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Title: One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools

Abstract

Thermal stratification phenomena are very common in pool type reactor systems, such as the liquid-salt cooled Advanced High Temperature Reactor (AHTR) and liquid-metal cooled fast reactor systems such as the Sodium Fast Reactor (SFR). It is important to accurately predict the temperature and density distributions both for design optimation and accident analysis. Current major reactor system analysis codes such as RELAP5 (for LWR’s, and recently extended to analyze high temperature reactors), TRAC (for LWR’s), and SASSYS (for liquid metal fast reactors) only provide lumped-volume based models which can only give very approximate results and can only handle simple cases with one mixing source. While 2-D or 3-D CFD methods can be used to analyze simple configurations, these methods require very fine grid resolution to resolve thin substructures such as jets and wall boundaries, yet such fine grid resolution is difficult or impossible to provide for studying the reactor response to transients due to computational expense. Therefore, new methods are needed to support design optimization and safety analysis of Generation IV pool type reactor systems. Previous scaling has shown that stratified mixing processes in large stably stratified enclosures can be described using one-dimensional differential equations, with the vertical transport by freemore » and wall jets modeled using standard integral techniques. This allows very large reductions in computational effort compared to three-dimensional numerical modeling of turbulent mixing in large enclosures. The BMIX++ (Berkeley mechanistic MIXing code in C++) code was originally developed at UC Berkeley to implement such ideas. This code solves mixing and heat transfer problems in stably stratified enclosures. The code uses a Lagrangian approach to solve 1-D transient governing equations for the ambient fluid and uses analytical or 1-D integral models to compute substructures. By including liquid salt properties, BMIX++ code is extended to analyze liquid salt pool systems in the current AHTR design, to provide an example of its application. Similar analysis is possible for liquid-metal cooled reactors. The current AHTR baseline design uses a large buffer salt tank to provide more thermal inertial and safety margin. Reactor vessel, intermediate heat exchangers, pool reactor auxiliary cooling system heat exchangers (PHX), and direct reactor auxiliary cooling system heat exchangers (DHX) are all immerged in the buffer salt pool. These structures provide major driving sources for vertical mixing and thermal stratification. Predication of the temperature distribution within the buffer salt tank directly affects the major safety systems design, such as the PHX and DHX, safety analysis results, and structure thermal stresses analysis. The BMIX++ code is used to predict mixing and thermal stratification in this pool system. This example shows the potential of 1-D analysis methods and BMIX++ to be included in system analysis codes for pool type of Gen-IV reactor systems.« less

Authors:
;
Publication Date:
Research Org.:
Idaho National Laboratory (INL)
Sponsoring Org.:
USDOE
OSTI Identifier:
915518
Report Number(s):
INL/CON-06-12026
TRN: US0805000
DOE Contract Number:
DE-AC07-99ID-13727
Resource Type:
Conference
Resource Relation:
Conference: NURETH-12,Pittsburgh, PA,10/04/2007,10/07/2007
Country of Publication:
United States
Language:
English
Subject:
22 - GENERAL STUDIES OF NUCLEAR REACTORS; COOLANTS; COOLING SYSTEMS; DESIGN; DIFFERENTIAL EQUATIONS; FAST REACTORS; HEAT EXCHANGERS; HEAT TRANSFER; LIQUID METALS; POOL TYPE REACTORS; REACTOR VESSELS; SAFETY; SAFETY MARGINS; STRATIFICATION; TEMPERATURE DISTRIBUTION; THERMAL STRESSES; AHTR; One-dimensional; Pool Reactors; SFR; Thermal Stratification

Citation Formats

Haihua Zhao, and Per F. Peterson. One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools. United States: N. p., 2007. Web.
Haihua Zhao, & Per F. Peterson. One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools. United States.
Haihua Zhao, and Per F. Peterson. Mon . "One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools". United States. doi:. https://www.osti.gov/servlets/purl/915518.
@article{osti_915518,
title = {One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools},
author = {Haihua Zhao and Per F. Peterson},
abstractNote = {Thermal stratification phenomena are very common in pool type reactor systems, such as the liquid-salt cooled Advanced High Temperature Reactor (AHTR) and liquid-metal cooled fast reactor systems such as the Sodium Fast Reactor (SFR). It is important to accurately predict the temperature and density distributions both for design optimation and accident analysis. Current major reactor system analysis codes such as RELAP5 (for LWR’s, and recently extended to analyze high temperature reactors), TRAC (for LWR’s), and SASSYS (for liquid metal fast reactors) only provide lumped-volume based models which can only give very approximate results and can only handle simple cases with one mixing source. While 2-D or 3-D CFD methods can be used to analyze simple configurations, these methods require very fine grid resolution to resolve thin substructures such as jets and wall boundaries, yet such fine grid resolution is difficult or impossible to provide for studying the reactor response to transients due to computational expense. Therefore, new methods are needed to support design optimization and safety analysis of Generation IV pool type reactor systems. Previous scaling has shown that stratified mixing processes in large stably stratified enclosures can be described using one-dimensional differential equations, with the vertical transport by free and wall jets modeled using standard integral techniques. This allows very large reductions in computational effort compared to three-dimensional numerical modeling of turbulent mixing in large enclosures. The BMIX++ (Berkeley mechanistic MIXing code in C++) code was originally developed at UC Berkeley to implement such ideas. This code solves mixing and heat transfer problems in stably stratified enclosures. The code uses a Lagrangian approach to solve 1-D transient governing equations for the ambient fluid and uses analytical or 1-D integral models to compute substructures. By including liquid salt properties, BMIX++ code is extended to analyze liquid salt pool systems in the current AHTR design, to provide an example of its application. Similar analysis is possible for liquid-metal cooled reactors. The current AHTR baseline design uses a large buffer salt tank to provide more thermal inertial and safety margin. Reactor vessel, intermediate heat exchangers, pool reactor auxiliary cooling system heat exchangers (PHX), and direct reactor auxiliary cooling system heat exchangers (DHX) are all immerged in the buffer salt pool. These structures provide major driving sources for vertical mixing and thermal stratification. Predication of the temperature distribution within the buffer salt tank directly affects the major safety systems design, such as the PHX and DHX, safety analysis results, and structure thermal stresses analysis. The BMIX++ code is used to predict mixing and thermal stratification in this pool system. This example shows the potential of 1-D analysis methods and BMIX++ to be included in system analysis codes for pool type of Gen-IV reactor systems.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Oct 01 00:00:00 EDT 2007},
month = {Mon Oct 01 00:00:00 EDT 2007}
}

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  • The suppression pool in a boiling water reactor (BWR) plant not only is the major heat sink within the containment system, but also provides the major emergency cooling water for the reactor core. In several accident scenarios, such as a loss-of-coolant accident and extended station blackout, thermal stratification tends to form in the pool after the initial rapid venting stage. Accurately predicting the pool stratification phenomenon is important because it affects the peak containment pressure; the pool temperature distribution also affects the NPSHa (available net positive suction head) and therefore the performance of the Emergency Core Cooling System and Reactormore » Core Isolation Cooling System pumps that draw cooling water back to the core. Current safety analysis codes use zero dimensional (0-D) lumped parameter models to calculate the energy and mass balance in the pool; therefore, they have large uncertainties in the prediction of scenarios in which stratification and mixing are important. While three-dimensional (3-D) computational fluid dynamics (CFD) methods can be used to analyze realistic 3-D configurations, these methods normally require very fine grid resolution to resolve thin substructures such as jets and wall boundaries, resulting in a long simulation time. For mixing in stably stratified large enclosures, the BMIX++ code (Berkeley mechanistic MIXing code in C++) has been developed to implement a highly efficient analysis method for stratification where the ambient fluid volume is represented by one-dimensional (1-D) transient partial differential equations and substructures (such as free or wall jets) are modeled with 1-D integral models. This allows very large reductions in computational effort compared to multi-dimensional CFD modeling. One heat-up experiment performed at the Finland POOLEX facility, which was designed to study phenomena relevant to Nordic design BWR suppression pool including thermal stratification and mixing, is used for validation. Comparisons between the BMIX++, GOTHIC, and CFD calculations against the POOLEX experimental data are discussed in detail.« less
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