www.inl.govHTTF Analyses Using RELAP5-3DPaul D. Bayless

RELAP5 International Users SeminarSeptember 2010

22

Outline

• HTTF description• Initial scoping analyses• Steady state and transient simulations• Code user observations

33

High Temperature Test Facility (HTTF)• Integral experiment being built at Oregon State University• Electrically-heated, scaled model of a high temperature gas reactor

– Reference is the MHTGR (prismatic blocks)– Large ceramic block representing core and reflectors – ¼ length scale– Prototypic coolant inlet (259°C) and outlet (687°C) temperatures– Less than scaled power– Maximum pressure of ~700 kPa

• Primary focus is on depressurized conduction cooldown transient

44

Initial Scoping Studies• Reference reactor simulations• Simulations using a scaled-down MHTGR model• Concerns with laminar flow and initial structure temperatures

55

MHTGR RELAP5-3D Scoping Model Features• Three systems

– Primary coolant– Reactor cavity– Reactor cavity cooling system (RCCS)

• Coolant gaps between the core blocks modeled• Each ring modeled separately• 2-D (radial/axial) conduction in all vertical heat structures• Conduction between fuel blocks and to adjacent reflector blocks• Radiation across gaps between reflector rings• Radiation from core barrel to vessel to RCCS• Core barrel divided azimuthally

66

MHTGR Reactor Vessel Core Region Cross SectionReactor vessel

Core barrel

Coolant channels

Central reflector

Fuel blocks

Side reflector

Control rod channels

77

MHTGR RELAP5-3D Core Region Radial Nodalization

Reactor vessel

Core barrel

Coolant channels

Central reflector

Fuel blocks

Side reflector

Coolant gaps

88

Fuel Block Unit Cell

Coolant hole

Fuel

MHTGR RELAP5-3D

99

130,

175

132,134,136

158 140, 160,145, 162,150 164,

166 115

100

105

250 295255

110

200

120

170

125

Reactor Vessel Nodalization

1010

Reactor Cavity and RCCS Nodalization

950900 940900

945

930955

960

925970

980 920

1111

Base Calculation Set• Steady state• Low pressure conduction cooldown (LPCC)

– 10-s forced depressurization to atmospheric pressure– Both reactor inlet and outlet open to He-filled volumes

• Conduction cooldown with intact coolant system– 60-s flow coastdown– Reactor inlet closed– Reactor outlet pressure reduced over 4-hr period– Three outlet pressures

• Normal operation (~6.3 MPa)• 3.0 MPa• 0.7 MPa

1212

Base Calculations – Peak Fuel Temperature

1313

Base Calculations – Peak Vessel Temperature

1414

Base Calculations – RCCS Heat Removal

1515

Base Calculations – Axial Conduction Effect

1616

MHTGR/HTTF Sensitivity Calculations• 25% power case

– Nominal coolant temperatures– Transient response uninteresting, no heatup

• 10% power case– Nominal coolant temperatures; laminar flow in bypass channels– Full power decay heat– Transient response similar to reference plant but with higher,

earlier temperature peaks• ¼ scale model

– Nominal coolant temperatures– Laminar flow in all flow channels– Core and reflector temperatures much higher than reference plant– Much higher transient fuel temperatures

1717

HTTF RELAP5-3D Model Description• Same components and approach as for MHTGR• No gaps between core and reflectors• All coolant holes are open at both ends without flow restrictions

– Loss coefficients adjusted to provide 11% core bypass flow• Control rod holes in reflectors modeled separately from solid regions• Radial heat transfer by conduction in core, central and side reflectors• Simplified model of the RCCS

1818

HTTF RELAP5-3D core region radial nodalization

Reactor vessel

Core barrel

Coolant channels

Central reflector

Core region

Permanent reflector

Coolant gaps

Heater rodCoolant hole

Side reflector

1919

HTTF Model Initial Unit Cells

Coolant channel

Heater rod

Ceramic

Reflector Core

Helium gap

2020

HTTF RELAP5-3D Model Unit Cells

Coolant channel

Ceramic

Reflector Core

Heater rod

Radiation

2121

Initial Steady State Calculations• Initial HTTF power was ~600 kW, but calculations showed that the

power needed to be >1250 kW to get turbulent flow in the core cooling channels

• Facility power subsequently upgraded to 2.2 MW• Sensitivity calculations looked at different reflector cooling hole

geometries to investigate effect on initial temperature and bypass flow rate

• Cooling hole geometry still being determined

2222

Transient Boundary Conditions• Decay power (compared to MHTGR)

– Power factor of 1/32– Time factor of 1/2

• Scram at transient initiation• Power held constant until decay power drops below 2.2 MW• 10-s depressurization in depressurized conduction cooldown (DCC)• 60-s flow coastdown in pressurized conduction cooldown (PCC)

2323

DCC Core Average Temperatures

2424

DCC Radial Temperature Profile (1)

2525

DCC Radial Temperature Profile (2)

2626

Peak Fuel Temperature Comparison

2727

Reactor Vessel Average Temperatures

2828

Heat Removal and Generation

2929

Transient Calculation Observations• Temperature response seemed reasonable and representative• Not a significant difference between DCC and PCC calculations

3030

Code User Observations• These studies exercised new or seldom-used models in the code

– 2-D conduction– Control variable-driven heat flux boundary condition on a heat

structure– Decoupled heat structures

• Code shortcomings– Inability to model both conduction and radiation from a heat

structure surface– No 2-D conduction in structures with an imposed boundary

condition