Modal Decay Research Articles

Pulsed neutron experiments on subcritical heavy water natural uranium lattices

Prompt neutron decay in several subcritical D 2O-moderated natural uranium lattices has been investigated by the pulsed technique. Employing various detector positions inside the multiplying medium we achieved a separation of flux harmonics by means of a Fourier transform. From the modal decay curves we obtained the prompt neutron decay constant α as a function of geometrical buckling B 2. In the lower buckling region, B 2 < 25 m −2, experimental α vs. B 2 curves agree very well with simple two-group calculations on the basis of lattice parameters determined in previous exponential experiments ( Meister, 1961). Deviations for higher B 2 are shown to be due to spectrum shifts towards lower neutron energies. In addition, we determined the change of radial buckling Δμ 2 due to cadmium rods fully inserted into the lattice. The results are in agreement with stationary experiments as well as two-group calculations. With single fuel rods withdrawn from the lattice, however, the corresponding radial buckling change Δμ 2 turns out to be strongly dependent on the buckling of the axial flux distribution.