Year
2025
Abstract
From the viewpoint of integration of the 3Ss, especially in the construction of innovative next-generation nuclear power systems using HALEU (High Assay Low Enrichment Uranium), it is necessary to fulfill all the requirements of 3Ss from the design stage. However, systems that monitor only one of the 3Ss have limited applicability and no comprehensive system that ensures all 3Ss have been developed. Therefore, this study proposes a new measurement method for nuclear material monitoring systems that can simultaneously monitor criticality, detection of nuclear material theft, and safeguards metrology.
The active neutron method with DD neutron source is applied as the monitoring system. The Rossi-α method is used for criticality monitoring, and the neutron coincidence counting (the conventional method or newly developed methods which use the result of Rossi-α method) is used to weigh nuclear material (which also serves as nuclear material theft detection). One of the new methods applies the principle of the threshold energy neutron analysis to improve the signal-to-noise ratio.
The experiments were conducted to test the simultaneous applicability of the Rossi-α method and the neutron coincidence counting, as well as the validity of the new neutron coincidence counting methods. A HALEU which contained about 350g of U-235 with an enrichment of about 20% and a DD neutron source, both owned by the Kyoto University Critical Assembly (KUCA), were used for the experiment. The HALEU in the sample was sequentially reduced, and each effective multiplication factor and coincidence neutron counting rate (doubles) were determined.
As a result, the experimental values of the effective multiplication factor were in general agreement with the result of the MCNP simulation. In addition, linearity was confirmed between the amount of U-235 and the doubles. Therefore, the simultaneous applicability of these methods and the validity of the new nuclear material accountancy methods were confirmed.