Polyvalent fission product elements

As can be seen from the Gibbs partial free energies of formation shown in Fig. 3.14., the alkaline earths, the earth metals and the rare earth elements in contact with UO2 are stable as oxides. From these data it can be concluded that the fission product isotopes belonging to these elements are present in the fuel in their usual oxidation states or, in macrochemical expression, as double oxides with UO2. 

UsuaUy, such double oxides are highly temperature-resistent and it can be expected that these fission products will show virtually no thermal-induced migration in the fuel matrix. This assumption has been confirmed by numerous measurements that show a distribution dependent on the bumup profile of the fuel. Most of the information in this area has been gained by examining fast breeder reactor fuels (Kley-kamp, 1985), i. e. materials which had been irradiated at distinctly higher fuel temperatures than LWR fuels. For this reason, thermal-induced migration of the polyvalent fission products in LWR fuels, with their distinctly lower temperatures during reactor operation, can be ruled out. 

Although zirconium oxide shows only a limited solubility in the UO2 matrix, fission product zirconium seems to be homogeneously distributed in the fuel, presumably due to an enhancement in Zr02 solubility in the presence of rare earth oxides. That means that, consistent with the results of thermodynamic calculations (see Fig. 3.14.), zirconium is present in the Zr(IV) state. Up to very high linear heat ratings, there is no noticeable zirconium redistribution in the temperature gradient. 

Despite the limited solubility of BaO in UO2 (quite in contrast to SrO, which is highly soluble), barium seems to be homogeneously distributed in the irradiated fuel matrix, presumably as Ba(II) in the UO2 lattice. Barium can also be incorporated into the perovskite-type grey phase which was detected predominantly in high-burnup fast breeder reactor fuels however, the significance of this phase in irradiated LWR fuels seems to be questionable. The chemical state of fission product barium in the oxide fuel apparently depends strongly on the stoichiometry of 

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In the case when defective fuel rods are present in the reactor core, the BWR reactor water contains the other fission products and the activation products released from the fuel in concentrations well below those of fission product iodine. This applies as well for fission product cesium, which is retained on the ion exchangers of the reactor water cleanup system with a decontamination factor of about 100. As far as it is known, cesium in the reactor water is present as the Cs ion, whereas large proportions of most of the polyvalent fission products and of the actinides are attached to the corrosion product particles suspended in the water as yet, there is no detailed knowledge on the chemical state of these elements (i. e., adsorbed to the surfaces or incorporated into the Fe203 lattice). It was reported that the strontium isotopes as well as Np appear in the reactor water in the dissolved cationic state, while Tc was found in the reactor water as a dissolved anionic species, most likely Tc04 (Lin and Holloway, 1972). According to James (1988), discrete fuel particles were not detected in the BWR reactor water. 

The radionuclides incorporated into the oxide layers, which lead to a radiation field in the surrounding area, are mainly the activated corrosion product nuclides, above all Co and Co. Out of the fission products present in the primary coolant during plant operation with failed fuel rods in the reactor core, iodine and cesium isotopes are not deposited into the surface oxide layers this reactor experience is consistent with the general chemical properties of these elements which do not allow the formation of insoluble compounds under the prevailing conditions (with the sole exception of Agl, see Section 4.3.3.1.2.). On the other hand, fission product elements that are able to form insoluble compounds (such as oxides, hydroxides or ferrites) in the primary coolant are incorporated almost quantitatively into the contamination layers (see Section 4.3.3.1.4.). However, because of the usually low concentrations of polyvalent fission products in the primary coolant, only in very rare cases will these radionculides make a measurable contribution to the total contamination level for this reason, they will not be treated in this context. 

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