Fission release from failed fuel rods

The release of fission products from failed fuel rods to the primary circuit... 

According to Hiittig et al. (1990), the amount of uranium released from defective fuel rods and deposited on in-core surfaces can be assessed from the coolant activity levels of various short-lived fission products such as I, I, Cs, calculating their source strengths under the assumption of a direct and instantaneous release to the coolant. Though the releases of these isotopes from failed fuel rods are quite small (due to their short halflives), such data can provide only an upper limit for the uranium contamination if there are simultaneously fuel rod failures in... 

In general, release of actinide isotopes from failed fuel rods and their subsequent behavior in the primary circuit is very similar to that of certain fission products, e. g. of cerium isotopes. For this reason, the y-emitting cerium isotopes which can easily be measured in the coolant by y spectrometry, can serve as a suitable indicator for early recognition of higher releases of actinides to the coolant. The release behavior of the actinides from failed mixed-oxide fuel rods to the coolant is almost identical to that from uranium fuel rods. This means that in both cases the U Pu... 

As was discussed above, the release of fission products from the fuel rods which failed during the burst, refill and reflood phases of the loss-of-coolant design basis accident is limited to their gap inventories due to the comparatively low maximum... 

III-l. The main source of radiation in a nuclear power plant under accident conditions for which precautionary design measures are adopted consists of radioactive fission products. These are released either from the fuel elements or from the various systems and equipment in which they are normally retained. Examples of accidents in which there may be a release of fission products from the fuel elements are loss of coolant accidents and reactivity accidents in which the fuel cladding may fail due to overpressurization or overheating of the cladding material. Another example of an accident in which fission products may be released from the fuel rods is a accident in handling spent fuel, which may result in a mechanical failure of the fuel cladding from the impact of a fuel element that is dropped. The most volatile radionuclides usually dominate the accident source term (the release to or from the reactor containment). Recommendations and guidance on the assessment of accidents are presented in Section 4 of Ref. [III-l]. 

The activation products of the coolant, with the sole exception of N, are not of substantial importance in plant operation in some cases, however, they have to be taken into consideration environmentally following release of off-gas or waste water. The fission products and the fuel activation products represent by far the greatest proportion of the radionuclide inventory in the reactor, from the viewpoint of radioactivity as well as from that of radiotoxicity. However, with the exception of severe accidents (which will be treated in Part C), during plant operation they are reliably confined within the fuel rods, so that only the very small amounts released from failed rods to the primary coolant are of interest in this context. Finally, the activated corrosion products are the origin of the buildup of radiation dose rates at the surfaces of the circuits, which potentially complicate the performance of operational procedures, in particular of inspection and repair work. 

All the observations made up to now do not give any indication of a noticeable difference in the defect behavior between UO2 fuel, on the one hand, and mixed-oxide fuels on the other. No specific mixed-oxide defect mechanisms have been observed the release mechanisms of fission products from the failed fuel rods are virtually identical for both types of fuel. It is clear that the low plutonium content in the mixed-oxide fuel does not significantly influence the physico-chemical properties of the fuel material (Goll et al., 1993). 

With regard to the fission product isotopes of Zr, Nb, Ru, Ce, Pr, La it can be assumed that they are not liberated directly from the fuel of a failed rod but that they are released by leaching of the fuel pellets and that the amount transported to the coolant is equivalent to the amount of fuel released from the defective rod. On the other hand, a direct release is assumed for strontium and barium, with the investigations suggesting a release rate lower by a factor of 10 than that of cesium (Beslu et al., 1984). 

Figure 4.11. Release mechanisms of fission products from a failed fuel rod (Neeb and Schuster, 1979)...

The mechanisms just discussed do not explain the striking shift of the iodine isotope ratio observed in the course of the spiking. As was mentioned above, the source strength ratio I changes from 3 to 4 during constant-load operation to 10 to 15 near the spike maximum the other, shorter-lived iodine isotopes show a depletion which is even more pronounced. This ageing of the fission product mixture which is released from the failed fuel rod can be basically explained by the following ... 

The major cesium isotope in the coolant (like in the fuel) is Cs, accompanied by Cs, which is produced from the stable fission product Cs by neutron capture (thermal neutron cross section 3 10 cm ) therefore, this radionuclide is of greater significance the higher the bumup level of the failed fuel rod. In addition to these two main isotopes, the very long-lived pure P" emitter Cs appears in the coolant as well as produced from it by neutron capture (thermal neutron cross section 8 10" cm ) their activity concentrations in the coolant also depend on the bumup of the defective fuel rods. Finally, the short-lived Cs has to be mentioned from the dependence of its source strength on the decay constant (see Fig. 4.3.) it can be concluded that this isotope is not released itself from the fuel rod, but that it is generated in the coolant by decay of its precursor Xe. Cesium isotopes with mass numbers beyond 138 are usually not detected in the coolant apparently, because of their short halflives they decay completely on the way from the fuel rod to the coolant. 

The isotopic composition of fission product iodine present in the BWR reactor water in the case of failed fuel rods in the reactor core is quite similar to that in the PWR primary coolant. Since the iodine purification factor of the reactor water cleanup system is on the order of 100, i. e. virtually identical to that of the PWR primary coolant purification system, this similarity in isotopic composition demonstrates that the release mechanisms of iodine isotopes from the failed fuel rods to the water phase are virtually identical under both PWR and BWR operating conditions. On the other hand, the resulting chemical state of fission product iodine in the BWR reactor water is quite different from that in the PWR primary coolant. The BWR reactor water usually does not contain chemical additives (with the possible exception of a hydrogen addition, see below) as a result of water radioly-... 

In PWR plants equipped with both a cold-leg and a hot-leg injection of the emergency coolant, a fraction of the fission products released from the failed fuel rods will be washed down by the downward water flow. Thus, it will be transported back to the water phase inside the reactor pressure vessel and, finally, to the containment sump water. Since the extent of this type of retention of fission products depends strongly on the contact time between the steam flow and the downward flow of the liquid emergency coolant, it is only difficult to quantify. It can be assumed that Csl (and other iodides) will be trapped almost completely in the water phase for this reason, a 90% retention of the halogens and alkalis and a 99% retention of the so-called solid fission products has been assumed in the German Storfall-Berechnungsgrundlagen . For the h fraction in the steam flow a similar degree of washout can be expected experiments performed under conditions similar to those in the relevant LOCA period have yielded h washout fractions of about 92% at 25 C and about 96% at 85 °C water temperature (Kabat, 1980). 

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