Physics of Thermal Reactors

Evaluation of ENDF/B-IV Data for Tightly Packed UOa-Water Lattices, W. O. Olsdti, F. /. Wheeler (EG G Idaho) 

The ENDF/B-IV data set has been evaluated for use in highly undermoderated reactor cores. The critical experiments analyzed are those Originially used in the design of the Power Burst Facility (PBF) reactor core. 

These were performed by Phillips Petroleum and G.E. in 1964 and 1967. The lattices have nonmoderator-to-moderator ratios of 1.9 and 2.3, and H/U ratios of 2.7 and 2.3, respectively. The fuel ro are UO pellets clad in stainless steel, surrounded by thin aluminum sleeves to simulate the Zr insulators in the PBF core. These rods are described in Table I. The rods are placed in a square mesh of 15.656-mm 0.616-in.) and is,l89-mm (0.598-in.) pitch for the 1.9 and 2.3 ratio cells, respectively. The experimental arrays are approximately right circular cylinders, with radial water reflectors, containing 900 to 1568 pins. These vary firom simple clean arrays to configurations containing mocked-up test areas, control rods, voids, etc. Tite water height was adjusted to achieve criticality. 

90-group pin-cell calculations were made with SCAMP (an internal EG G code) in the S approximation, using an equivalent cylindrical cell with seml-isotropic return at the outer boundary. This yielded homogemzed fuel cell constants. Cross sections were coU sed to 33- and 

4-group sets. Collapsing to 4 groups was very sensitive to detail, requiring accurate geometrical models and leakage terms. 

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The measurements mentioned have been used for two purposes to develop semiempirical calculatlonal schemes, recipes , for the design of real reactor and to provide experimental information with which theory can be compared, ip the hope of arriving, eventually, at a real understanding bf the physics of thermal reactors. A serious problem that arises is that of the possible difference between results of measurements of a particular quantity when made in a critical assembhf and in a subcritical assembly. For example, recent work at the Savannah River liaboratory has indicated significant differences between values of the material buckling obtained from exponential and critical assemblies moderated by heavy water. A similar effect does not seem to have been observed in assemblies moderated by graphite or ordinary water. 

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