Environmental Engineering Reference
In-Depth Information
approach would be much better, but there is practically no such basis for
understanding except at the very rudimentary level. Previous attempts to
model and predict these effects have been unsuccessful due to the com-
plexity of the interactions between the models of the various effects which
lead to severe non-convergence problems. Major programs are currently
underway funded by the U.S. Department of Energy (DOE) and industry
to try to understand and model fuel behavior on a phenomenological basis
using improved convergence algorithms (for instance the Consortium for
Advanced Simulation of Light Water Reactors (CASL) and the Nuclear
Energy Advanced Modeling and Simulation (NEAMS) Program).
Besides operating under extended burnup, the known operating limits
of nuclear fuel cladding are being exceeded by high thermal duty condi-
tions. This arises from the desire of nuclear utilities to get the most electrical
generating capacity out of their current plants while still using a minimal
amount of fuel. These higher thermal operating limits (in kilowatts per
meter) increase the temperature of the cladding which, in turn, increases
the build-up of crud on the surfaces, further increasing the cladding temper-
ature and its oxidation rate and hydride content. Higher thermal rates also
increase fretting issues between the fuel rods and their support structures
causing unintended wear and fuel cladding failures. The added vibration is
due not only to increased stress due to fl ow variations, but also to relaxation
of the grid structures that support the fuel rods. In addition, there is a trend
to move to slightly higher pH values by adding more lithium to the primary
coolant. These higher lithium levels affect the corrosion rate of the fuel
cladding though they also seem to inhibit stress corrosion cracking (SCC)
of steam generator alloys in the primary circuit (EPRI, 2012).
Besides the more recent concern with RIA-type accidents and the ability
to store used nuclear fuel for long times under both wet (spent fuel stor-
age pool) and dry (spent fuel storage casks and perhaps internment in a
fi nal repository) conditions, there is the continuing concern of how fuel
which has experienced high burnups will respond to such classic accident
scenarios as large and small break loss of coolant accidents (LOCAs) due
to loss in ductility and strength. Current knowledge of both oxidation layer
thickness and hydride content is gained entirely through empirical testing.
Development of new alloys is based entirely on an empirical approach with
very little theoretical guidance.
Areas of research for zirconium (and any other metal) alloy claddings
include the phenomenological understanding of:
The oxidation behavior of zirconium alloys at all temperature condi-
tions (including high temperature accidents).
The hydriding of zirconium alloys at all temperature conditions.
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