EPR – Loss of Offsite Power with Total Failure of Diesels.
Computational tools and scenario description.
To perform the analysis of the scenario Loss of Offsite Power, severe accident codes were used. The aim of the workshop was to compare the results between MAAP4 (AREVA specialists) and MELCOR codes for the prepared accident scenario. SARWUT team conducted simulations with the use of the MELCOR code – the computer code for severe accidents and the design basis accidents analyses. It is used by the regulatory bodies and research institutions. The code is fully integrated with primary purpose to model the progression of the accidents in LWRs including the estimation of the core behavior during its melting and the response of the containment to the severe accident conditions MELCOR modeling is general and flexible, making use of a control volume (nodal) approach with the mechanistic and also parametric models. In contrary MAAP4 is the severe accident code most widely used by nuclear utilities and vendors because of its short run time and reduced requirements for code expertise, it requires minimal computational time with NPP simplified geometry model. The codes aim is in general identical to become more best-estimate and predict the progression of the severe accident.
The Loss of Offside Power with the Total Failure of Diesels (LOOP650) is the most severe accident scenario that can be faced in the EPR reactor. It includes total unavailability of the active systems (safety injection systems – LHSI, MHSI), that are depending on the power supply. Additionally the severe accident dedicated systems – like Emergency Diesel Generators (EDG) and Station Black-Out diesels (SBO) are also assumed not participating in the scenario. Due to postulated unavailability of all diesel generators no feedwater to Steam Generators (SG)s is available, and no safety injection means are available, except the four accumulators. The depressurization of the system is done by the opening of the Pressurizer Discharge System (PDS) when maximum Core Outlet gas Temperature (TCOTmax ) is 650 ◦C. During the accident simulation only the in-vessel phase is considered. The available systems are: accumulators (x4) Pressurizer Safety Valves (PSV and PDS) and SG Safety Valves (MSRT & MSSV)
MAAP model that was prepared by the AREVA specialists and scenario results were supplied to the SARWUT Team with model description in the form of the representation of the nodalization as well as detailed boundary and initial conditions. The model is divided into separate zones in the RCS primary and secondary side (Figure 2). The level of detail of the model is adequate to predict? the relevant changes of the parameters is the system, such as temperatures and pressures. Core is divided into 32 axial nodes (28 active core) and 5 rings, what gives the complete sketch of the reactor core with its initial composition. There are two loops representing 4 loops in reality, where three are merged into one with corresponding three times larger flow areas and volumes.
MELCOR EPR model was prepared in the similar manner as the model from the MAAP4 code. The loops were divided into two (simulating merged and single one with pressurizer). The reactor core was divided into 3 control volumes (for core channel) and one for core bypass. Core model for COR package was divided into 19 axial levels and 6 radial rings simulating different areas of the core and lower plenum and over 100 Heat Structures representing the Reactor Pressure Vessel internals. The important element of the model were control functions simulating the opening and closing of the available valves and systems, which allowed to adjust the set points (like PDS opening). Additionally there were non-active Engineered Safety Features added to the model, like Passive Autocatalytic Recombiners (PARs) responsible for the hydrogen removal from the containment volume and preventing the hydrogen deflagration.
Results and conclusion.
The Scenario LOOP650 MELCOR and MAAP4 results are in good agreement with each other, with time shift (MELCOR events quicker in time) in the main events including the reactor core degradation, melting and the following core relocation and RPV failure. This implicates the main difference between codes, which is in the mode of the core melting, degradation and relocation.
All of the relevant events were properly simulated (all of the pressure, temperature peaks and connected phenomena). The comparison showed how important is the continuous verification and validation of the code for the purposes of the safety analysis and the understating of the implemented models in the code. Moreover it is recommended to perform detailed sensitivity analysis.