Degradation of failed fuel rod is a situation where the leakage path(s) through the damaged cladding increases to the point where the fuel itself is dispersed into the primary system (Strasser et al., 2008). This may occur if the rods degrade to such a point that the water contacts the fuel pellet, par­ticularly if the contact also involves active flow of the water over exposed fuel pellets, one example being a large axial cladding crack. Steam will not be able to cause fuel washout while water can by oxidising the fuel grain boundaries thereby causing disintegration f the fuel grains. Normally, util­ities are much more concerned about fuel washout than high iodine and noble gas release. This is because it may take up to ten years to clean the core from the tramp uranium resulting from the fuel dissolution, while the high iodine and noble gas activities released from the failed rod will be elimi­nated when the failed rod is extracted from the core.

Degradation has historically been more of an issue in BWRs than in PWRs (Strasser et al. , 2008). Failed rods in PWRs may degrade, but the amount of dispersed fuel is lower than in a BWR. The rationale may be that the coolant chemistry in a PWR is more reducing than in BWRs. During the period 1992-93, six plants in the United States and Europe were forced into unscheduled outages because of concerns about failed Zr-sponge liner fuel (IAEA, no. 388, 1998). This is a liner produced from Zr sponge material to which no alloying elements have been added; its major impurities are oxygen (about 600-900 wt. ppm) and iron (about

150-500 wt. ppm). In all these cases, the very high off-gas activities and significant loss of fuel pellet material resulted from only one or two failed rods. Other plants in the United States and Europe also elected to shut down during and slightly after this interval to remove failed fuel assem­blies and avoid the risk of large residual contamination from tramp ura­nium. More recently, the risk of degradation and residual contamination has been reduced by the use of corrosion-resistant liners in BWR fuel to the extent that forced and voluntary outages are less common.

Two different types of degradation scenarios have been identified, namely the development of two different types of cracks (Strasser et al, 2008):

1. Transversal breaks (also called guillotine cuts or circumferential break) occurring in BWRs, PWRs and VVERs.

2. Long axial cracks (axial splits), which can occur in BWRs due to the movement of control blades but may also occur in PWRs that are sub­jected to significant control rod movements during operation. Axial split is a term introduced by GE and represents a failed rod that either has an off-gas level larger than 5000 jiCi/s (185 MBq/s) or a total crack length that is larger than 152 mm (6 inches).

Transversal breaks in BWRs — normally occur in low to intermediate bur — nup rods in the bottom part of the rod with a primary failure in the upper part of the rod (see Fig. 5.6) (Strasser et al. , 2008). The primary defect will allow water/steam to gain access to the rod interior (1 in Fig. 5.6) where the steam will oxidize the fuel clad inner surface forming a zirconium oxide the thickness of which will decrease with distance from the primary defect (2 in Fig. 5.6). At the same time a hydrogen partial pressure is being built up in the pellet-cladding gap. At a critical distance from the primary defect, the steam partial pressure will be insufficient to protect the clad inner surface

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from hydrogen ingress thus causing secondary hydriding (3 in Fig. 5.6) (e. g. Olander et al., 1997). If the hydride precipitates along the whole fuel clad circumference, the fuel rod may fracture transversally due to the hydride embrittlement effect (4 in Fig. 5.6).

Transversal break in PWRs/VVERs — are caused by a mechanistic develop­ment similar to that of BWRs (Strasser et al, 2008). However, the second­ary hydride defects tend to form in the upper part of a PWR/VVER rod. The processes involved in developing a transversal break in a PWR rod are shown in Fig. 5.7 .

1. Axial cracks in BWRs — Formation of long axial cracks has three prereq­uisites, (Strasser et al, 2008):

la. A sharp primary defect such as a PCI crack or cracks in hydride blisters formed due to a primary defect. However, in this case the hydride blister is very local and does not exist along the whole fuel clad circumference, as seen in formation of transversal breaks.

lb. A fuel cladding hydrogen content larger than the hydrogen solid solubility.

lc. A stress intensity (K,) at the crack tip above the critical value for crack extension. K, will increase with clad tensile stress level which in turn depends on:

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‘2

5.7 Schematic description of the events resulting in transversal break formation (Strasser et al., 2008).

1c1. The initial pellet-cladding gap prior to the power ramp, which depends on:

lcla. Burnup since the gap is decreasing with increased bur — nup due to fuel swelling and fuel clad creep-down. This is the reason that axial cracks do not form in low burnup fuel since the fuel pellet-clad gap is so large.

lclb. The corrosion properties of the cladding inner surface (Edsinger, 2000). The pellet-cladding gap decreases if the corrosion properties of the cladding inner surface are poor, resulting in formation of a thick porous oxide layer in the failed rod. The decrease in gap is related to the zir­conium oxide having a larger specific volume than that of the zirconium metal. It also turns out that, if the corro­sion resistance of the cladding inner surface is poor, then formed oxide is less dense due to the many cracks and pores which will decrease the pellet-cladding gap further. The first type of Zr-liner materials used in the nuclear industry were non-alloyed with very poor corrosion prop­erties. Once it was realized that the corrosion properties of the Zr liner have a large impact on the tendency to form axial cracks in failed fuel, all fuel vendors did alloy their liners to improve the corrosion resistance. However, it is important to ensure that the alloying additions will not degrade the PCI performance of the fuel cladding.

1c2. The magnitude of the rod power increase.

The axial split formation is schematically shown in Fig. 5.8 (Strasser et al, 2008). Initially, the control rod is inserted during the time when the primary defect occurs (1 in Fig. 5.8). The same scenario as for transversal breaks in BWRs occurs, but the secondary hydrides are distributed to several fuel clad locations which means that each hydride becomes too small to encom­pass the whole fuel clad circumference (2 in Fig. 5.8). The tensile stresses in the cladding which are necessary for crack propagation result from a power increase in the failed rod, for example, when a control rod adjacent to the failed rod is pulled out of the core. This will increase the temperature in the fuel stack resulting in a thermal increase of the pellet diameter. If these stresses become large enough the sharp defect may propagate if the result­ing K: exceeds the critical value for crack propagation (3 in Fig. 5.8). It is proposed that the mechanism for crack propagation forming an axial split is a delayed hydrogen cracking (DHC) type failure process (see e. g. Efsing & Pettersson, 1998; Edsinger, 2000; Lysell et al, 2000 for more details). The lower bounds of the crack velocities are in the range 4 x 10-8-5 x 10-7 ms-1 based on assumed constant growth rates in the time between first detection of the defect and removal of the fuel (Strasser et al, 2008).

5.8 Schematic showing the events resulting in axial split formation. The numbers in the figure relate to the sequence of the different events that may lead to an axial crack as described in the text (Strasser et al., 2008).

Axial cracks in PWRs/VVERs — Long axial cracks do not form in PWRs as readily as in BWRs (Strasser et al, 2008). The reason for the difference is that in PWRs, the power regulation is done slowly and without pronounced increases in local power by decreasing the boron coolant concentration, while power regulation in BWRs is done by a combination of control rod movements and variations in coolant flow, with the control blade move­ments leading to rapid increases in local power. However, axial cracks may form in PWRs/VVERs by essentially the same mechanism as formation of long axial cracks in BWRs due to (Strasser et al., 2008 ):

• A class II transient and/or

• Due to control rod movements in load-following plants.

5.3 Materials performance during accidents

Having considered normal operating conditions, we now move on to cover accident scenarios.