OECD/NEA PBMR Coupled Neutronics/Thermal-hydraulics Transients Benchmark - The PBMR-400 Core Design

In co-operation with PBMR Pty Ltd, Penn State University (PSU)

Background and purpose

This international benchmark, concerns 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 has motivated the testing of existing methods for 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 includes defining appropriate benchmarks to verify and validate the new methods in computer codes.

The scope of the benchmark is 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.

The benchmark exercise has the following objectives:

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%
> = 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
Core outlet temperature 900°C.
Core inlet temperature
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 results in an increase in 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:

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.
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.

Frederik Reitsma,  Kostadin Ivanov,Tom Downar, Han de Haas, Sonat Sen, Gerhard Strydom, Ramatsemela Mphahlele, Bismark Tyobeka, Volkan Seker, Hans D Gougar: PBMR Coupled Neutronics/Thermal Hydraulics Transient Benchmark - The PBMR-400 Core Design, Benchmark Definition, Draft V03, published by the NEA in 2005.

Material available to participants on CD-ROM


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Last reviewed: 15 June 2011