Environmental Engineering Reference
In-Depth Information
(a)
(b)
(c)
5.5 Schematics showing how the pellet-cladding gap may change
over burnup. (a) low burnup - a signifi cant pellet-cladding gap exists;
(b) intermediate burnup - no pellet-cladding gap; (c) high burnup in a
high power rod with signifi cant fi ssion gas release - reopening of the
pellet-cladding gap (Strasser et al ., 2010a).
150-500 wt.ppm). In all these cases, the very high off-gas activities and
signifi cant 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 assem-
blies and avoid the risk of large residual contamination from tramp ura-
nium. 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 identifi ed, 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 sub-
jected to signifi cant 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
￿ ￿ ￿ ￿ ￿ ￿
Ci/s (185 MBq/s) or a total crack
length that is larger than 152 mm (6 inches).
μ
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 insuffi cient to protect the clad inner surface
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