Buoyancy-driven flow excursions in fuel assemblies Page: 4 of 23
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St = - 0.0065 (1)
GCP Tbl - Tsat )
By definition, the design-basis accident must be more limiting than other postulated
accidents. One of these other accidents is the Loss of Pumping Accident (LOPA), in
which a line break interrupts the supply of secondary cooling water. The secondary line
break trips the power to the primary AC motors for the pumps that supply primary
coolant. As a result, the primary pumps coast down and the coolant flow rate gradually
drops, until the emergency cooling water supply is activated. This stabilizes the coolant
flow rate until the backup DC pump motors become flooded with secondary cooling
water. When these motors flood, the primary pumps eventually coast to a stop. A
previous analysis indicated that the most limiting condition would probably occur during
the coast down of the DC pump motors . This analysis used a modified Stanton
number criterion based on the results of flow excursion tests conducted in a single-
channel test rig :
St = 0.0025 (2)
The purpose of this study was to generate a low flow power limit criterion based on
results of flow excursion tests performed in a multiple-channel test rig . This test rig,
called SPRIHTE, or SRS Prototypic Rig for Heat Transfer Experiments, was operated by
the Savannah River Site heat transfer test facility.
Description of Test Facility and Tests
The SPRIHTE test rig, depicted in Figure 1, was prototypic of a Savannah River fuel
assembly in that it consisted of concentric tubes separated by spacer ribs. Coolant flowed
in the four subchannels between each pair of adjacent tubes. As the figure shows, the
SPRIHTE rig contained two heated tubes and two unheated tubes, which correspond to
the fuel and target tubes of a fuel assembly. The rig differed from an actual fuel
assembly, however, in that the there was no outer housing. The purge channel between
the outer target tube and this outer housing was replaced by two external bypass tubes
located on opposite sides of the heated section.
The test rig also differed from a fuel assembly in that only the middle two tubes of the
test rig, corresponding to the inner and outer fuel tubes of the reactor assembly, were
heated. In a fuel assembly, both the fuel and the target tubes generate heat. The outer
heater tube of the test rig received 60% of the electrical power, and the inner heater tube
received 40%. The heater tubes were constructed to give an axial power profile
prototypic of a fuel assembly. The maximum power was located about three-quarters of
the distance from the top of the heated section and was 1.43 times the average power.
Overall flow rates to the SPRIHTE rig were measured using ultrasonic and turbine flow
meters; the ultrasonic flow meters measured high flow rates and the turbine flow meters
measured low flow rates. Individual channel and subchannel flow rates were not
metered. The rig was fully instrumented to measure subchannel coolant and heater wall
temperatures. Wall and subchannel coolant temperatures were measured using
thermocouples; inlet and outlet fluid temperatures were measured using resistance
temperature devices (RTD's). Inlet and outlet pressures also were measured.
Measurement uncertainties were 2-3% for the ultrasonic flow meters, about 2% for the
turbine flow meters, 1.0*C or-less for the thermocouples, 0.5*C for the RTD's, and 0.01
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Laurinat, J.E.; Paul, P.K. & Menna, J.D. Buoyancy-driven flow excursions in fuel assemblies, article, December 31, 1995; Aiken, South Carolina. (digital.library.unt.edu/ark:/67531/metadc724038/m1/4/: accessed June 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.