Normal Shutdown Transient

The results of over 1 year of continuous, on-line acoustic emission (AE) structural surveillance of high temperature / high pressure steam headers, gained on 2 M-scale 600MW supercritical multifuel ENEL power units in normal operation, are presented in the paper. The influence of background noise, the correlation between plant operating conditions (steady load, load variations, startup / shutdown transients) and AE activity and the diagnostic evaluation of recorded AE events are also discussed. 

The SSC-K code was used for assessment of the inherent safety features in the KALIMER conceptual design. The SSC-K aims not only at extensive analysis capability and flexibility, but also at sufficiently fast running to simulate long transients in a reasonable amount of computer time. The code is capable of handling a wide range of transients, including normal operational transients, shutdown heat removal transients, and hypothetical ATWS events. The SSC-K code is currently used as the main tool for system transient analysis in the KALIMER development. 

A variable speed positive displacement pump can minimize crystal attrition. The speed range must satisfy the flow requirements during start up and transients. Below a critical speed, crystals will settle out of the slurry. The minimum pump operating speed should be clamped above this rate for normal operation, and a line flush interlocked to stopping the pump for emergency shutdown. 

When the control rods are inserted in variants A, B and C, the core temperatures drop in the following minutes from normal operating temperatures to near uniform temperatures, nearly equal to the core Inlet temperature. During this transient period, the stress fields In the core components go from operating stresses to shutdown stresses. 

DBE-2 is a Heat Transport System (HTS) transient without control rod trip, requiring reactor shutdown with the reserve shutdown control equipment (RSCE), and core cooling with the Shutdown Cooling System (SCS). During the first 5 minutes of the event the core temperatures rise by a maximum of about 80 C (150°F) above the normal operating levels. After this initial phase, the core gradually cools down. 

Throughout this transient, the temperature gradients will be, for the most part, between those that exist during normal operation and at shutdown. 

The engineer has more than 40 hours of practical training with simnlators, to acquaint him with the running of the plant nnder normal operation, startup, shutdown, etc. The training covers safety systems, transients and abnormal sitnations, and shows the interrelationship between the parameters and operational conditions which will be experienced. These are demonstrated on the simulator, which is able to display the relevant parameters of the plant. In many cases, the simulator is a replica of the nuclear plant control room. 

B. The flywheels will be designed to withstand normal operating conditions, anticipated transients, and the largest mechanistic pipe break size remaining after application of leak before break as described in Section 3.6, combined with the Safe Shutdown Earthquake. 

The SSWS cools the Component Cooling Water System (CCWS) through the Component Cooling Water Heat Exchangers and rejects the heat to the ultimate heat sink during normal, transient, and accident conditions. The CCWS in turn provides cooling water to those safety-related components necessary to achieve a safe reactor shutdown, as well as to various non-safety reactor auxiliary components. 

Emergency core decay heat removal— Provide core decay heat removal during transients, accidents, or whenever the normal heat removal paths are lost. This heat removal function is available at RCS conditions including shutdowns. During refueling operations, when the IRWST is drained into the refueling cavity, other passive means of core decay heat removal are utilized. 

To maintain the specified degree of snbcritioality for an indefinite period of time after shutdown, additional means as provided in the design may be used, such as the use of boronated water or other poisons if the temperature, xenon concentration or other transient reactivity effects cannot be compensated for by normal reactivity control devices. 

In some countries, pilot operated relief valves, (PORVs) and block valves were originally designed as non-safety components intended for operation under normal conditions. The PORVs were provided for pressure control of the RCS during normal operation and transients. The block valves were installed because of expected leakage from the PORVs. The valves were not required for safe shutdown or to mitigate the consequences of accidents. 

The results of the analysis demonstrate that the peak reactor coolant system pressure reached during the transient is less than that which causes stresses to exceed the faulted condition stress limits of the ASME Code, Section III. Also, the peak cladding surface temperature is considerably less than 1482°C. These results represent the most limiting conditions with respect to the locked rotor event or the pump shaft break. With the reactor tripped, a stable plant condition is eventually attained. Normal plant shutdown may then proceed. 

Thermal effects and loads during normal operating or shutdown conditions, based on the most critical transient or steady-state condition... 

On three occasions in Summer 1989, the reactor was stopped by automatic emergency shutdown, the negative reactivity threshold (-10 pcm) being exceeded. This reactivity variation was very fast first a minimum after 50 ms followed by an increasing oscillation, and then a decrease, caused by the control rod drop, 200 ms after the start of the transient. The first two events were thought to be spurious (a neutronic chamber fault) and the reactor was restarted. The normal plant instrumentation did not allow proper recording of the transient so following the second trip special instrumentation was instiled. After the third trip, the reactor was shut down in order to identify the cause of the events. 

A period of up to three days must elapse before the reactivity returns to the value it had before the shutdown. If it is a requirement that it be possible to start the reactor up again at any time during this period, a high percentage of excess reactivity has to be built into it in order to overcome the xenon transient. This is known as xenon override capacity. In normal operation this built-in reactivity excess has to be held down by a large equivalent negative reactivity in the form of control rods or some other mechanism. 

Shutdown systems have a separate acceptance criterion. Modem PHWRs have two independent, redundant and diverse shutdown systems with separate logic and reactivity devices from the control system and from each other [5]. Each system, on its own, must be capable of shutting the reactor down after any accident, independently of the mitigation provided by the normal reactivity control devices. In general, two diverse trip parameters are required on each shutdown system for each accident over the range of operating conditions (unless it is impracticable or detrimental to safety to provide dual parameter coverage). As a result, it is not required to perform analysis of either transients or accidents without shutdown [6],... 

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