AN EVOLUTIONARY FUEL ASSEMBLY DESIGN FOR HIGH POWER DENSITY BWRs
Author(s)
Karahan, A.; Buongiorno, Jacopo; Kazimi, Mujid S.
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Other Contributors
Massachusetts Institute of Technology. Nuclear Fuel Cycle Program
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Show full item recordAbstract
An evolutionary BWR fuel assembly design was studied as a means to increase the power
density of current and future BWR cores. The new assembly concept is based on replacing
four traditional assemblies and large water gap regions with a single large assembly. The
traditional BWR cylindrical UO[subscript 2]-fuelled Zr-clad fuel pin design is retained, but the pins are
arranged on a 22×22 square lattice. There are 384 fuel pins with 9.6 mm diameter within a
large assembly. Twenty-five water rods with 27 mm diameter maintain the moderating power
and accommodate as many finger-type control rods. The total number and positions of the
control rod drive mechanisms are not changed, so existing BWRs can be retrofitted with the
new fuel assembly. The technical characteristics of the large fuel assembly were evaluated
through a systematic comparison with a traditional 9×9 fuel assembly. The pressure, inlet
subcooling and average exit quality of the new core were kept equal to the reference values.
Thus the power uprate is accommodated by an increase of the core mass flow rate. The
findings are as follows:
- VIPRE subchannel analysis suggests that, due to its higher fuel to coolant heat transfer
area and coolant flow area, the large assembly can operate at a power density 20% higher
than the traditional assembly while maintaining the same margin to dryout.
- CASMO 2D neutronic analysis indicates that the large assembly can sustain an 18-
month irradiation cycle (at uprated power) with 3-batch refueling, <5wt% enrichment
with <60 MWD/kg average discharge burnup. Also, the void and fuel temperature
reactivity coefficients are both negative and close to those of the traditional BWR core.
- The susceptibility of the large assembly core to thermalhydraulic/neutronic oscillations
of the density-wave type was explored with an in-house code. It was found that, while
well within regulatory limits, the flow oscillation decay ratio of the large assembly core is
higher than that of the traditional assembly core. The higher core wide decay ratio of
the large assembly core is due to its somewhat higher (more negative) void reactivity
coefficient.
- The pressure drop in the uprated core is 17 % higher than in the reference core, and the
flow is 20% higher; therefore, larger pumps will be needed.
- FRAPCON analysis suggests that the thermo-mechanical performance (e.g., fuel
temperature, fission gas release, hoop stress and strain, clad oxidation) of the fuel pins in
the large assembly is similar to that of the reference assembly fuel pins.
- A conceptual mechanical design of the large fuel assembly and its supporting structure
was developed. It was found that the water rods and lower tie plate can be used as the
main structural element of the assembly, with horizontal support being provided by the
top fuel guide plate and core plate assembly, and vertical support being provided by the
fuel support duct, which also supports the finger-type control rods.
Date issued
2007-07Publisher
Massachusetts Institute of Technology. Center for Advanced Nuclear Energy Systems. Nuclear Fuel Cycle Program
Series/Report no.
MIT-NFC;TR-092