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Nuclear Fuel Cycle (NFC) Technology and Policy Program
Evaluation of High
Power Density
Annular Fuel for
Korean OPR-1000
Reactor: Final Report
March 2008 – January 2010
Principal Investigator: M.S. Kazimi
Co-Principal Investigator: P. Hejzlar
Contributors:
Liang Zhang, Andrew Lerch, Bo Feng, Edward E. Pilat,
and Mujid S. Kazimi
MIT-NFC-TR-114 (March 2010)
Abstract
Earlier MIT studies have shown that internally and externally cooled annular fuel makes it
possible to significantly increase (by up to 50%) power density in the standard Westinghouse
PWR while maintaining or increasing safety margin. The present project has evaluated feasibility
of 20% higher power density for the Korean OPR-1000 reactor having annular fuel assemblies of
different dimensions and operating conditions than the standard Westinghouse PWR used in
previous MIT analyses. The most important difference is keeping the coolant flow rate fixed at
the reference value in this study.
Core physics parameters of an equilibrium core for an OPR-1000 type reactor with annular fuel
were found to be similar to those of a solid pellet core, even with a 20% uprate. This suggests
that reactor physics considerations peculiar to annular fuel do not provide any impediment to its
use. The proposed annular fuel assemblies are composed of 7.5% and 6.5% U-235 enriched fuel
rods, and burnable poisons with various Gd2O3 weight percentages ranging from 4% to 16%. The
analyses included calculation of cycle length, critical boron concentration, radial and axial power
distribution, cycle length, critical boron concentration, radial and axial power distribution,
temperature coefficient and shutdown margin. MIT core calculations were performed using the
CASMO-SIMULATE code package, which was initially benchmarked against MIT’s MCODE
combination of MCNP and ORIGEN, then against the deterministic code TRITON. Final
benchmarking was performed via an MIT model of Ulchin Nuclear unit 5 with solid fuel. This
model, developed using CASMO-4 and SIMULATE-3, closely matched KAERI’s results for
Cycles 01–03, but showed some differences in lifetime and critical boron concentration for Cycle
04. Given the good agreement for Cycles 01 through 03, it was felt that the differences in Cycle
04 may be due to an incorrect loading used in one of the fuel types whose definition was unclear.
DNBR evaluation of an annular fueled core using VIPRE-01 for the whole core showed that the
original KAERI annular fuel design has larger MDNBR margin than the solid fuel at 100%
power. Assuming an unchanged core flow rate and equal conductances for the inner and outer
gaps, however, this design cannot achieve power uprate to 120% even with a reduced core inlet
temperature. The MDNBR in the inner channel is too small. This problem arises because the
diameter of the inner channel does not allow sufficient flow rate through it. Use of KAERIs
suggested gap conductances (3500/7000 for inner/outer) significantly alleviates this problem,
allowing an uprate to 117.55%. Search was then performed to identify a better optimized design
that could achieve 20% power uprate. This might be accomplished through fine-tuning of the rod
dimensions by slightly increasing both inner channel and outer channel diameters, while keeping
the fuel to moderator ratio fixed, but the modified design requires reducing the gap between the
rods to 1mm, which may challenge manufacturing feasibility. An alternative design with slightly
larger gap was shown to also achieve 120% power with good MDNBR margin, but requires grids
with higher grid loss coefficient and results in a slight increase of core pressure drop.
A proposed 14x14 annular fuel design was also analyzed using a VIPRE-01 model. The results
of this design are not promising in terms of MDNBR, even after changing the width and height
of the corner cruciform guide tube. The designs of 12x12 and 14x14 annular fuel with reduced
inner gap conductance and increased outer gap conductance were also evaluated. The largest
possible power uprate is 17.55% for the 12x12 annular fuel, close but still less than the target of
20%.
A number of ancillary thermal-hydraulic issues were also investigated. Evaluation of MDNBR
sensitivity to manufacturing tolerances showed that the new proposed design can accommodate
typical manufacturing tolerances. Partial blockage of the inner channel by debris, and the impact
of corrosion and crud growth were analyzed. Although this is a hypothetical scenario because the
inlet debris filter is of much smaller size than the inner channel diameter, it has been shown that
the inner channel can accommodate a blockage up to 43% of its flow area before MNDBR falls
below the 1.3 limit. MDNBR results in the presence of corrosion and crud growth show that the
impact of crud and ZrO2 buildup does not reduce MDNBR margin below the 1.3 limit, as long as
the thickness is less than
74μm~94μm.
Thermomechanical behavior or the solid and annular fuel rod designs was compared using
several modifications of the NRCs FRAPCON code developed at MIT. Designs for the annular
fuel rod were analyzed and compared to the solid fuel rod and to the fuel rod safety limits given
by the NRC. The annular fuel design given by KAERI exhibits a better performance than the
solid rod because it is capable of achieving 70 MWd/kg burnup at both 100% and 120% power.
However, the inner clad exceeds the ZrO2 oxidation limit of 17%. The proposed 14x14 design
by KAERI was satisfactory from a mechanical point of view at 100% power, however, due to the
thermal hydraulic problems, this design was not pursued further. The most important result of
this analysis is that the heat flux split between inner and outer channels is nowhere near even;
initially it is higher through the outer clad, but by 10 MWd/kgU the heat flux through the inner
clad far exceeds that through the outer clad. This imbalance is at least partly responsible for the
excessive oxidation and clad temperature observed in the calculations, as well as the DNBR
problems with the inner channel.
Thus, the largest possible power uprate that appears justifiable from the present study is 17.55%
for the 12x12 annular fuel, close to but still less than the target of 20%. This limit arises largely
from DNB considerations in the inner channel of the annular fuel. However, given that our
analyses have applied conservative assumptions to cover the transient effects and uncertainties,
and that our methods have not been calibrated as much for the Korean reference assembly
designs (like grid effects, and power distribution factors), this 2.45% difference in the uprate
level may simply amount to added conservatism.
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