Cheol Ho Pyeon

Abstract Experimental studies on the uranium — and thorium-loaded accelerator — driven system (ADS) are being conducted for basic research of nuclear transmuta­tion analyses with the combined use of the core at the Kyoto University Critical Assembly (KUCA) and the fixed-field alternating gradient (FFAG; 100 MeV pro­tons) accelerator in the Kyoto University Research Reactor Institute. The ADS experiments with 100 MeV protons were carried out to investigate the neutronic characteristics of ADS, and the static and kinetic parameters were accurately analyzed through both the measurements and the Monte Carlo simulations of reactor physics parameters. An upcoming ADS at KUCA could be composed of highly enriched uranium fuel and Pb-Bi material, and the reaction rate ratio analyses ([1] [2] [3] [4]Np and [5]Am) of nuclear transmutation could be conducted in the ADS (hard spectrum core) at KUCA. The neutronic characteristics of Pb-Bi are expected to be examined through reactor physics experiments at KUCA with the use of solid Pb-Bi materials at the target and in the core.

Keywords 100 MeV protons • ADS • FFAG accelerator • KUCA • Spallation neutrons • Tungsten target

9.1 Introduction

The accelerator-driven system (ADS) has been considered as an innovative system for the nuclear transmutation of minor actinides and long-lived fission products with the use of spallation neutrons obtained from the injection of high-energy protons into a heavy metal target. At the Kyoto University Critical Assembly (KUCA), a series of ADS experiments [1—5] was carried out by coupling with the fixed-field alternating gradient (FFAG) accelerator [6—8], and the spallation neutrons generated by 100 MeV protons from the FFAG accelerator were success­fully injected into uranium — [1, 2, 4] and thorium-loaded [5, 7] cores.

In the ADS facility at KUCA, reactor physics experiments are being carried out to study the neutronic characteristics through the measurements of reactor physics parameters, including reaction rates, neutron spectrum, neutron multiplication, subcriticality, and neutron decay constant. Among these, neutron multiplication was considered as an important index to recognize the number of fission neutrons in the core induced by the external neutron source.

The mockup experiments [5] of thorium-loaded ADS carried out by varying the neutron spectrum and the external neutron source were aimed at investigating the influence of different neutron profiles on thorium capture reactions and the prompt and delayed neutron behaviors in the subcritical system. The results provided important effects of the neutron spectrum and the external neutron source on both static and kinetic parameters: the effect of the neutron spectrum was investigated by varying the moderator material in the fuel region, and that of external neutron source by injecting separately 14 MeV neutrons and 100 MeV protons into the thorium-loaded core varying the moderator. Before the subcritical experiments, a thorium plate irradiation experiment was carried out in the KUCA core to analyze the thorium capture and fission reactions in the critical system as a reference of the subcritical system, although the feasibility of Th capture and U fission reac­tions could be examined in the subcritical state.

In this chapter, experimental results of the uranium — and thorium-loaded ADS are shown. Accuracy was evaluated through the comparison between the experi­ments and the calculations of the Monte Carlo analyses through the MCNPX [9] code with ENDF/B-VII.0 [10], JENDL/HE-2007 [11], and JENDL/D-99 [12] libraries. The ADS static and kinetic experiments at KUCA are presented in Sect. 9.2, the results and discussion of the experiments and calculations in Sect. 9.3, and the conclusion of the study in Sect. 9.4.