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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 23 The Argon is pumped to the Reactor Vessel through a 2" header that is routed to the Primary System. Downstream of the two Reactor Coolant Boundary isolation valves, the piping turns into a circular header which parcels out into 24, ½" OD tubes, symmetrically arranged around a circumferential configuration, to be directed to various sectors of the Primary System. Each of these tubes is provided with a manual flow control valve, to equalize the gas flowrate through the various pipes. After penetrating the Reactor Roof, the tubes deeply plunge into the primary coolant at the Riser Channels level, and end up with a gas sparger, located 500 mm above the top of Core region, and shaped so that the outlet of the gas stream is directed upwards. Scope of the Argon injection is to enhance circulation of the primary coolant, based on the creation of a differential floating force between the primary coolant in the gas injection region and the upper region. The gas, after exiting the distribution spargers, is fractionated into a myriad of small bubbles. The resulting effect is one of swelling of the primary coolant volume, so that the apparent density of the Lead-Bismuth eutectic lowers, and the primary coolant is pushed upwards, creating an empty space which is immediately occupied by the coolant from the Core. After accomplishing its mission, the Argon is delivered to the cover gas space above the liquid metal surface. From the above-mentioned circumferential header, another ½” OD tube provides a small gas flow rate to the integrated purification unit (approximately 1 liter per second at normal conditions) to develop a driving force to overcome the pressure loss through the filter unit, allowing it to work. At the compressors outlet the excess gas line is used to temporarily store a portion of the cover gas, should the cover gas space pressure exceed the control band high (+ 50 mbar). The flow to/from the Buffer Vessel is dictated by two on-off valves (one respectively), located on the compressor discharge line. The Gas Buffer Vessel also serves to supply an extra-pressure at the system start-up, to overcome the primary coolant hold-up contained in the gas injection pipes, and to allow gas decay prior to routing the gas to the Waste Gas System (WGS). Downstream of the compressors there is a Hydrogen/Steam and Oxygen Injection Unit which is periodically operated to adjust the concentration of Oxygen in the cover gas. All system components are shielded against the radiation coming from the cover gas. All effluents, gaseous and liquid, are routed to the waste systems for storage and reprocessing. The circuit is accurately sealed to avoid external air to enter the sections under vacuum, and gas to be released to the environment in the pressurized portion. 3.2.6 Secondary Coolant System The Secondary Coolant System (SCS) (see Figure 3.2-9) basically consists of two independent identical subsystems (main loops), each one feeding a couple of IHXs and 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 24 provided with a filling/drain purification section. Consequently each loop is sized to cope with 50 % of the RCS heat duty during full power operation. The decay heat can be rejected to the ultimate heat sink via fuel assemblies → primary lead-bismuth coolant → intermediate heat exchangers → secondary organic diathermic oil coolant → air coolers → external atmosphere This flow path relies on natural circulation only, i.e. on heat transfer by natural mechanisms, consistent with the passive-safety philosophy, which is a principle of the XADS design. It relies on natural circulation of the primary lead-bismuth coolant inside the Primary Vessel (which takes place also in the absence of the argon gas injection), natural circulation of the secondary organic fluid coolant (which takes place by virtue of the secondary loops layout, provided that atmosphere air circulation is guaranteed across the air coolers), natural circulation of the atmospheric air. Each independent secondary coolant loops are designed to remove 100% decay heat. This design choice minimizes the number of accident scenario for which this decay heat removal path is unavailable. Each SCS loop mainly consists of: • Two Intermediate Heat Exchangers (IHXs), arranged in parallel; • Three Air Coolers (ACs), arranged in series; • One recirculation pump; • One expansion tank. The loop cold header branches into two identical lines close to the reactor vessel, feeding the IHXs; the flow rate to each IHX shall be balanced (mainly relying on an adequate piping and lay out), so as to ensure the same working conditions for each component. At the outlet of each individual IHX the oil is collected into a common hot header and routed to the AC building to feed the three ACs arranged in series. The portion of the system physically included between the penetration of the Reactor roof to the sealing area close to the border of the Reactor Building is enveloped in a Guard-Pipe. This Guard Pipe has been provided as a barrier to prevent fire associated to the oil autoignition at high temperature in the containment due to a postulated break in the SCS boundary, but it also performs the function of containment boundary. The SCS penetrates the containment (guard pipe boundary) to enter into the Reactor Vessel; hence the SCS portion outside of the Reactor Vessel is to be considered as a portion of a closed loop outside containment whilst the reminder portion is to be considered as a portion of a closed loop inside containment. In the Air Coolers the reactor power is transferred from the oil to the atmosphere. By flowing through the tubes of the Air Coolers, the oil temperature is gradually reduced down to the DNU 020/1
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