Automatic depressurization

In some BWR transient scenarios, the high pressure injection systems are postulated to fail. To make use of the low pressure injection system, it is necessary to depressurize the reactor coolant system, a function performed by the automatic depressurization system (ADS). In the scenario considered, ADS actuation is manual because the signals for automatic initiation of the system are not present. 

The AP600 passive safety system includes subsystems for safety injection, residual heat removal, containment cooling, and control room habitability under emergency conditions. Several of these aspects are in existing nuclear plants such as accumulators, isolation condensers as natural-circulation closed loop heat removal systems (in early BWRs), automatic depressurization systems (ADS - in BWRs) and spargers (in BWRs). 

Answer Figure 15.5.2-2 shows an automatic depressurization valve that opens on excessive conditions to release the steam which passes through a turbine to drive an electric generator to provide makeup water and operate other cooling equipment. After the steam has cooled in the low pressure turbine it discharges under water in the cooling reservoir and condensed. The reservoir cools by evaporation. 

Fig. 37. Schematic representation of the in-containment passive safety injection system (PS1S). 1RWST = in-containment refueling water storage tank. PRHR-HX = passive residual heat removal heat exchanger. ADS = automatic depressurization system (four stages). (Westinghouse)...

Core make-up keep core flooding High pressure coolant injection system (2) Low pressure coolant injection system (2) Accumulator tank (2) High pressure coolant injection system (2) Low pressure coolant injection system (2) Rehieling tank (1) Core makeup water tank (2) Automatic depressurization system (2) Accumulator tank (2j Containment vessel water (Water-filled containment vessel) (1)... 

The AP600 automatic depressurization system comprises 16 valves divided into four depressurization stages. These valves are installed in the reactor coolant system at three different locations. The valves... 

At the Vapore facility located at ENEA Casacda Research Centre, testing of the Automatic Depressurization System (ADS) of AP-600 reactor coolant system is in progress. 

LOCA (primary) automatic initiation of containment isolation coolant injection to the RPV if necessary, automatic depressurization to get low pressure coolant inj. system in operation, combined with elimination of below core large nozzles, facilitates reliable core flooding... 

Automatic Depressurization System (ADS) Passive Reduces pressure for passive safety injection... 

Automatic Depressurization System a Actuation mode (manual/automatic) - Automatic b Side location (primary/secondary circuit)- Primary... 

The Automatic Depressurization System Test is a full sized simulation of one of the two AP600 depressurization system flowpaths from the pressurizer which will duplicate or conservatively bound the operating conditions of the AP600 ADS valves, sparger, and quench tank. The ADS test is being performed at ENEA s VAPORE Test Facility in Casaccia, Italy, and will be conducted in two parts. Phase A and Phase B. 

The primary functions of the Nuclear Boiler System (NBS) are (1) to deliver steam from the RPV to Ae turbine main steam systan (TMSS), (2) to deliver feedwater from the condensate and feedwater system (C FS) to the RPV, (3) to provide overpressure protection of the RCPB, (4) to provide automatic depressurization of the RPV in the event of a LOCA where the RPV does not depressurize rapidly, and (5) with the exception of monitoring the neutron flux, to provide the instrumentation necessary for monitoring conditions in the RPV such as RPV pressure, metal temperature, and water level instrumentation. 

The Automatic Depressurization Subsystem (ADS) consists of the eight SRVs and six depressurization valves (DPVs) and their associated instrumentation and controls. The ADS quickly depressurizes the RPV in sufficient time for the Gravity-Driven Cooling System (GDCS) injecting flow to replenish core coolant to maintain core temperature below design limits in the event of a LOCA. It also maintains the reactor depressurized for continued operation of GDCS after an accident without need for power. 

Implemented system (Name) Automatic Depressurization System... 

Compared with current commercial LWR designs a number of safety-grade systems have been eliminated the control rods and the safety injection boron system are replaced by the density locks, the automatic depressurization system is not required, the auxiliary feedwater supply system for RHR is replaced by the reactor pool, the containment heat removal and containment spray systems are replaced by the passive cooling of the reactor pool. The safety-grade closed cooling water stem, HVAC sterns, and a.c. power supply systems have been replaced by non-safety-grade systems, allowing major simplification of the plant. 

Automatic depressurization system (ADS) X - five valves - total flow area of 0.057 m ... 

Supression pool DS Automatic depressurization (Borated water) system (4 trains, 8 valves)... 

Accident mitigation can be achieved by using the passive safety systems such as Passive Reactor Shutdown System (PRSS), Pressure Balanced Injection System (PBIS) and Containment Water Cooling System (CWCS), and active Automatic Depressurization System (ADS). 

Steam condensing Containment spray Suppression pool cooling High-pressure core spray (HPCS) system Low-pressure core spray (LPCS) system Automatic depressurization... 

Selected safety/relief valves are associated with the automatic depressurization of the primary system under assumed LOCA conditions. These valves have two independent logic channels powered from different power sources, either of which can initiate depressurization. Valves open automatically and remain open imtil the pressiue falls to a preset closure pressure. These valves open automatically upon signals of high drywell pressure and low reactor water level and confirmation of one LPCI function of the RHR system or LPCS system running. Initiation signals need not be simultaneous. The valves remain open until the primary system pressure is reduced to a point where the LPCI function of the RHR system and/or the LPCS system can adequately cool the core. The initiation of automatic depressurization is delayed from 90 to 120 s to allow the operator to terminate the initiation should the HPCS system initiation and acceptable reactor vessel level have been confirmed. 

The valves used for automatic depressurization can be manually power actuated to open at any pressure. The signal for manual power actuation is from redundant control room switches from different power sources. 

In the unlikely event that the RHR shutdown suction line is unavailable during reactor shutdown to cool reactor water and during the period when the LPCI function of the RHR system and/or the LPCS system pumps are injecting water into the reactor vessel, safety/relief valves used for automatic depressurization can be used to pass water from the reactor vessel to the suppression pool via valve discharge lines. For this to occur, the reactor vessel floods to a level above the vessel main steam line nozzles, selected safety/relief valves are opened from the control room to pass reactor water to the suppression pool. 

The ECCS comprises the LPCl function of the RHR system, HPCS and LPCS systems, and automatic depressurization of the primary system. The ECCS is designed to perform the following functions ... 

The operation of the automatic depressurization function, the HPCS system, and two LPCl loops of the RHR system (failure of division 1)... 

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