Simulation of SBWR startup transient and stability Page: 6 of 20
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2.1.2 Saturation at the Inlet to the Chimney
the saturation temperature in the chimney varies along its height of 10 m, with the lowest at the top.
During Phases I and II, the coolant in the chimney remains subcooled. Migration of the vapor
bubbles to the chimney results in condensation in the subcooled liquid and a consequent heating of
the coolant. This process of heat transfer to the subcooled fluid in the chimney through bubble
condensation continues and the fluid in the chimney gradually reaches saturation at different axial
levels. The transition from Phase II to Phase III takes place when the fluid at the bottom of the
chimney is saturated.
2.1.3 Minimum Npch Condition
As the startup proceeds, the control rods are withdrawn and core power increases. The chimney is
in saturation state with vapor generation taking place and the reactor may go into loop type
oscillations. Lahey  has described a stability map for a BWR based on linear stability analysis as
shown in Figure 1. The abscissa represents the Phase Change Number (Zuber Number) Npch, which
is proportional to power. The ordinate represents the Subcooling Number, Nsub which is proportional
to core inlet subcooling. As Figure 1 indicates, there is a minimum Npch below which the reactor
is stable with respect to density waves. Thus, the transition between phases III and IV can be defined
in terms of the minimum Npch.
2.2 Simulation of the SBWR Startup Using the RAMONA-4B Code
2.2.1 The RAMONA-4B Code
The SBWR startup transient has been simulated using the RAMONA-4B code developed at BNL
[10,11]. The code models all the important components in the reactor pressure vessel (RPV), such
as the reactor core, downcomer, lower plenum, upper plenum and riser, jet or internal pumps, steam
separators and dryers, and the steam dome, as well as the control and plant protection systems, steam
lines, balance of plant (BOP), and containment. SBWR specific components such as the isolation
condenser (IC) and the standby liquid control system (SLCS) have also been modeled. The neutron
kinetics is modeled with a time-dependent 3D diffusion theory with one and half group of prompt
neutrons and six groups of delayed neutrons for a maximum of 804 neutronic channels each with a
maximum of 24 axial nodes. Local thermal-hydraulic feedback is taken into account in terms of the
changes in nodal two-group cross sections due to the local void, fuel temperature, and moderator
temperature. The two-phase thermal-hydraulics in the core is modeled via a drift-flux formulation
with flow reversal capability for nonequilibrium, nonhomogeneous flow through multiple (up to
200) parallel coolant channels.
The SBWR specific models implemented into RAMONA-4B are the isolation condenser (IC),
standby liquid control system (SLCS), and local boron transport. The boron transport model solves
the boron transport equation in each of thermal-hydraulic cells in the RPV by means of standard
donor-cell differencing with flow reversal logics. The detailed description of the various RAMONA-
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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. (https://digital.library.unt.edu/ark:/67531/metadc702243/m1/6/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.