Real-time BWR accident simulation

9 January 2020

Nuclear utilities can achieve a higher standard of containment safety by applying real-time models for severe accident analysis, says Gill Grady.

DUE TO INDUSTRY AND REGULATORY focus on severe accident scenarios since Fukushima, it has become necessary to provide both operators and plant technical support staff with severe accident training. In the nuclear industry, the MAAP (Modular Accident Analysis Program) code developed by EPRI has been a standard for severe accident analysis for more than two decades throughout the world.

With MAAP, it is possible to simulate a wide range of severe accident phenomena and mitigation measures, including fuel deformation and meltdown, hydrogen generation and combustion, direct-containment heating, rapid pressurisation due to steaming, core concrete interactions, and fission product releases, transport, and deposition.

Recently, GSE has worked with customers to implement MAAP for design basis containment model for several boiling water reactor (BWR) plants. It does not model the Reactor Coolant Systems necessary to do the core melt progression of a severe accident simulator. However, the MAAP design basis containment model will support all design basis tests and training by utilising the parameter file from the probabalistic risk assessment (PRA) group with actual plant structural design data.

The trend now is to use MAAP in the full scope simulator for more a more comprehensive containment building model as well as for severe accident training. The main issue is that MAAP was never designed or intended for use in real-time simulation.

Two major challenges with using MAAP are:

  1. Creating real-time operations and solution of the calculation schemes in MAAP. Real-time operation is critical because the operator needs to understand the time frames in which he or she must respond to a plant event.
  2. Capturing all internal variables for “repeatability”, so that when the instructor resets the simulator it will behave precisely as it did in the previous run. The ability to recreate and rerun a scenario is critical from an operator training perspective.

Recently, a large utility commissioned GSE Systems to replace the BWR design-basis containment model in its current simulator and to implement a replacement containment model that would provide real-time simulation and training capabilities. The customer was eager to improve performance for containment safety systems and to derive results for theoretical loss-of-coolant-accidents (LOCAs).

The plant simulators, previously built by GSE, included the company’s JTopmeret two-phase modelling software, which is designed for balance of plant (BOP) systems and is flexible enough to accommodate a variety of systems, including large volumes such as the containment. The design basis containment upgrade, modelled by GSE’s implementation of MAAP for the primary containment and JTopmeret for the secondary containment, was integrated with the existing simulator models. GSE replaced the containment model, which consisted of JTopmeret turbine building, drywell, chilled water system, fuel pool cooling, containment atmospheric control, and dilution system, and implemented an overall upgrade of JTopmeret for the primary containment reactor building model.

The simulator application of MAAP was derived from the same design files that the plant’s safety analysis group uses. An important element of the project was matching the plant data so that GSE could closely represent the real-life scenarios with the simulator. In order to use the most recent version of MAAP and to increase confidence in the simulation results, the plant operator provided a MAAP parameter file for conversion. Once complete, GSE verified and validated the updated parameter file to ensure that any discrepancies could be explained by the theoretical physics of the power plant.

Interface protocols were then used to connect boundary variables to different systems, so that separate simulator software products could communicate effectively with one another. This was accomplished by using GSE SimExec software to integrate the model components and create a centralised communication platform.

Some of the improved capabilities of MAAP over traditional containment modelling tools included:

  • New containment break malfunctions
  • Hardened vent plant modifications
  • Interlock door between RX and turbine building
  • Drywell floodup
  • New steam leak malfunctions to the turbine building
  • Ex-vessel steam generation and hydrogen formation
  • Ex-vessel heat transport, water inventories, and containment cooling
  • Fission product transport and deposition in containment
  • Containment failure or venting and depressurisation
  • New containment model supporting all design basis tests and training

MAAP containment has it limits when it comes to simulating real-time plant data as it was designed to simulate LOCA and non-LOCA transients over long periods of time. Some finessing was needed to reach the goal, but GSE’s simulation tools enabled development of a real-time containment simulation and the plant operator noted immediate improvements.

Gill Grady, Senior vice president, GSE Systems

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