Shutdown spiking

Observations in some PWR plants showed that no shutdown spiking occurs when defective fuel rods are present exclusively in regions of low heat ratings, e. g. at the outermost periphery of the reactor core. 

As was discussed above, the maximum iodine activity concentrations attained in the course of a PWR shutdown spiking apparently depend on the type, position and number of defects present in the reactor core. When these parameters are... 

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. 

In evaluating fuel rod failures from the presence of these fission products in the primary coolant, it has to be considered that some of them are also formed by neutron activation of metallic core materials, e. g. Zr/ Nb, Mo/ Tc, Sb. Determining which of these two production mechanisms is responsible for the coolant activity of the isotopes just mentioned is often difficult. In some cases, the shutdown spiking can be used to identify their origin from failed fuel rods however, in most cases this effect is not unequivocal since the radioactive corrosion products also show such a spiking (see Section 4.4.). 

Some observations made at operating PWR plants have suggested the idea of influencing the primary circuit dose rates by appropriate modalities of the reactor shutdown procedures, e. g. in the course of refuelling, either to achieve minimum corrosion product redistribution during the shutdown spiking or to achieve a certain decontamination of the out-of-core surfaces. In this context, different procedures carried out at PWR plants have been reported however, when evaluating the results obtained one has to bear in mind that in some of the reported procedures it is not possible to correlate cause and effect properly therefore, the possibility of pure coincidence cannot always be ruled out. 

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