 31% thermal efficiency, or 142 tU at 90% capacity factor, hence 80% of the input compared with a typical LWR above. This is 17.9 tU/TWh.

Considering just how much of the original uranium is actually used: 0.7% fissile U-235 is in natural U (Unat), on above figures 0.49% of Unat goes into fuel as the fissile part, 0.39% is actually fissioned, and in addition about half that much U-238 turned into Pu-239 is fissioned, giving about a 0.6% utilization of the original Unat. [Back]

References

1. Heavy Water Reactors: Status and Projected Development, Technical Reports Series No. 407, International Atomic Energy Agency, 2002, STI/DOC/010/407 (ISBN: 9201115024). PHWR data is taken from Chapter 6, HWR Fuel Cycles [Back]

credit to : www.iaea.org