Radioactivity noble gases

The radiation dose from being in or near a "cloud" of aiibome radioactivity can be calculated if the radionuclide concentration in the cloud is known. While radioactive noble gases may be inhaled, they are not retained in the body, hence, most of their dose contribution is by cloud radiation. 

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. 

In Fig. 18.6 various possibilities of produeing radioactive noble gases, suitable for preparation of emanating sourees and investigation of emanating power, are listed. The isotopes of radon are applied most frequently. They are all produeed by decay of radium isotopes, whieh are formed by deeay of thorium isotopes. Therefore, either the radium isotopes or the thorium isotopes may be incorporated into the solid... 

Figure 18.6. Production of radioactive noble gases for application as emanating sources.

During the accident at the 850 MW Three Mile Island-2 nuclear reactor on March 28, 1979, an amount of Xe comparable to the Chernobyl incident was released. It is estimated that approximately 10 " Bq of radioxenon was released when fuel rods partially melted and radioactive noble gases vented into the atmosphere and into the reactor building through a pressure relief valve that stuck open. Elevated Xe levels were detected as far away as 375 km in Albany, NY where a peak atmospheric concentration of approximately 144 Bq/m was measured. ... 

The group of radionuclides to be considered are the ones produced by nuclear explosions and the ones present in the irradiated reactor fuel. This group comprises several hundred radionuclides, but only a limited number of them contributes significantly to human exposure. These would normally include fission products and activation products. Radioactive noble gases, e.g. Kr, Xe, are not considered since they are unlikely to contribute significantly to internal exposure via the food chain. 

For the analysis of radionuclides in airborne dust, the dust is collected on a filter using a dust sampler. Iodine in the air is collected on an active carbon filter using a dust sampler. Tritium exists in the form of vapour (HTO) or gas (HT) in the air. The HTO is absorbed on silica gel HT is changed to HTO using a palladium catalyst and then the HTO absorbed on silica gel. Radioactive noble gases such as Kr are absorbed on an active-carbon trap cooled with liquid nitrogen. 

The temperature dependences of the Henry s Law coefficients of the different gases listed in Table 3.6 are quite variable (Fig. 3.11). Helium, the least soluble noble gas, has very little solubility temperature dependence between 0 and 30 °C. On the other hand, Kr, the second most soluble of the non-radioactive noble gases, is much less soluble at higher temperatures. More details about gas solubilities are presented in the chapter on air-sea gas exchange (Chapter 10). Another notable aspect of the temperature dependence of the gas solubilities is that they are not linear. Thus, mixing between parcels of water of different temperatures at saturation equilibrium with the atmosphere results in a mixture that is supersaturated. This effect has been observed for noble gases in the ocean and may ultimately have a utility as a tracer of mixing across density horizons. 

In spite of the experimental difficulties there are at least two convincing reasons why radioactive noble gases are measured and applied to study transport in aquatic environments. 

The natural time scales of the dynamics of oceans and ground waters agree with the time-range assessable by radioactive noble gases and which hardly can be approached by any other tracer. 

Besides atmospheric sources radioactive noble gases, in particular Ar and Ar, are produced in rocks an minerals (Loosli 1983, Loosli et al. 2000). The Ar isotopes are produced by natural nuclear reactions in the solids of the aquifer matrix, from where the generated nuclei tend to emanate and to accumulate in the surrounding ground waters. The subsurface production sets a natural limit down to which the radioisotope can be... 

In contrast to other natural radioactive noble gases, radon is not of atmospheric origin. Rn is produced within the a decay series of U by disintegration of Ra. Rn and °Rn are produced in the other two a decay series of and Th. But their half-lives of 4 and 56 s respectively are far too short to make them applicable in environmental research. Due to its chemical inertness and its large enough half-life Rn emanates from the solids of the aquifer matrix and accumulates in the ground water. 

The presence of radioactive noble gases in the off-gas is monitored continuously. Abrupt increases occur when fuel pins are penetrated. By considering the differ t half-lives and activities it is oftoi possible to make a rough differ tiation between tramp uranium, old leaks, pinholes and large new leaks in the fuel. 

Activated charcoal beds are used to capture volatile materials and delay the discharge of radioactive noble gases. Wet-scrubbers are used to capture acids and entrained liquids. In-line filters and wet-scrubbers should be located in the penthouse of the faeility. Scrubbers should have an average flow of 20-30 L/min and should run continuously in any hood that evaporates mineral acids. The individual hood washdown system should have a separate switch to wash down the entire ductwork at the end of the day or at the end of a process. 

The reactor coolant and containment atmosphere sampling systems used for post-accident sampling conditions shall permit sampling of the reactor coolant and containment atmosphere without personnel exceeding their individual dose limits. The systems shall permit analyses for radioactive noble gases, iodine, cesium and nonvolatile isotopes, and also boron and chlorides. 

For the measurement of radioactive releases, 80 stations monitor air particulates on a daily basis - of those, 40 also measure radioactive noble gases directly in the atmosphere. 

The IMS network has 80 radionuclide stations that collect radioactive particulates on air filters. In addition, 40 of these also measure radioactive noble gases directly. There are also 16 laboratories around the world that are certified... 

One recent practical use of such reactions has been the removal of radioactive noble gases (such as Rn and Xe) from the gaseous effluents of nuclear power plants by converting them to nonvolatile ions and compounds. However, attempts to synthesize compounds of the three lightest noble gases. He, Ne, and Ar, have thus far been unsuccessful. 

The radiation releases consisted primarily of radioactive noble gases. This type of release had little health impact, because these gases quickly dissipated into the atmosphere and because they are biologically inert, did not have a transfer mechanism to get into the food chain. As a result, there was little concern that these would pose a long-term risk to the local population (Kemeny, 1979). 

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