11.1.1 Quantities and units

A unique characteristic of radiation protection is the full international harmonization of the relevant quantities and units. This has been achieved under the influence of the International Commission of Radiation Measurements and Units (ICRU), a sister organization to ICRP (ICRU, 1938, 1954, 1962), and is unique vis-a-vis other pollutants.

NPPs are characterized by the presence of radioactive substances, the amount of which is described by the quantity termed activity and measured in the unit termed becquerel — although, in the past, the unit curie was and still is widely used. One becquerel represents an extremely small activity; for instance, one becquerel is the activity of potassium (which is a long — lived, naturally radioactive element) contained in less than one-tenth of one banana! (Conversely, 1 curie represents a significant activity as it equates to 37 thousand million becquerels.) Although varying among plants, NPPs currently discharge into the environment an average activity of around a hundred million million becquerels (or terabecquerels, TBq) per kilowatt year of electrical energy produced, mainly of short-lived radioactive noble gases.

NPP materials with activity emit radiation that may expose both workers and members of the public and may be delivered from outside the person’s body (external exposure) or by radioactive substances arising from those materials that may be incorporated into the body via inhalation or inges­tion, or through open wounds or the skin (internal exposure). The potential health consequences on people caused by their exposure depend on the amount of radiation received, and also on the types of radiation involved and the organs exposed.

The amount of radiation is measured in terms of the quantity termed the radiation dose, and that received by human tissues is termed the absorbed dose and is assessed in units called grays (in the past, the unit rad was used) or in its sub-multiple, the milligray. One milligray is approximately the lowest annual dose absorbed by a human being due to exposure to natural background radiation.

Different types of radiation have different effectiveness to induce damage and, therefore, the absorbed dose has to be weighted by radiation weighting factors, wr, to take into account the effectiveness of various radiation types. The resulting weighted quantity is termed the equivalent dose.

Table 11.1 (ICRP, 2007a) presents the currently recommended radiation weighting factors, wR, which gives a general idea of the radio-efficiency of the various radiation types. It is noted that the main radiation types in NPP exposures, which are у rays, i. e. photons, and в particles, i. e., electrons, have a weighting factor equal to 1. The weighting factor for neutrons can be high depending on their energy, but exposure to neutrons is important for some NPP equipment but normally not for people.

Similarly, different organs and tissues have different sensitivity to radia­tion exposure, and therefore the equivalent dose has to be weighted by tissue weighting factors, wT, to take into account the various sensitivities to radiation of various organs and tissues. The quantity resulting from

cases the absorbed dose in gray or milligray should be used. Radiation protection standards usually contain universally agreed nominal coeffi­cients or factors for converting activity and particle fluence into absorbed dose and also equivalent dose and effective dose.

Figure 11.1 (adapted from ICRP, 2007a) illustrates the interrelation among the radiation protection quantities, including the nominal conver­sion factors.

The radiation protection quantities are not directly measurable. Instruments for assessing doses in people or in the ambient environment are usually calibrated against physical operational quantities rigorously defined by ICRU and incorporated in international standards. These are the personal dose equivalent and the ambient dose equivalent. They are also expressed in sieverts and, numerically, they approximately correspond to the radiation protection quantities. The operational quantities are formally used at NPPs for verification of compliance with standards.

For reasons of simplification, this chapter will use the term dose to mean generally and indistinctly any dose quantity and will express this quantity mainly in the unit millisievert (mSv).