Failed fuel rod

The general aim of the Mol 7C e3q>eriments in the BR2 reactor was to investigate the failure events and the inherent coolability conditions in a fuel bundle subjected to a local (partial) coolant flow blockage in the fissile zone. Operation of reactors with small number of failed fuel rods is desirable for economic reasons, but safety must nevei be prejudiced. Therefore the behaviour of failed fuel during accident is of primary interest and fuel simulation codes are essential in establishing an operational set of safety measures. The present MOL7C experiments fit clearly the current need for improved techniques which should help establish operational limits for fuel rods and develop higher performance materials. 

Diffusion of fission gases in UO2 is very sensitive to the stoichiometry of the material, increasing considerably even at a slight excess of oxygen. An increase of the diffusion coefficient by about 4 orders of magnitude was observed as the O U ratio increased from 1.997 to 2.02 (see the review paper by Lawrence, 1978). This effect is of particular importance in operationally failed fuel rods when the UO2 pellets are superficially oxidized by steam which has ingressed into the free volume of the rod (see Section 4.3.2.). 

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). 

Figure 4.6. Comparison between failed fuel rod estimation and results of refuelling inspections...

The escape rate coefllcients of the iodine and noble gas isotopes identified in one PWR or BWR plant can be directly applied to other plants of the same type, provided that the essential conditions, for example, fuel rod linear heat ratings, are comparable or can be corrected for. By this means it became possible to evaluate the number of failed fuel rods in the core of an operating reactor on the basis of the fission product activity concentrations measured in the coolant early attempts in this area were reported by Schuster et al. (1977). Although these estimates were based only on empirical data, they permitted a rather trustworthy prediction, as can be seen from Fig. 4.6., where the predicted numbers of failed fuel rods are compared with those detected in the course of post-cycle examinations. These techniques have been considerably improved, on the basis of experimental results as well as of model development and calculations. Because of the great number of parameters influencing the escape of fission products from defective fuel rods, evaluation of the number and type of defects from the measured activity concentration of fission products in the primary coolant is a difficult task which can be performed reliably only by specialists with considerable experience in this field. Attempts were, therefore, imdertaken to develop computerized expert systems that could be applied routinely. Lewis et al. (1992) described the development of such a system by means of which information can be obtained on the number of defects and their... 

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... 

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

The maximum level of the activity concentrations in the coolant is probably reached at the moment when the gap of the failed fuel rod is filled with water and when there is no further movement of the liquid front and no convection within the liquid phase. After this point, additional fission products may reach the leak position and escape to the coolant by diffusion in the liquid phase only, which within the gap is probably a comparatively slow process and does not cause a significant further increase of the activity concentrations in the coolant, which are already high at this moment. Therefore, the activity concentrations in the coolant begin to decrease at a rate which corresponds to that effected by the action of the purification system. Following a reduction in the coolant pressure, however, an additional fraction of the liquid phase can be transported from inside the rod to the coolant by the action of temporary pressure differences, leading to the formation of the secondary depressurization spikes as shown in Fig. 4.9. When the reactor is started up again after the shutdown period, water which still remained in the gap of defective fuel rods, containing dissolved fission products, is transported out to the coolant, forced by the increase in temperature of the fuel pellets. 

A comparison of the gap inventory of a failed fuel rod of about 5 10 atoms with the steady-state source strength from such a fuel rod of approximately 2 10 atoms/s shows a low steady-state release coefficient on the order of 10 s (as related to the gap inventory). This means that, without continuing supply from the fuel pellet, the gap inventory would need about 30 days to be emptied by steady-state release. One possible reason for this slow process is the transport through the long and narrow pathways within the gap. In addition, from such a... 

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 techniques described result in the identification of the assemblies containing failed fuel rods however, the identification of the failed rod itself is not possible. Frequently, in a fuel assembly identified by one of the techniques mentioned above. 

In order to replace failed fuel rods in an assembly lattice, the defective rod has to be identified. Detection of the failed rod in the assembly usually is done by ultrasonic testing, a change in signal intensity indicating the presence of liquid water in the rod. The techniques currently available can be applied to both PWR and BWR assemblies without disassembling the fuel assembly under consideration and without rod withdrawal their sensitivity does not depend on fuel bumup or other parameters. 

The fission product noble gas isotopes Xe and Xe are the predominant radionuclides in the primary coolant of a pressurized water reactor when failed fuel rods are present in the reactor core, in particular when the coolant degasification system is not in operation. Appearance of additional defective fuel rods in the reactor core results in a prompt response of the activity concentrations of the xenon isotopes in the coolant. For this reason, periodic analyses at comparatively short time intervals are required in order to initiate countermeasures in time, e. g. starting operation of the degasification system, whenever necessary. 

If one assumes a release kinetics of from the failed fuel rod similar to that of then its source strength should be directly proportional to the burnup of the failed fuel rod, since the fuel inventory of this isotope in the time periods discussed here increases linearily with burnup and does not reach an equilibrium state. On the basis of this assumption, one can calculate that, with a fuel burnup of the failed rod of 20MWd/kg U and a steady-state activity concentration in the primary coolant of 500 MBq/Mg, the corresponding value will be on the order of only 1 Bq/Mg. 

The production of iodine isotopes in the primary coolant from their precursors, which themselves have been released from the defective fuel rod, does not play a significant role compared to the direct release of iodine isotopes from the failed fuel rod. The only exception seems to be with its longer-lived precursor Te a differentiation of both release pathways is possible by using the fact that directly released from the fuel will decay in the coolant with its own halflife of 2.28 hours, whereas present in the coolant in a decay equilibrium with Te will decay with the halflife of this radionuclide of 78.0 hours. 

When failed fuel rods are present in the reactor core, fission product cesium isotopes will also appear in the primary coolant in significant activity concentrations. The high solubility of the cesium compounds deposited in the gap of the fuel rod facilitates the transport to the coolant which, however, is only possible via the liquid phase. This means that under constant-load operating conditions a significant cesium transport will only occur when such fuel rod failures are present in the core that allow a direct contact between fuel and liquid coolant in addition, the shutdown spiking results in a considerable cesium transport to the coolant with almost all types of fuel rod defects. The comparatively low cesium retention on the primary circuit purification resins which are saturated with LiOH occasionally leads to the buildup of activity concentrations of cesium isotopes in the coolant on the same order of magnitude as that of the iodine isotopes 1 and 1, even at comparatively low cesium source strengths or those which are not constant over time. 

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. 

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