Patent Number: 046577270
Section: summary

BACKGROUND OF THE INVENTION In response to the accident at Three Mile Island Unit 2, the federal government has required the development of symptom-related emergency operating procedures by utilities which operate nuclear power plants. The main objective of such procedures is to ensure that proper mitigating actions are implemented by licensed operators in the event of an accident. In addition, the criteria for implementing such mitigating actions identified in the emergency operating procedures must be based solely upon indications or symptoms observable by licensed operators using instrumentation within the control room of a nuclear station. The federal government has required the development and installation of instrumentation which can be used by operators in the control room to identify symptoms which are indicative of transient or accident conditions. Prior to the accident at Three Mile Island Unit 2, the typical response by an operator in a control room during a transient was to attempt to identify the event in progress and then to locate and implement a specific event mitigating procedure associated with the identified event. Since the accident at Three Mile Island Unit 2, the federal government has required utilities to develop emergency operating procedures which can by used by operators to mitigate emergency events independent of identification of their causes. The development of these symptom-related emergency operating procedures was intended to remove the possible errors an operator might make in trying to identify an event prior to attempting to mitigate it. Errors in event identification are possible by trained and licensed operators due to the vast numbers and combinations of possible indications which could appear on control room instrumentation due to different types of transients and accidents. The federal government and nuclear industry have acknowledged that the accident at Three Mile Island Unit 2 could have been mitigated had the operators utilized built-in plant features and responded to indications which identified a need for mitigating actions, rather than attempting to identify the event and to implement the proper mitigating procedures for the named event. The means to classify emergency events so as to properly activate state and local governments has not in general improved in the same manner as emergency operating procedures since the accident at Three Mile Island Unit 2. Current federal regulations regarding event classification specify that four levels of emergency events be established and referenced in the emergency response plans for nuclear generating stations. Current federal guidance documents present general emergency class descriptions and detailed lists of example initiating conditions which were considered to be representative of each specific emergency class. As a result of following this guidance utilities have typically developed means to classify emergency events within nuclear power stations by requiring the licensed operator to identify the event in progress and to correlate the identified event to the listing associated with each emergency class presented in the federal guidance. This approach to emergency classification is currently employed by the majority of operating nuclear power stations and is consistent with federal requirements and guidance. This method of event classification requires the correct identification of the emergency event to ensure proper classification and to facilitate appropriate responses by state and local governments to ensure public protection. The result of this approach has been to introduce or maintain the existence of the same source of error, i.e., failure to properly identify the event, which was most significant in the accident at Three Mile Island Unit 2. SUMMARY OF INVENTION Emergency classification must be independent of the event cause and subsequent development so as to minimize the effect of potential errors associated with nuclear plant operators' responses to perceived causes of events which may not be correct and consistent with protecting the general public. Emergency event classification methods and procedures must be consistent with emergency operating procedures. The present invention provides the means to classify emergency events which is independent of event identification, which confirms the area of hazard and auguments emergency operating procedures developed by the utilities operating nuclear generating stations subsequent to the accident at Three Mile Island Unit 2, and constitutes a major improvement in public safety. In addition, the present invention utilizes a computer to accomplish the comparison of symptoms of an emergency event to a logic matrix which corresponds to the various classifications for emergency events, and thus greatly assists operators in event classification. The present invention classifies actual and potential emergency conditions at commercial nuclear generating stations. This emergency event classification process is directly related to three barriers to fission product release to the environment. Since 1970, the design of nuclear power plants has been governed by the General Design Criteria specified in the Code of Federal Regulations, Title 10, Part 50, Appendix A. In particular, Criteria Nos. 10, 13, 14, and 16 identify three fission product barriers which are intented to prevent the uncontrolled release of radioactive material to the environment. The three fission product barriers are the reactor core (intergrity of the fuel cladding), the reactor coolant system pressure boundary, and the containment. The functional integrity of any of the three boundaries is sufficient to prevent the uncontrolled release of radioactive material to the environment. The loss of function any one barrier constitutes a significant reduction in the level of safety of a nuclear generating station. The implementation of the process requires rigorous analysis to determine appropriate values which characterize the functional integrity of the fission product barriers. This process establishes functional performance criteria for each fission product barrier such that the symptoms associated with degradation of any single barrier, or with degradation of several barriers or combinations of barriers are unambiguous, unique and identifiable. A computer is programmed to use the indications of barrier degradation to identify the magnitude of the hazard to the general public which exists when these indications are present. Using the computer, the nuclear power plant operator properly classifies emergencies at nuclear stations and automatically advises offsite authorities of the hazard associated with plant emergency conditions. Definitions regarding the operability of each fission product barrier are included in the license to operate each nuclear generating station. However, the definitions associated with conditions stated in operating licenses are appropriate only during plant operation and cannot be directly utilized to define emergency event classes. The process of implementation of the fission product barrier approach to emergency event classification is the determination of and use of functional indications in a nuclear generating station's control room of fission product barrier integrity. The process is a unique concept which is applicable to all power reactors which are designed in accordance with the Code of Federal Regulations, Title 10, Part 50, Appendix A, General Design Criteria. The process directly relates the loss of fission product barrier function to a specific emergency event classification. Events which are off-normal and which do not represent a loss of function of any of the three fission product barriers, but are of sufficient interest to warrant activation of a nuclear plant''s emergency plan, are classified as a "Unusual Events". Events resulting in the loss of function of one fission product barrier are classified as "Alerts". Events resulting in the loss of function of two fission product barriers are classified as "Site Area Emergencies". Events resulting in the loss of all three fission product barriers are classified as "General Emergencies". The criteria stated within a nuclear generating station's emergency plan for fission product barrier function are properly determined using the present invention to provide margin between limiting conditions for operation associated with characteristics of fission product barrier function (the normal upper limit of certain critical parameters monitored during normal plant operation) and minimum criteria for emergency plan activation and event classification. Such a margin precludes activation of the emergency plan due to simply exceeding a limiting condition for operation associated with a fission product barrier. Additionally, proper selection of the functional criteria for barrier function according to the present invention allows the licensed operator to recognize in a nuclear plant's control room the display of unique symptoms associated with the breach of individual fission product barriers, and all the possible combinations of breaches of fission product barriers. The operator can then readily identify the functional status of all fission product barriers employing the computer, whose output is verified by indications available within each plant's control room. The main objective of emergency preparedness efforts for commercial nuclear generating stations is to limit the radiation dose which might be received by the general public in the event of an accident having offsite radiological consequences. Federal emergency preparedness regulations require classification of any emergency condition according to a graded system commensurate with the hazard presented to the public. Radiation dose to the general public located in the vicinity of a nuclear generating station can be characterized approximately in the following relationship: EQU DOSE.alpha.f(.phi.,t.sub.o,u/p,t.sub.d).times.f(w).times.f(s).times.f(L.sub .F).times.f(L.sub.RCS).times.f(L.sub.C) where, during an event, relatively fixed functions are: f(.phi.,t.sub.o,u/p,t.sub.d) is the source term, a value related to the amount of radioactive material which exists in the fuel, the form of that material and the volatility of the material. The source term is related to reactor power, (.phi.); to the time of operation, (t.sub.o); to fuel characteristics, (u/p); and time after shutdown, (t.sub.d); among other factors; PA1 f(w) is a function related to meteorological conditions at the time of release of radioactive material; PA1 f(s) is a function of site demographic characteristics; PA1 f(L.sub.F) is a function of the leak rate of the reactor fuel cladding; PA1 f(L.sub.RCS) is a function of the leak rate of the reactor coolant system; PA1 f(L.sub.C) is a function of the leak rate of containment systems. And where relatively variable functions are: The above relationship is a qualitative relationship which illustrates how the barrier functions relate to offsite dose calculations, and is not intended to present a strict quantitative relationship. The reactor fuel is contained within the fuel cladding within the reactor coolant system, which is itself contained within the containment system. The fuel cladding (essentially a metallic shell which encapsulates the fuel), the reactor coolant system and the containment system constitute three barriers to release of radioactive material from the reactor fuel to the environment. The complete functioning of any one of these three barriers presents a sufficient obstacle to prevent the release of radioactive materials to the environment. If the numerical value of any one of the three functions f(L.sub.F), f(L.sub.RCS), or f(L.sub.C), presented above is equal to zero or is very nearly zero, the offsite radiation dose to the general public is also very nearly zero or equal to zero since all of the other functions have finite values. The values of f(.phi., t.sub.o,u/p,t.sub.d) and f(s) are determined by considerations unrelated to any nuclear power plant transient or accident and can be considered constants. f(w) may be determined by weather characteristics; it cannot be changed by any action by nuclear station operators and can be considered a constant for purposes of implementation of the present system. The only terms which are influenced by specific transients or emergency events are f(L.sub.F), f(L.sub.RCS), and f(L.sub.C). To present a signficant hazard to the public an accident must significantly influence the leak rate, i.e., the means by which and rate at which radioactive materials are being transported across any of the three barriers. The operators of nuclear power stations are required to determine the magnitude of hazard to the health and safety of the public which exists during any emergency situation and are required to classify the hazard into four categories, progressing from the less serious to the most serious hazard to the public. While it would seem that the quantification of f(L.sub.F), f(L.sub.RCS), and f(L.sub.C) would significantly aid in the determination of the hazard level, the representation or calculation of each of these functions in an absolute manner is extraordinarily difficult due to the complexity of the processes and the number of parameters which relate to status of each barrier. It is possible, however, to empirically assess whether each barrier exists (is functional) and is adequate to provide the degree of protection sought. To accomplish this assessment it is necessary to develop a functional definition of each fission product barrier and then, during any event, to determine if the function is being maintained. This determination of functional requirements for the fission product barriers constitutes the first part of the process of implementation of the fission product barrier emergency event classification and response system. The steps taken, in real time, by the nuclear power plant operator and the computer to utilize nuclear plant instrumentation to identify the functional status of fission product barriers so as to classify an event is the second part of the subject process. Considerations associated with quantification of the function of the fission product barriers, f(L.sub.F), f(L.sub.RCS), and f(L.sub.C), are now discussed. f(L.sub.F) Function of the Reactor Fuel as a Fission Product Barrier The fuel of a light water nuclear power reactor is comprised of ceramic pellets which are enclosed in metal tubes, referred to as fuel cladding. The length of these tubes is typically between 10 and 14 feet, with a length of 12 feet being most common. Each tube is a pressure vessel and as such is a leak-tight enclosure. Operational limitations are placed upon nuclear power plants so as to prevent the degradation of the integrity of the fuel cladding. During operation of the reactor the fuel material is transformed by the fission process to yield a distribution of various elements (fission products) some of which are gaseous. A portion of the gaseous fission products migrates to the void between the fuel matrix and the cladding tube and mixes with inert gases located in that void area or gap. Individual fuel rods are verified to be leak tight upon initial fabrication. During operation within a reactor, mechanical wear, internal pressure generation, metal fatigue and creep due to pressure variations result in minor degradation of the fuel cladding. During accident conditions, changes in the environment around the fuel due to changes in coolant flow rate, temperature and pressure may result in significant stresses upon the cladding material such that rupture, local melting, and chemical interactions with other materials can occur which may result in the loss of integrity of individual tubes containing fuel. The loss of that integrity results in the transport of radioactive material from within the tube to the area outside of the tube, i.e., to the reactor coolant system in which all fuel is contained. Typical power reactors contain tens of thousands of individual fuel tubes assembled in bundles called fuel elements. Due to the number of such fuel tubes it is not possible to monitor each tube for indication of pressure or any other physical parameter so as to determine the integrity of individual tubes. Integrity is inferred by the determination of the concentration of radioactive materials within the reactor coolant system using plant instrumentation. Since the only source of fission products is from the fuel, the detection of fission products within the reactor coolant system indicates the existence of degradation of one or more fuel tubes. Low levels of degradation have been considered in the design of nuclear power plants and limitations regarding the maximum allowable reactor coolant system radioactive materials concentrations have been established for each reactor so as to assure adequate functioning of the fuel cladding as a barrier to significant radioactive material transport and possible release of radioactive material to the environment during normal operation. The establishment of a numerical value for the concentation of radioactive material in the reactor coolant system in effect establishes criteria for consideration that the integrity of the fuel cladding as a barrier to fission product release is adequate and therefore that the barrier is functional. The invention establishes the criteria for selecting a particular radioactive material concentration as an indication that the fuel barrier is functional for the purpose of emergency event classification. The amount of radioactive material located within the fuel cladding is overwhelmingly the largest source of radioactive material within a nuclear power plant. The gaseous fission products located within the gap between the fuel pellets and the cladding inner surface amount to millions of curies of radioactive material. The allowable concentration of radioactive material present within the reactor coolant may amount to thousands of curies with normal concentrations being typically a few hundred curies or less. The process establishes the functional definition of the fuel as a fission product barrier by examining in detail many characteristics of a specific nuclear power plant related to plant systems, equipment performance characteristics, equipment locations, operating procedures, instrumentation displays within the control room, accident analysis results, and normal plant operating limitations. f(L.sub.RCS) Function of the Reactor Coolant System as a Fission Product Barrier The reactor coolant system of a nuclear power plant operates at high temperature and pressure so as to provide thermal energy in the form of heated steam to a turbine generator for the purpose of generating electricity. The reactor coolant system contains the reactor coolant water which has an average temperature in the range of 550 degrees Fahrenheit. All reactor coolant systems include instrumentation to inform the operator in the control room that adequate coolant inventory exists within the system. Minor leakage from the reactor coolant system occurs due primarily to the large number of mechanical seals on the system and the high differential pressure across those seals. The normal reactor coolant inventory is maintained by pumping systems which provide additional coolant to the system as necessary to maintain proper inventory. In the event of significant degradation of the reactor coolant system pressure boundary, additional pumping systems may be required to operate to maintain an adequate coolant inventory within the reactor coolant system. The reactor coolant system is located within a pressure vessel referred to as the containment. The environmental conditions and concentration of radioactive materials within the containment are monitored by the reactor operator. Degradation of the reactor coolant system pressure boundary results in the transport of reactor coolant to areas within the plant which would not normally experience the existence of radioactive materials in the concentration normally present within the reactor coolant system. In addition, since reactor coolant is at a high temperature, the transport of a significant amount of reactor coolant across the reactor coolant system pressure boundary results in significant energy transfer as well as the transfer of mass. Since the transfer of energy and mass will be to an enclosed system, the environmental conditions, radioactive material concentrations and mass of liquid located within the enclosed recipient system will reflect the transport of reactor coolant. The function of the reactor coolant system as a fission product barrier is manifested by the absence of the transport of radioactive materials, energy and mass into enclosed systems of a nuclear power plant which are designed to contain significant leakage or rupture of reactor coolant. In addition the ability to maintain reactor coolant inventory employing normal operating systems is also indicative of the integrity of the reactor coolant system and its ability to function as a fission product barrier. The specific characteristics of a breach of the reactor coolant system fission product barrier coupled with the operating characteristics of the reactor coolant system determine the specific combination of indications which will be manifested. Regardless of the type of failure or type of reactor considered, the invention establishes a characteristic description of symptoms exhibited by a loss of function of the fission product barrier associated with the reactor coolant system. f(L.sub.C) Function of the Containment as a Fission Product Barrier The containment of a nuclear power plant is a large pressure vessel designed to contain the energy and mass release resulting from a major rupture of the reactor coolant system, among other considerations. The environmental conditions within the containment provide information to the nuclear plant operator relative to the function of the reactor coolant system as a fission product barrier. Similarly the environmental conditions in the buildings which surround or are contiguous with the containment provide information regarding the integrity of the containment, and thus of its ability to function as a fission product barrier. Such buildings are equipped with monitoring equipment which functions continuously to inform the operator of the concentration of radioactive materials within those areas. The radioactive material concentration within plant buildings is low in comparison with the radioactive material concentration in containment. The identification of the presence of significant radioactive material concentrations beyond the containment fission product barrier is indicative of a degraded barrier. The transport of radioactive material to areas beyond the containment boundary constitutes the lack of containment and lack of functioning of the containment as a fission product barrier. Federal requirements for nuclear power stations specify enclosure of the reactor coolant system totally within the containment. The process compares concentrations of radioactive material within containment during an emergency event and the concentration of radioactive material being transported to the environment to define the function of containment as a fission product barrier.