Main steam isolation valve

Figure 6,1-2 is a simplified illustration of a BWR. The pressure of the moderator-cooling water at about half the pressure in a PWR forms steam as it flows upward through the core. Steam passes through a moisture separator (shown as vertical lines just above the core) exits the containment through Main Steam Isolation Valves (MSIV) drives the turbine and generates electricity. After the steam is cooled by the turbine, it is condensed, and pumped back to the reactor by the feedwater pump. 

In validation process, the semi-dynamic simulation tests were also performed additionally to investigate the integrity for system requirement. By use of the plant simulators, their response to the transients were verified such as LOCA, LOP A, and Main Steam Isolation Valve Closure etc. 

After the installation of the system, the whole system was tested to prove the installation integrity. This covered test items as same as the factory tests. In the pre-operation test, the system experienced more than 10 initiation against the transients such as load rejection at 20%, 50%, 75% and 100% power, LOPA at 20% power, plant trip at 50% power and Main Steam Isolation Valve Closure at 100% etc.. 

USNRC RG 1.96 (June 1976) Design of main steam isolation valve leakage control systems for boiling water reactor nuclear power plants . USNRC RG 1.106 (March 1977) Thermal overload protection for electric motors on motor-operated valves . 

AR375 1.96 Design of main steam isolation valve leakage control systems for boiling water reactor nuclear... 

Main Steam Isolation Valve Leakage Control System (BWR)... 

D Radiological Consequences of a Design Basis Loss-of-Coolant Accident Leakage From Main Steam Isolation Valve Leakage Control System (BWR)... 

Main Steam Isolation Valve 8 Disasse mble 4 Outage 2 2 2 2 2... 

Inadvertent closure of main steam isolation valves T... 

C. The Main Steam Isolation Valves are designed to isolate the main steam lines and the steam generators as required for maintenance. 

Each of the four main steam lines is provided with a power-actuated Main Steam Isolation Valve designed to stop flow from either direction when it is tripped closed. Each valve is located outside containment and is provided with means of actuation from the Engineered Safety Features Actuation System, meeting the requirements of IEEE Standard 279. 

The main steam isolation system consists of the Main Steam Isolation Valves and their associated controls and instrumentation. The Main Steam Isolation Valves are remotely operated valves designed to either fail closed or be guaranteed to close upon receipt of a Main Steam Isolation Signal. The Main Steam Isolation Valves can be monitored and controlled locally and in the control room. 

A steam line break inside containment would result in a pressure rise in the containment. Reverse flow protection is also achieved through the Main Steam Isolation Valves. To achieve reverse flow protection in the case of the main steam pipe rupture, the valve is fully closed within 5.0 seconds from receipt of the initiating signal. 

A. The Main Steam Isolation Valves are capable of isolating the steam generators within 5.0 seconds after receiving a signal g from the Engineered Safety Features Actuation System. In the event of a steam line break, this action prevents continuous uncontrolled steam release from more than one steam generator. Protection is offered for breaks inside or outside the containment. 

B. The Main Steam Isolation Valves, their operators, and associated circuitry are Seismic Category I, and are protected against missiles and the effect of high-energy line breaks. 

All main steam isolation valves are designed, fabricated, tested, and installed in accordance with the codes and standards identified in Chapter 10. Ii... 

There are two main steam isolation valves (MSIVs) welded into each of the two MSLs. On each MSL there is one MSIV in the containment and one MSIV outside the containment. The MSIVs are Y-pattem globe valves. The Y-pattem configuration permits the inlet and outlet flow passages to be streamlined to minimize pressure drop during normal steam flow. 

Loss of the main heat sink Loss of normal feedwater supply Glosure of the main steam isolation valves Stuck-open safety-relief valve... 

BWR MAIN STEAM ISOLATION VALVE LEAKAGE CONTROL SYSTEMS... 

In contrast, there are disadvantages to automatic isolation of EFW. If both channels of the controlling isolation logic systems were to spontaneously actuate either during normal operation or in the course of a transient, the availability of EFW would be lost and the main steam isolation valves would close. Most newer plants use turbine-driven main feedwater pumps. Thus, main feedwater would also be lost, resulting in complete loss of the secondary heat sink. Capability to lock-out the isolation logic is necessary to preclude such scenarios. 

A pressure relief function is used to control large pressure transients. This system will operate safety/relief valves following closure of the main steam isolation valves or the sudden closure of the turbine admission or stop valves and failure of the turbine bypass system to relieve the excess pressure. For this fimction, the safety/relief valves discharge steam from the steam lines inside the drywell to the suppression chamber. Each safety/ relief valve is operated from its own overpressure signal for the relief fimction, and by direct spring action for the safety function. 

Nuclear system isolation by isolation valve closure in process lines penetrating the containment barrier is based on two-out-of-four logic for main steam isolation valves and a one-out-of-two taken twice logic for remainder of nuclear system isolation function. 

An inverter supplied from either the AC emergency bus or the DC battery bus provides AC power for the scram solenoid pilot valves and the main steam isolation valves. 

Main steam isolation valve (MSIV) closure either due to presence of radiation in main steam line or break of MSL... 

Excessive water hammer on the feedwater line has not been experienced so far in Japan In view of the USI A-1, however, J nozzles have been installed on the steam generator feedwater ring Operating procedures have been accordingly improved, and it has been reflected on the simulator training In addition, paying attention to the potential possibilities of water hammer of such area as the vicinity of main steam isolation valves, it is made as practice to take the water hammer load into design consideration... 

Improved and qualified equipment on secondary side for fast cooldown functions. Break preclusion quality of the steam lines up to the steam safety valves, the steam relief station and the main steam isolation valve (MSIV) ... 

At the present time, two types of valves are installed on each WWER steam generator line (2 safety valves, 1 relief valve). The safety valves on the WWER have the possibility of adjusting the secondary pressure from 84 bars to 40 bars. The relief valve can do it from 74 bars to 1 bar. The relief valves in WWER-440/213 NPPs cannot be used in all situation because they are located downstream of the main steam isolation valves (MSIV). The existing probabilistic studies have pointed out some transients for which, in case of a failure of the relief valves, it was necessary to be able to use the safety valves at low pressure to cool the primary circuit. 

In addition there are French studies on a leak before break (LBB) approach, similar to the American ones (GDC 4). This concept LBB is used in the German-French project EPR for the main primary coolant loops and for the short section of main secondary loop between the external side of containment and the main steam isolation valves. 

Inadvertent closure of main steam isolation valves (results in, and is therefore bounded by, a turbine trip). This is fault 4.3.4 in the fault schedule. 

Main steam isolation valve monorail hoists. 

Each main steam line goes from its steam generator and passes through the containment boundary. The first valve encountered in each line is the main steam isolation valve, which is outside the containment boundary each isolation valve has a bypass valve in parallel with it. Beyond the main steam isolation valve in each of the two steam lines are the six steam safety valves every one of these safety valves vents to atmosphere through a discharge pipe and vent stack. Downstream of the safety valves is the power-operated atmospheric relief valve, which vents to atmosphere through a vent pipe and silencer. The operation of the power-operated relief valves is automatically controlled by steam line pressure during plant operations the power-operated relief... 

The main steam isolation valve and the associated bypass valve on each main steam line must both close automatically on receipt of an S-signal . 

The main steam isolation valve and the associated bypass valve on each main steam line both close automatieally to isolate the seeondary side of each of the steam generators and thereby prevent the imeontrolled blowdown of more than one steam generator on reeeipt of an S-signal from one of the following triggers (Section 7.3.1.2.10 of Reference 6.1) ... 

The annex/auxiliary buildings nonradioactive HVAC system serves the nonradioactive personnel and equipment areas, electrical equipment rooms, clean corridors, the ancillary diesel generator room and demineralised water deoxygenating room in the annex building, and the main steam isolation valve eompartments, reactor trip switchgear rooms, and piping and electrical penetration areas in the auxiliary building. 

III-IO. In some plants, leakage control systems have been added between the main steam isolation valves to limit the escape of radioactive material by this path. 

Adopting the large capacity main steam isolation valve (MSIV) reduces the number of main steam (MS) lines and the number of MSIVs. The number of residual heat removal (RHR) systems is reduced to one line by substituting a clean up water system (CUW) for an RHR because of the use of the large capacity CUW heat exchanger adopted in large capacity BWRs. 

Malfunction of Main Steam Isolation Valve (MSIV). 

To avoid additional penetrations to the reactor inlet plenum, the start-up system piping is connected to one of the four heat transport piping connections to the reactor. The connection point is within the containment building, between the reactor and main steam isolation valves and the feed-water isolation valves. The start-up system can be isolated from the reactor by means of valves located on the start-up system piping at the connection points. 

Figure 20.18 ECCS of NuScale (Reyes, 2012). DHRIV, decay heat removal isolation valve MSIV, main steam isolation valve RX, reactor, MFIV, main feedwater isolation valve DHRHX, decay heat removal heat exchanger, DHRIV, decay heat removal isolation valve.

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