Earthquake safety systems

Some of the stringent requirements on the storage pools are eff < 0.95 (even if unused fuel elements are introduced), earthquake safety, no possible water loss, water level automatically kept constant, adequate leakage and radiation monitoring systems, water tenqjerature < 65°C, acceptably low radiation level in working areas, etc. 

In the context of NPPs, the differences between Safety systems and Industrial systems concern, for safety systems, the guaranty of operation in all circumstances, even in case of failure or after an accident like a limited fire or an earthquake. 

Seismic action can cause serious accidents to industrial plants as shown in several occasions. The actual worldwide situation of major-hazard plants against earthquakes should be considered as critical. For instance, in Italy about 30% of industrial plants with major-accident hazards are located in areas with a high seismic risk. In addition, in case of a seismic event, the earthquake can induce the simultaneous damage of different apparatus, whose effects can be amplified because of the failure of safety systems or the simultaneous generation of multiple accidental chains. 

Such blockage might occur rapidly under emergency conditions such as an earthquake. Therefore, a second safety system has been provided for the Hanford Production Reactors. In order to minimize the possibility of the control medium not being able to enter the reactor, this system depends only on gravity for insertion and is fluid in nature. 

The criteria applied in the design of the Reactor Coolant System supports are that the specific function of the supported equipment be achieved during all normal, earthquake, safety valve actuation and Branch Line Pipe Break (BLPB) conditions. (BLPB includes feedwater line breaks and all loss-of-coolant-accident conditions resulting from breaks not eliminated by leak-before-break analysis in piping to branch nozzles of the reactor coolant system.) Specifically, the supports are designed to support and restrain the Reactor Coolant System components under the combined Safe Shutdown Earthquake and Branch Line Pipe Break loadings in accordance with the stress and deflection limits of Section III, ASME Code. 

The set of PlEs identified should include all the events which conld arise from outside the plant which could challenge nuclear safety, including naturally occurring and human induced events. These external initiating events could lead to an internal initiating event and failure of some of the safety system equipment that would be needed to provide protection from the event. Eor example, an earthquake could lead to plant equipment failures in addition to the loss of off-site power. 

The equipment and structures of the safety systems are designed for an earthquake of 8 points on the MSK-64 scale. 

All safety systems in the VVER-440 concept are divided into three, or in certain sections four, separate and independent redundant subsystems, each of which meets the requirements caused by loss-of-coolant accidents or other disturbances. The various circuits of the redundancy subsystems are located in physically separate areas and supplied with electrical power from separate diesel—backed sources. This separation principle provides reliable protection against external and internal influences, such as earthquakes, airplane crashes, explosions, flooding and fire, etc. 

Credible accidents exclude acts-of-war, sabotage, and large falling objects earthquakes which are severe enough to prohibit entry of poison from both safety systems are also considered Incredible. 

Each safety system must back up the other system for as many failure and accident conditions as possible There may be certain accidents such as a severe earthquake. In which one system Is disabled, rendering It useless In either a primary or backup capacity, therefore, the two systems shall be different In design. If possible, to reduce the probability of both systems being disabled by one accident. 

The HVAC system for the control room area is designed as seismic category I and will result in loss of system functional performance capability in the event of loss of offsite power concurrent with safety shutdown earthquake. The system ductwork is leak tested in accordance with ASME N509. 

An independent poison ball safety system is provided to assure that the reactor is driven and maintained subcritical in the event of failure of part or all of the Horisontal Hod System due, for example, to an earthquake, or in the event of actuation of the emergency raw-water cooling system which would result from primary loop depressurisation. Upon receipt of a trip signal, the balls are released to fall by gravity into vertical channels in the active sone of the reactor Excess hopper capacity is provided to assure that the balls fill the channels to a level above the top of the active zone. 

Earthquakes not only may impact the reliability of safety systems but also influence operator performance ... 

The layout of safety systems should be such that the minimum required capability is maintained in the event of a failure in one train of protection or in the event of needing to survive any internal and/or external hazards (e.g. earthquake, fire and flooding). 

Our present discussions relate only to the laboratory testing of safety-related secondary systems, as are employed in critical areas such as areas of emergency power supply and reactor power control supply etc. of a nuclear power plant (NPP) according to IEEE 344 and lEC 60980. There are other codes also but IEEE 344 is referred to more commonly. Basically, all such codes are meant for an NPP but they can be applied to other critical applications or installations that are prone to earthquakes. 

The first step-in plant-system and accident-sequence analysis is the identification of earthquake-induced initiating events. This is done by reviewing the internal analysis initiating events to identify initiating events relevant to seismic risk. For example. Table 5,1 -5 shows the initiating events that were used in the Seismic Safety Margins Research Program for a PWR plant (Smith et al., 1981)... 

Using the method described, all the "safety-related" valves in the system will be qualified for operability during a seismic event, except those that need not function during the earthquake. These methods conseirvatively simulate the seismic event and will ensure that the active valves perform their "safety-related" function when necessary. 

A general agreement exists that classification of systems, structures and components of a plant from the point of view of safety and from the point of view of resistance to external actions (earthquake, and so on) is necessary to make decisions on the following ... 

These building structures are passive safety features and therefore perform passive safety functions. The only SSC whose mis-operation can directly affect the safety function of these structures is the shield door hydraulic system [a non-safety-reiated (NSR) system], which is used to lower and raise the massive shield doors connecting Rooms 108 and 109, and Room 109 and the Zone 2A canyon. Mis-opeiation of this system with the Room 109/Zone 2A shield door down and radioactive waste in Room 109, can present a radiation hazard to workers at the north end of Room 112. The Zone 2A canyon structures, including the associated shielding windows, and the Room 109 structures are the only HCF SSCs that are required to maintain their safety functions following an earthquake. 

Table 4.4-4 provides performance criteria needed to demonstrate that the functional requirements for the Zone 1 and Zone 2A ventilation exhaust systems are met No operational events can affect the ability of the HEPA and charcoal filters to perform their safety functions, since these functions ahe only required when the HCF ventilation system is operating. Furthermore, failure of filter bank inlet or outlet dampers to remain open during ventilation system operation will essentially stop the flow of Zone 1/Zone 2A exhaust air to the HCF stack. The only events that could affect the ability of the charcoal filters to perform their safety function are a fire in the MER or an external event such as an earthquake or aircraft crash that would destroy the MER. The only events that could affect the ability of the HEPA filters to perform their safety function are similar events. 

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