Measurement and Analysis of Fission Rate Distributions of ²³⁷Np and ²³⁸U in a Polyethylene System with a 65 MeV Neutron Source

Abstract

Fission rates of ²³⁷Np and ²³⁸U in a polyethylene (CH₂) system were measured with a 65MeV quasi monoenergetic neutron source. Relative fission rate distributions dependent on polyethylene thickness up to about 70 cm were obtained for both nuclides with the experimental error within 7%. The present experiment was analyzed by the NMTC/JAERI code that has been employed for designing accelerator-driven transmutation systems. The fission rates of both ²³⁷Np and ²³⁸U calculated by the NMTC/JAERI did not agree with the experimental ones. The C/E values for both were about 2.0 at 71.8cm of polyethylene thickness when both experimental and calculated values were normalized to 1.0 at 0.0 cm of polyethylene thickness. A sensitivity analysis of the NMTC/JAERI was performed by changing cross sections and angular distributions of hydrogen and carbon and by employing three options of the intra-nuclear cascade/evaporation calculation of the NMTC/JAERI. The disagreement of the NMTC/JAERI calculation with the experimental values was partially improved by increasing the nonelastic-scattering cross section of carbon and by broadening the elastic-scattering angular distribution of carbon.

KEYWORDS:

• transmutation
• accelerator-driven reactor
• MeV range 10–100
• neutron sources
• neutrons
• fission rate
• neptunium 237
• uranium 238
• polyethylene
• hydrogen
• carbon
• experimental data
• errors
• NMTC/JAERI
• MORSE-CG