Design and Analysis of the PB-AHTR using RELAP5

1
Design and Analysis of the PB-AHTR using RELAP5
Cristhian Galvez, Nicolas Zweibaum, Per
Peterson Thermal Hydraulics Laboratory Department
of Nuclear Engineering University of California,
Berkeley 2010 RELAP 5 International Users
Seminar

2
Outline

• Introduction
• Overview of Plant Design
• Modeling needs
• Plant system-gtprocess modeling breakdown
• Solution methodology
• Results
• Conclusion Future Work

3
Introduction

• The Pebble Bed Advanced High Temperature Reactor
  (PB-AHTR) is a pebble fueled, fluoride-salt
  cooled, 900-MWt reactor under development at UC
  Berkeley.
• Design features large thermal margins to fuel
  damage. Thermal limits are imposed by metallic
  primary loop structures. Peak core outlet
  temperature is the parameter of interest

4
Overview of Plant Design Diagram
5
Overview of Plant Design 3D render
6
Coolant Flow diagram

• Primary heat removal system composed of 4
  Intermediate Heat Exchangers (IHX).
• Passive decay heat removal mechanism accomplished
  through 8 Direct Reactor Auxiliary Cooling System
  (DRACS). Heat is absorbed by the Direct Heat
  Exchanger (DHX), which is similar in design to
  the IHX and rejects heat to the environment
  through air-cooled Natural Draft Heat Exchangers
  (NDHX)

7
Annular-type Core
Annular core and components diagram
Lateral cross section
8
Annular Pebble Bed core design
Radially and axially zoned pebble bed core
PREX-2 filled with 129,840 pebbles

• Radially-zoned injection of buoyant pebbles
• Alternative injection of seed and blanket pebbles
  (axial zoning)
• Pebble Recirculation Experiment (PREX-2), 42
  actual core size, high density polyethylene
  spheres, dry

9
Channel-type core
Pebble channel assembly core and components
Elevation view and lateral cross section
10
Channel Pebble Bed core design
Baseline design for lower half of PCA showing
configuration of pebble channels
11
Fuel and Coolant
RELAP5-3D pebble fuel model description from
pebble center (left) to pebble surface (right)
for the annular pebble design

• Flibe Primary Coolant (Li2BeF2)
• Excellent heat transfer
• Transparent, clean fluoride salt
• Boiling point 1400ºC
• Reacts very slowly in air
• No energy source to pressurize containment

12
Active cooling system Intermediate Heat
Exchanger (IHX) and pumps

• IHX
• Tube and shell, disk and doughnut baffled heat
  exchanger
• Primary coolant (Flibe) on tube side,
  Intermediate coolant (Flinabe) on shell side
• Forced convection on external and internal side
  driven by active centrifugal conventional pumps
• Derived from MSBR heat exchanger design

13
Passive cooling system (DRACS) DHX, NDHX,
Fluidic diode

• NDHX
• Tube and shell helical heat exchanger
• Natural circulation coolant (Flinabe) on tube
  side, Natural draft coolant (Air) on shell side
• Radiation heat transfer important

• Fluidic Diode
• Low resistance during forward flow, high
  resistance during reverse flow
• Passive operation

• DHX
• Tube and shell heat exchanger
• Forced primary coolant (Flibe) on shell side,
  Natural circulation coolant (Flinabe) on tube
  side
• Radiation heat transfer possibly important

14
Reactivity control Feedback mechanisms

• Fuel and Moderator Temperature Feedback
• Monte Carlo studies performed to determine fuel
  and moderator temperature reactivity feedback
  coefficients
• Study was done for various fuel-burn up levels,
  however RELAP5 analysis assumes average burn-up

15
Reactivity control Shutdown rod

• Shutdown-rod design
• Neutrally buoyant rod remains above the core
  during normal operation at typical coolant
  temperatures, but looses buoyancy and sinks into
  rod channel during above-normal coolant
  temperatures during transients
• Analytical and experimental work to determine rod
  insertion speed and rod worth

16
Analysis Objectives
Design and analysis of the PB-AHTR requires
investigation employing analytical, computational
and experimental tools

• Steady state Mass, Pressure and Temperature
  distribution
• Transient Peak core outlet temperature
• Safety system performance
• Decay heat removal system performance
• Experiment design analysis

In order to obtain variables of interest and
capture important phenomena, a methodology to
breakdown the system and model it is used
17
System-process breakdown Core
Core
Coolant
Reflector
Fuel
1-? liquid pebble bed void volume
Solid spherical
Solid cylindrical
COEnergy
COMass
COMom
COEnergy
COEnergy
-Energy generation -Conduction
-Continuity
-Convection
-Form loss -Friction loss
-Convection -Conduction
Not available in current version of RELAP
18
System-process breakdown Active Cooling
Active cooling
Secondary Pump
Intermediate Heat Exchanger IHX
Primary Pump
1-? liquid tube side volume
Solid cylindrical
1-? liquid pump volume
1-? liquid shell side volume
1-? liquid pump volume
COEnergy
COMom
COMom
COMass
COMom
COEnergy
-Conduction
-Continuity
-Convection
-Form loss -Friction loss
-Momentum addition
-Momentum addition
19
System-process breakdown Passive Cooling
Passive cooling
Direct Heat Exchanger DHX
Natural Draft Heat Exchanger NDHX
Fluidic diode
1-? liquid tube side volume
1-? liquid shell side volume
1-? gas shell side volume
1-? liquid diode volume
1-? liquid tube side volume
COMom
COMass
COMom
COEnergy
-Form loss -Friction loss
-Continuity
-Convection
-Form loss -Friction loss
20
RELAP5-3D Model

• Evolutionary steps taken to deal with modeling
  gaps
• 1st Input heat and flow loss coefficients
  manually
• Only valid for steady state calculations
• 2nd Input heat and flow loss coefficients
  manually as a function of time with
  self-consistent heat / flow loss coeficients and
  mass flow history
• Approximation for transient
• 3rd Manipulate existing LWR options in RELAP5-3D
  to add user-input factors to replicate
  correlation using multipliers (fouling factor for
  h and internal junction form loss for f)
• Better approximation but still incomplete since
  power exponents of Re and Pr do not exactly match
  with available correlations coded in RELAP5-3D
  (Shah ESDU cross flow)
• 4th Implement pebble bed correlations into
  source code

21
Annular Core RELAP5-3D Model
Geometrical Configuration of the Core and the
RELAP5-3D Model

• 1/8 symmetric core modeled
• 3 multi-dimensional axial zones inlet,
  mid-section and outlet
• Active mesh inlet 47, mid-section81, outlet32
• Fixed T,P at coolant sources and fixed P at
  coolant sinks
• Power distribution resulting from coupling
  studies with MCNP5

22
Annular core flow distrubition
RELAP5-3D Model
Core Diagram
COMSOL FEM Multiphysics Model
23
Outlet Temperature Parametric Analysis
(a)
(c)
(b)
(d)
Inlets and Outlets Distributions in the Bottom,
Mid-Section and Upper Core
24
Outlet Temperature Distributions
Temperature distributions of the outlets in
different model variations
25
Best Model Variation
Best model variation sketch and simulation result

• ?T97K, optimal difference

26
Channel Core RELAP5-3D Model
27
Transient Description

• Several transients are analyzed, but focus of
  this study is Loss of Forced Circulation (LOFC)
  and Loss of Heat Sink (LOHS)
• LOFC involves the trip of the primary pumps, LOHS
  involves the trip of the intermediate pumps
• Both transients are evaluated under a different
  assumed safety system response
• Normal scram immediately after primary or
  intermediate pumps. Shutdown rod bank inserted.
• Failure to actively scram reactor with shutdown
  rods. Passive, buoyancy driven shutdown rod
  insertion occurs. Scram accomplished after a
  delay
• Failure to scram reactor with either system.
  Power reactivity coefficient is the only
  mechanism present to shutdown the reactor

28
Transient Results Loss of Forced Circulation

• Fast loss of primary flow at t 1000 s. Passive
  shutdown rod insert 32 s after transient
  initiation. Average fuel and core outlet coolant
  temperatures rise to acceptable levels

29
Transient Results Loss of Forced Circulation

• Fast loss of primary flow at t 1000 s. Flow
  within the Direct Heat Exchanger passively
  inverts shortly after the transient initiation.
  Steady state natural circulation for decay heat
  removal is rapidly obtained. Temperatures in
  metallic heat exchanger remain acceptable during
  severe transient

30
Transient Results Loss of Heat Sink

• Fast loss of intermediate flow at t 1000 s.
  Passive shutdown rod insert 32 s after transient
  initiation. Coolant temperatures rise to
  acceptable levels

31
Transient Results Loss of Heat Sink

• Fast loss of intermediate flow at t 1000 s.
  Intermediate coolant flow is quickly reduced to
  negligible amounts. Thermal reactivity feedback
  shuts down the reactor quicker in the case of
  LOHS transients vs. LOFC transients.

32
Conclusions

• RELAP5-3D model matches well with its analytical
  results, confidence in model exists for steady
  and transient conditions
• Passive and inherent reactor control mechanism
  perform well under postulated transients and
  maintain temperatures well below thermal damage
  limits for fuel (1600 oC) and metallic
  structures (765 oC) for Hastelloy 800 H
• Model provides preliminary insights on passive
  safety performance of the PB-AHTR. Additional
  work is necessary in order to consider other
  limiting cases such as 1) partial loss of flow 2)
  partial core flow blockage 3) partial heat
  exchanger flow blockage
• Need to configure the annular core model for
  transient simulations with optimized coolant
  outlet geometric distribution