| Summary: | Two new fuel assembly designs for light water reactors using advanced mixed-oxide
fuels have been proposed to reduce the radiotoxicity of used nuclear fuel discharged
from nuclear power plants. The research efforts of this thesis are the first to consider
the effects of burnup on advanced mixed-oxide fuel assembly performance and thermal
safety margin over an assembly?s expected operational burnup lifetime. In order to accomplish
this, a new burnup-dependent thermal-hydraulic analysis methodology has
been developed. The new methodology models many of the effects of burnup on an
assembly design by including burnup-dependent variations in fuel pin relative power
from neutronic calculations, assembly power reductions due to fissile content depletion
and core reshuffling, and fuel material thermal-physical properties. Additionally,
a text-based coupling method is developed to facilitate the exchange of information
between the neutronic code DRAGON and thermal-hydraulic code VIPRE-01. The
new methodology effectively covers the entire assembly burnup lifetime and evaluates
the thermal-hydraulic performance against ANS Condition I, II, and III events with
respect to the minimum departure from nucleate boiling ratio, peak cladding temperatures,
and fuel centerline temperatures.
A comprehensive literature survey on the thermal conductivity of posed fuel materials
with burnup-dependence has been carried out to model the advanced materials
in the thermal-hydraulic code VIPRE-01. Where documented conductivity values are not available, a simplified method for estimating the thermal conductivity has
been developed. The new thermal conductivity models are based on established
FRAPCON-3 fuel property models used in the nuclear industry, with small adjustments
having been made to account for actinide additions.
Steady-state and transient thermal-hydraulic analyses are performed with VIPRE-
01 for a reference UO2 assembly design, and two advanced mixed-oxide fuel assembly
designs using the new burnup-dependent thermal-hydraulic analysis methodology. All
three designs maintain a sufficiently large thermal margin with respect to the minimum
departure from nucleate boiling ratio, and maximum cladding and fuel temperatures
during partial and complete loss-of-flow accident scenarios. The presence of a
thin (Am,Zr)O2 outer layer on the fuel pellet in the two advanced mixed-oxide fuel
assembly designs increases maximum fuel temperatures during transient conditions,
but does not otherwise greatly compromise the thermal margin of the new designs.
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