Annular plenum

The alternative to using a single plenum and adjusting the gas flow with the distributor plate is the use of a split plenum. Here, the gas flow is regulated separately through a central and annular plenum. The air can be supplied by a single blower but separate controls are necessary for each plenum. If independent control of the gas flow to each plenum is required, then two blowers are necessary. 

The annular reactor core consists of fuel elements, graphite reflector elements, plenum elements, reactivity control material, and neutron startup sources. Each of these components is described below. 

The cold 1 piping enters the reactor vessel at the same level as the hot leg nozzles, and the 260°C return flow is directed downwards to the reactor core inlet via the down- comer. On its way down, the flow velocity is increased in a siphon breaker arrangement with open connections to the pressurizer. The siphon breaker is intended to prevent siphoning ofT too much reactor pool water inventory in the hypothetical event of a cold leg rupture. During normal operation, the siphon breaker does not affect the water circulation. At the bottom of the annular downcomer the return flow enters the reactor core inlet plenum. 

The ACS, shaped like a flat-bottom cylindrical glass, provides the support for the core instrumentation and forms the inner wall of the annular riser of the primary water. The ACS is open at the top. The water within it is part of the Intermediate Plenum and this helps to limit the primary water inventory in the reactor module to a minimum. The ACS is flanged to and suspended from the top of the Iimer Vessel for easy rranoval to allow standard fuel handling. 

Core type and dimensions Annular cylinder Inner/outer effective radius -16.41/111.83 cm Uniform composition fuel No control elements in the core no solid reflector elements Lattice - hexagonal P/D -1.36 Fuel rod cladding outer diameter - 1.56 cm Cladding thickness - 0.13 cm Fuel smear density - 75% Active fuel length -125 cm Fission gas plenum length - 1.25 cm. 

The coolant is directed downward through a venturi and through heat exchangers devoted to decay heat removal. The flow path continues down through the annular downcomer region to the lower plenum and then back to the core. 

To hold more fissile gas with a shorter gas plenum at the ends of the fuel rod, and to decrease the highest temperature of fuel pellets as much as possible, annular fuel pellets are adopted. The outside diameter of the rod is 8.19 mm, whereas the diameter of the inner gaseous space is 1.5 mm. 31 OS is selected as the cladding material and other stracture material. 

There are 354 fuel pins in the core and each fuel pin has its own annular coolant channel. Taking into account the fuel pin diameter, the gap between fuel and clad, the clad thickness, and coolant channel thickness, the coolant annulus has an outer and inner radius of 1.1575 cm and 0.8885 cm respectively. The core fueled region is 0.664 m tall. The 354 fuel channels are modeled as six separate flow paths designed to represent radial variation in power although not utilized, since radial power variation has not been calculated. The coolant channel in contact with the fueled portion of the pin is represented with 10 equal height volumes to adequately model axial power profiles (plus space in the top volume for fission gas plenum). 

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