Fission products regional deposition

Table IX. Regional Deposition of Fission Products by Nuclear Event ...

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

The value of for iodine depends to some extent on the adsorptive properties of the containment surfaces, which may vary with manufacture and conditions of use. However, several series of experiments have shown that the value is likely to be in the region around 10 m/sec for metal surfaces, and perhaps half this where concrete surfaces predominate (76). Very steamy, wet conditions make very little difference to the value of V. Rather similar behavior is to be expected of any other fission products which can deposit in molecular form or are attached to particles too small to deposit by gravitation. Values of of this magnitude lead to dejK>sition hal lives of a few hours for typical large containments. But in the ease of iodine there is a limit to the reduction of gas-bome iodine set by the presence of methyl iodide and also by the possibility of iodine desorbing from the walls. 

Cosmos-954 A satellite of the USSR, Cosmos-954, reentered the atmosphere over Northwest Canada on January 24, 1978. The satellite was believed to have contained 20 kg of highly enriched uranium. Released fission products were estimated since the reactor worked for 128 days at 100 kW. 75% of the particulate radionuclides were estimated to be dispersed in the atmosphere and 25% to be deposited on uninhabited regions in Northeast Canada. The vaporized radionuclides, and Cs, were dispersed in the atmosphere. The collective effective dose was 20 man-Sv in the world. 

The appearance of non-volatile fission products or actinide isotopes in the coolant can indicate the presence of fuel rod defects with a direct contact between the fuel and liquid water. This can occur with large-sized defects, in particular in comparatively cold regions of the fuel rod at the vertical or horizontal periphery of the reactor core. However, any statement in this regard can only be based on radionuclides that are not present in the coolant as a remnant from preceding transients this means that in a PWR Cs or Cs are not appropriate indicators for such fuel rod failures. The requirements are in principle fulfilled by Np, which is a reliable indicator for defects with fuel-to-water contact, as are ruthenium and cerium isotopes, as well. However, because of the complex behavior of these radionuclides in the coolant (adsorption on suspended corrosion products and deposition on primary circuit surfaces), only qualitative assessments can be made, which means that a quantitative evaluation of the number of fuel rods showing... 

The largest fraction of radionuclides by far enters the containment attached to aerosol particles which are carried by the steam flow. As was shown in Table 7.3., according to thermodynamic calculations oxides and metals are the predominant species of the refractory aerosols generated in the reactor pressure vessel, and one has to assume that at the lower temperatures prevaihng in the containment no significant changes in the chemical nature of these primary aerosols will occur. The primary aerosol particles will be coated by layers of more volatile substances such as CsOH and Csl which are deposited in regions of lower temperatures in the primary system other fission product species may also be attached to the aerosol surfaces by condensation, chemical reactions and, probably to a lesser extent, by physi- and chemisorption. 

Earth, the content of uranium minerals relative to was therefore higher. The age of the Oklo deposit has been established at 1.74 x 10 years. Calculation indicates that the content of the Oklo pitchblende 1.74 x 10 years ago was about 3 %. At this concentration of fissile material, water suffusing the ore deposit could have brought regions of the deposit to criticality, and a slow or intermittent chain reaction could have ensued. It is believed that other concentrated uranium ore deposits could have achieved supercriticality in the presence of water as a neutron moderator (2-3) x 10 years ago. Such chain reactions conceivably played an important part in early geological events. It is interesting to note that the fission products produced at Oklo over an estimated period of 10 years are still retained in the rock in which they were formed more than a billion years ago [63]. 

It was also assumed that the production ratios of the three radionuclides were the same as the ratios given for weapon fission yields (12). For each of the three storms and the reported nuclear explosions for the period, an analysis of the fractional deposition by event was made for each collection site along, and by regions of, the California coast. Based on these assumptions, the fraction of 140Ba from the most recent atmospheric nuclear explosion before each storm was 1.00. The 140Ba deposition on storm date was extrapolated to the explosion date. The initial... 

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