Nuclear fission reactors are not just a source of heat for power production. They are also an abundant source of neutrons. As neutrons have no charge, they have the ability to insert themselves in the nuclei of a wide variety of elements, sometimes turning the material into a radioactive isotope of the original material that will then decay into another element.

Canada as one of the primary sources of isotopes for medical and industrial use because the type of nuclear reactor they have built there (light water reactors) makes nuclear transmutation easier to do. They therefore make the isotopes and sell them around the world.

Depleted uranium is a by-product of the nuclear fuel industry. It gets its name because it's been stripped, or depleted, of most of its content of Uranium 235 - which is used as fuel in nuclear reactors. It is uranium 238 - an alpha emitter.

Depleted uranium is used in the production of anti-tank weapons and bullets because it's an extremely dense metal; a shell tipped with DU can rip through a heavily armoured tank! There is considerable controversy over whether it is safe or ethical to use such shells.

Many media sites have considerable information on this but it arouses strong feelings and therefore many of the accounts will be biased one way or the other. Always look at who wrote an article and think why they did it. Also always try to find a range of views and look for use of good sound scientific principles in any argument.
 
 

Disposal of nuclear waste

High Level Waste

High Level Waste includes spent fuel and highly radioactive liquids generated during reprocessing operations. The latter is all stored at Sellafield in high-integrity stainless steel tanks fitted with cooling coils to remove the heat generated. Management and disposal of this waste is difficult due to the high levels of radioactivity, the very long half-lives of some of the radionuclides present and the heat continually generated as a result of decay processes.
 
Current practice is to store these wastes, encapsulated in glass, in air cooled steel containers for 50 years to allow the heat generated to reduce to manageable levels, and then finally dispose of them in a deep mine.

Intermediate Level Waste

Intermediate Level Waste includes fuel element claddings removed prior to reprocessing, various sludges and ion exchange resins from fuel storage pond water treatment; concentrates of liquid waste streams; heavily contaminated scrap equipment; plutonium contaminated materials and graphite sleeves and steel components from AGR fuel assemblies. Large volumes of Intermediate Level Waste are expected from decommissioning operations are expected in the coming years.

Because of the wide range of Intermediate Level Waste sources many different forms of conditioning and packaging are required.
 
 

Low Level Waste

A lot of material classified as Low Level Waste, may in fact not be radioactive at all, but it is potentially radioactive through being in an active/contaminated area.

The low levels of radioactivity and the short-lives of the contaminants mean this waste is relatively harmless if handled properly. However, any site used for Low Level Waste disposal will need to be subject to land use restrictions for around 300 years after the site is closed. There is also always a risk of environmental problems if water leaching through the waste site finds its way into surface and ground waters.
 
 

Medical uses of nuclear radiation

Radioactive tracers

A radioactive tracer is very useful to doctors. A small amount of radioisotope is made to replace a non-radioactive isotope of an element in a compound that normally performs a task in the body. The path of that radioisotope or one of its daughter nuclei (product of the decay) is then monitored by detecting the emitted nuclear radiation.
 

The type of radiation emitted by a medical tracer is very important. Gamma emitters are ideal because of their low ionizing power. But gamma emission often follows closely on alpha or beta emission making the isotope unsuitable. Ideally we need to find a pure gamma emitter (one with a metastable nucleus). The nucleus should not be likely to emit a or b after emissionof the g ray either so decay to the next atom in the radioactive decay series should involve a long half-life. The energy of the g-ray emitted should be suitable for viewing with a gamma-camera (energy range 100 - 400 keV gives optimum detection, is easy to collimate and has low attenuation in the body).

For example:  

Technetium 99m

Technetium 99m is a very useful radioisotope. It is the most commonly used one in hospitals because it is ideal to use with a gamma camera and only emits gamma rays.
 
 

Technetium 99m is a very useful radionuclide in gamma imaging because:

• it only emits gamma rays and these are of an energy that is easily detected by a gamma camera (140keV).
• It has an ideal half life (six hours) which is long enough for diagnostic procedures to take place but short enough for the patient not to be inconvenienced unduly by remaining 'radioactive' for too long a period after the investigation. (See graph above)
• It is suitable not only for use alone but also for attachment to a wide range of compounds for tracer experiments.
• It can easily be produced 'in situ' using a 'cow' as it is the daughter nucleus of the decay of Molybdenum 99 and is easily separated from the parent (more hazardous beta-emitter) by a saline flush. A fresh supply is brought to the hospital fortnightly and then the 'cow' is 'milked' as and when required.

The gamma camera allows an image to be formed on a screen that shows how intensely gamma rays are being emitted from a particular part of the body. A Technetium-99m antimony sulphide colloid can be used as a radioactive marker to examine the function of the lymph nodes.

It is carefully injected into the drainage areas to be visualised.
 

A 'butterfly' (so-called because of its appearance) can be used to ensure that an injection of radioactive material is inserted into the correct site. It allows the doctor or nurse to check that the needle is inserted properly before the radioactive fluid is pumped into the patient. The fluid needs to be injected into the blood stream. If it goes into tissue it will be concentrated in a small area instead of being distributed throughout the blood (a large volume!) - it could therefore cause tissue damage.

The radionuclide in the syringe is shielded to protect the person giving the injection. It is important to minimise the dose of personnel working with radioactive materials as each time they are exposed to radiation they increase the odds on their contracting cancer at a later date.

(Above two images from CALRAD an interactive educational package designed by an educational consortium of several Universities - I came across it via the Open University module S803)
 
 

The gamma ray is electromagnetic radiation of very high penetration power. Therefore more rays exit the body and are available for detection than interact with the patient's tissue. The patient is now emitting gamma rays!

The doctor then gets the patient to wait for a while. This will give the tracer-chemical a chance to accumulate in the parts of the body the non-radioactive chemical would have done normally.

Cancerous cells divide more frequently than non-cancerous ones and therefore a 'hot-spot' of high activity results from any cancerous growth. A gamma camera is scheduled to scan the area of interest approximately four hours after the patient has been injected with the radioactive tracer. This gives it time to circulate and accumulate in 'hot-spots' of rapid cell division. A second scan is then taken within the 24-hour period and compared to the first to diagnose suspicion of overactive lymph node activity.
 
 

PET (Positron Emission Tomography)

PET is an imaging technique with many uses. It can be used for:

• bone imaging,
• monitoring tumour metabolism,
• monitoring the function of the heart : cardiac perfusion and myocardial blood-flow monitoring,
• studying fatty acid metabolism

When a positron is emitted it is soon annihilated and a pair of gamma rays is produced.
 
 

These gamma rays fly off in opposite directions and can be viewed using a PET Scanner. This is a special type of gamma camera that is designed to count only gamma rays that are produced in pairs. It is linked to a computer that can deduce where the annihilation of the positron took place.

There are many PET sites on the WWW but care has to be taken when searching because of confusion with 'pet animals'! (inclusion of the word 'gamma' usually overcomes this)

A patient is injected with a radioactive tracer that emits gamma rays. The gamma ray is electromagnetic radiation of very high penetration power. Therefore more rays exit the body and are available for detection than interact with the patient's tissue. These can be detected by a gamma camera and the concentration of radioactive tracer in various parts of the body can be ascertained.

Gamma rays cannot be focused by refraction (like light rays can be) therefore a lead collimator is used to direct rays from a point on the patient towards a single point on a sodium iodide crystal. The collimator absorbs g-rays emanating from other parts of the body before they activate the crystal ensuring that only rays from the relevant part of the body hit the crystal

A crystal of sodium iodide fluoresces when a gamma ray interacts with one of its orbital electrons, promoting it to a higher energy level. On its return to ground state a photon is emitted. If this energy is in the visible region a flash of light is seen.  

This light is detected using a photomultiplier and changed into an electrical output so that it can be displayed on a monitor.The electrical output can be translated into colour coded graphical display using electronic circuitry and the camera display can be made on a monitor and videoed. Hard copies of the scans can then be taken and compared. The electrical output can be translated into colour coded graphical display using electronic circuitry.
 
 

Image of heart, which has had a myocardial infarction (heart attack). The arrow points to areas that have been damaged by the attack, indicating "dead" myocardial tissue. Therefore, the patient will not benefit from heart surgery, but may have other forms of treatment prescribed.	Image of a normal heart         Images taken from:      http://www.biomed.org/pet.html#what	Note that in this scan only the red/yellow/white/black has been used. the rest of the colours haven't been programmed in.

Above image taken from: http://www.sielimaging.com/images/gamma_cam1.jpg

Radiation Therapy

Tumours can be treated using gamma or X-rays. Cells that divide rapidly are more prone to damage by high-energy electromagnetic radiation. This means that tumour cells are more radiosensitive than their normal counterparts. By carefully aiming the rays at the tumour (gamma-ray beams directed from a multitude of angles that result in the maximum gamma ray intensity within the tumour) the harmful effect of the ionising radiation is kept to a minimum in the surrounding tissue. This kind of treatment is most hazardous when a brain tumour is being irradiated and the surrounding tissue is vital for normal brain function. Several treatments are usually given over a time period of several weeks to minimise the unpleasant side effects (most commonly nausea, sickness, and tiredness). New methods of delivering radiation treatment have been developed -

• Interstitial radiation involves implanting radioactive chemicals (termed seeds) directly into a tumour.

• Stereotactic radiosurgery delivers a high, single dose of radiation to a small, well-defined area.

http://www.times-archive.co.uk/JohnDiamond/ (An excellent resource for bringing home the 'human side' of treatments - select the article published 22^nd July 2000 - he describes what its like on the receiving end of treatment requiring radioactive wires inserted into the neck)

http://www.salu.net/sites/r/rtsideffects/ (general information on possible side effects of treatment)
 

http://www.radiotherapy.com/education/index.htm

http://www.oncolink.com/specialty/rad_onc/general/xrt_intro.html

http://www.oncolink.com/pdq_html/6/engl/600071.html

http://www.biomed.org/pet.html#what

http://www.valley-radiotherapy.com/treatment/specific/index.asp