The MAAP5 code enhancements build on the substantial MAAP4 code enhancements to assess the approach to, and progression of, severe MAAP Services and PWR RCS Coolant Loop Viewaccidents 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 Probabilistic Risk/Safety Assessments (PRAs/PSAs), Severe Accident Management Guidelines (SAMGs), Emergency Operating Procedures (EOPs) and full scope control room simulators.
The substantial enhancements in the current MAAP5 code are summarized below:
- Natural circulation flows in the core, the hot leg and steam generators, before and after the core is uncovered
- Improved computation to handle a greater range of transients with one-dimensional and point kinetics neutronics models
- Increased capability for Steam Generator Tube Ruptures (SGTRs), Main Steam Line Breaks (MSLBs), Loss-Of-Coolant Accidents (LOCAs) analyses which require better steam generator models
- Improved models for the lower plenum debris pool response including detailed metal layer to wall heat transfer and heavy metal layer formation in the bottom of the lower plenum. It also has a detailed ex-vessel heat transfer model (nucleate boiling and critical heat flux as a function of azimuthal angle) for in-vessel retention evaluations
- A spent fuel pool model capable of modeling severe accidents in a spent fuel pool. The model is capable of calculating fuel uncovery, spent fuel heat up and degradation, Zr oxidation, hydrogen combustion events, Zr fires due to Zr + air interactions, etc.
- The PWR Reactor Coolant System (RCS) models have continued to advance as the requirements for MAAP have evolved.
The focus of the MAAP5 RCS model is to provide a fast-running, best-estimate representation of plant response to all types of plant accident conditions. The goal is to consistently describe the physical processes associated with the integral system response to plant upset conditions, especially those that can progress to severe accident conditions. MAAP5 models are not intended to replace other software codes that deal with large break guillotine ruptures, which require analysis of the rapid flow reversal within the core, however, it is intended to provide a best estimate description of the core, reactor coolant system, steam generators and containment needed for engineering assessments, including best estimate evaluations of operator procedures and success criteria for PSAs.
Of particular note in MAAP5 is the ability of the primary system model to accommodate independent coolant loop response for PWR designs which can have 1, 2, 3 or 4 steam generator loops. MAAP5 models the responses of each steam generator (including the number of tubes that are plugged in each generator) depending on the feedwater flows to the generators and their individual steaming rates. This multiple generator representation can assess the mid-loop operation status for steam generator repair scenarios.
MAAP5 contains both a point-kinetics and a one-dimensional core neutronics model. As part of this, MAAP5 models the boron distribution within the RCS. This capability is combined with natural circulation flows due to density differences between 1) the core and the downcomer, and 2) the hot leg and cold leg side of the steam generator tubes. This provides an integral representation of the RCS and core response when natural circulation flows are important, such as for Anticipated Transient Without Scram (ATWS) conditions.
The MAAP5 containment model builds on the MAAP4 Generalized Containment Model. It extends the capabilities into several new applications, including Design Basis Accident (DBA). This model has been extensively benchmarked with containment experiments with the full scale tests from HDR (PWR) and Marviken (BWR) being some of the most important. Not only do these tests represent the containment response to DBA conditions, but the HDR tests also demonstrate the conditions that would 1) cause hydrogen stratification and 2) the conditions that would result in global mixing to eliminate stratification.
The MAAP5 containment model has been enhanced so that it can be used for Design Basis Accident (DBA) analyses. This model will support either a single node or multi-node analysis. It can also be used to assess the margin in the DBA calculation that results from assuming no contribution from forced convection as a result of the pipe break. The model also addresses the thermal resistance due to paint layers on the walls. The containment model includes all the fission product isotopes needed to perform Alternate Source Term (AST) evaluations which are coupled with MAAP5 such that the in-plant and ex-plant doses and dose rates can be calculated in a single run. It also contains models for the aerosol transport and deposition mechanisms to assess the retention capabilities.
The containment model also includes hydrogen burn models to assess the extent of localized burning (for those conditions where the containment atmosphere is not inerted) that could occur in the containment for severe accidents. Thus MAAP5 can be used to evaluate the equipment survivability envelope for such conditions.
Additionally, the containment model now has the capability to model all aspects of an accident where the integrity of the spent fuel pool can be challenged. 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.