J Blomgren

Elastic neutron scattering at 96 MeV from {sup 12}C and {sup 208}Pb

A facility for detection of scattered neutrons in the energy interval 50-130 MeV, SCANDAL, has recently been installed at the 20-180 MeV neutron beam line of the The Svedberg Laboratory, Uppsala. Elastic neutron scattering from {sup 12}C and {sup 208}Pb has been studied at 96 MeV in the 10 deg. -70 deg. interval. The achieved energy resolution, 3.7 MeV, is about an order of magnitude better than for any previous experiment above 65 MeV incident energy. The present experiment represents the highest neutron energy where the ground state has been resolved from the first excited state in neutron scattering. A novel method for normalization of the absolute scale of the cross section has been used. The estimated normalization uncertainty, 3%, is unprecedented for a neutron-induced differential cross section measurement on a nuclear target. The results are compared with modern optical model predictions based on phenomenology or microscopic nuclear theory.

MCNPX simulations of the SCANDAL setup for measurement of neutron scattering cross section at 175 MeV

The Scattered Nucleon Detection Assembly (SCANDAL) setup at The Svedberg Laboratory has been used to produce neutron elastic scattering cross section data at 175MeV for bismuth and iron. This work ...

NEUTRON-DETECTION BASED MONITORING OF VOID EFFECTS IN BOILING WATER REACTORS

The ratio between the thermal and fast neutron flux in a BWR (Boiling Water Reactor) depends on the void fraction. The density of the steam-water mixture present in the core determines the efficiency of the moderation of fast neutrons born in fission, and therefore the void fraction could be determined by means of a simultaneous measurement of the thermal and fast neutron fluxes. Such measurement could also be used to investigate channel bow of the nuclear fuel bundles surrounding the detector because of sensitivity of the thermal flux to geometry changes. Calculations have been performed with lattice codes to study the behavior of the void traction correlation to the ratio of the thermal and fast neutron flux. The results prove the correlation to be nearly linear and robust. The rate of change of the correlation is insensitive to standard reactor operating parameters such as control rods and burnable absorbers; the sensitivity of the ratio to void fraction changes primarily depends on the geometry of the fuel bundles.

TOMOGRAPHY OF CANISTERS FOR SPENT NUCLEAR FUEL

Sweden and Finland are preparing for final deposition of spent nuclear power fuel. The adopted method is to encapsulate spent nuclear fuel in copper canisters filled with iron before deposition in a deep bedrock repository. The canisters will have a diameter of about one metre, which makes examination of the content in sealed canisters virtually impossible with any known technique today. Two methods for tomography of sealed canisters have been studied, high-energy neutron tomography and cosmic-ray muon tomography. Monte Carlo simulations using MCNPX have shown that it would indeed be possible to produce images of good resolution of thick massive objects, like these canisters, using high-energy neutrons. The cost for installing such a method would, however, be very high. GEANT simulations, supported by experimental tests, indicate that tomography using the natural flux of cosmic-ray muons results in images of lower quality, but to a much more modest cost, acceptable to the application.

A Novel Fast Neutron Detector for Nuclear Data Measurements

Accelerator driven system will use a heavy element target such as lead. Many calculations are available to simulate high-energy spallation neutron induced reactions, but little data are available for comparison with the simulations. In order to constrain the simulation tools we have measured (n,Xn) double differential cross section on different targets at The Svedberg Laboratory, Uppsala, Sweden. For neutron energy above 40 MeV, we have developed a novel detector, CLODIA, based on proton recoil and drift chambers to determine neutron energy. CLODIA (Chamber for LOcalization with DrIft and Amplification) is able to track recoil protons with energy up to 90 MeV with spatial resolution of about one millimeter and a detection efficiency of 99% for each drift chamber. Using CLODIA coupled with the SCANDAL set-up, we have been able to measure double differential (n,Xn) cross section on lead and iron for incident neutron energy in the 40-95 MeV energy region.