Radiation surveys, performed by the Health Physics Department, are carried out both on and off the site. These are an important method of controlling radia­tion as they decide the zone classifications on the site and establish that no undue discharges are taking or have taken place. They are also a method of detect­ing unexpected increases in radiation intensity before radiation exposures have been incurred.

The introduction of maximum permissible dose limits implies a requirement to measure the dose received by the individual and there are two possible methods whereby this requirement can be fulfilled. If detailed surveys are undertaken, as just described, of the radiation fields present in all work areas then, with a knowledge of the time workers have spent in each location, an assessment can be made of their accumulated doses. Such a monitoring regime, al­though providing useful information for dose control is extremely difficult to implement in a nuclear power station environment. Therefore, each individual is issued with a dosemeter to be worn whenever access is required to radiation areas. The dosemeter is gen­erally just a sampling device and does not measure the doses to the individual body organs of interest. It does, however, give an assessment of the. radia­tion exposure to the body in the vicinity of the dose­meter. The doses to the various body parts have to be assessed or separately measured by additional dose — meters if these are exposed to a significantly different extent.

The principal control system jusl. rdescribed is based on the time spent in radiation areas of specific dose rates. However, an additional factor which is also important in the total dose equation is the distance parameter.

If one considers a point source of radiation, e. g., a piece of irradiated swarf or a commercial radioactive source, then the dose rate varies as the inverse square of the distance from the source. That is, if the dis­tance from a point source is doubled then the dose rate reduces by a factor of four; conversely, if the distance is halved, the dose rate increases fourfold. With linear sources, e. g., pipework, the dose rate varies directly with distance; for other shapes of source the exact relationship can be very complex but in all cases the principle is the same, i. e., to reduce the accu­mulated dose, the distance from the source must be increased.

It may not always be practicable to have large distances between personnel and the radiation source they may be working with so, as well as restricting the time, the effective distance is increased with the introduction of shielding. The type of shielding de­pending on the type and strength of the radiation, as previously discussed.