Sources of Radioactivity

11.90. For reactors cooled (and moderated) by light water, essentially all the radioactivity present in the effluent from a nuclear power plant originates in the reactor vessel from two sources: (1) the escape of fission products from the fuel into the coolant water and (2) activation by neutrons of corrosion and erosion products, e. g., iron, chromium, nickel, and cobalt, and other substances, e. g., boron (in boric acid), dissolved or suspended in the water. Gaseous radioactive isotopes of oxygen, nitrogen, and argon, in particular, are also formed by neutron activation of these elements in the water molecule and in dissolved air. Most of these isotopes have fairly short half-lives, and they will have largely decayed to stable nuclides before being discharged to the atmosphere. However, although these radioactive species are a negligible environmental hazard, they must not be overlooked in connection with the protection of plant workers.

11.91. Fuel rods are clad with zircaloy to prevent escape of fission products, but in a small fraction of the rods, typically below 0.1 percent, toward the end of their operating life small holes and cracks may develop in the cladding as a result of welding defects or localized corrosion. Con­sequently, some fission products will escape into the coolant. These consist mainly of the gaseous radioisotopes of krypton and xenon and of the readily volatile element iodine. In addition, solid fission products are extracted to a small extent by the high-temperature water. Hence, radioactive fission products can be found in both gaseous and liquid effluents from a reactor installation. Corrosion and erosion products from the reactor vessel, cool­ant pumps, steam generator, and piping are inevitably present in the water and these are activated by neutrons from the reactor core (§7.78).

11.92. Another important source of radioactivity in the water coolant is tritium, the radioactive isotope of hydrogen. About one fission in 104 of uranium-235 is a ternary fission in which tritium is one of the products; the other two products are heavier nuclides similar to ordinary (binary) fission products. Some tritium is also formed by the interaction of neutrons with boron in the control elements of BWRs. (The control rods of PWRs do not contain boron.) Thus, tritium can escape into the coolant through cladding defects in both fuel rods and control elements. Tritium, is also formed directly in the coolant by the capture of neutrons by deuterium, i. e., heavy hydrogen, nuclei that are normally present in water. In PWRs, the major sources of tritium are the reactions of neutrons with boron (in the boric acid) used as a chemical shim (§5.187) and with lithium (in lithium hydroxide) used to control the pH of the water. Regardless of its origin, most of the tritium in the reactor coolant is found in the form of НТО (§6.14); a small proportion occurs as HT gas but this is of little significance.