Fission product condensation

Less-refractory fission products condense later onto the surface of the particles. Those with gaseous precursors, for example Sr and 137Cs, condense as they are formed by decay of their parent nuclides. The... 

Immediately after having left the hot region of the melting reactor core, the low-volatility fission products condense to form aerosols. The chemical state of the individual elements in the freshly formed aerosols may be either metallic or oxides, depending on the prevailing oxidation potential of the system. Thermodynamic calculations (Wichner and Spence, 1985) yielded the data given in Table 7.3., together with those for the volatilized core structural materials (see Section 7.3.1.2.). 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

Computer sensitivity studies show that hole size strongly affects the fraction of fission products released from the containment. The failure location determines mitigation due to release into another building in which condensation and particulate removal occur. The quantity released depends on the time of containment fails relative to reactor vessel failure. If containment integrity is maintained for several hours after core melt, then natural and engineered mechanisms (e.g., deposition, condensation, and filtration) can significantly reduce the quantity and radioactivity of the aerosols released to the atmosphere. 

Appropriate 1-benzopyrans can undergo ring fission and condensation with 1,2-benzenediamines to afford 3-o-hydroxybenzyl-2(lF/)-quinoxalinones or related products. The following examples illustrate this somewhat specialized procedure. 

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. 

If we apply these equations to the condensation of a fission product in a cooling nuclear fireball, we must deal with sources from radioactive growth, sinks from radioactive decay, and dynamic conditions of temperature drop. In the simple case of radioactive decay... 

Elements from selenium through the middle rare earths will be present in the mixed fission product population they exhibit a wide variety of volatilities (1). The elements Y, Zr, and Nb and the rare earth oxides are high boiling and condensable at low partial pressures, whereas the noble gases, and the alkali metals Mo, Tc, Pd, Ag, Cd, Sn, Sb, Te, Ru, and perhaps Rh, are very volatile in a relative sense Sr and Ba are predicted to be of refractory or intermediate behavior. 

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 differences in condensation history of the three alkaline-earth element fission products allows examination of their radioactivity ratio as a method for determining fractionation. The recent atmospheric... 

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. 

Recovery of metals such as copper, the operation of batteries (cells) in portable electronic equipment, the reprocessing of fission products in the nuclear power industry and a very wide range of gas-phase processes catalysed by condensed phase materials are applied chemical processes, other than PTC, in which chemical reactions are coupled to mass transport within phases, or across phase boundaries. Their mechanistic investigation requires special techniques, instrumentation and skills covered here in Chapter 5, but not usually encountered in undergraduate chemistry degrees. Electrochemistry generally involves reactions at phase boundaries, so there are connections here between Chapter 5 (Reaction kinetics in multiphase systems) and Chapter 6 (Electrochemical methods of investigating reaction mechanisms). 

The particle size of a fission aerosol, and the distribution of fission products between particulate and vapour phases, depends on the mechanism of release to the atmosphere. In a weapons explosion, some physicochemical fractionation of radionuclides may occur, particularly if the explosion is near the ground. Everything in the vicinity is vapourised by the heat of the explosion, but within less than a minute the fireball cools to a temperature in the range 1000-2000°C, and refractory materials such as metal oxides and silicates condense to form particles (Glasstone Dolan, 1977). Refractory fission products, and plutonium, are incorporated in these particles. 

Table 2.12 shows washout ratios of radioactive and stable nuclides as measured in the UK. Fission products from distant bomb tests become attached to natural condensation nuclei in the atmosphere, and enter the accumulation mode of particle sizes (approximately 0.02 to 0.2 pm diameter). Washout ratios in the range 250typically present as particles in the 1-5 pm range, outside the normal accumulation mode, and this explains... 

Horner et al. (9) have observed that in dissolution of irradiated fuels residues, ruthenium present as fission product Ru metal, destroys the Ce(lV) probably through formation of volatile RuO followed by its decomposition to solid Ru(>2 and return from the dissolver condenser to the solution. The Ru02 is then again oxidized to RUO4. Elimination of this problem will require removal of ruthenium from the solution. It should be noted that other fission products such as iodine are oxidized to high oxidation states by Ce(lV) and also will consume Ce(lV). 

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. 

Radioactive gases created by neutron flux activation, such as A41, and certain radioactive fission product gases (Xe, Kr), are not easily removed by conventional approaches. The noble gases may be condensed and adsorbed on activated charcoal at extremely low temperatures. The cost of such systems per cubic foot of treated air, is so high that the method is feasible only for small volumes. Another approach for such volumes is compression and storage of the gases in chambers, for times of sufficient length to permit these isotopes to decay. 

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