3. DESCRIPTION OF BENCHMARK
This international benchmark, concerns Pebble-Bed Modular Reactor (PBMR) coupled neutronics/thermal hydraulics transients based on the PBMR-400MW design. The deterministic neutronics, thermal-hydraulics and transient analysis tools and methods available to design and analyse PBMRs lag, in many cases, 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 given 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:
- Establish a standard benchmark for coupled codes (neutronics/thermal-hydraulics) for PBMR design
- 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
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 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 benchmark is divided into Phases and Exercises as follows:
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