Loss of offsite power

Two types of initiators are internal and external. Internal initiators result from failures within a plant or the plant s support utilities. Thus, vessel rupture, human error, cooling failure, and loss of offsite power are internal events. All others are external events earthquakes, tornados, fires (external or internal), and floods (external or internal). Event trees can be used to analyze either type of initiator. 

An obvious example is the situation which can occur such that a loss of offsite power makes some power buses unavailable for RCS heat removal. In addition, this loss-of-power mitiator affects the availability of the remaining systems, because emergency power becomes the only source of electric power. 

Loss of offsite power at nuclear power plants is addressed in EPRI NP-2301, 1982 giving data on the frequency of offsite power loss and subsequent recoveiy at nuclear power plants. Data analysis includes point estimate frequency with confidence limits, assuming a constant rate of occurrence. Recovery time is analyzed with a lognormal distribution for the time to recover. 

Four onsite power diesel generators that start automatically on loss of offsite power to reduce the frequency of the loss of station AC by an order of magnitude compared with ( , NDl 1 -6. This with the diesel driven auxiliary feedwater pump provides ample recovery imie... 

The ESDs were then translated into associated event trees. A generic event tree was developed for all initiators not involving LOCA. The generic transient event tree for each category of the transient initiators and loss of offsite power were specialized by the impact of the initiators on the safety and support systems, from the success criteria of the mitigating systems, and the initiator-specific human actions which were modeled in the fault trees. 

Ten front line systems and 5 support systems were modeled using about 700 basic events. A generic transient event tree with 215 sequences was developed and quantified for 14 different transients and three loss-of-offsite-power events. Four LOCA event trees were de I and... 

Select 1 and type a title for the problem the default title is Station Blackout from Loss of Offsite Power, unless you want to use this enter your title. 

In April 1982, a data workshop was held to evaluate, discuss, and critique data in order to establish a consensus generic data set for the USNRC-RES National Reliability Evaluation Program (NREP). The data set contains component failure rates and probability estimates for loss of coolant accidents, transients, loss of offsite power events, and human errors that could be applied consistently across the nuclear power industry as screening values for initial identification of dominant accident sequences in PRAs. This data set was used in the development of guidance documents for the performance of PRAs. 

Automatic and manual initiation demands In addition to test demands, equipment may be activated or deactivated by intended or spurious signals. These signals include losses of offsite power and normal shutdowns or startups. Different components react to different signals or sets of signals, depending on their functions for each system. The... 

The base scenarios are classified by initiating events or safety system availability which may impact on an accident progression. Left hand side of Fig. 2 represents sample database structure of analysis results for a loss of offsite power initiating event which can be applied to the OPR-1000 plant. There are 12 base scenarios depending on the operation status ofplant safety features. The operation status of safety systems are ... 

As the German nuclear power plants are designed to withstand the impact of external explosions and aircraft crashes, loads due to high winds are not expected to he important risk contributors. Nevertheless, outdoor switchyards and transformers are susceptible to wind driven missiles. If such equipment fads, loss of offsite power event sequences (transients typically analyzed as part of the plant internal events) might result. Similar simations can also arise fiwm an increased rainwater, sea salt spray, sand, or dust loading. 

The possible consequences of a flooding depend on the question whether it is hmited to the outdoor plant area or water actually penetrates into buildings. In the first case, the dominant consequence is apotential loss of offsite power. In case water should penetrate into safety related huildings/rooms, there is additionally a potential for a radioactive release or even a loss of the residual heat removal system. 

The operation of the affected plants has primarily been impaired by loss of offsite power and by short-circuits due to increased moisture ingress. 

In the case of a bush or forest fire, there is generally sufficient time for the operators to bring the reactor into a safe state and to take appropriate protection measures. The potential impacts of external fires are the intake of smoke via the ventilation system and a loss of offsite power. 

In the LTOP mode, each SCS relief valve is designed to protect the reactor vessel given a single failure in addition to the failure that i iitiated the pressure transient. The event initiating the pressure transient is considered to result from either an operator error or equipment malfunction. The SCS relief valve system is independent of a loss of offsite power. Each SCS relief valve is a self actuating spring-loaded liquid relief valve which does not require control circuitry. The valve opens when the RCS pressure exceeds its setpoint. 

Following certain DBEs (feedwater line break, small break LOCA, steam line break, or loss of offsite power), shutdown cooling can be initiated with RCS hot leg conditions which exceed the normal shutdown cooling initiation temperature of 350 F. However, shutdown cooling will never be initiated at conditions which exceed the design temperature of the SCS components. 

Loss of elcctnc sources (partial)--> loss of offsite power... 

LOSS OF OFFSITE POWER SUBSEQUENTIAL SMALL LOCA... 

DIESEL GENERATOR LOADING PROBLEMS RELATED TO SIS RESET ON LOSS OF OFFSITE POWER... 

These breakers are provided for abnormal scenarios such as Loss-Of-Offsite Power and Station Blackout when it is necessary to isolate the Division I II 4160 VAC buses from the Permanent Non-safety buses. Double breakers (normally open) are also provided for the 4160 VAC standby transformer which feeds each of the Class IE buses to maintain independence when they are not being used during maintenance. No single failure can prevent operation of the minimum number of required safety loads. See CESSAR-DC, Sections 8.3.1.2.1, 8.3.1.2.3 and 8.3.1.2.5 for a... 

Generic Safety Issue (GSI) A-09 in NUREG-0933 (Reference 1), addresses the issue of assuring that the reactor can attain safe shutdown after incurring an anticipated transient with a failure of the Reactor Trip System (RTS). An ATWS is an expected operational transient (such as a loss of feedwater, loss of condenser vacuum, or loss of offsite power to the reactor) which is accompanied by a failure of the RTS to shut down the reactor. 

There are two approaches to assuring the reliability of the safety-related system Class IE power supplies for future plants. The first approach is to restrict the connection of primarily safety loads to Class IE power supplies. [In previous designs, non-safety electrical equipment was connected to Class lE power supplies (i.e., the emergency diesel generators) to provide a source of power during Loss-Of-Offsite-Power (LOOP) events.]... 

Specifically, a second level of voltage protection should be provided for Class IE equipment in addition to the existing protection based on detecting the complete loss of offsite power to the Class IE buses. The second level should have two separate time delays before alerting the control room operator and automatically separating the Class IE buses from the offsite... 

Typical onsite electrical distribution systems for plants use diesel generators as an emergency source of power. These emergency power sources supply safety-related equipment, which is used to prevent or mitigate accidents, in the event of a loss of offsite power. 

Two different, non-class IE, power sources (one from each Permanent Non-Safety-Related Bus X or Y) are available for post-accident sampling. During a loss of offsite power, an alternate power supply is available to meet the 3 hour recommendation of NUREG-0737 for post-accident sampling and analysis. 

Generic Safety Issue (GSI) II.G.l in NUREG-0737 (Reference 1), addresses the reliability of the emergency power source which is used for the pressurizer relief (PORVs) and block valves and for the pressurizer level indication in the event of loss-of-offsite power. 

The TMI accident demonstrated the need for reliable pressurizer equipment (e.g., the ability to open or close the PORVs as necessary). Moreover, power supplies used to provide power for the pressurizer PORVs, block valves and level indication may not have been qualified to present stringent post-accident requirements. Several concerns were identified regarding the adequacy of the power supplies for the pressurizer equipment particularly with respect to the loss-of-offsite power event [see NUREG-0737]. NUREG-0737 identifies new guidance to assure adequate power for the pressurizer equipment consistent with the requirements of 10 CFR 50 Appendix A, (Reference 2) (GDCs 10, 14, 15, 17 and 20). 

The acceptance criterion for the resolution of GSI II.G.l is that, in the event of a loss-of-offsite power, the above pressurizer equipment shall be supplied with uninterrupted electrical power consistent with the guidance given in NUREG-0737 and the requirements of 10 CFR 50 Appendix A (GDCs 10, 14, 15, 17 and 20). 

In summary, the safety-related SDS and PAMI are powered from emergency onsite power and can sustain a total loss of offsite power (among other design basis events) and remain functional. Therefore, since the intent of the guidance given in NUREG-0737 has been met and thus the requirements of 10 CFR 50 Appendix A, this issue is resolved for the System 80+ Standard Design. 

Loss of offsite power Reactor protection system functions Diesel generator system functions... 

Accident initiators based on plant s specific design. These are called special initiators such as loss of offsite power and loss of service water. 

NUREG/CR-3992, "Collection and evaluation of complete and partial losses of offsite power at nuclear power plants. ... 

USNRC Information Notice 86-87, "Loss of offsite power upon an automatic bus transfer. ... 

Events which result in a loss of offsite power require reliable emergency diesel generators (EDG) to supply all necessary safety systems with power to make possible a safe shutdown of the plant. It is also shoYm in most PSAs that the starting up reliability of the EDGs has a high level of importance to reduce the core damage frequency. 

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