The risk analysis of the electrical Cross-tie for extended Station Blackout in Multi-Unit site
Introduction
After the Fukushima accident in Japan, the Multi-Unit risk assessment (MURA) became important in Probabilistic Risk Assessment (PRA). The conventional PRA only considers a single unit risk assessment so far. Thus, many countries who have several NPPs in a site actively develop the method of the multi-unit risk assessment. In this paper, we will discuss the development of PRA methodology for the Multi-Unit risk assessment (MURA) from a conventional single unit PRA. To examine the developed methodology in the probabilistic approach, we used the AIMS-PSA CODE and developed the KU-PRA model based on the KAERI Pilot model. The KU-PRA model included many SBO aspects such as a recent failure data with modeling of Emergency Diesel Generator (EDG), Common Cause Failures (CCFs) of Alpha factor method, occupancy factors of Alternative Alternating Current Diesel Generator (AAC-DG) and of the EDG crosstie and others. We selected the EDG crosstie option to verify the suggested methodology.
The initiating event of Station Blackout (SBO) for the NPP unit is a combination of a Loss of Offsite Power (LOOP) and loss of its EDGs. It is considered as one of the main contributors to the core damage risk (Volkanovski and Prošek, 2013). The SBO event has received a lot of attention from the world nuclear regulators (U.S. NRC, 2013; U.S. NRC, 2003), and NPP utilities (Lee et al., 2014). The SBO event can be extended (Ex-SBO) when the loss of the shared AAC-DG comes along with the LOOP and loss of EDGs. Most of NPP site have Multi-units in one site. It makes the NPP possible to share the Structures, Systems, and Components (SSCs) among the units.
The electrical cross-tie was proposed so that the non-impacted unit can donate one of its dedicated (EDG) to the unit under Ex-SBO of multi-unit NPP in one site (Shen et al., 2014). The risk of Ex-SBO re-evaluated with the EDG crosstie option (Jung et al., 2003) in a systematic approach based on multi-unit dependency factors (Schroer and Modarres, 2013). The identification of crosstie issues was addressed in previous relevant research (Ha et al., 2017). In this research, we developed a detailed approach to evaluate the risk of sharing the onsite AC power and quantified the extended SBO risk and its impact on Core Damage Frequency (CDF). The current CDF of a single unit (Unit-1) by SBO is reevaluated considering the impact of multi-unit features (EDG crosstie).
From the risk management point of view, the unit under the extended SBO with EDG crosstie will benefit from an onsite additional AC source from the non-impacted unit. The donor unit also has its regulatory requirements in the availability of its redundant onsite EDGs sources in different operational modes and with limiting conditions of technical specifications (Shen et al., 2014, U.S.NRC, 2012). In the extended SBO, the core damage needs to be prevented by employing any available AC sources at the site and EDG crosstie may be adopted with conditions.
We start to discuss an indication of a single unit PRA and SBO and how to develop the KU PRA model from the KAERI pilot model (Han et al., 2018) in section 2. Then, we elaborate on the methodology and modeling of the crosstie option and address aspects related to the SBO and Ex-SBO in section 3. It starts with Human Reliability Analyses (HRAs) treatment for SBO and Ex-SBO (3.1). Then, it addresses the operator action in DC Battery load shedding to extend the available time to the operator from 3 h to 17 h to carry the crosstie task (3.2). After that, the summary of the HRA results as human failure probabilities are presented for the actions related to the SBO and Ex-SBO (3.3). Sections 3.4 and 3.5 include a review of the multi-unit topic in literature and present the modeling of Multi-unit LOOP and Multi-unit SBO initiators in the KU pilot model with modeling of occupancy factors of both AAC-DG and EDG crosstie as multi-unit dependency factors.
To identify the impact of the additional onsite AC power (redundancy) on the reduction of the SBO risk, we examine the risk of SBO and CDF by adding third and fourth EDG to the KU PRA model. First, we will discuss the results of SBO and CDF from the developed KU PRA Model for a single unit (Section 4.1). Then, we will compare the impact of the addition of third and fourth EDGs into the SBO and CDF for a single unit with the reflection of CCFs between DGs in section 4.2. At last, we continue to discuss the results of the single unit PRA with the impact of Multi-unit features (2 Units/Site, 3 Units/Site and 4 Units/Site) in section 4.3.
Section snippets
Single unit PRA of station blackout
In order to develop the scenario of the SBO event in the probabilistic approach, it’s important to know the related NPP design and to understand the plant response to the different scenarios. The safety functions of the front-line barriers (systems) required to respond to the initiating event are the key features of the design and used for the modeling of the accident sequences of the PRA.
In any initiating event, the main fundamental safety functions need to be maintained as: Control of the
Methodology and modelling of the crosstie option
The impacted unit is the unit that has a LOOP and both of its onsite dedicated EDGs fail to start and run, leading to an event of SBO. In addition to SBO, when the shared AAC-DG between units as a coping DG for the SBO also fails to start and run (or it could be unavailable because it is occupied by another unit that also under SBO), this situation will lead to the event of extended SBO for the impacted unit. After LOOP and SBO, it is expected that one of the Turbine- Driven Auxiliary Feed
Results of the developed KU PRA model for a single unit
For the developed KU PRA pilot model, the CDF of a single unit is found to be 1.36E-05 per year. The CDF related to the SBO with LOOP accident has a value of 8.50E-06 per year.
From the developed model, the SBO and LOOP are found to be the highest contributors to the CDF and represent about 62.42% of the total CDF. It can be recognized that the CDF and SBO risks are highly increased with the developed PRA model as a reason for adding all failures modes and unavailability due to test and
Conclusions
According to conventional safety regulations, each NPP unit must be independent with dedicated safety features. Thus, most of the risk analysis was performed for a single unit. However, as the Fukushima accident expends safety concerns from the single unit to the multi-units, the conventional PRA with a single unit became not enough to represent the risk of the site with the multi-units. The concept of the electric cross-tie between NPP units for SBO was proposed to reduce core damage risk in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank Khalifa University of Science and Technology (KUST), Korea Atomic Energy Research Institute (KAERI) and Federal Authority for Nuclear Regulation (FANR) for their support in the research and providing the AIMS PSA code with KAERI Pilot model.
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