Buoyancy-driven flow excursions in fuel assemblies Page: 3 of 23
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BUOYANCY-DRIVEN FLOW EXCURSIONS
IN FUEL ASSEMBLIES (U)
James E. Laurinat
Pran K. Paul
John D. Menna
Westinghouse Savannah River Company
Savannah River Technology Center
Aiken, SC 29802
A power limit criterion was developed for a postulated Loss of Pumping Accident
(LOPA) in one of the recently shut down heavy water production reactors at the
Savannah River Site. These reactors were cooled by recirculating moderator downward
through channels in cylindrical fuel tubes. Powers were limited to prevent a flow
excursion from occurring in one or more of these parallel channels.
During full-power operation, limits prevented a boiling flow excursion from taking place.
At low flow rates, during the addition of emergency cooling water, buoyant forces
reverse the flow in one of the coolant channels before boiling occurs. As power increases
beyond the point of flow reversal, the maximum wall temperature approaches the fluid
saturation temperature, and a thermal excursion occurs.
The power limit criterion for low flow rates was the onset of flow reversal. To determine
conditions for flow reversal, tests were performed in a mock-up of a fuel assembly that
contained two electrically heated concentric tubes surrounded by three flow channels.
These tests were modeled using a finite difference thermal-hydraulic code. According to
code calculations, flow reversed in the outer flow channel before the maximum wall
temperature reached the local fluid saturation temperature. Thermal excursions occurred
when the maximum wall temperature approximately equaled the saturation temperature.
For a postulated LOPA, the flow reversal criterion for emergency cooling water addition
was more limiting than the boiling excursion criterion for full power operation. This
criterion limited powers to 37% of historical levels.
Power limits for the heavy water production reactors at the Savannah River Site were set
to prevent a postulated Loss of Coolant Accident (LOCA) . In this design-basis
accident, a line break interrupts the supply of primary coolant, causing the reactor to
scram. Power limits were set to prevent a parallel channel flow excursion during the two
to three seconds immediately following the scram. This flow excursion occurs when the
pressure drop reaches a minimum in one flow channel due to boiling in that channel. If
the coolant flow rate decreases further, the boiling diverts flow to adjacent channels, so
that the affected channel dries out and undergoes a thermal excursion. The limiting
criterion for a boiling excursion was a lower bound  to the Saha-Zuber correlation for
onset of significant void generation . The lower bound to the Saha-Zuber correlation,
which accounts for uncertainties in the correlation, takes the form of a Stanton number,
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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/m1/3/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.