A Systematic Study on the Neutron Skyshine from Nuclear Facilities—Part I. Monte Carlo Analysis of Neutron Propagation in Air-over-Ground Environment from a Monoenergetic Source

The skyshine of monoenergetic neutrons directed upward from sources both as a vertically collimated beam and as a point isotropic cone fixed on the ground has been calculated systematically by a Monte Carlo method for distances up to ∼2 km from the source. The energy of the neutrons ranged from 14 MeV to thermal. The calculated skyshine spectra approach an approximate equilibrium having an approximate 1/E dependence in the keV region beyond about a few hundred metres from the source. The total neutron flux Φ(r) and dose D(r) at a distance r from a source are well represented by a simple formula, and D(r) = Q_Dexp(-r/λ_D)/r, and the constants , and λ_D are only dependent on the source-neutron energy. In respect to the dependence of , and Q_D on the upward aperture, θ_s, of the cone source and λ_D change very little with θ_s, but and Q_D increase with θ_s, when θ_s is larger than 30 deg. This simple formula was applied to evaluate the experimental results of skyshine neutron doses from a fast-neutron source-reactor facility and showed nice agreement.