Pebble Bed Modular Reactor (PBMR) Coupled Neutronics/Thermal-hydraulics Transients Benchmark - PBMR-400 Core Design

The international benchmark concerned pebble bed modular reactor (PBMR) coupled neutronics/thermal-hydraulics transients based on the PBMR-400MW design. In many cases the deterministic neutronics, thermal-hydraulics and transient analysis tools and methods available to design and analyse PBMRs lag behind the state of the art compared to other reactor technologies. This motivated the testing of existing methods for high-temperature gas-cooled reactors (HTGRs), but also the development of more accurate and efficient tools to analyse the neutronics and thermal-hydraulic behaviour for the design and safety evaluations of the PBMR. In addition to the development of new methods, this included defining appropriate benchmarks to verify and validate the new methods in computer codes.

The scope of the benchmark was to establish well-defined problems, based on a common set of cross-sections, to compare methods and tools in core simulation and thermal-hydraulics analysis with a specific focus on transient events through a set of multi-dimensional computational test problems.

Publications and reports
  • establish a standard benchmark for coupled codes (neutronics/thermal-hydraulics) for PBMR designs
  • code-to-code comparison using a common cross-section library
  • obtain a detailed understanding of the events and the processes
  • benefit from different approaches, understanding limitations and approximations
Major design and operating characteristics of the PBMR
PBMR characteristic Value
Installed thermal capacity 400 MW(t)
Installed electric capacity 165MW(e)
Load following capability 100-40-100%
Availability                       >= 95%
Core configuration Vertical with fixed centre graphite reflector
Fuel TRISO ceramic coated U-235 in graphite spheres
Primary coolant Helium
Primary coolant pressure 9MPa
Moderator Graphite
Core outlet temperature 900°C.
Core inlet temperature 500°C
Cycle type Direct
Number of circuits 1
Cycle efficiency >= 41%
Emergency planning zone 400 meters

The PBMR functions under a direct Brayton cycle with primary coolant helium flowing downward through the core and exiting at 900°C. The helium then enters the turbine relinquishing energy to drive the electric generator and compressors. After leaving the turbine, the helium then passes consecutively through the LP primary side of the recuperator, then the pre-cooler, the low-pressure compressor, intercooler, high-pressure compressor and then on to the HP secondary side of the recuperator before re-entering the reactor vessel at 500°C. Power is adjusted by regulating the mass flow rate of gas inside the primary circuit. This is achieved by a combination of compressor bypass and system pressure changes. Increasing the pressure results in an increase in mass flow rate, which increases the power removed from the core. Power reduction is achieved by removing gas from the circuit. A Helium Inventory Control System is used to provide an increase or decrease in system pressure.


The PBMR-400 benchmark consists of phases, each consisting of different exercises.

Phase I: Steady state benchmark calculational cases

  • Exercise 1: Neutronics solution with fixed cross-sections
  • Exercise 2: Thermal-hydraulic solution with given power/heat sources
  • Exercise 3: Combined neutronics thermal-hydraulics calculation – starting condition for the transients

Phase II: Transient benchmark

  • Exercise 1: Depressurised loss of forced cooling (DLOFC) without SCRAM
  • Exercise 2: Depressurised loss of forced cooling (DLOFC) with SCRAM
  • Exercise 3: Pressurised loss of forced cooling (PLOFC) with SCRAM
  • Exercise 4: 100-40-100 load follow
  • Exercise 5: Fast reactivity insertion – control rod withdrawal (CRW) and control rod ejection (CRE) scenarios at hot full power conditions
  • Exercise 6: Cold helium inlet

Material available on CD-ROM

Data related to PBMR400 can be requested from the NEA Databank.

  • proceedings from the first workshop (PBMRT1)
  • updated benchmark specifications document – Draft V103, dated 1 September 2005
  • simplified cross-section set "OECD-PBMR400-Simplified.XS"
  • multi-dimensional interpolation routine "lint5d.for"
  • sample Excel spreadsheets to report results for transient cases
  • sample Excel spreadsheets to report the steady state case results
  • library “OECD-PBMR400.XS” that contains cross-sections as a function of fuel and moderator temperatures; xenon concentrations, and fast and thermal buckling values (42 MB)
  • library “OECD-PBMR400-NoBuck.XS” that contains no buckling dependencies, only fuel and moderator temperature and xenon concentration dependencies (5 MB)


Ian Hill