Evaluations of the generalized area and the α/ν methods of interpreting pulsed neutron measurements for subcritical reactivity

The Generalized Area and α/ν Methods of interpretation of pulsed-neutron measurements for subcritical reactivity were found to minimize the error caused by source-induced harmonics and kinetic distortion. The use of multiple neutron detectors distributed over the core of the reactor in the pulsed source measurements is recommended for increased accuracy of interpretation. The measured data are reduced to a reported value of the subcritical reactivity by the use of numerical solutions to the reactor eigenvalue problems. In the Generalized Area Method, the numerical solution provides an estimate of the static adjoint function which is used to weight the prompt and delayed neutron flux integrals measured in the experiment. These weighted integrals are then used to form the subcritical reactivity. In the α/ν Method, the static eigenequation solved by numerical methods is transformed to a time eigenequation to provide a bridge between the measured decay constant of the fundamental mode and the subcritical static reactivity. Also, the transformation provides a means of normalizing the reported static reactivity. The evaluations were performed by applying both methods to numerical data generated by one-dimensional, space--time diffusion theory for which k/sub eff/ was known precisely. The Generalized Area Method was found to deduce reasonably accurate reactivity values from simulations of data from pulsed-neutron experiments in both large and small reflected reactors. The errors from the true reactivity ranged from +3% to -7% in rho at k/sub eff/ = 0.9. However, the single-detector Sjoestrand analysis failed badly for all pulsing simulations except for that of the small reactor with the source at the center of the core. Errors from the true reactivity ranged from +90% to -260% in rho at k/sub eff/ = 0.9. The α/ν Method was applicable only to the small-reactor simulation where the error was -1% in rho at k/sub eff/ = 0.9