MEASUREMENTS OF THE SPATIAL AND ENERGY DISTRIBUTION OF THERMAL NEUTRONS IN URANIUM, HEAVY WATER LATTICES

Technical Report · Mon Aug 20 00:00:00 EDT 1962

OSTI ID:4772727

Brown, P S; Thompson, T J; Kaplan, I; Profio, A E

Intracell activity distributions were measured in three natural U, heavy water lattices of 1.0l0 in. dia., Al clad iods on triangular spacings of 4.5 in., 5.0 in., and 5.75 in., respectively, and in a U, heavy water lattice of 0.25 in., 1.03% U/sup 235/, Al-clad rods on a triangular spacing of 1.25 in. The distributions were measured with bare and Cd-covered foils of Au, Lu, and Eu. The Au was used as a 1/v absorber to measure the thermal neutron density distribution. Because the activation cross sections of Lu and Eu depart considerably from 1/v behavior, their activation depends strongly on the thermal neutron energy spectrum. Hence, they were used to make integral measurements of the change in the neutron energy spectrum with position in the lattice cell. A method was developed for treating the partial absorption, by Cd covers, of neutrons at the 0.46 ev Eu resonance, and it was found possible to correct the Eu activations to energy cutoffs just above and just below the resonance. The measured activity distributions were compared with those computed with the THERMOS code. In the natural U lattices, THERMOS gave excellent agreement with the measured Au activity distributions and very good agreement with the Lu and Eu distributions, indicating that THERMOS gives a good estimate of the spatial and energy distribution of thermal neutrons in these lattices. In the enriched lattice, THERMOS gave a large overestimate of the activity dip in the fuel for all three detectors. The discrepancy was attributed to a breakdown in the Wigner- Seitz cylindrical cell approximation at small cell radii. However, the measured ratios of Lu and Eu activity to Au activity were in good agreement with the THERMOS values, indicating that THERMOS gave a good estimate of the degree of spectral hardening. Neutron temperature calculations were made from the data by using Westcott effective cross sections. The temperature changes so calculated agreed well with those predicted by THERMOS. Disadvantage factors calculated by the Amouyal-Benoist-Horowitz (ABH) method were in excellent agreement with the measured values in the natural U lattices. The agreement was not as good in the enriched lattice because of an expected breakdown in the ABH method at small cell radii. Values of the thermal utilization obtained from experiment, from THERMOS, and with the ABH method weie in excellent agreement for all the lattices studied. Radial and axial buckling measurements made with Lu were in excellent agreement with similar measurements made with Au, indicating that the thermal neutron spectrum was uniform throughout the lattice tank. Measurements of intracell Au activity distributions made in off-centcr cells differed only slightly from those made in the central cell of the lattice, indicating that the radial flux distribution was almost completely separable into a macroscopic J/sub 0/ and a microscopic cell distribution. (auth)

• Research Organization: Massachusetts Inst. of Tech., Cambridge
• NSA Number: NSA-16-032362
• OSTI ID: 4772727
• Report Number(s): MITNE-17; NYO-10205
• Country of Publication: United States
• Language: English

Related Subjects

ABSORPTION
ACTIVATION
ALUMINUM
BUCKLING
CADMIUM
COATING
CROSS SECTIONS
DENSITY
DISTRIBUTION
ENERGY
ENERGY LEVELS
EUROPIUM
FOILS
GOLD
HEAVY WATER
LUTETIUM
MEASURED VALUES
MECHANICAL STRUCTURES
METALS
NEUTRON FLUX
NUMERICALS
PHYSICS
PROGRAMMING
RADIOACTIVITY
RESONANCE
RODS
TEMPERATURE
THERMAL NEUTRONS
URANIUM
URANIUM 235
WIGNER-SEITZ METHOD
ZONES