Hironori Kumanomido

Active neutron multiplication method for fuel lattices in water

In this paper an active neutron multiplication method is studied in laboratory experiments aiming at establishing and improving nondestructive measurement methods for a spent light water reactor fuel bundle in water. The fuel rods used in the experiments consist of 1.0-cm-diam UO{sub 2} pellets enriched to 1 to 3 wt% {sup 235}U, clad in 1.18-cm-o.d. aluminum tubes. The rods are arranged in square arrays spaced to form a 1.52-cm lattice. The analysis is carried out by two- of three-dimensional, three-energy-group diffusion calculations. A simple empirical expression for the correlation between the neutron flux and the effective neutron multiplication factor k{sub eff} is proposed in which a constant term is added to the well-known formula for one-point subcritical flux. The new expression has been found to be applicable to high-precision measurements. Through an experimental study of the correlation between the neutron flux profile and the neutron source response to a detector and studies of the new correlation expression, an improved measurement system is presented that is suitable for more precise measurements of k{sub eff}.

Calibration of Burnup Monitor Installed in Rokkasho Reprocessing Plant

Rokkasho Reprocessing Plant uses burnup credit for criticality control at the Spent Fuel Storage Facility (SFSF) and the Dissolution Facility. A burnup monitor measures nondestructively burnup value of a spent fuel assembly and guarantees the credit for burnup. For practical reasons, a standard radiation source is not used in calibration of the burnup monitor, but the burnup values of many spent fuel assemblies are measured based on operator-declared burnup values. This paper describes the concept of burnup credit, the burnup monitor, and the calibration method. It is concluded, from the results of calibration tests, that the calibration method is valid.

Intra-pellet neutron flux distribution measurements in LWR critical lattices

We have developed inexpensive and easy-handling measurement methods on intra-pellet neutron flux. A foil activation method with metallic foils, which were fabricated by punching out technique and etching technique to reduce fabrication error and positioning error, was used for the intra-pellet neutron flux distribution measurement. The developed method was applied to measure intra-pellet neutron flux distributions in a reduced–moderation light water reactor (LWR) lattices, and uncertainty of the distributions was estimated to be 1% to 2%. Measured values were analyzed with a continuous energy Monte Carlo code. Comparison of measurements and analyses revealed that the developed method is useful for the validation of an advanced fuel design method considering neutron behavior in fuel pellets.

Void reactivity evaluation by modified conversion ratio measurements in LWR critical experiments

We have developed a void reactivity evaluation method by using modified conversion ratio measurements in a light water reactor (LWR) critical lattice. Assembly-wise void reactivity is evaluated from the “finite neutron multiplication factor”, k*, deduced from the modified conversion ratio of each fuel rod. The distributions of modified conversion ratio and k* on a reduced-moderation LWR lattice, for which the improvement of negative void reactivity is a serious issue, were measured. Measured values were analyzed with a continuous-energy Monte Carlo method. The measurements and analyses agreed within the measurement uncertainty. The developed method is useful for validating the nuclear design methodology concerning void reactivity.