Fuel rod failures

Fuel rod failures, associated with the thermomechanical interaction of fuel with claddings, are mainly typical of reactivity initiated accidents. The... 

If the coolant undergoes a phase change (boiling), a different approach must be used since the design limit is clad heat flux, not clad temperature. The CHF is used as the limit to prevent film boiling and fuel rod failure. Figure 22.29 shows the heat balance. 

The pathway of fission gases from the interior of the fuel pellet to the rod plenum leads through a network of cracks and tunnels which are formed in the course of fuel operation. The comparatively long time needed to cover this distance results in an extensive decay of the shorter-lived isotopes, which means that the gas mixture reaching the plenum is mainly composed of stable krypton and xenon isotopes, of Kr and of a fraction of the initial amount of Xe. Short-lived isotopes of the fission product noble gases which reach the pellet - cladding gap by fission-induced recoil are superimposed upon this mixture. This isotopic composition is of interest in the event of fuel rod failure when fission products are released from the fuel rod to the primary coolant, a topic that will be treated in more detail in Section 4.3.2. 

Besides the fuel, the closed chemical system fuel rod includes the fuel pellet -cladding gap and the upper and lower gas plenum, which are filled with helium overpressure (about 2 MPa at ambient temperature) in the course of fuel rod fabrication. During reactor operation, a certain gap inventory of radionuclides is generated which is of interest in the event of an operational fuel rod failure as well as in a loss-of-coolant accident. In typical LWR fuel rods, this gap inventory is mainly formed by fission product recoil from the fuel pellets. According to Wise (1985), one quarter of the fission fragments generated within a recoil length i from the... 

The occurrence of fuel rod failures is often characterized by the initial appearance of very small perforations (pinholes). After prolonged operation of a Zircaloy-cladded defective rod, secondary hydride failures may develop, caused by hydrogen which is radiolytically generated from water vapor entering the rod via the primary defect. Such secondary failures may appear at a certain distance away from the primary defect (e. g. 1 m above it). Fretting failures also increase in size with time often they are located in the lower region of the fuel rod. Thus, the size of the failure as well as its location can differ considerably and it is important to identify these parameters already during the relevant fuel cycle. 

In severe cases, fuel rod failures may lead to fuel losses from the rod, with their magnitude depending on the size of the defect and on the question of whether liquid water or only steam has entered the rod. Operational experience has shown that after one year of prolonged operation of a failed rod one has to expect the following fuel losses to the coolant (Assmann and Stehle, 1984 Beslu et al., 1984) ... 

During steady-state operation of the reactor and as long as there is no change in the number and type of fuel rod failures, the source strengths of the iodine isotopes and their activity ratios are constant over time. This observation suggests that the amount of fission product iodine deposited in the gap shows an equilib-... 

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... 

The appearance of non-volatile fission products or actinide isotopes in the coolant can indicate the presence of fuel rod defects with a direct contact between the fuel and liquid water. This can occur with large-sized defects, in particular in comparatively cold regions of the fuel rod at the vertical or horizontal periphery of the reactor core. However, any statement in this regard can only be based on radionuclides that are not present in the coolant as a remnant from preceding transients this means that in a PWR Cs or Cs are not appropriate indicators for such fuel rod failures. The requirements are in principle fulfilled by Np, which is a reliable indicator for defects with fuel-to-water contact, as are ruthenium and cerium isotopes, as well. However, because of the complex behavior of these radionuclides in the coolant (adsorption on suspended corrosion products and deposition on primary circuit surfaces), only qualitative assessments can be made, which means that a quantitative evaluation of the number of fuel rods showing... 

As was discussed in Section 4.3.1.1., the fission product noble gas isotopes "Kr, Kr, Kr, Xe and Xe (besides the fission product iodine isotopes) are valuable indicators for evaluating the number and type of fuel rod failures present in the reactor core. 

The level of the activity concentrations of the fission product iodine isotopes in the primary coolant depends on the number and the size of the fuel rod failures in the reactor core. The isotopes I and are always the main contributors to the total iodine activity, whereas the shorter-lived isotopes and are only... 

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.). 

Radiochemical analyses of PWR primary coolant show that the major fraction of the neptunium and plutonium traces in the coolant is usually associated with the corrosion product suspended solids and that it can be removed from the coolant by filtering. Only in cases of very low corrosion product concentrations in the coolant were significant proportions of the transuranium elements observed to be present in a dissolved (i. e. non-filtrable) form. As yet, it is not known whether mixed oxide formation between magnetite-type oxides and these elements is responsible for this behavior or whether the actinide traces are adsorbed by van de Waals forces onto the large surface areas of the finely dispersed suspended solids. Under constantload operation conditions and as long as no additional fuel rod failures occur, the activity ratio Pu Co in the corrosion products remains virtually constant over time, thus indicating a similar behavior of these different elements in the coolant. 

Condition II faults, at worst, result in a reactor trip with the plant being capable of returning to operation. Condition II events are not expected to result in fuel rod failures, reactor coolant system failures, or secondary system over-pressurisation. 

Accident analysis shows that at the NPP blackout accident, the ferritic-martensitic steel cladding remains at temperatures above 900 K for a rather long time. Therefore, reliable test data are necessary on the strength and corrosion properties of this steel at high temperatures to make final conclusions about fuel rod failure... 

The reliability, safety, and operability of existing reactors, although acceptable, are affected by these design features. Narrow coolant channels decrease the coolant volume available to receive heat from the core. Increased fuel rod failure rates... 

Figure 2.6. A fuel rod failure, probably caused by different expansions of the uranium and the can.

The fuel rod design of the Super LWR follows those of BWRs and PWRs. It is designed for a high density UO2 pellet. The coolant pressure of 25 MPa is significantly higher than the 7.0 MPa of BWRs or the 15.4 MPa PWRs. Therefore, PCMI needs to be considered as one of the major fuel rod failure mechanisms. 

Fuel rod failure modes and associated fuel rod design criteria were established. As an example of the fuel rod design, the parameters were determined with those criteria including thermo-hydrodynamic and thermo-mechanical considerations. 

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