Measurements of Thermal Neutron Spectra in Heterogeneous Lattices by Foil Activation

Thermal neutron spectra have been measured with good spatial resolution within a unit cell in several H₂O-moderated natural uranium lattices and in one graphite-moderated lattice. The H₂O-moderated lattices had water-to-uranium volume ratios of 1:1, 2:1, and 3:1, with fuel rod diameters of 2.54 cm. Dysprosium-164, ¹⁵¹Eu, ¹⁷⁶Lu, and ¹¹⁵In were used as detectors, and both activation ratios and unfolded spectra are compared with THERMOS code calculations. The agreement between the results and the calculations is satisfactory; however, the agreement in the water regions is generally much better than in the fuel regions of the H₂O-moderated lattices. In the graphite lattice, the agreement of results with THERMOS calculations using a free gas kernel is poor, while calculations with a crystalline kernel show better agreement.