Thermal hydraulic code validation benchmark for high temperature gas-cooled reactors using HTTF data (HTGR T/H)
High Temperature Test Facility (HTTF) at Oregon State University. Image: Oregon State University, USA


Accurate modelling and simulation tools for thermal hydraulic calculations are a key element needed to efficiently develop and deploy new advanced reactors including small modular reactors (SMRs) and microreactors. Uncertainties in modelling and simulation can have significant safety and economic implications. Well defined benchmark problems that include code-to-code comparisons as well as comparisons to measured data are an efficient mean to quantify accuracy and identify sources of uncertainties in thermal hydraulic calculations.

A few years ago, the Coupled Neutronic/Thermal-Fluid Benchmark of the Modular High Temperature Gas-cooled Reactor (MHTGR) 350 MW Core was launched with the goal to perform code-to-code comparisons for a prismatic modular high-temperature gas-cooled reactor design. This benchmark successfully identified differences between solutions of different multi-physics codes (Coupled Neutronic/Thermal-Fluid). In the meantime, national laboratories along with several commercial entities expressed interest in a simpler, single physics (thermal hydraulics), but still integral effects benchmark for advanced gas-cooled reactor code validation that includes comparison to experimental data.

Scope of work

This benchmark activity consists of a multi-stage code-to-code-to-data thermal hydraulics code validation benchmark based on data measured at the High Temperature Test Facility (HTTF) at Oregon State University (OSU).

HTTF is a scaled integral effects experiment designed to investigate transient behavior in high-temperature gas-cooled nuclear reactors with prismatic fuel and reflector blocks. It is a one-quarter-scale model of the general atomics’ MHTGR. Several tests have been completed at the HTTF, including depressurised conduction cooldown (DCC) and pressurised conduction cooldown (PCC) transients for which high-quality measured data is available. This data is suitable for a thermal hydraulics code validation benchmark for codes for gas-cooled reactor simulations.

Benchmark problems and exercises

Three benchmark problems are proposed, each capturing different  physical phenomena. These problems are intended to be solved with systems codes, computational fluid dynamics (CFD) codes, and/or coupled systems codes/CFD models.  

  • Problem 1 - Lower Plenum Mixing: One area with relatively low knowledge level on HTGR thermal hydraulics is gas behavior in the lower plenum. This problem uses CFD or coupled systems code/CFD models to assess how hot and cold helium mix in the lower plenum. This can provide insight into hot streaking, temperature measurements downstream from the lower plenum, and applicability of different turbulence models. This problem is based on HTTF experiment PG-28.
  • Problem 2 - DCC: The DCC is a transient that occurs when the coolant pressure boundary is ruptured. As coolant escapes to containment, the system enters decay heat mode, and after an initial temperature rise, the core cools down over many hours through a combination of conduction and radiation heat transfer. This problem is intended to be solved with systems codes or coupled systems code/CFD models and is based on HTTF experiment PG-29.
  • Problem 3 - PCC: The PCC is a transient that occurs when forced flow of coolant stops but the pressure boundary remains intact. As forced circulation stops and the reactor enters decay heat mode, intracore natural circulation patterns begin to develop, helping redistribute heat from the hottest parts of the core to the coldest ones. The primary cooldown mechanisms here are still conduction and radiation heat transfer. This problem is intended to be solved with systems codes or coupled systems code/CFD models and is based on HTTF experiment PG-27.

The following table provides a summary of the problems, experiments, and their toolsets.

  CFD Systems codes CFD-Systems code coupling
Lower Plenum Mixing (PG-28) X   X
DCC (PG-29)   X X
PCC (PG-27)   X X


The different experiments and physical phenomena in the HTTF define different problems in the benchmark. These problems are then further broken down into exercises. Exercises are intended to provide different sets of conditions and analysis for these problems, as well as opportunities for code-to-code comparison, code-to-data comparison (validation), and understanding of relationships between error and uncertainty in HTTF, and error and uncertainty in the MHTGR-350.

Exercise 1: Fixed boundary conditions

The HTTF facility is equipped with more than 500 instruments. While these instruments provide high fidelity data for temperature distributions and evolutions during investigated transients, some of the boundary conditions (in particular helium mass flow rate in the primary loop and thermal properties for the core ceramic) have significant uncertainties. The first proposed exercise will include well defined boundary conditions so that all participants can establish a “base solution” that can be compared to other participants solution. This will help identify differences between solutions of different codes.

Exercise 2: Best estimate boundary conditions

In the second exercise, participants can use their best estimation of the boundary conditions, e.g. instead of following prescribed helium mas flow rates and valve positions, they can implement models of the helium blower and valve control systems to determine helium mass flow rate and valve position, respectively. This exercise will allow to compare different code solutions to measured data as assess output uncertainties (code validation).

Exercise 3: Error scaling form HTTF to MHTGR

During the design of any experiment, the designer must assess how closely the experiment reproduces the physics compared to the full-size installation the experiment attempts to represent.

One of the challenges of this step is to bound the error prediction of a model, when applied to a facility yet to be built, starting from the comparison of the model prediction with a set of experimental results. This problem is the validation extrapolation step. Since the HTTF is a scaled-down model of the MHTGR (for which geometry and model descriptions are available through the mentioned MHTGR benchmark), the combination of HTTF models with measured data and MHTGR models provide an opportunity to assess the validation extrapolation for different codes. The solution to compare for this problem is the quantification of the uncertainty in a simulation for the MHTGR plant knowing the uncertainty of a simulation for the HTTF facility (assessed in Exercise 2). Participants can provide solutions using their scaling methodologies such as Dynamic System Scaling (DSS), representativity, Physical Coverage Mapping (PCM), etc.

Benchmark organisation

This benchmark activity is part of the benchmark activities on reactor single- and multi-physics of the Working Party on Scientific Issues and Uncertainty Analysis of Reactor Systems (WPRS) and is supervised by the Expert Group on Reactor Core Thermal-Hydraulics and Mechanics (EGTHM).


  • Benchmark co-ordinator: Aaron EPINEY, Idaho National Laboratory, USA
  • Problem-specific co-ordinators:
    • Problem 1 (Lower Plenum Mixing): Izabela GUTOWSKA, Oregon State University, United States
    • Problem 2 (DCC): Robert KILE, Idaho National Laboratory 
    • Problem 3 (PCC): Thanh HUA, Argonne National Laboratory, United States

Participants: All NEA member countries HTGR T/H Participants' working area

Benchmark participation

Access to the benchmark is open to all OECD/NEA member countries and requires only acceptance of the benchmark conditions.

Please express your interest by sending a signed copy of the conditions form  to the WPRS Secretariat.



WPRS Secretariat