General radiation Flashcards
The degree of biological damage caused by radiation is affected by:
The size of the absorbed dose - Internal dose rates may well be more of a problem than the external dose rates. Inhalation of radioactive dust and fumes is extremely dangerous
Cumulative dose - If a large dose is received over a protracted period, there may be no immediate or early signs of injury. Damage may have occurred, however, and it may manifest later in the irradiated individual or perhaps in the individual’s descendants
Type of radiation - As stated previously, the extent of biological damage is dependent on the type of radiation involved
Type of tissues receiving the dose - For example the most sensitive tissues are the gonads, the lymphatic system, bone marrow, lining of the small intestine and the eye lens. Least sensitive are connective tissues, tendons, bone and nerve cells etc.
Age of the Person Irradiated - Children and young adults are the most susceptible groups
Damage to eye - If radiation affects molecules of DNA (deoxyribonucleic acid), the hereditary material in living cells, it may cause a permanent change called mutation.
what is radioactivity
This is the phenomenon by which unstable isotopes1
of some atoms break down to form
a more stable isotope of a different atom by expelling a small amount of matter from the
nucleus (centre) of the unstable atom. Although there are several ways in which this can
occur, by far the most dominant are by alpha emissions or by beta emissions. Shortly
after an alpha or beta emission has occurred it is usually, but not always, followed by a
gamma emission. Radioactive materials continue to undergo this process, often many
millions of times per second until all the original unstable atoms have changed into the
new stable atoms whereupon the radioactive material ceases to exist. Radioactivity
cannot be destroyed other than by allowing it to decay away
what is half life
The time taken for a
radioactive source to reduce to half its original quantity is known as the half life. If a
radioactive material is burned in a fire, the equivalent amount of radioactivity will still exist
in the smoke and the ash.
The time taken for a
radioactive source to reduce to half its original quantity is known as the half life. If a
radioactive material is burned in a fire, the equivalent amount of radioactivity will still exist
in the smoke and the ash.
When measuring radiation (alpha beta, gamma or X-ray) there are two properties which
need to be classified
the activity (or strength) of the source and the dose (or amount) of ionising energy which is being absorbed by the body
There are two principal hazards which arise from radioactivity, regardless of the type of
radiation. These are:
• irradiation – which presents an external risk (i.e. from outside the body)
• contamination – which presents both an internal (i.e. inside the body) and
possibly an external risk.
Irradiation
Irradiation is mainly a problem with materials which emit gamma radiation or from X-ray
machines. Ionising energy is radiated out from the source and passes through a person’s
body. As it does so, some of the energy is absorbed by the body tissues and the ionising properties can cause chemical changes in human cells. This can lead to damage and
possibly disease. The source material however, never enters the body but a “hazardous
materials exposure” has occurred and a radiation dose will be received.
Radioactive contamination
Contamination is a potential problem with any radioactive material except electrically
generated X-radiation. If a material which contains radioactive isotopes are in a form which
is easily dispersed (i.e. dusts, powders, liquids, gases) the radioactive substance can
become attached to the exterior of the body by direct contact or airborne dispersion (e.g.
dust, spray, mist etc.). It may also enter the body through inhalation, ingestion or through
an open cut or wound. In this sense, internal radioactive contamination poses much the
same threat as any other chemical toxin or “hazardous materials exposure”. Once inside
the body, alpha and beta emissions, which were not considered high risk in terms of
external contamination, may produce damaging ionising radiation directly into the cells of
the lymph system, blood and internal organs.
It is important for emergency responders to make a distinction between sealed (closed)
sources of radioactivity and unsealed (open) sources of radioactivity.
Sealed sources – A sealed source is a radioactive source that is encapsulated
into a solid material, usually metal. The encapsulation is intended to prevent
the escape of radioactive material while allowing the radioactive energy to pass
through. Sealed sources are designed to withstand rough handling and elevated
temperatures without releasing the radioactive material. Because the radioactive
source substance is encapsulated or plated onto a surface, sealed sources do
not present a contamination hazard under normal conditions, however, they can
present an irradiation hazard.
• Unsealed sources – Unsealed sources consist of powders, liquids or
sometimes gases which contain radioactive elements and which could easily be
released from their containers through leaks and spillages and dispersed into the
environment. The main hazard with unsealed sources is contamination although
there may also be a significant irradiation hazard from the bulk material
• Shielding – Both sealed and unsealed sources are generally stored or
transported in such a way that they are ‘shielded’ by solid materials, usually their
containers. These prevent or limit irradiation hazards. If a source’s shielding is
removed or damaged the radioactive hazards are increased.
The damage caused by radiation may be divided into two different categories
deterministic
stochastic
derministic
Deterministic effects are those which occur at a relatively high dose and the severity of
the effect is proportional to the dose. In all cases it is necessary to exceed a threshold
dose before the effect is experienced at all. The most common effects in this category
are skin reddening, hair loss, impaired fertility, lowered blood count, nausea, vomiting and
diarrhoea. The threshold for detectable deterministic effects is about 100mSv. At this level
no symptoms would be exhibited but tests on blood may start to show signs of damage.
As dose levels increase the severity of effects and the rapidity of their onset increases.
Doses above 5,000 millisieverts in a short period of time are life threatening.
stocastic
Stochastic effects are those where the probability of experiencing the effect is
proportional to the dose but the severity of the effect is independent of the dose. The
most common effect in this category is cancer, The likelihood of contracting cancer
increases with the dose but the severity of the disease is the same irrespective of the
dose that caused it. Genetic effects are also believed to be stochastic although these
have never been demonstrated in humans. As such it is assumed that any level of dose of
radiation carries some risk and therefore all doses need to be kept as low as possible
effects at certain dose rate levels
Dose Effect Comments 5 Sieverts (5,000 mSv, or 5,000,000 μSv) Probable lethal dose Very dependent on rate of delivery and health of individual 3 Sieverts (3,000 mSv or 3,000,000 μSv) Erythema (skin reddening) May not appear for several days 3 Sieverts (3,000 mSv or 3,000,000 μSv) Depilation (Hair loss) Temporary between 3 and 7 Sv; permanent above 7 Sv 1 Sievert (1,000 mSv, or 1,000,000 μSv) Threshold for radiation sickness Dependent upon other factors e.g. health, rate of delivery, skin type etc 700 mSv (700,000 μSv) Threshold for temporary sterility Can be permanent at higher doses in excess of 3 Sv 100 mSv (100,000 μSv) Chromosomal changes in blood cells detectable. Small increase in existing cancer risk Minimum dose at which any physical changes can be detected. No noticeable effects by the person receiving the dose 5 mSv (5000 μSv) Very small increase in overall cancer risk No immediate observable effects
dose constraint will usually be significantly lower than the legal dose limits. Fire
and Rescue Services should consider any possible operations not involving situations
immediately threatening to life where they may wish to impose a dose constraint below
the legal annual limit. This could possibly be the case at a protracted incident or if it was
considered possible that crews may have to attend more than one radiation incident
within a 12 month period.
It is recommended that a dose constraint of 5 mSv per incident is introduced at
operational incidents. The reasons for this level of constraint are:
• it corresponds to the alarm setting on the Electronic Personal Dosemeters,
supplied through the Fire and Rescue Service National Resilience project, which
would naturally prompt staff to leave the Hazard Zone
• it is in line with dose reference levels used by the ambulance service
• it does not legally preclude female firefighters from entering the Hazard Zone as
it is less than 13 mSv
• if a firefighter were to receive a dose in excess of one third of any formal dose
limit (i.e. 1/3 of 20 mSv), the employer must conduct an investigation into the
circumstances. This would equate to approximately 6 mSv for a whole body
dose as measured by an EPD and by using 5 mSv as the dose constraint level
this should avoid crossing this reporting threshold.
