FUEL ASSEMBLY GROUPS

A region denotes a grouping fuel assemblies that was loaded at the same time... 

The second step shown in the figure involves the calculation of isotopic contents using the decay and depletion steps of the SAS2H calculational sequence of SCALE. Calculations are performed for each assembly group based on the initial fresh fuel content and operating history of the group. Output consists of calculated isotopic contents for each of a number of user-specified time steps. 

The reactor core is composed of fuel rods with square cross-section. At the comer there are fins that are spiral with respect to longitudinal axis of the fuel rod. Fuel composition is uranium-zirconium alloy with - 20 % enrichment by U235. Fuel cladding is made of zirconium alloy. Fuel rods are grouped in fuel assemblies (FA). Burnable rods placed in FAs and absorber rods moving outside fuel channels are used to compensate for reactivity change in the core. 

The entrance gate monitor (ENGM) is a passive neutron coincidence collar detector permanently installed at the entrance to the fresh fuel transfer route of a fast breeder reactor (Hashimoto et al. 1994 Iwamoto et al. 1997). The detector head has four groups of He with six tubes each. Fresh fuel assemblies entering the reactor facility must pass through the ENGM so that their Pu content can be verified. Therefore, the ENGM is the system that verifies the amount of fresh fissile fuel in an assembly and serves as the first detector in a sequence of detector systems that follow the movement of fuel assemblies within the reactor facility. 

A group of four fuel assemblies, surrounding a cruciform control rod, makes up a core module unit. The control rod blades and control rod drives for the BWR 90 are of a well-proven design. The cruciform rod is based on solid steel blades that are welded together. Holes filled with B4C as neutron absorber are drilled horizontally in the blades. In the top of the rod, the absorber consists of Hafnium which makes the rod tip more "grey" and provides for a long service life. 

Control and protection system (CPS) X Six control rod groups (10-16 rods m each fuel assembly), two 3-tram sets m protective subsystem... 

Low-enriched (2 %) UO] fuel pins in a light-water-moderated iattice were used to construct 20 critical loadings, that simulated a variety of close-packed LWR fuel storage conHgurations. The critical loadings consisted of nine LWR-type fuel assemblies (clusters) grouped in a 3 X 3 array. Each fuel cluster consisted of a 14 X 14 matrix of 1.21-cm-o.d. low-enriched UO] fuel pins spaced on 1.64-cm centers. The fuel pins are clad with aluminum and have an active length of about 1,6 m. 

Criticality analysis of the proposed Maine Yankee nt fuel racks was done with two methodologies NITAWL-KENO with the 123-group XSDRN library and LEOPARD-RODWORTH-PDQ (Refs. 6, 7, and 8), with four- up collapsed cross sections. In both methods, a two-dimensional (x-y) model of the rack was constructed froin a basic unit of the rack. The basic unit of the rack is one-quarter of a fuel assembly and rack unit. Cali ulations are done assuming no leakage in either axial or radial direction i.e., reflecting boundary conditions. In addition, the models for Maine Yankee s spent fuel racks employed worst-case" mechanical uncertainties with fresh fuel at 68°F to bound reactivity eHects. 

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