Simulation of SBWR startup transient and stability Page: 2 of 20
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flow in the downcomer and the core oscillates in phase. Finally, density wave oscillations of higher
frequencies can occur at higher power-to-flow conditions.
Although oscillations during startup has been demonstrated in the laboratory scale experiments, the
two st r t tgaghafural circulation Dodewaard reactor during 1992 have not shown any type of
instability . However, there are sufficient differences between the SBWR, Dodewaard reactor and
the labot6ry0 expdinients, so that the conclusions made from these tests can not be applied to the
SBWR directly. Assessment of the RAMONA-4B code developed at Brookhaven National
Laboratory'(BNL) with these tests will provide the confidence for the simulation of the SBWR
startup. A simulation of startup conditions for the SBWR has been performed at BNL using the
RAMONA-4B code. This code uses a 3D neutron kinetics with a drift flux model for two-phase flow
thermal-hydraulics. Since information on the spatial and temporal movements of the control rods
are not available from the GE report, in order to simulate the SBWR startup, a time-dependent power
ramp profile has been imposed as a boundary condition, and a thermal-hydraulic only calculation
was performed with four parallel coolant channels represention of the reactor core. Thermal-
hydraulic calculations over a long period of time (5~10 hours) are needed for a startup simulation.
Simulations of the SBWR startup were also performed by GE using the TRACG code .
2. SIMULATION OF THE SBWR STARTUP TRANSIENT
2.1 SBWR Startup Transient Phases
A Phenomenon Identification and Ranking Table (PIRT) was developed at BNL for the SBWR
startup transient . According to the PIRT, the startup transient can be divided into four distinct
phases. Table I shows the four different phases of the startup process. Events within each phase is
separated by specific thermal-hydraulic conditions of the reactor. A brief description of these phases
and their transitions will be given below.
2.1.1 Onset of Net Vapor Generation (NVG)
Due to the hydrostatic head, there is an axial variation of pressure and corresponding saturation
temperature, along the core height of 2.7 m. During the early part of Phase I, the pressure at the
steam dome does not change significantly and hence the axial profile of saturation temperature
remains unchanged. Within this period, single phase natural circulation flow prevails in the core until
subcooled vapor generation begins. The saturation temperature in the core is lowest at the top and
the liquid temperature is the highest in this region. Therefore, subcooled vapor generation begins in
this region. During the startup, the boundary of subcooled vapor generation moves toward the core
entrance. However, the subcooled condition in the core is unable to sustain the vapor generation,
which results in condensation of the bubbles after detachment. During this process, the coolant
temperature within the core increases and the flow is enhanced. Onset of Net Vapor Generation
(NVG) is associated with sustained bubble concentration in the channels and a consequent increase
in buoyancy induced flow. NVG separates Phases I and II as shown in Table I.
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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/m1/2/: accessed February 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.