Emergency Diesel Generators (EDG's)
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Emergency Diesel Generator Diesel Generators as Emergency Power Sources 1. DIESEL GENERATORS AS 3. Why diesel engines are used as the EMERGENCY POWER SOURCES prime movers for emergency power generators instead of alternative Learning Objectives engine designs. The predominant means of supplying 4. An overview of the regulations, codes, onsite emergency (standby) electrical guides, and standards that establish power for nuclear power plants is the use of the design basis for these emergency emergency diesel generators (EDG's). power systems. Therefore, diesel generator sets are the specific focus of this NRC training course. 5. How the above documents are Upon completing this lesson students will translated into the licensee's understand the fundamental criteria used in application and design for a nuclear selection of emergency diesel generators power station. for onsite electric power supplies at nuclear power plants (NPP's). That will include an 6. The major components of a diesel overview of the primary applicable federal generator system, as well as some regulations, regulatory guides, codes, and considerations involving site facilities industry standards. that support EDG operation. In addition, this lesson will conclude with a NOTE: Many participants in this course will summary overview of the EDG and its already be very familiar with the regulatory associated systems and components, as criteria applicable to EDG's. However, the well as remarks on how they interface with NRC requires such documentation to be site facilities. For the EDG to be capable of part of the course because some attendees performing its design basis function, all of may be relatively new on the job. Even its on-skid and off-skid support systems those with considerable experience may and components must also meet their own benefit from a brief review of the underlying design basis functional requirements. documentation. This Chapter provides an overview of the fundamental requirements The primary objective of this lesson is to relevant to EDG's in nuclear service. For set the stage for later Chapters by giving both new and experienced staff it can serve students a fundamental understanding of: as a convenient reference. 1. The basic regulatory requirements 1.1 Regulatory Basis for Redundant, establishing the need for redundant Independent Power Systems power systems (onsite and offsite) for operating nuclear power plants. Federal regulations applicable to nuclear power plants, and hence onsite emergency 2. Three fundamental performance power supplies, originate in Title 10, Part requirements that emergency diesel 50 of the Code of Federal Regulations (10 generators (EDG's) must meet. CFR 50). This document is the successor Rev 1/11 1-1 of 12 USNRC HRTD Emergency Diesel Generator Diesel Generators as Emergency Power Sources to the Atomic Energy Commission (AEC) one "postulated" accident but regulations General Design Criteria of 10 July 1967, to require much more complex scenarios to which many early Nuclear Power Plants, be considered in selecting EDG systems. meaning those starting construction prior to 1972, were licensed. Supporting regulatory Independence is the absence of shared guides, codes, and industry standards used components that could result in the to implement these federal regulations are simultaneous failure of both units. That briefly described in this Chapter. includes physical and electrical separation, such that a transformer or cable tray fire, One of the most important nuclear power for example, would not impact operation of plant safety requirements is for redundant, the other emergency diesel generator. and independent, power systems. This is contained in 10 CFR 50 Appendix A, Redundancy is required to achieve the General Design Criterion (GDC) 17, which desired operational reliability, and also to specifically requires both off-site and onsite accommodate "down time" for testing and power systems "to permit functioning of maintenance. This means a dual EDG structures, systems, and components installation, where the required power is important to safety." available with either unit out of service. GDC 17 further states that onsite electric Testability is somewhat self-explanatory. power supplies, including the distribution EDG testing is discussed in a later Chapter. system, shall have sufficient independence, redundancy, and testability to perform their 1.2 Regulatory Guide (RG) 1.9 sets the safety functions assuming a single failure. Three Fundamental Performance This requires emergency power systems to Requirements for EDG's be designed such that failure of one will not adversely impact the other (more about that Regulatory Guide 1.9, Rev 4 (March 2007) later). Each electric power source must be is "Application and Testing of Safety- capable of providing the capacity and Related Diesel Generators in Nuclear capability to assure that: Power Plants" (new title for this edition) and evolved from AEC Safety Guide 9. A copy 1. "Fuel design limits and design conditions of RG 1.9, Rev 4 is included in this Manual of the reactor coolant pressure boundary as Appendix 2. It establishes the three are not exceeded" for any anticipated fundamental performance requirements every occurrences, and EDG must meet to perform its design function. They are as follows: 2. The "core is cooled and containment integrity and other vital functions are 1. The unit must be able to "...start and maintained in the event of postulated accelerate a number of large motor accidents." loads in rapid succession while main- taining voltage and frequency within A Loss of Offsite Power (LOOP) event is acceptable limits...." Rev 1/11 1-2 of 12 USNRC HRTD Emergency Diesel Generator Diesel Generators as Emergency Power Sources 2. The unit must also be able to "...provide power source to be “ready to accept loads” power promptly to engineered safety following loss of offsite power. So, two features if a loss of offsite power and an fundamental questions asked in the accident occur during the same time selection of the emergency onsite power period...." source are: 3. The unit must "...supply power 1. Based on the plant-specific accident continuously to the equipment needed analysis, how fast must electrical power to maintain the plant in a safe be restored to support Emergency Core condition...." Cooling System (ECCS) operation to prevent core damage (i.e., to keep from A later chapter will cover the challenge of exceeding peak fuel clad temperature)? complying with the first of these three Stated another way, how soon must the performance requirements (EDG loading). emergency electrical source be running Item (2) immediately above introduces the with breaker closed, ready to accept the requirement to cope with two events required step loads? By analysis, many (failures / accidents) that occur either of the reactor designs required power to simultaneously or with one following the be available within 15 to 30 seconds other. Finally, please note Item (3) has no after receiving a start signal. Although endurance time. "Continuously" is open- this time included the inherent delay for ended and effectively means "until normal protective circuits to sense the loss of power is restored" (however long it takes). power and initiate a start signal to the emergency power source, the bulk of it 1.3 Why DIESEL Generators? represented the start-up time necessary to spin up the generator, energize its There is no requirement that emergency field, and get it switched on line. diesel generators must be used for onsite electric power supplies at nuclear power To assure an acceptable margin of safety, plants. Hence, a natural question could be, the accident analysis for many reactor why have diesel generators been selected designs assumed emergency generator as the predominant means of supplying this power would be available within 10 power? Some other potential sources that seconds. NOTE: In some plants where could have been selected are: nuclear fuel upgrades have been implemented and new core damage • Gas Engine Generators accident analysis calculations performed, • Gas Turbine Generators they have frequently supported a • Steam Turbine generators corresponding increase in the delay time • Hydro Generators for emergency generator availability. However, it was not unusual to have the The answer to “why diesel generators” can containment integrity support systems (e.g. usually be found by looking at the time Containment Spray System) become the requirement set for the onsite emergency new limiting consideration. Rev 1/11 1-3 of 12 USNRC HRTD Emergency Diesel Generator Diesel Generators as Emergency Power Sources 2. Once the accident analysis has set the Criterion 38: Reliability and Testability time limitations on restoration of power of Engineered Safety Features. All to vital safety equipment, the only engineered safety features shall be remaining question is what power designed to provide high functional supplies are readily available that could reliability and ready testability. In reliably supply the needed power within determining the suitability of a facility for those time limitations? proposed site, the degree of reliance upon the acceptance of the inherent and With the exception of large gas engines, engineered safety afforded by the system, which have other onsite energy availability including the engineered safety features, and safety concerns, none of the sources will be influenced by the known and the listed above could reliably match the demonstrated performance