Fission product vapors

The specific activity of a debris sample will depend upon the thermal history experienced by the sample, the time after fission when the melted or heated soil was exposed to the fission product vapors, the time-temperature history of the exposure, and a number of undefined and complex parameters. 

If uniform mixing of the fission product vapors and volatilized materials results, the recondensed particles might be expected to have a constant specific activity of elements having similar boiling points. Note parenthetically that studies of fission-product incorporation into the metal and oxide products of vaporized iron wires (in which iron-metal spheres and iron-oxide irregulars are formed) indicate no simple relationship between specific activity and size. For example, a refractory element like zirconium is found most enriched in particles of intermediate size. This is probably in part caused by a concentration effect—i.e.y in these experiments the zirconium represented a mole fraction of about 10"9. As indicated earlier, the fission products are a minor constituent in the fireball, and a very complex pattern of incorporation can be anticipated, especially if coagulation with melted but unvaporized particles ensues. 

The kinetics of chemical interactions between fission product vapor and aerosols is mainly controlled by gas phase mass transport, by the kinetics of the chemical reaction, and by mass transport in the condensed phase. Another factor potentially influencing the kinetics of vapor deposition is that the heat liberated by condensation or by chemical reaction of vapor with aerosol must be disposed of. Because of their small masses, aerosol particles have only limited capacity for conducting away this heat, compared with the structures within the reactor coolant system. This problem may arise particularly in the deposition of water vapor onto aerosol particles which have been previously covered by hygroscopic or water-soluble compounds such as CsOH. 

In general, the results of these tests showed that significant interaction occurred between the fission product vapors and bulk material aerosols. The aerosol particles released from the fuel rod specimens as well as from the control rod specimens acted as centers for nucleation of the fission product vapors and induced sorption of fission products as well. Likewise, chemical reactions were observed between control rod aerosols and fission product vapors (e. g. formation of CdTe and Cdh). In addition, the results demonstrated that the concentration of fission products in the vapor is the determining parameter as regards the dominant interaction mechanism. At high concentrations (represented by simulant fuel specimens), chemical reaction and vapor condensation are the dominant mechanisms, whereas at low concentrations (represented by trace-irradiated fuel specimens) sorption phenomena predominate. 

In all cases studied, MELCOR predicted very low releases of fuel, control rod, cladding and structural materials. Fission products are released in accordance with their volatilities. Fission product vapor deposition is predominant on in-vessel structures, steam generator inlet water boxes and pressurizer. Revaporization of the most volatile species is significant from the in-vessel structures when core support plates f ai 1. 

Three accident sequences have been assumed to occur in a W-PWR 900 Mwe, three loop NPP. Sequences AB, V-LPIS and SGTR have been analyzed with the MELCOR code. The attention has been focused on thermal revaporation, and subsequent transport, of volatile fission product vapors (Cs(OH), Csl and TeO) condensed on surfaces. 

Table 4.1. Aerosol and Fission Product Vapors Released from Fuel in ST-1... 

Steam), and oi er input from CORCON. It contains a library of thermodynamic properties je energies from bich vapor pressures are calculated) for chemical species (mostly elements, oxides, and hydroxide that might be formed by fission products and other melt constituents. 

This paper deals mainly with the condensation of trace concentrations of radioactive vapor onto spherical particles of a substrate. For this situation the relation between the engineering approach, the molecular approach, and the fluid-dynamic approach are illustrated for several different cases of rate limitation. From these considerations criteria are derived for the use of basic physical and chemical parameters to predict the rate-controlling step or steps. Finally, the effect of changing temperature is considered and the groundwork is thereby laid for a kinetic approach to predicting fallout formation. The relation of these approaches to the escape of fission products from reactor fuel and to the deposition of radon and thoron daughters on dust particles in a uranium mine is indicated. 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

There are little data available which can be used to predict the rates of uptake of the different vaporized radioactive elements or oxides. Since such data are important to the application of any fallout prediction model based on kinetics, a program has been started at this laboratory to measure the rates of uptake of a selected group of fission-product oxides under conditions approximating those found in the cooling fireball. The data from these measurements will be useful, not only as input to fallout models, but also for discovering the mechanisms which govern the rates of uptake. 

In considering the operational safety and accident analyses of sodium-cooled fast reactors, similar information on the release of fission products from sodium is needed. Although the extent of vaporization can often be calculated from thermodynamic considerations (3, 4), appropriate transport models are required to describe the rate phenomena. In this chapter the results of an analytical and experimental investigation of cesium transport from sodium into flowing inert gases are presented. The limiting case of maximum release is also considered. 

Equation 13 reduces to the Rayeigh equation (3) when the ratio of the gas-phase diffusivities, , is unity. Since gas-phase diffusivity is inversely proportional to the square root of the reduced mass, in the case of fission product-sodium systems where sodium has the smallest molecular weight, the above diffusivity ratio is less than unity. Therefore, the Rayleigh equation, which was derived on the basis of equilibrium vaporization, in fact represents an upper limit for the fractional fission-... 

Equilibrium Vaporization. The cesium release results presented in this chapter may also be used to demonstrate our earlier conclusion that equilbirium vaporization represents the upper limit for the fractional fission-product release as a function of sodium vaporization. Figure 6 shows three cesium release curves. Curve A was calculated from the Rayleigh Equation in conjunction with the partial molar excess free energy of mixing of infinitely dilute cesium—sodium solutions reported... 

If only a few percent of the energy is expended in volatilizing the soil, several tens of thousand tons of soil will be entrained in the vapor cloud. The partial pressure of the vaporized soil constituents will then be of the order of a million times that of the fission products. There will... 

The size distribution of the radioactive debris containing the majority of the fission products may bear little relationship to the size distribution of the environmental soil. Vaporization, agglomeration, condensation, and coagulation will probably lead to particles smaller than and larger than those found in the soil. A striking demonstration of this is found in the size distribution of radioactive debris of a low yield explosion over an alluvial salt bed in Nevada (6). While the mean diameter of the pre-shot soil particles was about 6/, the prompt fallout contained many intensely radioactive particles of 1000/ or greater. 

The radioactivity ratio of potentially unfractionated fission product radionuclides in precipitation should be independent of the amounts of aerosol and water vapor removed from the air masses. For an air mass containing uniformly mixed radioaerosols from the same nuclear explosion, the ratios should be the same by time and collection-site latitude along the coast. The ratios at storm date may be calculated for depositions following a specifically known atmospheric nuclear explosion with known initial production quantities. The presence of longer lived radio-... 

For a nuclear weapon hurst in air. all materials in the fireball are vaporized. Condensation of fission products and other bomb materials is then governed by the saturation vapor pressures of the most abundant constituents. Primary debris can combine w ilh naturally-occurring aerosols, and almost all of (he fallout becomes tropospheric or stratospheric. If the weapon detonation takes place within a few hundred Icet of (either above or below) a land or water surface, large quaniilies of surface materials are drawn up or thrown into the air above Ihe place ol detonation. Condensation of radioactive nuclides in this material then leads in considerable quantities of local fallout, but some of the radioactivity still goes into tropospheric and stratospheric fallout. If the hurst occurs sufficiently fur underground, the surface is not bruken and no fallout results. 

In addition, Brambilla has claimed that both iodine and ruthenium are volatilized when the molten nitrate reacts with the oxide fuel (9). In contrast to this volatilization, other literature claims that both iodine and ruthenium will be found in the molten phase (6, 13). Avogadro and Wurm state that most of the fission products, other than the noble metals, are either volatile or soluble in the nitrate melt, even without addition of nitric acid vapor (12). In a later paper, however, Avogadro reports that iodine is stable as iodide in molten nitrates, and that ruthenium is partially soluble in the molten phase, and partially volatilizes, while the majority remains with the... 

It has been observed, however, that those fission products that are initially soluble in the nitrate melt remain so throughout addition of nitric-acid vapor and subsequent recovery of sodium diuranate. Those fission products that are initially insoluble in the nitrate appear to remain so, although it is expected that the addition of the nitric-acid vapor will shift the distribution ratio. 

Preliminary investigation has shown that most fission products are not soluable in alkali metal nitrate melts and that they are not dissolved by addition of 100% nitric acid vapor. If these characteristics are verified by further experiments, a fission product separation is easily envisioned. One could react the fuel with the molten nitrate, dissolve the uranate with the addition of 100% nitric acid, and separate the uranium from the remaining solids, which should consist of both plutonium dioxide and fission products. 

The sorption of the fission products Cs and Sr by the graphitic materials, from which the core and the fuel elements of High-Temperature Gas-Cooled Reactors (HTGRs) are made, is important for the prediction of fission product release in the case of an accident. Hilpert et al. [564, 565] determined, therefore, Cs and Sr partial pressures over such graphitic materials with different Cs and Sr concentrations. The vaporization enthalpies obtained showed a strong chemisorption of Cs and Sr by these materials. The vaporization enthalpy of Sr exceeds that of the pure Sr metal by about 210kJmol at 1500 K, if fuel element matrix graphite with a Sr concentration of 4.0 mmol kg is considered [564]. This value for Cs amounts to about 230 kJmol" at 1250 K for a similar concentration of 4.2 mmol kg[565]. In addition, sorption isotherms were evaluated. 

Waste can be permitted to boil within the tanks and the evolved vapors condensed and disposed of externally, or returned as liquid to the tank. Unstable conditions can result from this practice, however, for some fission products will concentrate in sludges at the bottom of the tank, and heat generated here can build up as superheat. Eventually the unstable system is disturbed, initiating rapid boiling, and steam is suddenly released at a rate in the order of ten to twenty times the normal rate. Temperatures as high as 176°C. have been observed at tank bottoms (Al). 

It should be noted that these decontamination factors are appreciably lower than those for liquid entrainment. The decontamination factors alone are defined as the ratio of fission product activity in the still pot to the fission product activity in the condensed vapor. The above equations apply simultaneously with Eqs. (1) and (2), but only to that portion of the activity which is strongly adsorbed on the suspended solids. 

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