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Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 25 cold leg temperature. The serial arrangement of the Air Coolers allows to fully exploiting their heat transfer capability, and provides wider control flexibility. The Air Coolers (see Figure 3.2-10) are provided with motor-operated variable speed fans, in order to select the desired heat removal rate during each plant operating condition. At low heat load, the air can flow on natural circulation and the flow rate can be controlled by means of movable dampers. The air exhausts are then discharged to the external atmosphere through a battery of stacks. The Expansion Tank is installed after the ACs upstream of the IHXs. This tank holds the oil volume fluctuations due to possible operating temperatures fluctuations and oil swelling from 60°C to 320°C. The recirculation pump, installed on the hot leg downstream of the IHXs, is sized to deliver 100% of the required loop flow rate. Each pump is connectable to the on-site power generation bus (Diesel Generator) to be automatically actuated in case of a Loss of Off-Site Power. Separate power buses, belonging to the Non-Class normally feed 1E AC Power Systems (NACS the SCS pumps, loop 1 and 2, 1E AC Power Systems (NACS). This redundancy ensures a defense-in-depth capability against the possibility of a reactor shutdown. On high radioactivity signal from one SCS loop the corresponding recirculation pump is automatically stopped in order to terminate contaminated material recirculation; beside, the ACs air path is intercepted in order to terminate the natural recirculation. The reactor shall be shut down and the other available SCS loop shall be used to drive the plant to a controlled state. On low RCS temperature signal a total air coolers shutdown is automatically initiated in order to limit primary coolant temperature decrease, to try to maintain just above the LBE freezing point. A safety-relief valve protects the SCS from the potential overpressure caused by secondary coolant thermal expansion. Before start up phase’s gas pockets shall be eliminated through adequate vent lines, on the upper parts of the secondary circuit, connected to the Expansion Tank. The Expansion Tank and all the main loop piping are thermally insulated. This sections; one for each SCS loop, include the equipment necessary for filling/draining of each loop. These sections are also utilized for a continuous feed and bleed to perform the required oil purification. More specifically an oil drain line is provided to connect each loop to a dedicated Storage Tank capable of containing all the drainable coolant oil. DNU 020/1
Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 26 In normal operation a continuous oil flow rate is spilled from the main loop through the same drain line and routed to the Storage Tank. At the same time, a filling pump delivers an equivalent oil flow rate from the Storage Tank to a pipe connection upstream of the secondary coolant pumps through a filter, in order to maintain a continuous stream of cleaned oil to feed the main loop. This feed and bleed operation is automatically controlled by the control loop of the expansion tank level. The Storage Tank is provided with connections for oil make-up and disposal. An electric heater is immersed in the tank to perform the initial heat up of the fresh oil, after tank filling, and to maintain the proper oil temperature and consequently to ensure the adequate viscosity for pumping. 3.2.7 Reactor Vessel Air Cooling System The Reactor Vessel Air Cooling System (RVACS), as shown in Figure 3.2-11, mainly consists of a number of air U tubes vertically located in the cylindrical annulus between the wall of the Reactor Cavity and the Guard Vessel; inside each tube air flows in natural circulation. Four equally sized stacks constitute the air inlet and outlet paths. Each stack contains an internal duct that is the path of the heated airflow to the atmosphere and an external annular duct connected to the intake openings. The air intakes and the outlet openings are laid out so that the pressure drops through the parallel pathways are balanced to the maximum extent, with no shortcuts for the exhaust air to re-enter in cycle. The intakes and the outlets are designed to operate during extreme weather conditions like winds, rain and dust and are protected by metal grids specially designed for anti-intrusion, in order to prevent external bodies from entering from the surrounding environment. The air flows downwards into the intake openings, passing through the annular ducts, until it gets to the level of the Reactor Building plan elevation slab, and enters the outermost of two concentric headers (outlet header). Inside the outlet header, the air turns vertically towards the Reactor Cavity, flowing into the U tubes, welded to the plenum itself. The air pipes are equally distributed to form two concentric pipe walls, housed in the annular space between the Guard Vessel and the Reactor Cavity wall. The outer rank is formed by the tubes in which flows the fresh air, while the inner rank is formed by the tubes in which flows the hot air that is the closest to the Guard Vessel. Each cold-side pipe is bent near the Reactor Cavity bottom, to effectively separate the cold from the hot tubes. A cylindrical, thermally insulated plate (Insulating Cylinder) separates the descending from the rising tubes, thus preventing the heat from being transferred directly from the hot to the cold tubes. Downstream of the bend, the tubes are routed vertically towards the inner duct. Along their ascending path, they are exposed to the radiating heat from the Guard Vessel. The ascending portion of the U tubes shall be externally black painted, to increase the DNU 020/1
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