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STEAM LINE BREAK ANALYSIS OF STEAM GENERATOR IN NUCLEAR
POWER PLANT USING RELAP5
Chang-Hoon Ha, Jong-In Kim and Moo-Yong Kim
DOOSAN Heavy Industries & Construction Co. LTD, 555 Guygok-Dong, Changwon, Gyeongnam,
641-792, Korea
1. Introduction
The steam generator in nuclear power plant has a function not only to supply continuously the high
quality steam for the turbine to produce electricity but also to sustain the pressure boundary to the high
primary pressure. Therefore, the component designer of steam generator should give consideration on the
structural integrity as well as the high performance for the design life. The steam generator goes through
numerous thermal hydraulic transients, such as pressure, flow rate and temperature and also the accident
postulated.
This study is to simulate the accident, especially in the steam line break (SLB) using RELAP5/MOD3
(herein called RELAP5) thermal hydraulic computer code. A RELAP5 (reference [1]) model was
developed to simulate the SLB of steam generator and to determine hydraulic loads across the internals.
Therefore, various thermal hydraulic parameters, such as pressure, flow rate, velocity and void fraction,
were obtained to evaluate the integrity of steam generator internals during the SLB accident.
CEFLASH-4B (reference [2]) computer code has been used to perform SLB analysis of steam generator
before. But preparing the input list of CEFLASH-4B is very difficult because the model consisted of
many nodes and flow paths. In addition, the initial condition of nodes and flow paths must be computed
using other computer program. As a result of this investigation a new analysis method which will be
conveniently used in thermal and hydraulic simulation of SLB with a good reliability, is developed in
this paper. The SLB analysis was performed using the RELAP5. The RELAP5 is the third major variant
of the RELAP5 advanced thermal-hydraulic code that was developed by the Idaho National Engineering
Laboratory (INEL) in 1979. Initially, this code was used principally by INEL analysts for understanding
Loss-of-Fluid Test and Semiscale Experimental Behaviour. Since then, the code has become widely
accepted throughout the world for analysing commercial and experimental reactor systems together with
their related scaled subsystems.
A RELAP5 model for the Replacement Steam Generator (RSG) of Sequoyah unit 1 (SQH#1) was
developed to analyse the SLB and system model preserved most of major features of the RSG including
component elevations, component volumes, flow rates and flow lengths.
2. Analytical Model and Condition
The RELAP5 equation set gives a two-fluid system simulation using a non-equilibrium, non-
homogeneous, six-equation representation. Constitutive models represent the interphase drag, the various
flow regimes in vertical and horizontal flow, wall friction, and interphase mass transfer. The code also
has the capability to simulate energy transfer to and from the fluid by the presence of slabs of material
adjacent to the fluid. Control systems and component models permit simulations of equipment
controllers, balance-of-plant equipment, and lumped-node representations of various processes. The
process of building a RELAP5 model is analogous to physically building the system that is being
modelled. Just as the physical system is composed of pipes, valves, pumps, and other components, so is
the RELAP5 model. Since the hydrodynamic simulation used within RELAP5 code is based on a one-
dimensional model of the transient flow for a steam-water non-condensable mixture, the numerical
solution scheme that is used results in a simple system representation using control volumes that are
connected by juncture.
The RELAP5 building blocks can be divided into four fundamental groups; thermal-hydraulic, heat
structures, trips, and control variables. The thermal-hydraulic group is composed of components
designed to simulate fluid passages and fluid handling equipment. Heat structures are designed to
simulate material mass and the interaction between the material mass and the fluid passage. Trips are
designed to simulate the signals that initiate equipment actions of various sorts (e.g., turning on a pump
at a desired time or causing a valve to open at one pressure but close at another). Finally, control systems
are designed to give the code additional modelling capability by allowing equipment control systems.
2.1 Analytical Model
The SQH#1 RSG is of the vertical, inverted U-tube, feed ring type and shown on Fig.1. The pressure
vessel incorporates a primary head, a main shell containing the heat transfer surface, and a enlarged
steam drum at the top. The heat transfer surface, which also provides the interface between the primary
and secondary systems, consists of a bundle of 4983, alloy 690, u-tubes hydraulically expanded into and
welded to the tubesheet. The tubesheet forms the pressure boundary between the primary and the
secondary sides. The primary fluid enters the primary head through one inlet nozzle, passes through the
tubes and leaves via the two outlet nozzles in the primary head. The primary head contains a divider plate
that separates incoming and outgoing primary fluid. The tube bundle is situated within a shroud which
defines the secondary side two-phase circulation flow paths. The steam is separated from the fluid in the
steam drum by separators and dryer. The remaining liquid drops to the can-deck and flows down the
downcomer, outside of the shroud, and enters the top of tubesheet.
Figure 1 The replacement steam generator of Sequoyah Unit 1
In developing the RELAP5 model, the principal modelling objective was to preserve all secondary-side
volumes and elevations. As a consequence, many of the flow areas within the unit are representative of
the average flow area and were obtained by dividing the flow volume in each section of the steam
generator by the flow length. On the primary-side, only the fluid volume contained within the tubes was
Steam Outlet Nozzle
Steam Dryer
Moisture Separator
Feedwater Nozzle &
Feed Ring
Shroud
U-Tube & Tube Bundle
Tubesheet
Primary Head
Downcomer Region Tube Support
191192
170-11176-1
180-J2
180
180-J1
171-J1
170-12174
174
178
174-J2
174-J1
171
171-J3
182
183
186189
190
184
185
176-1
174187
188
1
3
4
5
6
7
8
9
10 11
12
13
14
15
16
17
18
19
20
176-1 170-11
177107 109
106 110
2
3
4
5
6
7
8
9
10 21
2
3
4
5
6
7
8
9
10
preserved. The primary inlet and outlet plenum dimensions and volumes were not preserved since these
regions are considered to be non-active and do not affect results.
The secondary-side volumes are calculated as the total secondary-side volume excluded all major
internals. The downcomer volume includes all of the annulus outside of the shroud from the top of lower
shell to the tubesheet.
Referring to the RELAP5 nodalization diagram shown in figure 2, there are three pipe components in the
RELAP5 model. The components numbered 108, 170 and 176 are the three pipe components.
Component 108 represents primary fluid volume inside of tubes. The first and last part of component 108
represent fluid volume inside of tube within tubesheet at the hot side and cold side respectively.
(Upper part of SG) (Lower part of SG)
Figure 2 The schematic diagram of RELAP5 analytical model
The first 8 part of Component 170 represents secondary fluid volume between each tube support plates.
The 9th part of Component 170 represents U-bend region of tube, and the 10th and 11th part of
Component 170 represents riser, and The 12th part of Component 170 represents fluid volume inside
primary separator. The first part of Component 176 represent downcomer transition region and the other
parts of Component 176 represent lower downcomer. The separating region from top of separator to
dryer was modeled separator component. The separator component provides a simplified representation
of the actual process. In general, this component accepts the inlet flow, performs an idealized prescribed
separation of the liquid and vapor phases. When in the separating mode, it passes the vapor out the
separator outlet junction and passes the liquid out the liquid return junction. Single volume 174
represents the volume of fluid contained within the drum liquid region. The normal water level was input
as a void fraction of this volume. Single volume 178 represents the volume of fluid within drum and
outside of separator. Branch 180 represents the flow path from the separator outlet to dryer inlet. Single
volume 182 and 184 represent the volume above the dryer. Single junction 185 represents the steam
outlet and trip valve 191 represents the break line of steam to the containment building which is modeled
as single volume 192. Time dependant juncture 187 represents the feed water control valve to control the
water level. Cylindrical heat structures were used to model the heat transfer path between the primary
and secondary side.
2.2 Condition Analyzed
One power level, 100% power, was analyzed in this study and the steam generator operating parameters
for the 100% power level is taken from reference [3] and [4]. The normal water level is 500 inches above
the tubesheet. The operating parameters for 100% power level are shown below.
Table 1 Operation parameters for 100% power level
Primary Side Condition Secondary Side Condition
� Pressure : 15.513 MPa
� Inlet Temperature : 320.06 oC
� Outlet Temperature : 286.67 oC
� Flow Rate : 4.646E+03 kg/sec
� Heat Load : 866.904 kW
� Saturation Pressure : 6.17 MPa
� Steam Flow Rate : 477.532 kg/sec
� Feed Water Temperature : 224.33 oC
� Circulation Ratio : 4.97
� Water Level : 12.7 m
3. Method of Solution
The thermal hydraulic information presented in this paper was obtained using the RELAP5 which
calculate the behaviour of a reactor coolant system during transient and can be used for simulation of a
wide variety of hydraulic and thermal transients in both nuclear and non-nuclear systems involving
mixtures of steam, water, non-condensable, and solute.
The result of RELAP5 largely depends on user's approach made in the application of the code to the
specific system. Therefore the results of RELAP5 to the SQH#1 RSG model are deliberately reviewed as
described below.
The elevation variation check through the steam generator circulation loop is properly implemented by
reducing the height of last part of downcomer component, which make the re-entrance elevation of first
part of tube bundle to its realistic value.
Component pressures are also checked through the secondary system in order to prevent unrealistic
pressure drop due to Courant limit error which happens in the component with very small fluid volume
relative to other components. Heat structure input like tube thickness, tube inside diameter and overall
tube length within a fluid volume is also reversely checked by comparing the resulting transferred heat
with desired values.
Separator model is checked by reviewing whether it give proper liquid fall back path condition and
vapour outlet path condition. As a result, high quality steam condition is obtained in vapour outlet path
and subcooled water condition is obtained in the downcomer region which is downstream of liquid fall
back path.
The analysis results of RELAP5 are compared with those of CEFLASH-4B (reference 6). CEFLASH-4B
is a one-dimensional, two-phase, thermal-hydraulic code which calculates the time-dependant behaviour
of the fluid state resulting from a steam line rupture or an operational transient. The solution of
CEFLASH-4B proceeds by numerically integrating the momentum equation applied to the flow paths,
which is represented as juncture component of RELAP5, while maintaining mass and energy balances in
the nodes which are represented as volume and pipe in RELAP5. The conditions applied on each
analytical computer code, RELAP5 and CEFLASH-4B, are listed in Table 2.
Table 2 Conditions applied on analysis computer code
Condition RELAP5 CEFLASH-4B
Model Structure Based on Volume & Juncture Based on Node & Flow Path
Two-Phase Non Homogeneous Homogeneous
Critical & Choked Flow Henry-Fauske Flow
Moody Critical Flow
Modified Henry-Fauske/Moody
Flow
Opening time 0.0 sec 0.001 sec (1 ms)
Discharge Coefficient 1.0, 1.2 1.0
The three cases of applied condition for RELAP5 are investigated for various results and are shown on
Table 3.
Table 3 Cases of applied condition for RELAP5
Case I.D Flow Model Discharge Coefficient(DC)
DC 1.0 Henry-Fauske Flow 1.0
DC 1.2 Henry-Fauske Flow 1.2
MDC 1.0 Moody Critical Flow 1.0
4. Analytical Results
4.1. The Results of System Initialization
Before the SLB analysis, the initial condition of 100% power level is set up on RELAP5 model. When
the analytical system of RELAP5 model becomes a steady state, the trip valve opens to simulate the
steam pipe rupture. Table 4 shows the results of hydraulic parameters on the initial condition.
Table 4 Results of Initialization of 100% Power Condition for RELAP5 SLB Model
Operating Parameters From RELAP5 From Thermal Hydraulic
Performance Analysis Ratio
Steam Flow Rate (kg/sec) 476.5 477.5 0.998
Recirculation Flow Rate (kg/sec) 2359.5 2375.1 0.993
Heat Transfer Rate (kW) 8.55 E+05 8.67 E+05 0.986
Water Level (m) 12.7 12.7 1.00
4.2. Condition Analyzed
The method described in section 3 is used to obtain pressure loads on the steam generator internals which
include the steam dome, the separator deck, U-bend region and tube supports. Results are obtained for a
2.0 second blowdown simulation at 100% power. Peak pressure loads on steam generator internals are
presented Table 5.
Table 4 Results of Peak Pressure Loads (kPa)
RELAP5 CEFLASH-4B Location
DC 1.0 DC 1.2 MDC 1.0 MDC 1.0
Steam Dome 25.5 32.9 27.6 18.2
Separator Deck 66.4 84.7 68.7 79.1
U-Bend Region 20.5 23.8 21.0 25.3
Tube Bundle 98.7 106.0 100.0 97.6
Upper ATSG 14.3 16.3 14.6 19.8
Middle ATSG 14.3 16.1 14.5 18.4
Lower ATSG 11.2 11.7 11.4 19.4
Figure 3 and 4 present the pressure variation and the flow rate as a function of time for 100% power
case, respectively.
Pressure Variation at Steam Dome Pressure Variation at Separator
Figure 3 Pressure Variation at Upper SG
Flow Rate at Upper Tube Bundle Flow Rate at Lower Tube Bundle
Figure 4 Flow Rate Variation at Tube Bundle
5. Conclusion
The SLB analysis of SQH #1 RSG was successfully performed using RELAP5. As a result of this
investigation, it was found that the RELAP5 hydrodynamic model was well constructed because
RELAP5 hydrodynamic component represented actual hydrodynamic system component. Furthermore,
no additional calculation to get initial condition of RELAP5 hydraulic model is needed because the
initial condition can be obtained by steady state solution of the code itself.
The RELAP5 model developed for this analysis also can be used to perform the various hydraulic
performance analyses of steam generator. These hydraulic performance analyses include finding the
relationship of water level to moisture carry under, producing the thermal hydraulic responses for
pressure, temperature and flow rate inside of steam generator during the plant operation, and predicting
water level variation during power-up transient.
6. References
[1] Nuclear Safety Analysis Division, “RELAP5/MOD3.3 Code Manual, Volume I through VII”,
Information Systems Laboratories, Inc., Rockville, Maryland, Idaho Falls, Idaho, Prepared for the
Division of Systems Research Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory
Commission Washington, DC 20555, NUREG/CR-5535/Rev P3-Vol I through VII.
[2] Michonski, M., “Software Verification and Validation Report, CEFLASH-4B, Version f4b.1.1”,
VV-FE-0178-0, Transient thermal-hydraulic code used for the licensing analysis of blowdown
loads during a large break loss-of-coolant accident, ABB Combustion Engineering Nuclear
Operations, Windsor, Connecticut, October 1994.
[3] Basol, M., “Design Specification for Replacement Steam Generator for TVA Sequoyah Nuclear
Power Plant Unit 1”, Specification No. S1RSG-CD-C201, Rev.01, CE Nuclear Power LLC,
Chattanooga, TN, January 15, 2001.
[4] J.C. Lowry, “Thermal and Hydraulic design Data Report”, Prepared for Tennessee Valley
Authority by Westinghouse Electric Company, June 11, 2002.
[5] J.C. Lowry, “Thermal and Hydraulic Analysis Report”, Prepared for Tennessee Valley Authority
by Westinghouse Electric Company, August 27, 2002.
[6] J.R.Schwall, “Hydraulic Loads for Steam Line Break”, Westinghouse Electric Company, 2002.