MAAP - Modular Accident Analysis Program

The MAAP4 input file processor allows users the capability to simulate operator actions for LOCA and non-LOCA transients, thus allowing the code to simulate the full spectrum of plant response to all types of accidents.  Several utilities have even successfully modeled their entire Emergency Operating Procedures (EOPs) using MAAP4.  MAAP4 also includes a graphical interface, MAAP4-GRAAPH, to increase the analytical capability as well as enhance the review function.  Specifically, the user can interactively interface with the code during execution to modify the status of on-site power, pumps, valves, etc., as well as to directly observe the results.

The MAAP4 code builds on the substantial modeling enhancements over MAAP3B to assess the approach to, and progression of, severe accidents in BWRs and PWRs operated in the western hemisphere, and extends these to the plant-specific features that are presently represented in the state-of-the-art models for PRAs/PSAs, SAMGs, EOPs, and full scope control room simulators.  MAAP4 is a fast-running computer code capable of modeling an array of accident sequences in a short period of time, thus providing plant PRA staff a powerful tool to answer day to day issues that come up in the operation of a nuclear power plant.

The MAAP4 code has been used to perform severe accident analysis and associated severe accident phenomena, including hydrogen generation and combustion, direct-containment heating, rapid pressurization due to steaming, core concrete interactions, fission product releases, transport and deposition, etc.  The MAAP4 code is also used extensively in the probabilistic risk/safety assessment (PRA/PSA) arena as well for success criteria evaluations, human reliability analyses (HRA) and Level II source term evaluations, etc.

(EPRI owned and licensed computer software)

Background

Following the accident at Three Mile Island Unit 2, the nuclear power industry developed the MAAP (EPRI owned and licensed computer software) computer code as part of the industry degraded core rulemaking (IDCOR) program. Its objective was to provide a useful tool for analyzing the consequences of a wide range of postulated plant transients and severe accidents for current plant designs and Advanced Light Water Reactors (ALWRs). The code can predict the progression of accident scenarios to a safe, stable, coolable state within the core or it can predict the occurrence of vessel failure and model the containment performance with successful debris cooling or pressurization of containment to a pre-defined failure condition. MAAP5 is the latest version of the suite of MAAP computer codes designed specifically to perform accident and severe analyses for numerous nuclear plant designs.

Additional work to augment the code to include Mark 1 containment and BWR-specific models after the Fukushima continues.

Since the events at Fukushima, there has been an increased interest to expand current simulator capability to address severe accidents. The Modular Accident Analysis Program (MAAP), an Electric Power Research Institute (EPRI) owned and licensed computer software, was developed to simulate and study severe accidents. MAAP is an integral code simulating both containment and primary system during severe accidents. FAI has been under contract to improve MAAP models related to BWR primary system, lower plenum, instrument tubes, molten core concrete interaction and others in order to better follow the severe accident at Fukushima.

Simulators can be expanded to cover severe accidents by implementing the MAAP code into the existing simulator. This implementation using MAAP4 was done for Krsko in Slovenia and Ulchin in Korea. MAAP5 was implemented for Daya Bay in China and Kori in Korea.

MAAP5 PWR code can be a good thermal hydraulic engine for PC based simulator for severe accident training.

The MAAP5 PWR code is the latest generation of MAAP and it implements new models to calculate forced and natural circulation inside a reactor coolant system (RCS) with more detailed nodalization, point kinetic and 1-D neutronics models, features to address details of new advanced reactor designs such as AP1000 and EPR, and improved containment models.

Improvements were also made to include a steam header model with detailed steam dump logic so that the code can calculate initial RCS and steam generator responses after a reactor scram. In addition, MAAP5 has improved models for shutdown states such as modeling nozzle dams in the RCS loops, mid-loop operation, and reactor head open with the vessel submerged under the refueling water pool.

MAAP5 code can also calculate the ANS-3-5 transients required for simulators. These transients include a manual reactor trip, simultaneous trips of feed water pumps, simultaneous closure of all MSIVs, trip of any single reactor coolant pump (RCP), loss of coolant accidents, main steam line break, maximum power ramp, and maximum design load rejection.

The MAAP5 software package provides engineers with a tool to rapidly evaluate the progression of accidents in terms of the reactor core (is the fuel damaged or not?), the containment (is containment integrity being challenged?) and radiological consequences (do the dose rates inside the plant or in the population areas present concerns in terms of taking precautionary measures such as shelter or evacuation?).

MAAP5 also can model the progression of an accident in a plants’ spent fuel pool.  The MAAP5 code can calculate the time to boil away the pool water inventory, model the heatup and relocation of the spent fuel, the potential for the release of hydrogen from the spent fuel cladding due to Zr oxidation (due to steam and air) and the potential for any type of hydrogen combustion event in the spent fuel pool room/enclosure.  Due to the rapid computation speeds and its capability to model all types of reactor transients, Loss-Of-Coolant Accidents and loss of AC/DC power events (SBOs), MAAP5 is a powerful code that can be used in the development of a plant’s accident management strategies.

Originally developed by Fauske & Associates, LLC (FAI) as part of the Industry Degraded Core Rulemaking (IDCOR) program, FAI has developed and maintained the code under the sponsorship of the Electric Power Research Institute (EPRI) and the MAAP Users Group (MUG).

MAAP5, and its predecessor MAAP4, have been used solely by the nuclear industry throughout the world for more than two decades as an engineering tool for severe accident analysis and associated severe accident phenomena, including hydrogen generation and combustion, direct-containment heating, rapid pressurization due to steaming, core concrete interactions, fission product releases, transport and deposition etc.  The MAAP code is also used extensively in the PRA/PSA arena as well for success criteria evaluations, human reliability analyses (HRA) and Level II source term evaluations etc.