Intermediate reactor auxiliary cooling systems

Shutdown heat removal system (2) Intermediate reactor auxiliary cooling system (IRACS) Primary reactor auxiliary cooling system (PRACS)... 

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 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). 

The rated thermal output of MONJU [5.63, 5.64] is transported through the primary heat transport system (PHTS) and intermediate heat transport system (IHTS) loops to the steam generators. Shutdown heat removal is normally by forced circulation (FC) provided by pony motors associated with each of the loop pumps. Heat is rejected to air at the air blast heat exchanger of the intermediate reactor auxiliary cooling system (ACS) which branches off from each IHTS loop. Thus the auxiliary cooling system (ACS) of the Monju reactor is coupled with the secondary system which also has the role as decay heat removal system. 

A schematic of the reactor and cooling system is shown in Fig. 11.2. Two intermediate reactor auxiliary cooling systems (IRACSs) and one direct reactor auxiliary cooling system (DRAGS) have been applied as a decay heat removal system (DHRS) suitable for the two-loop cooling system and the adopted type of SG. These systems are passive type by natural circulation. 

S has several safety systems active, passive, and inherent (IAEA, 2003) (see Fig. 20.22). Active shutdown systems are (1) inserting reflectors by using gravitational force and (2) inserting black control rods. The passive safety system of 4S uses natural circulation in RVACS and Intermediate Reactor Auxiliary Cooling System (IRACS). In addition, inherent safety system uses Doppler effect via metallic fuel and large inventory of coolant. 

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. 

Direct reactor auxiliary cooling system (DRAGS) with immersed DHX in main vessel, intermediate sodium loop and Na-air heat exchangers. 

Six-control rod subassemblies made of 90% enriched B4C were used in JOYO MK-II and were located symmetrically in the third row. In 1994, one control rod was moved to the fifth row to provide a position for irradiation test assemblies with on-line instrumentation. Since then, the control rod subassemblies have been loaded asymmetrically. The JOYO cooling system has two primary sodium loops, two secondary loops and an auxiliary cooling system. The cooling system uses approximately 200 tons of sodium. In the MK-II core, sodium enters the core at 370°C at a flow rate of 1 100 tons/h/loop and exits the reactor vessel at 500°C. The maximum outlet temperature of a fuel subassembly is about 570 C. An intermediate heat exchanger (IHX) separates radioactive sodium in the primary system from non-radioactive... 

The SPINNOR and VSPINNOR plants adopt pool type Pb-Bi cooled fast reactors without intermediate heat exchanger. Centrifugal pump is adopted for primary circulation, which is, together with steam generator, placed inside the reactor vessel. Decay heat is removed with the use of a passive reactor vessel auxiliary cooling system (RVACS), as shown in Fig. XXVI-1. 

The reactor building is in the center of the plant. In addition to two HTR modules, it contains some auxiliary and ancillary systems. Components of the start-up and cool-down systems, the steam generator fast discharge systems, and the intermediate cooling systems are installed in the annex to the reactor building. The two modular units are separated from each other by a central service area. An outer protective shell encloses the inner building structure. It fulfills the requirements for protecting the reactor plant from external impacts (e.g. aircraft crash). 

An integral primary system layout is employed (Fig. 12.4), ie, reactor core, variable frequency submersible coolant pumps, intermediate heat exchanges, safety system heat exchangers, and cold trap filters. The reactor vessel is enclosed in a guard vessel. There are no auxiliary sodium systems in the primary circuit. The reactor core consists of fuel assemblies, boron shield assemblies, and absorber rods. The central part of the core consists of wrap-spaced hexagonal fuel assemblies and cells with absorber rods. The spent fuel is stored in the reactor vessel for up to 2 years, which facilitates spent fuel cooling and eliminates the need for spent fuel storage casks. Assemblies with boron carbide are placed behind the spent fuel to protect the reactor vessel. 

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