Noble gases production from radioactive decay

Emanation techniques are based on the production of radioactive noble gases by decay of mother nuclides or by nuclear reactions. The emanating power has been defined by Hahn as the fraction of radioactive noble gas escaping from a solid relative to the amount produced in the solid. It depends on the composition of Ihe solid, ils lallice structure and its spccihc smTace area. Reactions in the solid have a major inlluence. Further factors affecting the emanating power are the half-life of the noble gas radionuclide, its recoil energy and the temperature. 

The report on the disassembly and postoperative examination of the ARE pointed to the ease of removal of the noble gases and the deposition of certain noble metal fission products on metallic surfaces. It was also learned that because of the evolution of chemistry that occurs during radioactive decay, it is important to account for the kinetics of noble gas removal from the salt. 

The major exposure of the population to natural radiation arises from inhalation of the short-lived radioactive progeny of the radioactive noble gas radon-222, which in turn is a sixth-generation radioactive decay product of natural uranium. The amount of radon-222 present in the air depends on many factors (e.g., gas permeability in soil and rock, relative humidity, and barometric pressure) but is necessarily linked to the geological concentration of the uranium parent radionuclide. There is about an eightfold range of concentrations of uranium in different types of rocks and soils. 

A small percentage of the fuel elements in a water-cooled reactor release gaseous fission products to the coolant. The insoluble noble gases are collected and stored for radioactive decay prior to their release to the atmosphere. Calculate the required storage time such that the radioactivity levels of Xe and Kr in the released gas are equal. Assume fissions at constant power only in an average irradiation time of 2 years, and assume that these noble gas radionuclides are released to the coolant in the same proportion as they exist within the fuel. Obtain mass yields from Table 2.9. Twenty-three percent of the fissions at mass 85 produces Ki. 

The noble gases occur as minor constituents of the atmosphere (Table 17-1). Helium is also found as a component (up to 7%) in certain natural hydrocarbon gases in the United States. This helium undoubtedly originated from decay of radioactive elements in rocks, and certain radioactive minerals contain occluded helium which can be released on heating. All isotopes of radon are radioactive and are occasionally given specific names (e.g., actinon, thoron) derived from their source in the radioactive decay series 222Rn is normally obtained by pumping off the gas from radium chloride solutions. Ne, Ar, Kr and Xe are obtainable as products of fractionation of liquid air. 

Some 20% is assumed to enter the blood compartment. The ICRP biokinetic model for radium has the same general structure as that for strontium and uranium (see Figure 26.2-2). Bone is the critical organ with a biological half-life for radium in the range of 20 years. Since the decay of radium leads to the noble gas radon with a physical half-life of 3.8 days, most of the radioactivity of the decay product escapes from the body before further decays occur. 

---