Alternative high-level radiation sources for sewage and waste-water treatment

The choice of an energy source for the radiation treatment of waste-water or sludge is between an electron accelerator or a gamma-ray source of radioactive cobalt or caesium. A number of factors will affect the ultimate choice and the potential future adoption of radiation as a treatment technique. The present and future availability of radioactive sources of cobalt and caesium is closely linked to the rate of nuclear power development and the assumption by uranium fuel reprocessors of a role as radioactive caesium suppliers. Accelerators are industrial machines which could be readily produced to meet any conceivable market demand. For energy sources in the 20-30 kW range, electron accelerators appear to have an initial capital cost advantage of about seven and an operating cost advantage of two. While radioisotope sources are inherently more reliable, accelerators at voltages to 3 MeV have achieved a reliability level adequate to meet the demands of essentially continuous operations with moderate maintenance requirements. The application of either energy source to waste-water treatment will be significantly influenced by considerations of the relative penetration capability, energy density and physical geometrical constraints of each option. The greater range of the gamma rays and the lower energy density of the isotopic sources permit irradiation of a variety of target geometrics. The low penetration of electrons and the high-energy density of accelerators limit application of the latter to targets presented as thin films of several centimetres thickness. Any potential use of radiation must proceed from a clear definition of process objectives and critical comparison of the radiation energy options for that specific objective. (Author)