Radiation Safety and X-Ray Shield Design

10.5. Summary

In this chapter we have considered various problems of radiation safety, with emphasis on the design of the radiation shield. Since electron accelerators for food irradiation are limited to maximum kinetic energies of 10 MeV for direct electron irradiation, and 5 or 7.5 MeV for indirect x-ray irradiation, there is essentially no neutron production and no possibility of induced radioactivity. Consequently, the primary area of concern is providing protection against penetrating x-radiation.

The general problem of calculating the radiation dose at a particular location for a given shield design is quite complicated. Fortunately, it has proven to be possible to design adequate radiation shields using a simplified approach based on empirical source strengths and attenuation factors. The results of such analyses lead to three generally conservative guidelines for shield design in food irradiation facilities; these can be summarized as follows: (1) eleven feet of concrete in the forward direction; (2) eight feet of concrete in the side directions; and (3) any unshielded path for reflected x-rays should include at least three “bounces.” These guidelines should be used as the starting points. Once a preliminary facility design has been developed, more detailed calculations can then be performed using the techniques of this chapter, with shielding corrections to be made as necessary.

While this chapter has emphasized shield design, it should be noted that several additional precautions and procedures are required to prevent inadvertent or accidental radiation exposure. These include the use of safety interlocks, warning devices, appropriate radiation signs, training programs covering all important aspects of radiation safety, and monitoring of facility personnel.