**1. Introduction**

The fusion-fission hybrid energy reactor, consisting of a low-power magnetic confinement fusion assembly and a subcritical blanket, is one of the advanced reactors of applying fusion technology to solve the present energy crisis. Natural thorium contains one isotope 232Th. Thorium is a fertile element that can be applied in the conceptual blanket design of a fusion-fission hybrid reactor [1, 2]. The actual neutron spectrum in the subcritical blanket based on the Th/U fuel cycle is composed of fast and thermal spectra. The 232Th capture cross section at fast neutron is slightly larger than that of 238U, and 232Th is more suitable to breed 233U under fast spectrum. Since 232Th capture cross section for thermal neutron is about 2.7 times larger than that of 238U, the conversion rate in the Th/U fuel cycle is more than that in the U/Pu fuel cycle and the neutron economy of thorium is better. Moreover, the 233U capture cross section for thermal neutron is smaller than that of 239Pu and 233U needs to absorb neutrons many times to produce Pu and long-life Minor Actinides (MA, such as 237Np, 241Am, and 242Cm), whereas Pu and MA produced in the Th/U fuel cycle are one order of magnitude less than those in the U/Pu fuel cycle. Therefore, the Th/U fuel cycle is beneficial to reduce the long-life nuclear waste and prevent nuclear proliferation. The feasibility and reliability of the physical

design for the subcritical blanket based on thorium depend on the accuracy of 232Th nuclear data and calculational tool. It is essential to carry out the fusion neutronics experiments for validating the evaluated 232Th nuclear data and studying the breeding properties.

A small number of fusion neutronics experiments on thorium were carried out, and there exist essential differences between the calculations and experiments [3–5]. The 232Th fission rate with fast neutrons was determined by detecting the gamma rays emitted from 140Ba and 140La, and the calculated-to-experimental ratio was 0.9 based on ENDF/B-IV [4]. The thorium fission reaction rate in a metallic sphere setup was determined by absolute measurement of the gamma-emission from 143Ce, the experimental uncertainty was 5.2%, and the calculation to experiment ratio was 1.17 employing ENDF/B-IV [5].

The integral fusion neutronics benchmark experiments for macroscopic thorium assemblies with a D-T fusion neutron source were carried out at Institute of Nuclear Physics and Chemistry (INPC) [6–17]. The method for measuring integral 232Th reaction rate and its application in an experimental assembly were developed and investigated [6–8]. In this chapter, the progress in the fusion neutronics experiments for thorium assemblies is described. The overview of main results is presented. The thorium assemblies with a D-T fusion neutron source consist of a polyethylene shell, depleted uranium shell, and thorium oxide cylinder. The 232Th reaction rates in the assemblies and leakage neutron spectra are measured separately. The benchmark experiments on fuel and neutron breeding properties derived from the 232Th reaction rates in representative thorium assemblies are carried out and analyzed. The breeding properties are valuable to the breeding ratio in the conceptual design of subcritical blanket based on the Th/U fuel cycle. The experimental results are simulated by using the MC code with different evaluated data. The ratios of calculation to experimental values are analyzed.
