Environmental Engineering Reference
In-Depth Information
many other additives fall into both categories, attention is
directed to the aluminosilicate additives and chromia-base dop-
ants as they appear to be the closest to large-scale application.
2b3. Aluminosilicate additives consist of a mixture of SiO
2
and Al
2
O
3
and is offered by GNF. During the pellet sintering process, the
additive forms a glassy phase that collects on the grain bound-
aries. It appears that the Al-Si-O additive at the pellet grain
boundaries will chemically react with I, Cs and Cd, thus prevent-
ing these SCC-promoting elements from accessing the fuel clad
inner surface (Matsunaga
et al
., 2009 , 2010 ). Additive fuel has
been irradiated in commercial and test reactors in the US and
in Europe. Ramp tests under BWR conditions in the R-2 and
Halden reactors of segmented additives rods from commercial
reactors show excellent resistance to the PCI failure mechanism
(Davies
et al
., 1999 ).
2b4. Chromia (Cr
2
O
3
) is the dopant of greatest commercial signif-
icance in this class of additives. Two types of chromia-based
additives are being offered. The fi rst consists of Cr
2
O
3
in
UO
2
as offered by AREVA (Delafoy
et al
., 2003 ). The sec-
ond consists of Cr
2
O
3
and Al
2
O
3
in UO
2
as offered by
Westinghouse (Arborelius
et al
., 2005 ). Alumina is reported
to be used in the second form to minimize the effects of
chromium on the fi ssion cross-section of doped pellets while
enhancing grain growth. In both cases, chromia is expected
to reside largely within grains as interstitial Cr
3+
and as
insoluble Cr
2
O
3
depending on the concentration and tem-
perature. The alumina in the mixed Cr-Al-O dopant should
exist as a grain-boundary phase as in the Al-Si-O additive.
The cation dopants were developed to increase grain size
to reduce fi ssion gas release (FGR) at extended burnup(see
for example Delafoy
et al
., 2007). In addition to improved
FGR, chromia-based dopants are reported to improve PCI
resistance. Information available in this area is less exten-
sive for the chromia-based dopants than for the aluminosil-
icate additives. However, ramp tests indicate that the resis-
tance to PCI failures of fuel with chromia-based dopants
are improved relative to standard fuel in cladding without
PCI-resistant liners (Delafoy
et al
., 2007 ).
5.2.3 Cladding liftoff
If the rod internal pressure becomes larger than the reactor system pres-
sure, the fuel cladding may start to creep outwards (Fig. 5.5) (Strasser
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