Polyvalent fission products

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. 

The French Profip code, which was developed on the basis of these and other measurements, allows the calculation of the distribution of polyvalent fission products and actinides in the primary circuit and of the resulting radiation dose rates for rather simple geometric conditions. According to these calculations, the deposited fractions will be highest in the volume control system, while the main coolant piping will show considerably lower deposited amounts. The dose rates caused by... 

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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