By: Robert Reeves, Director, MAAP Services, Fauske & Associates, LLC (FAI)
Modular Accident Analysis Program (MAAP), which is owned by the Electric Power Research Institute (EPRI), is a computational tool that the nuclear industry has relied on for over 30 years to resolve not only severe accident issues, but also operational issues associated with nuclear power plants. Today, MAAP is primarily used within utility PRA (Probabilistic Risk Assessment) groups. Within PRA organizations, MAAP is typically used to perform thermal-hydraulic analyses in support of PRA Level I and Level II
For the Level II analyses, MAAP is used to assess the performance of the containment and its support systems, with the focus on accident sequences that could lead to a large breach early in the accident. The MAAP models of the Reactor Coolant System mass/energy releases, containment heat removal processes through multiple systems and heat exchangers and the containment heat-up phenomenology are all important for these analyses.
MAAP is a fast-running analytical tool suitable for predicting accident progression (i.e., core uncovery, damage, vessel breach, containment breach) and evaluating the timing and effectiveness of operator actions. MAAP represents a large number of interacting phenomena using simple, first-principle validated models (i.e., it is not restricted to just a few phenomena with very detailed models).
The MAAP5 version contains fully-integrated thermal-hydraulics (T/H) models for:
1. Primary system thermodynamics
2. Core heat-up, degradation and melting
3. Fission product release, transport, settling and heating
4. ESFs (engineered safety features)
5. Containment thermodynamics
6. Dose (in-plant and ex-plant) calculations
Many of these models have been updated since the release of MAAP4 and can provide more realistic predictions of the plant performance during a postulated accident.
A current issue within the nuclear industry is that the majority of nuclear plant PRA models are based on results from older versions of the MAAP4 computer code (i.e., MAAP4.06, MAAP4.07, etc.). There were few modeling differences associated within each minor version of MAAP4. However, over the past 10 years, EPRI has released a new version of MAAP called MAAP5 which has numerous improvements and new modeling capabilities associated with it. Significant differences are summarized below to assist in the recognition of the benefits of updating plant PRA models to use results from MAAP5.
1. MAAP4 development is no longer supported by EPRI. Thus, for any issues that arise in the future (e.g. SFP analyses, containment venting, low power shutdown analyses, etc.), MAAP4 will not have the modeling capabilities sufficient to address them. The only support EPRI currently provides associated with MAAP4 is direct utility support for user questions only.
2. The enhanced MAAP5 modeling capabilities of the PWR primary system provide a more realistic model and modeling capability to substantiate the analytical results.
– Due to the improved MAAP5 modeling of the primary system and its more realistic models, MAAP5 typically shows that for small LOCAs and station blackout accidents (with seal LOCAs), there is increased time to core uncovery and core damage and/or more time to implement actions to preclude such events.
– Higher containment gas temperatures and more detailed primary system nodalization and modeling capabilities yields lower primary system heat sink temperatures, resulting in less long term cesium iodine (CsI) re-vaporization in the primary system, thus potentially lower Level II source term releases.
– Other MAAP5 enhancements to the core melt progression modeling can impact long term fission product releases as well.
3. MAAP5 includes several model upgrades to assess the effects of severe accidents, including those affecting the spent fuel pool:
4. In terms of licensing, the majority of the US utilities have already purchased MAAP5 licenses; therefore, the only new cost associated with updating to MAAP5 is the upgrade of the existing MAAP parameter file to MAAP5. Depending on the scope and quality of the existing MAAP4 parameter file and its supporting documentation, the cost associated with such an upgrade is generally around $50K (not including add-ons such as dose, SFP model, neutronics, detailed design-basis containment model, or other improvements that are not directly applicable to most PRAs).
5. Additionally, almost all European and Asian MAAP users are already using MAAP5, and thus, they are indirectly in control of further MAAP5 development (in terms of prioritizing what improvements and model enhancements get funded by EPRI).
In summary, MAAP5 has more realistic models, both in terms of nodalization and phenomenological models, of the Reactor Coolant System, containment, support systems and spent fuel pool. These more realistic models generally all use relaxed equipment performance requirements and relaxed operator response times. However, there are differences in each plant’s thermal-hydraulic model and PRA model, making the exact benefit for a specific plant impossible to quantify a priori.
One final consideration is that if the PRA models success criteria and operator response times are based on MAAP4 or older versions, it is likely that these analyses are at least 10 years old, and there may have been changes to the plant design and operator response capabilities that are not reflected in those thermal-hydraulic analyses. An update, or, at the least, some selective benchmarking, is likely necessary to maintain consistency with current standards for the scope and quality of the PRA models.
For more information and discussion, contact Robert Reeves, firstname.lastname@example.org or (630) 887-5220. www.fauske.com