Shutdown leveling

Fig. 4. Hypothetical leakage curves to show effect of ionic concentration and flow rate, where the dashed line corresponds to the leakage shutdown level...

At background or shutdown levels the Lov Level Nemtron Flux Monitor System (Sub-Crltlcal Monitor) is used This system is eoi prlsed of two separate and identical channels > each composed of a chamber mounted on a screw drive mechanism and an amplifier-counting circuit indicating period and level ... 

A shutdown level-1 PSA has almost completed. The reliability of the decay heat removal system (DHRS) during the plant shutdown (refueling and maintenance) was evaluated. There are several system trains out of service because of the scheduled maintenance so that it reduces redundancy of the safety functions. We considered several candidates of accident management (AM) measures as an operator recovery action under accident condition. As a result of this study, it concluded that these AM candidates were effective for the DHRS to maintain high reliability in cooperation with a long grace period. 

Answer Prom shutdown level to full power level, there are many factors of ten difference in magnitude, yet it is important that the level of neutron flux be known at all times. Since no instrument can give meaningful indications over such a wide range directly, special adaptations are used such as shunts on the electrical circuitry, movement of the chamber into or out of the hig i flux region, and several overlapping instruments to cover the required rcuige. 

All of the promethium in the core decays to samarium and, since samarium is stable, the samarium builds up to a shutdown level and stays at that level. The level of samarium buildup depends on operating power because promethium is power dependent. 

HIPS is considered at just above the emergency shutdown level on the Layers of Protection Analysis (see Figures 10.2 and 10.3). 

Enhanced prevention A complementary medium LOD should be introduced (as far as practicable) within the residual risk category to interrupt sequences which would have the potential to cause serious core damage. This is coherent with the criteria and requirements related to off-site releases. For instance, such an LOD may be obttuned by means of a third shutdown level consisting of passive and active measures capable of bringing the reactor to a safe condition in case of postulated failure of the two basic shutdown systems (which comply with the (2a + b) LODs requirements). 

Reactor shutdown is assured by two independent and diverse basic shutdown systems which guarantee very high reliability so as to relegate shutdown failure into the domain of residual risk. In the domain of residual risk, however, the "Third Shutdown Level" becomes effective. It consists of a bundle of additional engineered safety features which are incorporated in the design as a result of extensive risk-minimization studies. The system consists of active and passive subsystems and is supported by beneficial natural core behaviour. For example, the following features of the absorber rod actuators are part of the "Third Shutdown Level" ... 

The "Third Shutdown Level" is capable of maintaining core integrity in the case of a postulated failure of the two basic shutdown systems. 

The overall goals of the planned update of the section on low power and shutdown Level 1 PSA in (FAK 2005a) are to provide more precise terms and definitions with particular reference to the different phases of low power and shutdown states in German nuclear power plants (with Boiling Water Reactor (BWR) or Pressurized Water Reactor (PWR)) as well as an adaptation and clarification of the examples for the analyses provided in the document. 

Elevation of low interface level shutdown (LILSD) is normally set at 150 mm from the bottom of the vessel however, it is always advisable to check the liquid residence time for LILSD. A too short liquid residence time will increase the chances of a gas breakthrough from the separator. Generally, 1 to 3 min residence time is adequate for the shutdown level. The same principle can be used to set the low liquid level shutdown (LLLSD). This setting also depends on tire type of separator being used. For the flooded weir separator, this level can be set either above or below the weir level, depending on the residence time requirement. 

High level shutdown 1900 1200 Water shutdown level should not be set above the weir height. 

In this accident, the steam was isolated from the reactor containing the unfinished batch and the agitator was switched ofiF. The steam used to heat the reactor was the exhaust from a steam turbine at 190 C but which rose to about 300°C when the plant was shutdown. The reactor walls below the liquid level fell to the same temperature as the liquid, around 160°C. The reactor walls above the liquid level remained hotter because of the high-temperature steam at shutdown (but now isolated). Heat then passed by conduction and radiation from the walls to the top layer of the stagnant liquid, which became hot enough for a runaway reaction to start (see Fig. 9.3). Once started in the upper layer, the reaction then propagated throughout the reactor. If the steam had been cooler, say, 180 C, the runaway could not have occurred. ... 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Fig. 5. Radioactivity after shutdown per watt of thermal power for A, a Hquid-metal fast breeder reactor, and for a D—T fusion reactor made of various stmctural materials B, HT-9 ferritic steel C, V-15Cr-5Ti vanadium—chromium—titanium alloy and D, siUcon carbide, SiC, showing the million-fold advantage of SiC over steel a day after shutdown. The radioactivity level after shutdown is also given for E, a SiC fusion reactor using the neutron reduced...

There are three basic requirements that Hquid level control devices are designed to satisfy alarm functions, pump/valve control, and transmitted output signal to track level continuously. Alarm devices provide warning or shutdown functions when process levels pass a predeterrnined point in the vessel pump /valve control devices turn on/off pumps or open/close valves at predeterrnined levels in the vessel and transmitters provide a proportional output signal over a predetermined span to send to a local meter or signal back to a control room. 

Table 2. Corrosion Products Contributing to PWR Shutdown Radiation Levels ...

Whereas addition of hydrogen to feedwater helps solve the O2 or ECP problem, other complications develop. An increase in shutdown radiation levels and up to a fivefold increase in operating steam plant radiation levels result from the increased volatiUty of the short-Hved radioactive product nitrogen-16, N, (7.1 s half-life) formed from the coolant passing through the core. Without H2 addition, the in the fluid leaving the reactor core is in the form of nitric acid, HNO with H2 addition, the forms ammonia, NH, which is more volatile than HNO, and thus is carried over with the steam going to the turbine. 

Water chemistry is important to the safe and reflable operation of a nuclear power plant. Improper conditions can lead to equipment and material failures which ia turn can lead to lengthy unscheduled shutdown periods for maintenance (qv) and repair operations. Water chemistry can also have an impact on the radiation levels duriag both power operations and shutdown periods. These affect the abiUty of personnel to perform plant functions. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

A typical break-even chart is used with production models to predict optimum production levels, break-even points, and shutdown conditions under various scenarios. These models tend to involve a reasonable amount of approximation. For example, sales revenue as a function of production level involves numerous variables and relationships that are not always weU known. Such charts, however, provide useful guides for production operations. 

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

Various protective instruments are used to provide a shutdown signal (to a fast-acting trip valve at the expander inlet) that senses various things, such as overspeed, lubricant pressure, bearing temperature, lubricant temperature, shaft runout, icing, lubricant level, thrustbearing load, and process variables such as sensitive temperatures, levels, pressures, etc. However, too many safety shutdown devices may lead to excessive nuisance shutdowns. 

Implement operating instructions to maintain liquid level above heating surface at all times Install automatic level control with low level alarm and shutdown of liquid withdrawal system to ensure liquid is above heating surface at all times... 

Whenever a motor is installed in a humid atmosphere and is switched on after a long shutdown, insulation resistance must be checked before energizing the motor. As a precaution, insulation resistance must be checked before a restart after a long shutdown, even in temperate conditions. If the insulation level is found to be below the recommended level as shown in equation (9.1) it must be made up as noted below. 

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