The employment at a nuclear site may involve certain risks during the activities (e.g. burning wounds, during treatment of caustic acids in a laboratory, ...).
However, at SCK·CEN, specific apparatus or materials, called radioactive materials, producing ionizing radiation, are used. The dangers inherent to this radiation require particular attention since :
this danger perhaps is new to people. Consequently, people may be not used to taking the appropriate protection measures ;
this danger is treacherous because the radiation can not be observed by our senses. The following contains useful information related to safety and health of protection at SCK·CEN.
What is radioactivity ?
Radioactivity is a notion which sounds familiar to us since a few years; the phenomenon is however already as old as the world. Since his creation, the man has been exposed to radioactive radiation coming from space (cosmic radiation) and of "radioactive materials present in the earth crust".
It is only after the second world war that the man has succeeded in producing considerable quantities of radioactive products. Any matter surrounding us is composed of elementary components; the smallest component is called an atom. Some atoms have the property of emitting rays, this phenomenon is called "radioactivity".
How to identify a radioactive material ?
Radioactive materials are identified by :
the denomination of the radioactive material (the isotope) , generally expressed symbolically by letters and a number :
* Cobalt-60 or Co-60 or 60Co
* Uranium-235 or U-235 or 235U
When the denomination of the material is known, one disposes of a data allowing to identify the nature of the emitted radiation.
the activity , which is a measure of the emitted quantity of radioactive rays. The activity of a quantity of radioactive material is expressed in Becquerel (Bq) (1 Bq = 1 desintegration/sec).
When the name of the radioactive material and its activity are known, one disposes of data allowing estimation of the risks presented by the radioactive material.
S.I. (International System) unit is " Becquerel", or Bq (formerly Curie : Ci)
1 Ci = 3.7 x 1010 Bq
ex 3.: 2(0) mrem = 2(0) x 10 µSv
ex 4.: 5(0) mSv = 5(0) x 100 mrem
Type of radiations
The type of radiation to be considered is ionizing radiation. This radiation is produced by radioactive materials, röntgen- or X-ray apparatus and particle accelerators.
Ionizing radiation differs from other, known types of radiation such as light and heat because they can not be observed with our senses.
Which types of radiation are there ?
The most important types of radioactive radiation are alpha (&apha;), beta (&béta;), X or gamma (γ) and neutron radiation.
alpha-rays are quickly retained by a few centimeters of air, a paper leaf, rubber gloves, the skin, ...;
beta-rays may penetrate a few meters in air, they are retained by a few millimeters of metal;
gamma-rays are very penetrating and will cover a large distance in the air before being absorbed; to stop them screens of heavy metals such as lead are used; to reduce the radiation by one tenth, a few centimeters are already required; X-rays are gamma-rays : they are less penetrating and are already weakened by a lead plate of a few millimeters. These rays are emitted by a radioactive material; in most cases, X- and gamma-rays are emitted simultaneously with alpha-and beta-rays. Alpha and beta radiation is generally already stopped by the walls of the container in which the material is packed.
Gamma radiation is more penetrating and passes through the walls of the container.
neutron radiation are very energetic "rays" and cover large distances before being stopped by very heavy and thick materials. This type of radiation may occur in the vicinity of reactors, accelerators, by spontaneous fission of certain plutonium isotopes and neutron sources, these are particular mixtures in which an alpha emitter is in contact with certain materials, e.g. 226Ra (radium-226) with 4Be (beryllium-4).
Neutrons are particularly dangerous for irradiation of the human body because they transfer their energy to the water, molecules constituting 80 % of the human body.
The following figure gives some idea of how it is :
Why is ionizing radiation dangerous ?
The human body is composed of organs (e.g. skin, eyes, intestines, blood, blood-forming organs, reproductive organs) which are built up by tissues (e.g. muscles, nerves, osseous tissues), the elementary components of whch are the cells.
By ionization, the metabolism of these cells can be affected by radiation thus causing damage to tissues and organs.
The damage is a function of :
the received dose ;
the irradiated organ or part of the human body
The risks may be divided into two categories : on the one hand external irradiation and on the other hand contamination; the latter may occur both externally as internally with respect to our body.
There is a risk for external irradiation when a part or the whole of the body is exposed to radiation emitted by apparatus or radioactive materia1s.
When a radioactive material enters into contact with the skin, the hair or any other external part of the body, one speaks about external contamination.
When a radioactive material is inhaled, ingested or penetrates via any other way into the body (e.g. via an injury), one speaks about internal contamination.
How can ionizing radiation be established ?
As ionizing radiation is dangerous, it has to be detected and measured. To this end, use can be made of :
Collective measuring apparatus : the measuring values of which are valid for anyone entering a measured space.
Individual measuring apparatus : only usable for each person individually.
Individual radiation measurement devices
The Thermoluminescent dose meter consists of a plastic holder containing pellets developing the physical phenomenon "thermoluminescence" upon an exposure to ionizing radiation. Such a dose meter gives an indication about the "dose" received.
An Alarm dose meter, also allows to give an indication about the received dose or an indication about the dose rate at which one is exposed. This meter gives an audible signal when the dose rate threshold or integrated dose threshold is exceeded.
"Finger dose meters" consist of a plastic ring. A thermoluminescent detector is placed in the seal of the ring. It is worn in principle with the seal as close as possible at and directed towards the radioactive source. This dose meter has to be worn whenever required by radiation control.
A "wrist dose meter" is of the same type as the above-mentioned thermoluminescent dose meter and is worn by means of a bracelet at the wrist at the instruction of radiation control.
Measuring devices for contamination
Contamination by radioactive materials can be detected by movable or permanent measuring apparatus.
Hand and foot monitors are installed at the entrance or exit of controlled areas in view of controlling alpha-beta or alpha emitter contamination or both. These devices must be used when leaving controlled areas. The instructions of the radiation control personnel must be followed.
> Hand and foot monitors must always be used when leaving controlled areas, when exceeding the alarm level, the radiation control agent must immediately be informed.
Besides this way of detecting external contamination there are other means of following up possible internal contamination.
Measurements can be realized with a "Total body counter" or with a lung counter as also control of the urine.
Golden rules for protection against irradiation and contamination
Any radioactive matter emits radiation. It has to be tried to minimize one's own dose load. The appropriate protection rules are the following :
the first important rule is the limitation of the exposure time . To this end, the duration of a work with radioactive materials must be precisely be planned.
the distance between source and body must be as large as possible
The dose rate changes quadratically with the distance to the source. At 1 m one receives 10 times less than at 30 cm. Consequently, a radioactive source must never be touched with the fingers but a pair of tongs or any other device allowing telemanipulation of the source must be used.
the appropriate protection may reduce radiation
The thickness and the nature of the screen depend on the type of radiation and on the source strength.
during your work,the number of sources must be limited to the strictly required minimum. Unnecessary sources involve an unnecessary exposure. Radioactive sources must always be placed safely in their containment systems.
a visible warning must always be placed on c ontainers with radioactive materials left without surveillance.
Any non encapsulated radioactive material may cause contamination. Unpermitted manipulations on an encapsulated source may cancel the density of the encapsulation thus giving rise to a non controlled dispersion and contamination with radioactive materials.
Contamination of your work environment may give rise to :
personal internal contamination ;
dispersion in the air or on the soil and to other areas involving risks for your colleagues and third persons.
A radioactive material penetrating into the body will give rise to a continuous body irradiation and dose during a longer time or not.
In view of limiting the risks, following rules are to be applied :
make use of the collective protection means during a work. Manipulations on radioactive materials have to be performed in ventilated areas or hoods and, if necessary, in a room with forced ventilation.
do not eat or drink, do not use cosmetic products or do not smoke in areas where radioactive products are being treated. Inform Radiation Control when you have injuries and have to work with radioactive materials or in controlled areas.
protection clothes must be worn in the controlled areas and during working with radioactive materials. This may go from a simple laboratory coat, with a coloured collar or not, until complete working-clothes and underwear. Contaminated protection clothes must never been left at places where they do not belong.
- wear protection gloves must be worn during working with radioactive materials or in controlled areas whenever specifically prescribed.
- in areas where temporarily an increased air contamination is measured, Radiation Control may require the use of personal protection means.
This may go from the use of a dust mask with air filter until a completely equipped diving suit.
- Contamination of hands and shoes and, if necessary, the working-clothes must be regularly controlled.
- if activities might involve a risk for skin contamination, it is advisable to wash at least hands; if necessary, to take a shower.
- waste must not be thrown at places where it does not belong.
- The rules related to radioactive waste must always be strictly followed.
Warning signals with specific indications
A board with the warning signal "radioactivity" and giving following special indications may be displayed at the access to a laboratory, an area or enclosure :
"Very high radiation intensity", only Physical Control is entitled to authorize access to such an area.
"High radiation intensity", access to or residence in such a laboratory or area is placed under the surveillance of Radiation Control.
"Ionizing radiation", continuous residence should be limited.
"Radioactive contamination danger", at places where non encapsulated sources are stored or used.
"Radioactive air contamination", when the contamination level of the air of the considered spaces exceeds a fixed limit.
Access to this space is only authorized under surveillance of Radiation Control and with the use of personal protection means.