Fission gas plenum

The active core consists of 181 fuel subassemblies with two enrichment zones, of which 85 with 21% Pu02 content are in the inner enrichment zone and 96 with 28% Pu02 content are in the outer enrichment zone. Each fuel subassembly consists of 217 helium bonded pins of 6.6 mm outside diameter. Each pin has 1000 mm column of MOX, 300 mm each of upper and lower depleted UO2 blanket columns and lower fission gas plenum (fig. 1). 

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. 

Coolant level drops to bottom of fission gas plenum -1.869 ... 

FISSION GAS PLENUM ACTIVE CORE FLOW DISTRIBUTOR HEAD... 

Fission gas plenum height above active core equal to 25% of active core height ... 

Rail transportability imposes a size limitation upon the reactor vessel and guard vessel of 6.1 m in diameter and 18.9 m in height [XXIII-25]. The fission gas plenum height is based upon an assumed conservative gas release from nitride fuel of 2.5% per atom % of bum-up. The fuel volume fraction was held fixed in the thermal hydraulic design analyses at the value of 0.215 determined by the core design. The fuel rod outer diameter and pitch-to-diameter ratio were varied to determine an optimum combination. Figure XXIII-5 shows the relationship... 

In the pin design for the first core loading a fission gas plenum was not foreseen because of both limited design experience and inadequate knowledge of fuel behavior under irradiation. As a result, the gas pressure reached 30 MPa and caused fuel leakage. Ten SAs at the peak bum-up values from 2.2 to 6.7% at. were selected for PIE. Failed fuel pins were found in six of the SAs. Two of these 6 SAs at bum-ups of 4.9% at. contained pins with cracked cladding. Each of the other four SAs, at bum-ups of 5.6, 6.1, 6.6, 6.75% at.,... 

Figure 5.2 shows a typical core fuel element (also called a fuel pin) and fuel assembly (also called a fuel subassembly). The core fuel element contains the core fuel, upper and lower axial blanket fuels, and a space called the fission gas plenum within a cladding tube. Then they are assembled as a fuel element bundle. The fuel assembly contains the fuel element bundle in a hexagonal assembly duct called a wrapper tube. 

The core consists of driver fuel assemblies, internal blanket assemblies, radial blanket assemblies, control rods, ultimate shutdown system (USS) assembly, gas expansion modules (OEMs), reflector assemblies, B4C shield assemblies, shield assemblies, and in-vessel storages (IVSs). There are no upper or lower axial blankets surrounding the core. A fission gas plenum is located above the fuel slug and sodium bond. The bottom of each fuel pin is a solid rod end plug for axial shielding. The reflector assemblies contain solid Inconel-600 rods. The control assemblies use a sliding bundle and a dashpot assembly within the same outer assembly structure as the other assembly types. 

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). 

The fission gas plenum to coolant channel structure induding conduction/convection from the gas plenum, conduction across the cladding, and convection to the coolant channel. 

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