Simulation of SBWR startup transient and stability Page: 1 of 20
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SIMULATION OF SBWR STARTUP TRANSIENT AND STABILITY
Hsiang S. Cheng, Hasna J. Khan, and Upendra S. Rohatgi
Brookhaven National Laboratory C ElVE D
Upton, New York 11973 JUN 17 1998
ABSTRACT 0ST 1
The Simplified Boiling Water Reactor (SBWR) designed by General Electric is a natural circulation
reactor with enhanced safety features for potential accidents. It has a strong coupling between power
and flow in the reactor core, hence the neutronic coupling with thermal-hydraulics is specially
important. The potential geysering instability during the early part of a SBWR startup at low flow,
low power and low pressure is of particular concern. The RAMONA-4B computer code developed
at Brookhaven National Laboratory (BNL) for the SBWR has been used to simulate a SBWR startup
transient and evaluate its stability, using a simplified four-channel representation of the reactor core
for the thermal-hydraulics. This transient was run for 20,000 sec (5.56 hrs) in order to cover the
essential aspect of the SBWR startup. The simulation showed that the SBWR startup was a very
challenging event to analyze as it required accurate modeling of the thermal-hydraulics at low
pressures. This analysis did not show any geysering instability during the startup, following the
startup procedure as proposed by GE.
The Simplified Boiling Water Reactor (SBWR) designed by General Electric is an advanced passive
design using natural circulation for coolant flow without any active pumps. Details of the design
for various safety features are available in the Standard Safety Analysis Report (SSAR) of the
SBWR . The startup procedure of the SBWR is of special importance since the low pressure, low
flow and low power prevailing in the early part of the transient can become a precursor to an
instability. In a natural circulation system like the SBWR, the core flow is strongly coupled to the
reactor core power. Startup of the reactor begins with slow heat-up of the coolant in the reactor
followed by removal of selected groups of control rods. Gradual increase of reactor power results
in single-phase natural circulation flow within the reactor, while the system pressure is still very low.
During this period, transition from a subcooled core to a saturated core with a subcooled chimney
may be accompanied by creation of large vapor bubbles which can initiate geysering type instability.
Certain combination of pressure, flow and thermal-hydraulic conditions was required for geysering
to occur in small scale laboratory experiments such as Aritomi et al. [2,3] and Wang et al. .
According to these research findings and supporting analyses by Paniagua et. Al. , out of phase
geysering in two parallel channels is limited to low-pressure and low-flow conditions. A subcooled
chimney allows condensation of the large bubbles leading to instability in the flow within the
channels. At higher power, a loop type instability may follow the geysering instability, where the
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Cheng, H.S.; Khan, H.J. & Rohatgi, U.S. Simulation of SBWR startup transient and stability, article, June 1, 1998; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc702243/m1/1/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.