Environmental Engineering Reference
In-Depth Information
Extension of the understanding of these effects to new fuel pellet mate-
rials (e.g. uranium nitride (UN)).
Effects of long-term wet and dry storage, as well as environmental con-
ditions in any potential disposal site, on the integrity of the fuel in terms
of its holdup of long-lived radioactive components (mainly U, Pu, Am,
Cm, and Np).
Interactive effects of non-homogeneous portions of the fuel (such as the
rim after high burnups) on the performance of the fuel and its interac-
tion with the cladding during upset events such as reactivity insertion
accidents (RIAs).
Understanding of these effects in a mechanistic way is at the frontier of
nuclear fuel research. Since the current level of burnup between 50 and 60
MWd/kgU is just about the practical and economic upper boundary of 5%
enriched U-235 fuel today, this is the limit of our empirical knowledge. There
is a need to extend this boundary if enrichments above 5% become accepted
or if higher density fuels (such as UN) come into use. Phenomenological
models, not correlation of empirical data, will be needed to allow predic-
tions to be made without the huge cost associated with totally empirical
approaches.
9.2.2 Getting longer life from zirconium alloys
Under current operating exposure times imposed by the fi ve weight percent
U-235 enrichment limit, the behavior of zirconium based fuel cladding alloys
is reasonably understood on an empirical, and even somewhat phenomeno-
logical, basis. This understanding is based on predictions of the oxide layer
and hydride content of the cladding, both of which affect its ability to with-
stand stresses due to normal and accident conditions. This does not mean
that there are not unknown issues with the current zirconium alloy systems.
Prediction of the cladding resistance to RIAs is not well understood; here
large amounts of heat are almost instantaneously imposed on the fuel which
causes rapid expansion of the pellets into the cladding resulting in clad-
ding failure. As the exposure of the fuel increases and these rapid expan-
sion effects become more pronounced and less well understood, the nuclear
fuel manufacturers are left with the choice of understanding the mechanical
behavior of highly exposed (and oxidized and hydrided) cladding to rapid
stresses imposed by the fuel either on an empirical or phenomenological
basis. The empirical basis used so far relies on the collection of immense
amounts of data covering almost any potential event during fuel operation,
a very time consuming and expensive approach. The phenomenological
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