Fission product radionuclide aerosols

Following an accident at a nuclear power station, great amounts of radioactive aerosols are emitted into the troposphere, because airborne fission product radionuclides interact with the environment and a carrier is responsible for their long-range transport and atmospheric diffusion. Radioactive nuclides, such as Ru, Te and Cs, characterise these aerosols. 

The particle size of a fission product aerosol and the distribution of fission products between particulate and vapour phases depend on the mechanism of release to the atmosphere. 

At distances of about 100 km from ground zero of the explosion site the aerosol particle size of fallout ranges from a few pm to a few 100 pm. Fission products with volatile precursors are enhanced by about a factor of 2 compared with refractory fission products. The fractionation is greater in the smaller particles (Hicks, 1982). 

As iodine ( I) was mainly (about 80%) present in the gas or vapour phase, the adsorption of gaseous species onto aerosol particles was strongly surface correlated (Papastefanou and Bondietti, 1987). These surface distributions for environmental aerosols usually peak at size fractions with particle diameters 0.4 pm (Seinfeld, 1986 Whitby et al., 1972). Therefore, 

Switzerland from 09 00 h April 30 to 09 00 h May 13, 1986 using an Andersen aerosol cascade impactor. Note that the low activities are due to averaging over the 14-day sampling period. 

---

Radioactive aerosols can be classified in the following categories (a) Radioactive aerosols associated with radioactive nuclides of cosmogenic origin, such as Be, Na, and (b) radon and thoron decay product aerosols associated with Po Pb, Pb, Pb, and °Po, (c) fission product radionuclide aerosols associated with Sr, Sr, Cs, Ru, isif 132-pg i40g (-(j) radioactive aerosols associated with the operation of high-energy ac-... 

Fig. 2.12. Mass size distribution of mine aerosol in an all-electric equipped mine (from Cantrell and Rubow, 1990). 5. Fission product radionuclide aerosols...

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-... 

The Chernobyl reactor accident (April 26,1986) produced elevated radioactivity in ambient air. lost et al. (1986) sampled aerosols with an Andersen impactor in Spiez (46°41 N, 7°39 E), Switzerland from April 30 to May 13, 1986 and in Zurich (47°23 N, 8°32 E), Switzerland from May 2 to 8,1986 with a Berner impactor, so soon after the accident. In Figure 2.13 (data from Spiez), it is shown that had a rather different activity size distribution from other fission product radionuclides such as Ru and Cs. Most I was found in the size fraction on the back-up filter (<0.47 pm), whereas Ru and Cs showed a pronounced maximum at 0.93 pm (geometric mean diameter). In Figure 2.14 (data from Zurich), a similar activity size distribution is shown with the maximum for I at 0.35 pm and at 0.71 pm for Ru, Te and Cs. Different activity size distributions were due to the different cutoff diameters of the aerosol sampling devices. The activity size distributions for Ru, Te and Cs are very similar to the concentration patterns of prominent ions, such as NH or and NOJ as determined by ion chromatography. 

Residence time of tropospheric aerosol particles associated with the radon decay product radionuclides Po and the fission product radionuclides Sr,... 

Apart from the activity ratios of the radon-222 decay product radionuclides, the residence time of tropospheric aerosols can be derived from the activity ratios of the fission product radionuclides released into the atmosphere during the explosions from nuclear weapons testing or nuclear reactor accidents, such as Sr/ Sr and " Ba/ Sr. These nuclide ratios are considered as nuclear clocks. The applicability of the radionuclide ratios depends on whether steady-state conditions hold at the time and place of measurement and on the kind of sample, whether surface air or precipitation (rain or snow), used for the radioisotope activity determination. 

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. 

Other activities of man have led to the distribution of appreciable amounts of radionuclides in the atmosphere and on the earth s surface. In the first place nuclear explosions and nuclear weapon tests have to be mentioned, by which Pu and fission products have been deposited on the earth, either directly or via the atmosphere in the form of fall-out. The amount of Pu released by nuclear weapon tests between 1958 and 1981 is estimated at 4.2 tons, of which 2.8 tons were dispersed in the atmosphere and 1.4 tons deposited locally. By underground nuclear explosions about 1.5 tons of Pu have been liberated. Radionuclides released into the air are mainly present in the form of aerosols. 

The release of fission products and structural materials from the overheated and, finally, molten reactor core represents the initial and, therefore, most important component of the source term. It provides vapor and aerosol sources for radionuclide transport in the reactor primary system and, further on, to the containment. 

The decontamination factors mentioned above may be affected over time by mechanisms which lead to a revolatilization of fission products from the pool water. When the bubbles break up at the pool surface, they produce new aerosol droplets by which non-volatile fission products can be carried to the containment atmosphere. Calculations using the Sparc code (Owczarski and Burk, 1991), however, showed that radionuclide entrainment caused by this mechanism is very small and, thus, can be ignored in source term analyses. Iodine, on the other hand, can be revolatilized if the pH of the aqueous phase is decreased as a consequence of water radiolysis. In the presence of alkalizing substances such as CsOH, such a decrease in pH leading to a lower I2 partition coefficient is not to be expected. The existing codes (such as Sparc, Supra, Busca) do not take chemical reactions into consideration and, thus, do not provide any information on chemical iodine revolatilization from the pool. 

Wassel, A. T, Hoseyni, J. L., Farr, J. L.jr., Oehlberg, R. N. Scrubbing of radionuclide aerosols in water pools. Proc. American Nuclear Society Meeting on Fission Product Behavior and Source Term Research. Snowbird, Utah, USA, 1984 Report EPRI NP-4113-SR (1985), p. 34/1-19... 

For the treatment of the behavior of the radionuclides in the containment, it seems reasonable to divide them into three groups aerosols, tellurium, and iodine. The fission product noble gases as the fourth group of elements are distributed homogeneously over the entire free volume of the containment after a short time interval and they are only subject to radioactive decay thus, their behavior will not be discussed in detail in the following. In spite of the considerable differences between PWR and BWR plants, the environmental conditions within the containment are in most cases not so different as to influence fundamentally the chemical reactions of the fission products. Therefore, the discussions in the following sections will apply to both types of LWR plants where there are significant differences they will be mentioned in the respective section. 

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. 

---