Assignment five

5.1  particles in the nucleus and diagrams of half life.

The nuclear model.

Atoms contain 3 sub atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

here are three main types of radiation, called alpha, beta and gamma radiation, which all have different properties. The half-life of a radioactive isotope is the time taken for half its radioactive atoms to decay. Nuclear equations describe what happens during alpha and beta decay.

Half-life

The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay.

The radioactivity of an object is measured by the number of nuclear decays it emits each second – the more it emits, the more radioactive it is.

Half-life

It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope.

There are two definitions of half-life, but they mean essentially the same thing. Half-life is the time taken for:

·         The number of nuclei of the radioactive isotope in a sample to halve

·         The count rate from a sample containing the radioactive isotope to fall to half its starting level

Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.

Graphs

Radioactivity decreases with time. It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time. The graph shows the decay curve for a radioactive substance.

5.2  results and graph of radioactive decay.

(1) Determination of the half-life of a Radioisotope

• The radioactivity from a radioisotope is measured over a period of time.
• Graphical or mathematical analysis is performed to calculate the time it takes for the radioactivity of the isotope to halve.
• For short-lived radioactive isotopes, the radioactivity is likely to be measured in terms of the count rate.
• Therefore the half-life will be the time it takes for the count rate to halve.
• An example of what this means is shown in the diagram below.
• The graph shows the rapid decay of a very unstable radioactive isotope in terms of count rate per minute (cpm) versus minutes.
• From the graph you can work out the time (half-life) it takes for half of the radioactive atoms to decay from the decrease in count rate.
• e.g. in terms of time elapsed, count rate ==> we get
• 0s, 400cpm ==> 10min, 200cpm ==> 20min, 100 cpm etc.
• clearly showing the half-life is 10 minutes.
• You need to practice these sort of calculations of half-life determination, radioactive residue left, and dating calculations (see below) using the multiple choice QUIZ (higher GCSE = AS GCE)

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(2) Using half-life data in hazard analysis or prediction of radioisotope residue

• From the half-life you can calculate how much of the radio-active atoms are left e.g. after one half-life, ¹/₂ is left, after two half-lives, ¹/₄ is left, after three half-lives, ¹/₈ is left in other words its a 'halving pattern' etc.
• Example Q: The half-life of a radioisotope is 10 hours. Starting with 2.5g, how much is left after 30 hours?
• 2.5g =10h=> 1.25g =10h=> 0.625g =10h=> 0.3125g (after total time of 30h)
• Another way to think - if the time elapsed is equal to a whole number of half-lives you can just divide the 30 h by 10 h, giving 3 half-lives.
• Therefore you just have to halve the amount three times!
• e.g. 2.5 ==> 1.25 ==> 0.625 ==> 0.3125g
• The half-life of a radioisotope has implications about its use and storage and disposal.
• If the half-life is known then the radioactivity of a source can be predicted in the future (see (1) above).
• Plutonium-244 produced in the nuclear power industry has a half-life of 40 000 years!
• Even after 80 000 years there is still a 1/4 of the dangerously radioactive material left.
• Quite simply, the storage of high level nuclear reactor radioactive waste is going to be quite a costly problem for many (thousands?) of years!
• Storage of waste containing these harmful substances must be stable for hundreds of thousands of years! So we have quite a storage problem for the 'geological time' future! see also dangers and background radiation.

Radioisotopes used as tracers must have short half-lives, particularly those used in medicine to avoid the patient being dangerously over exposed to the harmful radiation, but a long enough half-life to enable accurate measurement and monitoring of the tracer.

 

Archaeological dating with the isotope carbon-14

* Most carbon atoms are of the stable isotope carbon-12. A very small % of them are radioactive due to carbon-14 with a half-life of 5700 years. It decays by beta emission to stable nitrogen-14. Archaeologists can use any material containing carbon of 'organic living' origin to determine its age. This can be bone, wood, leather etc. and the technique is sometimes called radiocarbon-14 dating.

• When the 'carbon containing' material is in a living organism there is a constant interchange of carbon with the environment as food or carbon dioxide. This means the carbon-14 % remains constant. When the organism is dead the exchange stops and the carbon-14 content of the material begins to fall as it radioactively decays.
• Compared to when it was 'alive' ...
• if an object has ¹/₂ (¹/₂ of 1, 50%) of the expected carbon-14 it must be 5700 years old,
• if it only has ¹/₄ (¹/₂ of a ¹/₂, 25%) of the expected ¹⁴C left, the object it must be 11400 years old (5 700 + 5 700),

and if only ¹/₈ (¹/₂ of ¹/₄, 12.5%) of the ¹⁴C left it is 17100 years old (11 400 + 5700) etc. etc.

 5.3 how to calculate half life examples

 5.4 properties of ionising radiation

 Ionising radiation

Radioactive substances give out radiation all of the time. There are three types of nuclear radiation: alpha, beta and gamma. Alpha is the least penetrating, while gamma is the most penetrating. Nonetheless, all three are ionising radiation: they can knock electrons out of atoms and form charged particles.

Radiation can be harmful, but it can also be useful - the uses of radiation include to:

·         detect smoke

·         gauge the thickness of paper

·         treat cancer

·         sterilise medical equipment.

Types of radiation

Nuclear radiation comes from the nucleus of an atom. Substances that give out radiation are said to be radioactive. All radiation transfers energy. There are three types of nuclear radiation:

·         alpha

·         beta

·         gamma.

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.

The thicker the substance, the more radiation is absorbed. The three types of radiation penetrate materials in different ways.

Penetrative properties of different types of radiation

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

 

Type of radiation emitted & symbol	Nature of the radiation (higher only)	Nuclear Symbol (higher only)	Penetrating power, and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more penetrating)	Ionising power - the ability to remove electrons from atoms to form positive ions
Alpha	a helium nucleus of 2 protons and 2 neutrons, mass = 4, charge = +2		Low penetration, biggest mass and charge, stopped by a few cm of air or thin sheet of paper	Very high ionising power, the biggest mass and charge of the three radiation's, the biggest 'punch'!
Beta	high kinetic energy electrons, mass = 1/1850, charge = -1		Moderate penetration, 'middle' values of charge and mass,most stopped by a few mm of metals like aluminium	Moderate ionising power, with a smaller mass and charge than the alpha particle
Gamma	very high frequency electromagnetic radiation, mass = 0, charge = 0		Very highly penetrating, smallest mass and charge, most stopped by a thick layer of steel or concrete, but even a few cm of dense lead doesn't stop all of it!	The lowest ionising power of the three, gamma radiation carries no electric charge and has virtually no mass, so not much of a 'punch' when colliding with an atom

 

5.5   effects of radiation on cells, the dangers of isotopes.

 

Benefits and risks

Radiation that is not absorbed by the atmosphere reaches the Earth's surface and warms it, leading to the greenhouse effect. Some radiation, such as ultraviolet, exposes our skin to harmful rays and puts us at risk of developing skin cancer.

The atmosphere

Some radiation of the electromagnetic spectrum is absorbed by the atmosphere, but some is transmitted.

Light, some infrared, some ultraviolet, and microwaves, pass through the atmosphere and reaches the Earth’s surface. Gamma rays, X-rays, most of the ultraviolet and some of the infrared are absorbed by the atmosphere and do not reach the Earth’s surface.

Infrared

Infrared from the Sun reaches the Earth’s surface and warms it.

The warm Earth emits some infrared radiation, and some of this is absorbed by gases in the atmosphere. This is called the greenhouse effect. If there were no greenhouse effect, the Earth would be too cold for life as we know it.

Photosynthesis

Light from the Sun reaching the Earth’s surface provides the energy for plants to produce food by photosynthesis.

Photosynthesis replaces carbon dioxide in the atmosphere with oxygen. This reverses the process of respiration.

Microwaves

The atmosphere transmits microwaves, and these can be used to communicate with satellites.

Radiation and cell damage

Any radiation absorbed by living cells can damage them by heating them. However, ionising radiations are more likely to damage living cells. This is because photons of ionising radiation deliver much more energy. They can easily kill cells, and can also cause cancer by damaging the DNA in the nucleus of a cell.

Effects of microwaves

 

Microwaves in the environment may be harmful, but there is no agreement on this. They are not ionising, and so cannot cause cancer in the way that ultraviolet, X-rays or gamma rays do.

Microwave ovens work because the food contains water molecules which are made to vibrate by the microwaves. This means that food absorbs microwaves and gets hot. The microwaves cannot escape from the oven, because the metal case and the metal grid on the door reflect microwaves back into the oven.

Some people think that mobile phones, which transmit and receive microwaves, may be a health risk. This is not accepted by everyone, as the intensity of the microwaves is too low to damage tissues by heating, and microwaves are not ionising.

Ultraviolet

One health risk which is definitely present in our environment is ultraviolet, in sunlight. Not much of the ultraviolet reaching the Earth gets to us, because the ozone layer high up in the atmosphere absorbs most of it. In the summer, it is wise to use sun-screens and clothing to absorb ultraviolet, and prevent it reaching the sensitive cells of the skin.

 

4b. The Dangers of Radioactive Emissions - beware of ionising radiations from radio-isotopes!

The penetration trends and the effects of Ionisation from radioisotopes

All radioactive emissions are extremely dangerous to living organisms. When alpha, beta or gamma radioactive emissions hit living cells they cause ionisation (ionization) effects, they can kill cells directly or cause genetic damage eg to the DNA molecules. High radiation doses cause burn effects and can kill cells. However, low doses don't kill the cells, but if they are genetically damaged and can still replicate, these mutations can lead to the formation of cancerous cells and tumor development later. When alpha, beta and gamma radiation collide with neutral atoms or molecules they knock off electrons and convert them into charged or ionised particles (ions). Positive ions are formed on electron loss and negative ions are formed by electron gain. The positive ions maybe unstable and very reactive and cause other chemical changes in the cell molecules. The 3 radiations have different capacities to cause cell damage.

• If the radioactive source, a 'radionuclide', gets inside the body the 'danger' order is alpha > beta > gamma. The bigger the mass or charge of the particle, the bigger its ionising impact on atoms or molecule. BECAUSE the order of mass is 4 > ¹/₁₈₅₀ > 0, and for electric charge the order is 2+ > -1 > 0. If the radioisotope is in the body the radiation impacts directly on cells with the consequences described above.
• However, if the radioactive source is outside the body, the order danger is reversed to gamma > beta > alpha because the danger order follows the pattern of penetrating power. The smaller the mass and charge the more penetrating the radiation (reverse the order of above). Gamma and beta are the most penetrating and will reach vital organs in the body and be absorbed. Most gamma passes through soft tissue but some is inevitably absorbed by cells. Alpha radiation would not penetrate clothing and is highly unlikely to reach living cells.
• Because of the dangers of this ionising or atomic radiation, all workers and medical staff who are likely to be near radioactive or ionising sources must wear lapel radiation badges containing photographic film to monitor their exposure to radiation. The film is regularly developed and the darker the film the more radiation would have impacted on the person.
• Examples of precautions that can be taken include:
• Radiographers wear lead lined aprons and anyone else involved in radiotherapy cancer treatment must take particular precautions and radiation monitored.
• In nuclear fuel preparation and reprocessing, as much work is done using robotic control systems in behind steel, concrete, lead or thick lead glass panels for visual monitoring of the situation.
• In research laboratories, experiments are conducted in sealed fume cupboards at the laboratory side and technicians work through sealed whole arm gloves through a thick lead glass front. You can also reduce the pressure in the fume cupboard so there is no chance of pressure leakage out into the laboratory area.

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5.6   isotopes in the home and at work, good and bad points.

Alpha emitters

Because alpha particles are easily stopped, an alpha source is used in some smoke detectors. A sealed alpha source of Americium-241 (half-life 458 years, producing constant signal) sends a stream of alpha particles to a sensor across an air gap. Any smoke present will block the alpha particles and change the sensor signal, this change in signal triggers the alarm. Beta and gamma radiation would be of no use because the smoke particles would not stop them, no change in signal, no alarm triggered!

 Alpha sources are too readily absorbed to show up with a Geiger counter or other detector and so are not suitable for 'tracer' applications.

However, an alpha particle emitting isotope of radium (radium-233, half-life 11.4 days) can be directly injected in tiny quantities into tumorous tissue to directly irradiate and kill cancer cells, an excellent medical use of an alpha emitter. Since they are not very penetrating, there is less chance of damaging healthy cells.

 

 Most Beta particles are stopped by a few mm or cm of solid materials. The thicker the layer the more beta radiation is absorbed. A beta source is placed on one side of a sheet of material. A detector (e.g. a Geiger counter) is put on the other side and can monitor how much radiation gets through. The signal size depends on thickness of the sheet and it gets smaller as the sheet gets thicker. Therefore the signal can be used to monitor the sheet thickness. The half-life must be quite long so that change in the signal does not result from rapid decay.

 

This idea is used to control production lines of paper, plastic or steel sheeting. Before the sheet material passes through 'flattening' rollers, it passes between a beta source and detector. The detector signal is checked against that for a preset thickness. If the signal is too big the sheet is too thin and the rollers are moved apart to thicken the sheet. If the signal is too small the sheet is too thick and the rollers are moved closer together.

Uses of gamma radiation sources

 

 Gamma radiation is highly penetrating and so gamma sources are used where the radiation must be detected after passing through an appreciable thickness of material. This is used in various tracer situations and usually the half-life should be relatively short to avoid any health hazards.

A gamma emitting tracer can be added to the flow of water in a pipe and the outside of the pipes monitored with a Geiger counter. Any leaks would be detected by an increase in radiation reading. The flow of water in underground streams can be followed in a similar way.

 Radiotherapy: It seems ironic that the very radiation which causes cancer, can also be used to treat it. A beam of gamma radiation is directed onto the tumor site to kill the cancer cells. Unfortunately the radiation passes through the 'good' tissue too and kills or damages 'good' cells. Modern techniques use multiple rotating gamma sources that are focused on to the tumor. This means the surrounding 'good cells' are less frequently hit and minimises potential harmful side-effects on the rest of the body (e.g. sickness or other mutations). Radiotherapy also avoids the need for intrusive surgery which has its own risk factors. The gamma emitters used have relatively long half-lives to give the instrument a good working life.

Gamma radiation can be used in a non-destructive way to test the structure of a material.

In a sense it is an alternative to X-ray photography for more dense materials e.g.

It is used test the structure and quality of pipe welds.

A gamma source is placed inside the pipe and photographic paper wrapped around the weld.

If there is any gap or flaw in the weld, more gamma radiation gets through and shows up as increased exposure on the 'gamma-ray picture'.

Its better to find out the fault now, rather than later when it fractures, and has to be 'dug up' or retrieved from the bottom of the sea!

Because gamma radiation is so deadly and penetrating it can be used to sterilise surgical equipment or packaged food:

The radiation is deadly for bacteria even in the most microscopic pockets of apparently smooth and shiny stainless steel of surgical instruments.

It is very convenient for 'convenience' food!. After cooking and sealing in a plastic packet, you don't need to reopen to complete the sterilization to give it a long shelf-life!

 5.7 case study isotope. Good and bad points.

Technetium-99 is a gamma emitter (half-life 6 hours) and is used in medicine as a tracer.

In medical applications, in a suitable chemical form, the radioisotope is injected into the body and its 'movement' can be followed.

Time is allowed for the radioactive tracer to spread and its progress tracked with a detector outside the body.

The patient can be placed next to a 'detection screen' that shows where the radioactive tracer is.

The effective function of organs like the liver and digestion system can be checked.

Similarly, a patient can breathe in air with a gaseous gamma emitter in it, and the effectiveness and structure of the lungs can be checked.

The half-life must be relatively short so it does not linger in the body increasing the harmful effects of cell damage.

Technetium atoms can be incorporated into many organic chemicals called radiopharmaceuticals which can be used to monitor biochemical aspects of the bodies chemistry e.g. the functioning and performance of a particular organ.

 

5.8 controlling fission in a nuclear power station

Nuclear fission

Nuclear power reactors use a reaction called nuclear fission. The fission is a source of energy for the generation of power. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239.

Splitting atoms

Fission is another word for splitting. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors. Their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons.

When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:

·         The nucleus splits into two smaller nuclei – daughter nuclei, which are radioactive

·         Two or three more neutrons are released

·         Some energy is released

The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it moving too quickly.

Nuclear power

A power station makes electricity. Fossil fuel (coal, oil and gas) power stations and nuclear (uranium) power stations both use the same processes to make electricity. These are:

1.    Fuel produces heat and heat is used to boil water and is turned into steam

2.    Steam turns a turbine

3.    Turbine turns a generator and the generator makes electricity

4.    Electricity goes to the transformers to produce the correct voltage

The only difference between fossil fuel and nuclear power stations is how the water is heated. Fossil fuel power stations burn their fuel while a nuclear power station uses the fission of uranium to generate heat.

Uranium is a non-renewable energy resource.

Radioactive waste

All nuclear reactors produce radioactive waste. At present the most dangerous waste is sealed in glass-like blocks which are buried deep within rocks. Careless disposal of waste in the past has led to pollution of land, rivers and the ocean.

Nuclear reactors

As well as producing heat the nuclear reactor can be used to make other materials radioactive. The chain reaction inside the reactor releases neutrons. If a material is put into the reactor some of these neutrons may be absorbed by the nuclei of its atoms. This will make an atom's nucleus unstable which means it has become radioactive. These man-made radioisotopes are often then used astracers in hospitals to diagnose and treat patients or in industry to detect leaks in pipes.

Higher tier

The nuclear reactor is designed to allow a controlled chain reaction to take place. Each time a uranium nucleus splits up it releases energy and three neutrons. If all the neutrons are allowed to be absorbed by other uranium nuclei the chain reaction will spiral out of control causing an explosion. To control the energy released in the reactor moveable control rods are placed between the fuel rods. These control rods are made of boron which absorbs some of the neutrons so fewer neutrons are available to split uranium nuclei. The control rods are raised to increase and lowered to decrease the number of free neutrons.

 

5.9 controlling a fusion reaction

Nuclear fusion

Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens. Nuclear fusion can also be used as a source of energy.

The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei. Here is one nuclear fusion reaction that takes place:

A nuclear fusion reaction showing the nuclei involved

Nuclear fusion involves a deuterium and a tritium nucleus colliding and being forced together. Both nuclei are positively charged and therefore will repel each other. This is known as electrostatic repulsion. The nuclei have to get very close in order to collide, which is approximately a million millionth of a millimetre. If the nuclei are moving very fast then they can overcome the electrostatic repulsion. The hotter a molecule is, the faster it will move and the more likely it is to collide.

 

For a nuclear fusion reactor to work, the temperature and pressure would each have to be very high. These extremely high temperatures and pressures are very difficult to reproduce and are very expensive. As a result, fusion as an energy source is a long way off.

 

 

 

 

 

 

 

 

 

5.10 Case study of Chernobyl

 

Chernobyl, Ukraine

(Case study of a nuclear reactor explosion)

 

Location: The Chernobyl nuclear power plant is located in the Ukraine, formerly  part of the Union of Soviet Socialist Republics (USSR).

 

What happened?

• On 26 April 1986 there was a catastrophic explosion and fire at the Chernobyl nuclear power plant. A nine-tonne radioactive cloud of debris blasted into the atmosphere and spread over parts of the western USSR, Eastern Europe, and Scandinavia. This was the worst nuclear power accident in history.
• The fire was put out and the reactor encased in concrete by ‘liquidators’, who could work for only 40 seconds before they received a lethal dose of radiation, and many have since died. The surrounding people, animals, and land were badly contaminated.
• Perhaps 375 000 people were eventually evacuated from the area. But 10 days passed before any evacuation. Caesium-37 caused radiation throughout much of the northern hemisphere, and iodine-131 triggered thyroid cancer in local people, particularly children.

 

What has happened now?

• 160 000 sq km remain contaminated. Even sheep farms in the Lake District (UK) still face restrictions on selling their lambs because of continuing contamination of the vegetation.
• 1000 people stayed in Chernobyl because the power plant continued to generate much-needed electricity until it finally closed in December 2000.
• Some elderly people have moved back to farm the land because they have no other way of getting food. The crops they grow are contaminated.
• The Ukraine branch of Greenpeace estimates that more than 32,000 people have died and 1.25 million people are directly affected. Children are badly affected.
• All the evacuations, medical care etc. have cost over (US) $250 billion.
• The economies of local states have been devastated, economic development has almost ceased and the quality of life has deteriorated throughout the region.

 

The nuclear energy debate:

 

For	Against
Only very limited raw materials are needed, i.e. 50 tonnes of uranium per year compared with 540 tonnes of coal per hour needed for coal-fired stations.	It is not clear how safe it is. Globally there have been several leaks, with a serious explosion at Chernobyl in the Ukraine.
Oil and natural gas could be exhausted by the year 2030. Coal is difficult to obtain and dirty to use.	There is a large conservationist lobby which claims that one accident may kill many, and ruin an area of ground for hundreds of years.
Numerous safeguards make the risks of any accident minimal.	Many people think that renewable energy forms should be used rather than those that are non-renewable.
Nuclear waste is limited and can be stored underground.	Potential health risks i.e. high incidence of leukaemia.
Nearly all the money spent in Britain on energy research has been on nuclear power.	Nuclear waste can remain radioactive for many years. There are problems with reprocessing and then storing nuclear waste.
Nuclear energy schemes have the support of large firms and government departments.	The cost of decommissioning old power stations is extremely high.
Nuclear power is believed to contribute less than conventional fuels to the greenhouse effect and acid rain.	I

 

 

 

 

 

5.11 long-term effects of fission material on environment

The most long-lived radioactive wastes, including spent nuclear fuel, must be contained and isolated from humans and the environment for a very long time. Disposal of these wastes in engineered facilities, or repositories, located deep underground in suitable geologic formations is seen as the reference solution.[4] The International Panel on Fissile Materials has said:

It is widely accepted that spent nuclear fuel and high-level reprocessing and plutonium wastes require well-designed storage for periods ranging from tens of thousands to a million years, to minimize releases of the contained radioactivity into the environment. Safeguards are also required to ensure that neither plutonium nor highly enriched uranium is diverted to weapon use. There is general agreement that placing spent nuclear fuel in repositories hundreds of meters below the surface would be safer than indefinite storage of spent fuel on the surface.[5]

Common elements of repositories include the radioactive waste, the containers enclosing the waste, other engineered barriers or seals around the containers, the tunnels housing the containers, and the geologic makeup of the surrounding area.[6]

The ability of natural geologic barriers to isolate radioactive waste is demonstrated by the natural nuclear fission reactors at Oklo, Africa. During their long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the uranium ore body. This plutonium and the other transuranics remained immobile until the present day, a span of almost 2 billion years.[7] This is quite remarkable in view of the fact that ground water had ready access to the deposits and they were not in a chemically inert form, such as glass.

Despite a long-standing agreement among many experts that geological disposal can be safe, technologically feasible and environmentally sound, a large part of the general public in many countries remains skeptical.[8] One of the challenges facing the supporters of these efforts is to demonstrate confidently that a repository will contain wastes for so long that any releases that might take place in the future will pose no significant health or environmental risk.

Nuclear reprocessing does not eliminate the need for a repository, but reduces the volume, reduces the long term radiation hazard, and long term heat dissipation capacity needed. Reprocessing does not eliminate the political and community challenges to repository siting.[5]