Principles of radiation protection

Radiation protection is based on three principles: justification, optimization and dose limitation.
The principle of justification states that application of ionizing radiation is allowed only if the application has more benefit for the exposed individual or the society than the detriment of the exposition and all costs taking into account the costs of application and radiation protection. Activities with radiation danger can be applied only in those cases where other methods with similar costs and results are not available. Once the use of radiations is justified, the process involving radiation danger has to be optimized. Optimization means the use of available resources in order to reduce the risks connected to radiations. Radiation doses have to be reduced “As Low As Reasonably Achievable” which is usually cited as the ALARA principle. The excess dose due to the application and the release of radioactive materials has to be reduced by applying all reasonable methods. According to the limitation principle, the effective dose for individuals must remain below the recommended dose limits. The occupational dose limit is 20 mSv averaged over 5 years but not exceeding 50 mSv in any 1 year period. For the public, the dose limit is 1 mSv/year. The only exception where the dose limitations do not apply is radiotherapy. According to the ALARA principle, these dose limits should not be reached if there is a reasonable way to reduce the exposition.

Radiation protection regulation

The tree factors used to minimize the dose from a given radiation source are: time, distance and shielding. In practice, these three factors are observed simultaneously to minimize radiation exposure. The dose is proportional to the time of exposition, thus, reduction of time spent in an area of radiation risk is a straightforward way to reduce the dose. Reduction of time also means that only those activities are allowed in areas of ionizing radiation which are not possible to perform at other places. Exposition time can be reduced by careful planning of operations and, if necessary, by rehearsing the processes using non-radioactive materials. This is a cheap and easy way to reduce the dose, but usually the achievable dose reduction is not very significant. The dose rate follows the inverse square law in function of distance to source, i.e. D ~d^-2, where D is the dose and d is the distance from point source. This dependence on the distance can be used to suppress the dose rate by increasing the distance from the source. This strategy is used in practice when radioactive materials are moved by using pincers or manipulators. It is forbidden to hold radioactive samples in hand, because even weak sources can cause considerable dose on the hand.
The dose rate can be suppressed by using shielding materials between the radiation source and the environment. The shielding must be designed for a given type and intensity of radiation. Alpha particles can be effectively absorbed by a thin layer of material – even a sheet of paper – or some cm of air. Shielding of beta particles is possible by using plastic, glass, water of low-Z metals like aluminum. For X-rays and gamma radiation the use of high-Z metals is the adequate solution. Shielding of neutron radiation usually requires a sandwich-type shielding. If the neutrons have energy above the thermal energy range, a thermalizer layer with high hydrogen concentration is used. Then the thermalized neutrons can be effectively captured by materials like B, Cd, Gd. The neutron capture is followed by the emission of gamma radiation, thus, a third layer is needed to shield the gamma radiation.

1. General rules

The Instrument Responsible (IR) manages the work of visitors and others at the BNC instruments. The IR ensures the observance of procedures concerning general safety training and observance of the Work and Radiation Protection Rules of the Budapest Research Reactor. Visitors and experimenters are obliged to follow the instructions of the supervising person / local contact. Proper clothing (white coat, overshoes) has to be worn when staying in an area of radiation risk. Additionally, wearing a dose-rate monitor is obligatory. The Environmental Protection Office will issue a certificate on the acquired dose.

2. Entry and exit procedures of the Reactor Building

Entrance to the reactor building is only possible with a special badge. Following an inspection at the reactor’s gate, your ID card has to be given to the guard in exchange to a visitor proxy card marked with a letter „V”. Upon leaving the reactor, the visitor card will be interchanged with your ID card. Entrance to the reactor building is allowed only in the presence of the host person.

Before leaving the building the contamination monitor at the gate has to be used. If contamination is detected, the person on duty from the Radiation Protection Department has to be called and personally contacted. Henceforward his/her instructions are to be followed.

3. Entry and exit procedures of the Reactor Hall

Entry to the Reactor Hall is only possible on the 1st floor, in front of the control room through the interlock gate. Entry is controlled by operator on duty or the deputy operator. In order to be allowed to enter the reactor hall, a laboratory coat and overshoes must be worn by the visitor(s) who and must be accompanied by a supervising person. Before leaving the reactor hall, the coat must be hanged on the coat stand and the shoe cover disposed into the bin near the interlock gate. Before leaving the reactor hall checking for possible radioactive contamination is compulsory using the contamination monitor. Contamination has to be reported immediately to the operators in the control room. Except for the authorized personnel, it is forbidden to stay in the reactor hall when the reactor is being started, stopped or the operating power is being changed.

4. Rules for using the vertical beam ports

The safety of the experiments at the vertical beam ports and shielding around the beam port are the responsibility of the experiment responsible. Opening the beam is enabled by the radiation safety personnel on duty after the appropriate shielding, and, if needed, fencing are installed. The IR of each vertical channel must ensure that workers and visitors at the respective channels understand and respect these rules. Three indicator lamps are installed above the vertical channels indicating the channel status. If the green light is on, the channel is closed. If the green and white (in some cases yellow) lights are simultaneously on, it means that the opening of the beam port is allowed by the radiation safety group and the channel can be opened any time. Thus, the channel must be considered as open. If only the white light is on, it means that opening the channel has started but has not finished yet. If the red light is on, the channel is open. It is forbidden to enter the area behind the shielding while the beam port is open.

Rules for using horizontal channel instruments of BNC

According to the special geometry, location or usage of some instruments, the IR of the instrument may order and post to apply special radiation protection regulations at the respective instrument (however, within the limits of the above rules). Such rules to be respected must be discussed before the experiment.

References

ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4)