20 e Congrès de maîtrise des risques et de sûreté de fonctionnement - Saint-Malo 11-13 octobre 2016 RELIABILITY OF OFFSITE POWER OF NUCLEAR POWER PLANTS IN EVOLVING POWER SYSTEMS FIABILITÉ DE L’ALIMENTATION ÉLECTRIQUE EXTÉRIEURE DES CENTRALES NUCLÉAIRES DANS DES RÉSEAUX ÉLECTRIQUES EN ÉVOLUTION Pierre Henneaux Tractebel (ENGIE) 7, av Ariane B-1200 Bruxelles pierre.henneaux@tractebel.engie.com Pierre-Etienne Labeau Université libre de Bruxelles 50, av FD Roosevelt B-1050 Bruxelles pelabeau@ulb.ac.be John Mitchell Obama IMdR 12 avenue Raspail F-94250 Gentilly john.obama@imdr.eu Summary The safety of Nuclear Power Plants (NPPs) significantly relies on the reliability of offsite power. In particular, the Loss Of Offsite Power (LOOP) event is an important contributor to the total residual risk at NPPs. The Core Damage Frequency (CDF) is strongly related to the LOOP frequency. Probabilistic safety assessment take thus into account LOOP events and their occurrence frequency. The LOOP frequency is usually estimated from operational feedback: the maximum likelihood estimator of this frequency is given by the number of events observed during a time period divided by the number of reactor-years. Because a LOOP event is a rare event, a long time period (typically 15 years or more) must be used to reach a sufficient number of events. In our changing world, power systems are also strongly evolving. The validity of the operational feedback based on such long-term past periods to predict the future in this context is questionable. Moreover, statistics are usually aggregated for several NPPs so that statistics can rely on a sufficient number of events. It is thus assumed implicitly that the LOOP frequency does not differ strongly from one NPP to another, which is also a questionable assumption. A probabilistic methodology to compute the LOOP frequency of a NPP, able to consider the current state of the power system, is thus needed. The objective of this communication is to present such a methodology and to apply it to a study case to show that the LOOP frequency could change significantly when the generation mix evolves from a main share of fossil fuels to a large penetration of renewable energies. Résumé La sûreté des Centrales Nucléaires (CNs) repose fortement sur la fiabilité de l’alimentation électrique extérieure. En particulier, la perte de l’alimentation électrique extérieure (LOOP) est un contributeur important au risque résiduel dans les CNs. La fréquence de fusion de cœur (CDF) est fortement liée à la fréquence de LOOP. Les études probabilistes de sûreté prennent donc en compte les événements de type LOOP et leur fréquence d’occurrence. La fréquence de LOOP est habituellement estimée à partir du retour d’expérience : l’estimateur maximum de vraisemblance est donné par le nombre d’événements observés pendant une période temporelle divisé par le nombre d’années -réacteurs. Étant donné qu’un LOOP est un événement rare, une longue période temporelle (typiquement 15 années ou plus) doit être utilisée pour atteindre un nombre suffisant d’événements. Dans notre monde en mouvement, les réseaux électriques évoluent également fortement. La validité du retour d’expérience basé sur de telles périodes temporelles passées pour prédire le futur est discutable. De plus , les statistiques sont souvent agrégées pour plusieurs CNs de telle sorte que les statistiques puissent s’appuyer sur un nombre suffisant d’événements. Il est donc implicitement supposé que la fréquence de LOOP ne change pas significativement d’une centrale nucléaire à l’autre, ce qui est également une hypothèse discutable. Une méthodologie probabiliste pour calculer la fréquence de LOOP d’une CN, capable de prendre en compte l’état actuel du réseau électrique, est donc nécessaire. L’objectif de cette communication est de présenter une telle méthode et de l’appliquer à une étude de cas pour montrer que la fréquence de LOOP peut changer significativement lorsque le mix de production évolue des combustibles fossiles vers les énergies renouvelables. Introduction The safety of Nuclear Power Plants (NPPs) significantly relies on the reliability of offsite power. In particular, the Loss Of Offsite Power (LOOP) event is an important contributor to the total residual risk at NPPs: statistics show that the LOOP event represents usually approximately 70% of the Core Damage Frequency (CDF) (Eide et al., 2005). Indeed, the core of a NPP must be cooled down even when the nuclear fission reaction is stopped. This cooling relies on auxiliary equipment, especially primary pumps that are based on asynchronous motors. These motors require an important power to work properly (the auxiliary equipment typically consumes several tens of MWs). The availability of AC power to NPPs is thus essential for safe operations and accident recovery. The electrical grid, to which the NPP is connected, referred as “offsite power”, usually supplies this power. The LOOP event occurs when the plant loses offsite power, i.e. all connections to the external grid. In that case, emergency diesel generators can provide “onsite power”. The simultaneous failure of offsite power and onsite power is called a Station BlackOut (SBO). A SBO can lead to core damage as all cooling capacity may be lost due to the general defect in feeding auxiliary pieces of equipment with energy. LOOP events are usually classified in four categories (Eide et al., 2005): plant-centered, switchyard-centered, grid-related and weather-related. A plant-centered LOOP event is defined as “a LOOP event in which the design and operational characteristics of the nuclear power plant unit itself play the major role in the cause and duration of the LOOP”. A switchyard-centered LOOP event is defined as “a LOOP event in which the equipment or human-induced failures of equipment, in the switchyard play the major role in the LOOP”. A grid-related event is defined as “a LOOP event in which the initial failure occurs in the interconnected transmission grid that is outside the direct control of plant personnel”. Finally, a weather-related LOOP event is simply a LOOP event caused by severe or extreme weather. The overall LOOP frequency is given by the sum of these four individual Communication 2E /1 page 1/7