High Temperature Test Facility (HTTF)

The High Temperature Test Facility (HTTF) is a one-quarter scale integral test facility model of the Modular High Temperature Gas Reactor. The facility is capable of operating at temperatures like those expected in a loss of forced convection cooling accident. The nominal working fluid is helium although other gases can be used. The facility is configured to simulate a variety of postulated depressurized conduction cooldown (DCC), pressurized conduction cooldown (PCC) and normal operations events.


The purpose of the test facility is twofold: to validate the correlations that are currently uses to model relevant heat transfer and fluid flow phenomena required for the integrated simulation of the prismatic core phenomena; and generate results that can be used to validate different simulation methodologies applied in the gas cooled reactors modeling.

Facility Design

The facility is built in modular design to allow core reconfiguration from prismatic to pebble design.

In the current configuration, the graphite prismatic block structure in the MHTGR is simulated by ceramic blocks to capture prototypical core temperature profiles. The HTTF reactor core is built of 10 hexagonal core blocks that are made of a cast ceramic, Greencast 94-F. The reactor core is surrounded by reflectors on each side (2 upper reflectors, 3 bottom reflectors, and side reflectors). The side reflectors are made of a cast ceramic, ShotTech SiC 80 while the top and bottom reflectors are also made of Greencast 94-F. In addition, there are 3 separate structures designed to model the core exit chamber: lower plenum roof, lower plenum (the chamber that houses 163 support posts), and lower plenum floor.

The heating system is a network of electrically heated graphite rodlets/dog bones. The facility is equipped with the network of electrically heated graphite rods that produce approximately 2.2 MWth. There are 210 heater rods arranged in 10 heater banks, with 3 heater legs per heater bank. Each heater leg consists of 7 heater rods.

Detailed facility technical specification and instrumentation plan are available here:

Section view through the HTTF RPV and helium flow path
HTTF system CAD model


HTTF Core Block
Arrangement of coolant channels, bypass channels, and heater rods channels in the HTTF core block
HTTF Upper Head
HTTF upper head with instrumented guide tubes.



OSU HTTF at a glance


  • Integral Effects Test (IET) facility
  • Provides data for thermalhydraulic codes validation
  • Primarily designed to model the DCC transient
    • Variety of break size and location (upper, lower and cross-duct)
    • Four distinct phases of the transient (depressurization, lock-exchange flow,
      molecular diffusion and natural circulation)
    • Reactor Cavity Cooling System as boundary condition
  • Other scenarios can be examined: PCC and Normal operation
  • Working fluids: Helium and Nitrogen
  • Reference design: MHTGR
  • Facility Scaling
    • 1/4 length scale
    • 1/4 diameter scale
    • 1/8 pressure scale (0.8 MPa)
  • Temperatures: T max =1400˚C, T in =259˚C, T out =687˚C
  • Electrically Heated: max. Power ~2.2 MW
  • Operates under NQA-1 program

Tests Performed

Selected HTTF Test Acceptance Reports are available at https://www.osti.gov/

Test # Procedure # Test title Phenomena
1 PG-01 Pre-Operation


2 PG-02 Circulator and System Form Loss Characterization Characterization
3 PG-06 Facility Gas Conditioning Characterization
4 PG-07A Primary Loop and RCST Volume Determination Characterization
5 PG-08 Break Valve Characterization Characterization
6 PG-09

Steam Generator Secondary Side Volume Determination

7 PG-21

Lock Exchange Flow and Diffusion Test with 500°C average Gas Temp

8 PG-22

Lock Exchange Flow and Diffusion Test with 125°C average Gas Temp

9 PG-23

Lock Exchange Flow and Diffusion Test with 375°C average Gas Temp

10 PG-24

Lock Exchange Flow and Diffusion Test with 250°C average Gas Temp

11 PG-26

Low Power (<350kW) Double Ended Inlet-Outlet Crossover Duct Break, 2 Heaters

12 PG-27

Low Power (<350kW) Complete Loss of Flow, 2 Heaters

13 PG-28 Low Power (<350kW) Lower Plenum Mixing Mixing
14 PG-29

Low Power (<350kW) Double Ended Inlet-Outlet Crossover Duct Break, Hybrid Heater

15 PG-30

Low Power (<350kW) Lower Plenum Mixing, Constant Temperature

16 PG-31

Low Power (<350kW) Pressure Vessel Bottom Break with Restored Forced Convection Cooling

17 PG-32 Low Power (<350kW) Asymmetric Core Heatup Heatup
18 PG-33 Zero Power Long Term Cooldown Cooldown
19 PG-34

Low Power (<350kW) Asymmetric Core Heatup Full Hybrid Heater

20 PG-35

Low Power (<350kW)  Zero Power Crossover Duct Exchange Flow and Diffusion


Selected HTTF related papers

M.S. Theses and Ph.D. Dissertations

Journal Papers

  • Gutowska, I., Woods, B. G., & Cadell, S. R. (2019). CFD modeling of the OSU High
    Temperature Test Facility inlet plenum flow distribution during normal operation. Nuclear
    Engineering and Design, 353, 110216. https://doi.org/10.1016/j.nucengdes.2019.110216
  • Gradecka, M. J., & Woods, B. G. (2016). Development of thermal mixing enhancement
    method for lower plenum of the High Temperature Test Facility. Nuclear Engineering
    and Design, 305, 81–103. https://doi.org/10.1016/j.nucengdes.2016.02.043
  • Andre, M. A., Burns, R. A., Danehy, P. M., Cadell S. R., Woods B. G., and Bardet, P.
    M., “Velocimetry during depressurized conduction cooldown events in the
    HTTF," Nuclear Engineering and Design, 341, pp. 406-414, 2019.
  • Andre, M. A., Burns, R. A., Danehy, P. M., Cadell S. R., Woods B. G., and Bardet, P.
    M., “Development of N2O-MTV for low-speed flow and in-situ deployment to an
    integral effect test facility," Exp Fluids (2018) 59:14.

Conference Papers

  • Gutowska I., Halsted J., Schlamp N., Woods B. G, Manera A., Petrov V., Balestra P
    Building Conduction Cooldown Scenarios Experimental Validation Database for HTGRs,
    NURETH-19, Brussel, Belgium, March 2022
  • Gutowska I., Woods B. G, CFD Assessment of LOFA Intra Core Natural Circulation in
    the High Temperature Test Facility, NURETH-18, Portland, Oregon, August 2019.
  • Gutowska I., Cadell S. R., Woods B. G., CFD Air Ingress Analyses on High Temperature
    Test Facility, ANS Winter Meeting, Washington DC, US, October/November 2017).
  • Brumback, K. E., Woods, B. G., & Cadell, S. R. (2017). Overview of Shakedown Testing
    for the High Temperature Test Facility. Transactions of the American Nuclear Society,
    117, 1619.
  • Brumback, K. E., Cadell, S. R., and Woods, B. G., “Temperature and Flow
    Characteristics of the High Temperature Test Facility during Depressurized Conduction
    Cooldown Testing,” Proceedings, Advances in Thermal Hydraulics 2018, Orlando,
    FL, November 2018.
  • Andre, M. A., Bardet, Cadell S.R., and Woods B.G., “Velocimetry/Thermometry
    Data Fusion from DCC Tests in the HTTF,” Proceedings, International 18th International
    Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 18), Portland,
    Oregon, August, 2019.
  • Andre, M. A., Bardet, P. M., Danehy, P. M., Burns, R. A., Cadell S.R., and Woods
    B.G., “Non-Intrusive Velocity Measurements with MTV During DCC Event In the
    HTTF,” Proceedings, International 17th International Topical Meeting on
    Nuclear Reactor Thermal Hydraulics (NURETH 17), Xi’an, China, September, 2017.