Nuclear Radiation from Human Activity
Alchemists dreamt of changing one element into another. With the advent of nuclear reactors that is no longer just a dream! It is called artificial transmutation of elements and it has allowed us to 'make' elements that might have never before existed on our planet - the transuranic elements!
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.
The main world isotope suppliers are
Most medical radioisotopes made in nuclear reactors are sourced from relatively few research reactors, including:
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 primarily 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.
High Level Waste
This waste requires very heavy shielding as it is very radioactive.
The intense radioactive decay generates a large amount of heat. this needs to be carefully considered when thinking about storage and final disposal.
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.
Intermediate Level Waste
This waste also needs to be heavily shielded, as it can be extremely radioactive, but does not generate as much heat as High Level Waste. Some of the radioactive particles present in this waste may have very long half-lives and so require isolation for many thousands of years.
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
This waste tends to be low in radioactivity and high in bulk. It ranges from general rubbish (gloves, clothing, packaging, paper towels, over shoes, laboratory glassware, etc.) to some very low-level plutonium contaminated materials.
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.
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
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 α or β after emission of the γ-ray either so decay to the next atom in the radioactive decay series should involve a long half-life. The energy of the γ-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).
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.
The gamma camera (Click on the link for more info!) 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.
is carefully injected into the drainage areas to be visualised.
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 is an imaging technique with many uses. It can be used for:
It is not widely available because the equipment is very expensive. It is found in research establishments and requires highly qualified staff to carry out scans.
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)
(this site has a 'real-time' image of a beating heart being monitored by gamma radiation)
The Gamma Camera - Click here
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 22nd July 2000 - he describes what its like on the receiving end of treatment requiring radioactive wires inserted into the neck)
(general information on possible side effects of treatment)