11.93. The basic objective of the reactor radwaste system is to reduce the radioactivity in liquid and gaseous wastes to such levels that they can be safely discharged to the environment. The general procedures used for this purpose are natural decay and decontamination, either by evaporation or by demineralization. Natural decay is commonly used to decrease the activity of short-lived gaseous radionuclides, such as nitrogen-16 and sev­eral isotopes of krypton and xenon. Gaseous wastes are thus held up for a period of time, from 30 minutes to several days, before discharge.

11.94. Liquid wastes contain such a wide range of fission (and other) products that natural decay is of minor consequence. These wastes are therefore treated by a decontamination process. The aqueous waste so­lution may be evaporated and the vapor condensed. The condensate is essentially pure water which can be reused in the plant or discharged. In the alternative demineralization process, dissolved radioactive substances are extracted by passage through an ion-exchange resin. The evaporator residues and spent resins contain the removed radioactive material and are disposed of in a safe manner.

11.95. The choice between evaporation and demineralization depends on circumstances and may vary from one plant to another. The decontam­ination factor, i. e., the ratio of the activity concentration before to that after treatment, is 103 to 104 for evaporation but is only 10 to 102 for demineralization. On the other hand, installation and operation is less expensive for a demineralizer. If the liquid waste contains significant amounts of dissolved material (other than fission products), decontamination by evaporation is preferred to avoid rapid saturation of the ion-exchange resin.

11.96. Radwaste treatment systems differ to some extent for pressur — ized-water and boiling-water reactors. There also are some variations among reactor systems of the same type fabricated by different vendors. The descriptions that follow are thus intended to indicate the general principles of operation of radwaste systems.