www.inl.govRELAP5-3D Analyses Supporting HTTF DesignPaul D. Bayless

RELAP5 International Users SeminarJuly 2011

Outline

• HTTF description• Recent analyses• Code user observations

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

High Temperature Test Facility

Core Block Design (1)

HTTF Analysis Overview

• Objectives– Determine how well the High Temperature Test Facility (HTTF) will

serve as a code assessment data source for validating computer codes that will be used in licensing the NGNP

– Identify differences/distortions in the experiment facility design and response and evaluate their significance to code assessment

• Approach– Develop and use models of the Modular High-Temperature Gas-

cooled Reactor (MHTGR) to determine the steady state and transient response of the reference plant

– Develop and use models of the HTTF to determine the steady state and transient response of the experiment facility

Focus of Recent Analyses• Steady state behavior of the core• Assessing impact of operational distortions• Trying to design around distortions• Design simplification• Core design has been a moving target

HTTF RELAP5-3D Model Description• Three systems

– Primary coolant– Reactor cavity– Reactor cavity cooling system

• Central and side reflector regions divided into regions with or without coolant holes

• 2-D (radial/axial) conduction in all vertical heat structures• Heater block unit cell centered on the coolant channel• Radial conduction and radiation inside core barrel• Radiation from core barrel to vessel to RCCS• Coolant risers and dead volume between core barrel and vessel• Core barrel divided azimuthally

Modeling Assumptions• All coolant holes are open at both ends without flow restrictions

– Loss coefficients adjusted to provide 11% core bypass flow• 1-mm gap on either side of permanent reflector, but open to flow only

at the bottom• RCCS response estimated by applying constant temperature boundary

condition to the back side of the panels• Radiation is purely radial from the core to the RCCS• Radiation from top reflector to upper plenum shield is not modeled

132

175

140, 162 164,145, 166150

115

100

105

250 295255

110

200

120

170

125

Reactor Vessel Nodalization

HTTF RELAP5-3D core region radial nodalization (1)

Reactor vessel

Core barrel

Coolant channels

Central reflector

Core region

Permanent reflector

Coolant gaps

Heater rodCoolant hole

Side reflector

HTTF RELAP5-3D Model Unit Cells

Coolant channel

Ceramic

Reflector Core

Heater rod

Radiation

The Reflectors Are Too Hot• Problem: steady state calculations showed that there were significant

hot spots around the heater rods on the core periphery.• Solution: add more coolant holes, so that each heater rod has six

coolant holes around it.• Additional change: since the control rod holes were no longer needed

for cooling the core, they were moved farther away from the heater rods to simulate the core bypass flow paths, and more were added. The permanent side reflector was also merged with the side reflector.

Core Block Design (2)

HTTF RELAP5-3D Core Region Radial Nodalization (2)

Reactor vessel

Core barrel

Coolant channels

Central reflector

Core region

Permanent reflector

Coolant gap

Heater rodCoolant hole

Side reflector

Design Change Effect on Reflector Temperatures

Location Core Design 1 Core Design 2

Central reflector, inner 703 581

Central reflector, outer 663 627

Side reflector, inner 584 532

Side reflector, outer 505 383

Permanent side reflector 355 328

Axial Average Temperature (°C)

The Core Periphery Helium Is Too Cold• Problem: the coolant channels on the core periphery have less heat

input, so the core outlet temperature varies significantly across the core.

• Solution: either add inlet orifices to reduce the flow or reduce the channel diameter.– Inlet orifices are sized for turbulent flow during steady state, but will

operate in laminar flow during most transients. Required loss coefficients are high. Removing orifices simplifies operation and modeling, although it removes some flexibility.

– Three different core coolant channel diameters are required. How are these combined in a lumped channel model?

• Additional changes: the permanent side reflector was separated back out from the side reflector, the number of coolant risers was reduced and their width increased.

Sensitivity Calculations for Reflector Hole Sizes• Calculations run without orifices to determine bypass flow vs. hole size• Target is around nominal bypass flow of 11%.

Central reflector hole diameter (in.)Side reflector hole diameter (in.) 1.0 0.875 0.75 0.625

1.0 25.5 25.4 25.2 --

0.875 20.1 20.0 19.9 --

0.75 14.8 14.7 14.6 14.4

0.625 -- -- 9.7 9.6

Core Block Design (3)

HTTF RELAP5-3D Core Region Radial Nodalization (3)

Reactor vessel

Core barrel

Coolant channels

Central reflector

Core region

Permanent reflector

Coolant gaps

Heater rodCoolant hole

Side reflector

Core Region Modeling• Two core regions have combinations of three different coolant channel

diameters• Need to model these as a single flow channel• Approach

– Model the three size channels explicitly• Assign 1/6 heater to each channel that sees 1 heater rod• Assign 1/3 heater to each channel that sees 2 heater rods• Assign 1/2 heater to each channel that sees 3 heater rods• Determine the flow that provides a desired outlet temperature

– Model a single channel with the correct total flow area and heat input

• Specify the same flow rate determined above• Iterate on the hydraulic diameter to get the same pressure

drop

Steady State Results Parameter Core Design 2 Core Design 3Core power (MW) 2.2 2.2

Helium reactor vessel inlet temperature (°C) 258.6 258.6

Helium outlet plenum temperature (°C) 687.0 687.0

Total helium mass flow rate (kg/s) 0.965 0.965

Total core bypass flow (%) 11.0 11.6

Central reflector helium outlet temperature (°C) 522.1 514.2

Inner core ring helium outlet temperature (°C) 761.9 748.0

Middle core ring helium outlet temperature (°C) 807.1 738.4

Outer core ring helium outlet temperature (°C) 677.9 728.9

Side reflector helium outlet temperature (°C) 380.1 387.6

User Observations on the 2-D Conduction Model• Can simplify or eliminate conduction enclosures• It is a specific application of the reflood model

– Each axial node divided in two– User cannot control the axial meshing

• Cannot be used with a heat flux boundary condition• Cannot be used without a boundary volume• Uses the length of the right boundary volume as the structure height

– Can distort the axial conduction if more than one axial structure is attached to the same fluid volume

– Can distort the axial temperatures if the user chooses to divide a large, well-mixed volume into a 1-D stack of cells