Environmental Engineering Reference
In-Depth Information
Mechanical tests
- Mechanical tests of differently shaped samples from
the fuel cladding can be carried out in the hot cell. Some of the tests that can
be done with irradiated cladding are:
Room and elevated temperature axial tensile test yield strength (YS),
ultimate tensile strength (UTS), uniform elongation (UE) and total
elongation (TE).
Cladding room and elevated temperature ring tensile test (YS, UTS, UE
and TE).
Burst testing.
Cladding thermal creep rate test.
Hardness testing.
5.6
Future trends and research needs
Improved fuel reliability and operating economics are the driving forces for
changing operating conditions, while at the same time maintaining accept-
able margins to operating and regulatory safety limits. Table 5.1 gives the
trends for burnup (BU) achieved compared to regulatory limits in various
countries. An approximate conversion of BU to fl uence is 50 GWd/MT,
which is equivalent to about 1
10
22
n/cm
2
,
E
> 1 MeV (or about 17 dpa),
but this depends on many nuclear parameters such as enrichment, extent
of moderation and neutron energy spectrum. In general PWRs operate
to higher discharge burnups compared to BWRs because of higher PWR
power densities and neutron fl uxes, but the differences are decreasing with
time. There are some incentives to reach burnups of 60-70 GWd/MT batch
average, but the economic values of doing so are decreasing. The majority
of US plants and many in Europe have undergone power uprates from a
few per cent to up to 20%. This increases the number of fuel assemblies in
a core that operate at high power, thereby decreasing the margin to estab-
lished limits. In cooperation with utilities, fuel suppliers have operated lead
test assemblies (LTA) or lead use assemblies (LUA) to very high burnup,
in some cases approaching 100 GWd/MT peak rod exposure (Strasser in
Adamson
et al
., 2010 ).
As discussed in earlier sections, as burnup and fl uence become higher so
material properties and microstructure evolve. Examples include:
×
In PWRs it is found that Zircaloy-4 no longer meets corrosion and
hydriding needs therefore virtually all current PWR cladding uses a zir-
conium alloy containing Nb.
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