Loss Of Offsite Power Events Research Articles

Methodology for estimating the contribution of forest fires in loss of offsite power events

Forest fires are disasters caused by natural or human factors and have environmental as well as economic impacts such as loss of biodiversity, agricultural damage, and property damage. According to statistics, the number of forest fires and the area damaged are gradually increasing in Korea. In addition, there are cases where the units 1, 2, and 6 of the Hanul Nuclear Power Plant (NPP) have already been affected by forest fires. Korea has recently initiated research on forest fires under multi-unit probabilistic safety assessment (PSA) project. This paper deals with the impact on the loss of offsite power (LOOP), which is one of the NPP incidents and accidents affected by forest fires. To estimate the frequency of LOOP induced by forest fires, the PSA methodology for earthquakes, a representative external event, was referred.

The Method of Calculating the Frequency of the Initiating Event in a Dual-Unit Site with the Example of LOOP Events

In a nuclear power plant, the consequences of a multi-unit event occurring concurrently are more serious than those of a single-unit event. The first step in the probabilistic safety analysis of multi-units is to analyze the initiating events and calculate the frequency of initiating events for simultaneous events of multiple units. The difficulty in using the fault tree model is that the known data are all frequency data from a single unit and cannot be logically multiplied. In this paper, taking a dual unit as an example, we used the formula to convert the probability of failure of the second unit within 72 h and then build a fault tree model. After analyzing the results of the dual unit, the most frequent cut set was the common cause of failure of the main transformer and of the switching failure of the main and auxiliary external power. The final calculation of the frequency of simultaneous loss of off-site power events for the dual units within 72 h was 3.22 × 10−4/year. After comparing with the single-unit results, it was found that the common cause failure of each unit’s independent equipment was the main reason for the occurrence of a loss of off-site power. Shared equipment in a single unit was ranked low in all the cut sets (such as the stability of the external grid for the main and auxiliary power systems) but was ranked high in multiple units. The calculation results of the frequency of initiating events of double units were two orders of magnitude lower than those of a single unit. However, the consequences of simultaneous events of multiple units were higher than those of single reactors. Therefore, attention should be paid to the risk of a simultaneous loss of off-site power event of multiple units.

On the Estimation and Reduction of the Frequency of the Loss of Offsite Power Event

Loss of offsite power (LOOP) contributes significantly to the estimated core damage frequency (CDF) for the majority of nuclear power plants (NPPs). Based on experience from some near-miss events around the world and especially in Japan from Fukushima Daiichi accident and several other NPPs it was proven the how important power supply is for nuclear safety. Therefore comprehensive description of the LOOP initiating event is given in this paper. Specific experience of the NPP Krško (NEK) related operation is also presented with comparison to the U.S. NPPs’ approaches and experience. Statistical data are given to explain main characteristics (frequency and duration) of the LOOP event. Important historical trends are also identified. LOOP event frequency is often calculated using generic data because of statistical data base. Thereat, many important factors that influence LOOP are neglected, such as the specific power network configuration, load profiles, climate conditions, and ageing of the equipment. Several different approaches that take these factors into consideration are therefore presented and discussed. Familiarity with the actual state of the NPP, power network and weather conditions helps in the proper planning process of the test and maintenance (T&M) activities and reducing of the risk. This is significant for better online risk estimate. Due to the Fukushima accident, it is necessary to review the safety of the NPPs from new different angles. This paper also shortly reviews planned modifications in NEK that will increase the electric power supply availability (i.e., reduce LOOP event frequency. Other, worldwide solutions that can help to avoid or mitigate LOOP effects are briefly presented.

Management of loss of offsite power avoiding reactor trip

A loss of offsite power event in a nuclear power plant resulting from the loss of connections to the electrical grid can be effectively managed without resulting in an unscheduled reactor trip. We have developed effective procedures for managing such house load operations that will maintain a stable steam generator water level using the auxiliary feedwater system. The procedures are based on a first-principle energy balance with the heat flux from the primary to the secondary side represented through an average primary system temperature. The reduced order model has been validated through actual simulations of steam generator dynamics with the MARS-KS code. With the help of the reduced order model, various scenarios or tracks involving judicious combinations of reactor power, reactor coolant flowrate and steam generator feedwater flowrate that could support stable house load operations are simulated and summarized as an operating procedure in a multi-path event tree structure.

Analysis of Loss of Offsite Power Events at China’s Nuclear Power Plants

The function of the electric power system of nuclear power plants (NPPs) is to provide safe and reliable electricity for the equipment both in normal operation and accident conditions, and to provide emergency power for nuclear safety-related systems to maintain the safety of NPPs. Station blackout (SBO) occurs when loss of offsite power (LOOP) happens concurrently with unavailability of the onsite emergency alternating current (ac) power. LOOP is a precursor of SBO which rarely occurs but contributes significantly to reactor core damage frequency (CDF). Collecting and analyzing all LOOP events in NPPs of China from 1993 to 2017, this paper summarizes the common features of the LOOP events, and identifies the weaknesses and lessons learned from these events. Conclusions and experience feedback suggestions are put forward for improving the reliability of the offsite power supply of NPPs in China.

Safety Analysis of a 300-MW(electric) Offshore Floating Nuclear Power Plant in Marine Environment

The Offshore Floating Nuclear Plant (OFNP) integrates an advanced light water reactor into a cylindrical, double-hull, floating platform. It offers a series of potential benefits in economics and safety. The 300-MW(electric) version, named OFNP-300, uses an ocean-based direct reactor auxiliary cooling system (DRACS) to remove decay heat from the core passively and indefinitely during loss of feedwater or loss of off-site power events. In the ocean, the OFNP platform may roll during storms or even statically tilt following asymmetric flooding of underwater compartments. The effects of rolling motion and static tilt on the engineered safety systems are investigated in this paper using a RELAP5-3D (version 4.3.4) model of OFNP-300. The oscillations of the platform are described as the superposition of sinusoidal motions for the six degrees of freedom, i.e., heave, roll, pitch, yaw, sway, and surge. The plant’s thermal-hydraulic responses to two postulated accidents, i.e., loss-of-coolant accident (LOCA) and station blackout (SBO), are then studied in three scenarios: (a) a design-basis 100-yr storm, (b) a bounding scenario in which the platform is assumed to pitch and roll with an amplitude of 20 deg, and (c) a bounding scenario in which the platform experiences a static tilt of 30 deg. The results of the RELAP5 analysis show that the safety margins of OFNP-300 are not challenged in the aforementioned three postulated scenarios. From a thermal-hydraulic point of view, the pitch and roll motions affect the flow in the DRACS but have no negative effect on the temperatures in the core during LOCA and SBO. Static platform tilt is tolerable up to 45 deg, beyond which the emergency core cooling system can no longer function.

Analysis of loss of offsite power events reported in nuclear power plants

This paper presents the results of analysis of the loss of offsite power events (LOOP) in four databases of operational events. The screened databases include: the Gesellschaft für Anlagen und Reaktorsicherheit mbH (GRS) and Institut de Radioprotection et de Sûreté Nucléaire (IRSN) databases, the IAEA International Reporting System for Operating Experience (IRS) and the U.S. Licensee Event Reports (LER).In total 228 relevant loss of offsite power events were identified in the IRSN database, 190 in the GRS database, 120 in U.S. LER and 52 in IRS database. Identified events were classified in predefined categories.Obtained results show that the largest percentage of LOOP events is registered during On power operational mode and lasted for two minutes or more. The plant centered events is the main contributor to LOOP events identified in IRSN, GRS and IAEA IRS database. The switchyard centered events are the main contributor in events registered in the NRC LER database. The main type of failed equipment is switchyard failures in IRSN and IAEA IRS, main or secondary lines in NRC LER and busbar failures in GRS database.The dominant root cause for the LOOP events are human failures during test, inspection and maintenance followed by human failures due to the insufficient or wrong procedures. The largest number of LOOP events resulted in reactor trip followed by EDG start.The actions that can result in reduction of the number of LOOP events and minimize consequences on plant safety are identified and presented.

Statistical Analysis of Loss of Offsite Power Events

This paper presents the results of the statistical analysis of the loss of offsite power events (LOOP) registered in four reviewed databases. The reviewed databases include the IRSN (Institut de Radioprotection et de Sûreté Nucléaire) SAPIDE database and the GRS (Gesellschaft für Anlagen- und Reaktorsicherheit mbH) VERA database reviewed over the period from 1992 to 2011. The US NRC (Nuclear Regulatory Commission) Licensee Event Reports (LERs) database and the IAEA International Reporting System (IRS) database were screened for relevant events registered over the period from 1990 to 2013. The number of LOOP events in each year in the analysed period and mode of operation are assessed during the screening. The LOOP frequencies obtained for the French and German nuclear power plants (NPPs) during critical operation are of the same order of magnitude with the plant related events as a dominant contributor. A frequency of one LOOP event per shutdown year is obtained for German NPPs in shutdown mode of operation. For the US NPPs, the obtained LOOP frequency for critical and shutdown mode is comparable to the one assessed in NUREG/CR-6890. Decreasing trend is obtained for the LOOP events registered in three databases (IRSN, GRS, and NRC).

Integrated Power Recovery Using Markov Modeling

The solution to a Markov chain modeling electric power supply to critical equipment in a typical four-loop pressurized water reactor following a loss of offsite power event is compared with a convolution method. The standard “convolution integral” approach is described, and an alternative methodology based on a Markov model is illustrated.

Considerations for improving the reliability of the off-site power supply for nuclear generating stations

It is argued that improvements in plant distribution system analysis, maintenance procedures, work control, definition of ownership, provision of detailed operational procedures and improved training can reduce loss of offsite power events. These improvements are not simple to implement in many cases, but as maximum grid loads grow and control and protective relaying become more complicated, it is essential that the processes to ensure adequate analysis, training, maintenance, and work control be in place.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>