15 May 2023

Integration of MELCOR at the Bohunice FSS

The Melcor code is a widely used computer program used in the field of nuclear engineering for simulating and analyzing severe accidents in nuclear power plants. It provides a comprehensive set of models and algorithms to simulate the behavior of various thermal-hydraulic and physical phenomena that occur during severe accidents.

The primary purpose of using the Melcor code on nuclear simulators is to assess and improve the safety of nuclear power plants. By running simulations, engineers and researchers can study the response of a plant to hypothetical severe accident scenarios and evaluate the effectiveness of safety systems and mitigation strategies.

The Melcor code incorporates complex models for core heatup, fuel melting, fission product release, hydrogen production, containment behavior, and other phenomena relevant to severe accidents. It considers factors such as fluid dynamics, heat transfer, phase changes, chemical reactions, and material behavior under extreme conditions.

Using the Melcor code on nuclear simulators allows analysts to study a wide range of accident scenarios, including loss-of-coolant accidents, station blackout, and core meltdown. It helps in understanding the progression of accidents, the behavior of nuclear fuel, the release of radioactive materials, and the response of safety systems.

The simulation model of the Bohunice representative full-scope simulator was modified to include the MELCOR severe accident code to model the transients that occur after the top of the active zone dries out, defined when the exit temperature of the core exceeds 650 °C.

MELCOR module was integrated into the simulator executive sharing memory with the existing THOR thermohydraulic module. The purpose of this integration is to provide accident progress analysis, accident consequence analysis and reactor safety perspectives. The MELCOR model consists of the reactor pressure vessel (RPV) and pressurizer (PRZ) and an interface with THOR is placed at the connections to the RPV including the cold and hot leg connections. At each interface, THOR is used to control the injection or extraction rate of fluid and convective heat transfer between vessel wall and the hermetic zone is modeled. MELCOR does not replace THOR model of the RPV/PRZ but supplements it when a severe accident is predicted. A severe accident is presaged just prior to the start of a runaway oxidation, taken as 650 °C. At this demarcation, THOR determines the current conditions and creates a supplementary set of input. After starting MELCOR’s time advancement, THOR uses the boundaries to interface with MELCOR to provide cooling or pressure relief.

FAT tests at CORYS, Grenoble (© CORYS)
FAT tests at CORYS, Grenoble (© CORYS)

The THOR code has been extended to simulate passive autocatalytic recombiners (PARs). Controlling the parameters of the atmosphere in the containment is necessary from the point of view of the danger of burning the hydrogen mixture in the containment areas. PARs are distributed in the HZ premises based on the expected distribution of hydrogen during severe accidents and are modeled based on input data from a real unit.

Hydrogen generation by MELCOR (© VUJE)
Hydrogen generation by MELCOR (© VUJE)

The implementation of the core melting model in the simulator will ensure reliable simulation of selected phenomena associated with the management of a severe accident so that it is possible to train the permanent crew of the unit control room during emergency exercises.

By performing simulations with the Melcor code, engineers can identify potential vulnerabilities in plant design, assess the effectiveness of accident management strategies, and propose improvements to enhance the overall safety of nuclear power plants. The insights gained from these simulations can inform regulatory decision-making, emergency planning, and design modifications to mitigate the consequences of severe accidents.

In summary, the usage of the Melcor code on nuclear simulators provides a powerful tool for analyzing severe accidents, improving the safety of nuclear power plants, and ensuring the protection of public health and the environment.


Juraj Marec
VUJE, a.s.