Resumen de A proposed methodology for passive autocatalytic recombiners sizing and location in LWR containments
One of the main objectives of nuclear safety is to ensure the integrity of the containment building, which constitutes the last barrier designed to prevent radioactive material release to the environment. The hydrogen combustion in a post-accident containment represents one of the most significant hazards for that integrity.
The hydrogen is generated during a severe accident in different modes, as well as different quantities and is produced at various stages according to the different melting points of the materials. The early degradation of the core implies the generation of hydrogen by the exothermal oxidation reactions of Zircaloy of the fuel cladding with water and steam at high temperatures and its subsequent release inside the containment building. In the later phase, the oxidation of other metallic components or metals from the corium or debris also has to be taken into account.
An issue of major interest for hydrogen safety inside the nuclear environment are the mitigation measures for hydrogen combustion risk in case of severe accidents. The reduction of hydrogen concentration from the containment atmosphere during an accident can be achieved through the use of Passive Autocatalytic Recombiner (PAR). The use of this technology is the most extensively deployed strategy to consume the hydrogen before it could reach flammable concentrations during a severe accident, particularly during accidents with loss of electric power supply. The PAR implementation is under study in many operating NPPs to reduce the hydrogen concentration during severe accidents. The PARs remove hydrogen from the reactor containment by an exothermal reaction of hydrogen oxidation by the oxygen presented in the containment atmosphere with the use of metals as catalysts generating steam and heat. This method is entirely passive and requires studies that accurately predict the hydrogen pathways and 3D distribution to ensure an unimpaired PAR performance.
Given the need for more detailed multidimensional analysis of containment, some computational tools have been developed that permit thermo hydraulic analysis. The thermo-hydraulic GOTHIC code allows performing 3D detailed models and could achieve greater flexibility in designing operational strategies, containment systems, and evaluation of design basis accidents and severe accidents.
The methodology posed herein analyses the Passive Autocatalytic Recombiners implementation answering the regulatory requirements emerged after the accident in Fukushima Daichii. This PhD thesis comprises PAR sizing and location, and the hydrogen control during severe accidents by developing safety demonstration analyses, which include the implementation of optimised PARs configuration in several containment buildings (BWR, PWR-KWU, and PWR-W).
This methodology is divided into four steps. The step 1 – consists of the selection of the accidental scenarios simulated with a severe accident code (MAAP, MELCOR) to obtain mass and energy sources; step 2 – development of a 3D containment model with GOTHIC code,; step 3 – hydrogen distribution analysis in containment to determine the hydrogen pathways; step 4 – PAR location, implementation, and analysis of efficiency. After the number and location of these recombiners are defined, a demonstration of the effectiveness of the PAR system installation is required by comparing the sequences with and without recombiners, to quantify the reduction achieved in the combustion risk. If the hydrogen combustion risk or the recombination rates of each PAR are not acceptable, the process starts again, being an iterative methodology.
Given the results, the optimised PARs configurations are capable of managing the hydrogen released in the chosen sequences, decreasing the possibility of hydrogen combustion risk below the deflagration limits in all the containment compartments at the end of the transient. The fact of having very detailed 3D models allowed creating a strategy of implementation based on the hydrogen preferential pathways and areas of accumulation. Nevertheless, sudden and significant hydrogen releases that may happen in some scenarios might not be under control by the PARs performance. Also, the studies show that the PARs could be unable to recombine in the early period of a possible fast release, due to their inertia and occurrence of oxygen starvation conditions, failing to completely prevent the combustion risk for a limiting scenario with fast hydrogen release.
The proposed methodology provides a guideline for PARs implementation and establishes a useful reference for PARs configuration, capable of coping with hydrogen combustion risk. This methodology has proven to be accurate enough for analysing the PARs installation in the BWR Mark III, PWR-KWU, and PWR-W containment type.
