RVACS system

In the highly unlikely event that the IHTS becomes completely unavailable, the safety-related RVACS will passively remove decay heat from the reactor vessel. As the temperature of the reactor sodium and reactor vessel automatically rise, the radiant heat transfer across the argon gap to the contairunent vessel increases to accommodate the heat load. With the increase in containment vessel temperature, the heat transfer from the containment vessel to the atmospheric air surrounding the containment vessel increases. The RVACS system... 

Regarding the control of accidents within the design basis (DID Level 3 in Table 4), the design of AHTR incorporates a mechanical reactivity control and shutdown system based on control rods with the external drives, and two diverse decay heat removal systems, of which one is passive and one is active. The reference AHTR design uses passive reactor vessel auxiliary cooling (RVAC) systems similar to that developed for decay heat removal in the General Electric sodium cooled S-PRISM reactor. Different from its prototype, the RVAC system of the AHTR relies not only on the processes of convection and conduction but on the radiation also. 

In the 4S, the decay heat is removed by two systems consisting of the decay heat removal coil installed in the reactor (PRACS) and the natural air ventilation from outside the guard vessel (RVACS). The analysis considers the destruction of PRACS and the RVACS cooling stack by a large falling aircraft. In addition to this extreme severe condition, 50% of the cross sectional area of the RVACS stack is assumed to be blocked. 

RVACS reactor vessel auxiliary cooling system... 

To meet the passive safety requirements of the NGNP, the AHTR uses a reactor vessel auxiliary cooling system (RVACS) similar to that of S-PRISM. It may also use a direct reactor auxiliary cooling system (DRAGS) similar to what was used in the Experimental Breeder Reactor II to supplement the RVACS and reduce the reactor vessel temperature. 

Earlier, a sealing analysis for the passive decay heat cooling system suggested that the AHTR could operate at a thermal power of 2400 MW(t). A more sophisticated analysis was performed that indicates that 2400 MW(t) can indeed be achieved with reasonable RVACS capacity. The analysis showed that for a loss-of-forced-cooling accident (with scram), significant natural convection of the molten salt is established and the eore temperature peaks at only 1160°C, which occurs about 30 hours after the accident. The reactor vessel temperature peaks at 750°C after about 40 hours. This analysis, which did not include a DRAGS, indieates that a 2400 MW(t) AHTR ean easily survive this type of transient. 

The AHTR appears to have excellent safety attributes. The combined thermal capacity of the graphite core and the molten salt coolant pool offer a large time buffer to reactor transients. The effective transfer of heat to the reactor vessel increases the effectiveness of the RVACS and DRAGS to remove decay heat, and the excellent fission product retention characteristic of molten salt provides an extra barrier to radioactive releases. The low-pressure, chemically nonreactive coolant also greatly reduces the potential for overpressurization of the reactor containment building and provides an important additional barrier for fission product release. The most important design and safety issue with the AHTR may be the performance and reliability of the thermal blanket system, which must maintain the vessel within an acceptable temperature range. 

It is also possible to supplement the RVACS heat removal capacity using a direct reactor auxiliary cooling system (DRAGS) based on natural circulation of an intermediate coolant from bayonet heat exchangers in the reactor vessel to air-cooled heat exchangers. This type of DRAGS system was used in the Experimental Breeder Reactor II (EBR-II) with sodium-potasium as the intermediate coolant. There are a variety of potential intermediate coolants, several of which have been used extensively in industry for similar heat transfer applications. 

Fig. 2.10. Illustration of a combined RVACS/DRACS system for the AHTR.

Thermal inertia can slow the temperature rise in the reactor core however, decay heat removal is ultimately required. The thermal capacity of the 2400 MW(t) AHTR core is more than a factor of four greater than the 600 MW(t) GT-MHR, while maintaining a peak core temperature of 1160°C at 50 hours in the AHTR. The acceptable thermal power of the AHTR is then limited by the peak decay-heat removal capacity of the RVACS, potentially supplemented by a DRAGS. The 600 MW(t) GT-MHR reactor cavity cooling system (RCCS) has a peak capacity that matches the decay heat output at the time of peak core temperature 50 hours after loss of cooling. To achieve the same 50-h duration at 2400 MW(t), the AHTR RVACS/ DRAGS system must have a heat removal capacity four times that of the GT-MHR. 

RVACS/DRACS test loop (research). RVACS and DRAGS decay heat removal systems have been developed and tested for sodium-cooled fast reactors. However, the AHTR RVACS/DRACS will operate at significantly higher temperatures. Test loops are required to provide integrated experimental data to qualify design codes for higher temperatures. 

LOF Primary Reactor Auxiliary Cooling System (PRACS) Reactor Vessel Auxiliary Cooling System (RVACS) TOP Mechanical stop of hydraulic drive system of reflector... 

PRACS) Reactor Vessel Auxiliary Cooling System (RVACS) Passive 1 system... 

The thermal shutoff system (TSS), separate from the RPS, is designed to automatically shut off the EM pumps on high sodium temperature, in case the heat sink (IHTS) is lost and the RPS fails to scram. For this scenario it is assumed that the loss of heat sink event starts from full power and that neither the PCS nor the RPS can shut down the reactor so that the reactor is left with only RVACS to remove heat. 

The SG is cooled by circulating air over the exterior shell surface to provide an alternative method of reactor decay heat removal. The ACS supplements the RVACS and reduces the time required to cool the reactor system and IHTS to hot standby temperature following... 

AT.MR primary sodium and reactor vessel auxiliary cooling system (RVACS) air flow circuit using for heat removal. 

For heat removal from a shutdown reactor, two independent passive systems are provided, which are the reactor vessel auxiliary cooling system (RVACS) and the intermediate reactor auxiliary cooling system (IRACS). The RVACS is completely passive and removes shutdown heat from the surfaces of the guard vessel using natural circulation of air. There is no valve, vane, or damper in the flow path of the air therefore, the RVACS is always in operation, even when the reactor operates at rated power. Two stacks are provided to obtain a sufficient draft. 

The fully passive shutdown heat removal system (RVACS) based on natural air draft and natural circulation of sodium ... 

After the reactor shutdown, primary pumps trip with the flow coastdown facilitated by the SM system then natural circulation begins within the reactor vessel. Coolant temperature in the hot plenum (upper core region) decreases because of the shutdown. Only the RVACS with reduced draft removes decay heat by air convection. Temperatures in the primary boundary... 

RVACS Natural circulation of air. Enhancement of heat radiation from steel to the air. In the USA, ANL has conducted the tests and GE has designed the system for the PRISM reactor. The experiments were also conducted by CRIEPI in Japan. 

The reactor vessel auxiliary cooling system (RVACS) is a system for shutdown heat removal however, to keep the fully passive features, it is continuously operating even at normal operation of the reactor. The intermediate reactor auxiliary cooling system (IRACS) is a sodium loop with an air cooler for shutdown heat removal, arranged in series with the secondary sodium loop (Fig. XIV-2). 

If the IRACS fails completely, the RVACS is able to remove shutdown heat as a fully passive system of air convection. 

Note that RVACS Is always working with natural air circulation as a fully passive system. 

ACS - auxiliary cooling system RVACS, or RVACS + IRACS LOHS - Loss of heat sink UHS - Ultimate heat sink... 

RVACS - Reactor vessel auxiliary cooling system DHX - Direct heat exchanger PRACS - Primary reactor auxiliary cooling system RV -Reactor vessel... 

The protected loss of heat sink (PLOHS) event was simulated to predict the heat removal capability of the RVACS. PLOHS was assumed to be initiated by a loss of the external alternate current (AC) power, resulting in a total loss of AC power, because the 4S-LMR has no emergency AC power on-site. The steam/water system cannot remove the decay heat in this event. The primary coolant flow shifts to natural convection mode. The design heat removal capabilities of the PRACS and the RVACS are 2.5 MW(th) and 1 MW(th), respectively. 

A scheme of the 4S-LMR main heat transport system with the indication of heat removal path in normal operation and in accidents is given in Fig. XV-11. The reactor incorporates redundant passive decay heat removal systems. Specifically, a reactor vessel auxiliary cooling system (RVACS) is adopted in which the natural convection airflow removes the decay heat radiated through the guard vessel. The heat removal capability depends on the thermal radiation area. A specific (per thermal power) heat radiation area of small reactors is larger than that of medium sized or large reactors. It is expected that about 1% of the nominal power could be removed with the RVACS. 

RVACS Reactor Vessel Auxiliary Cooling System PLOHS Protected Loss Of Heat Sink... 

There are no special decay heat removal systems other than a reactor vessel air-cooling system (RVACS) that uses natural air draft for the heat sink ... 

Reactor is very simple and robust. It has small number of components. There are no moving components except for the control (6 pieces) and safety (1 piece) element drives these drives need to be actuated only seldom. The only special safety system is the Reactor Vessel Air Cooling System (RVACS) this system is passive. 

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