Kenji Konashi and Tsugio Yokoyama

Abstract A hydride target including minor actinides (MA) is able to enhance the transmutation rate in a fast breeding reactor (FBR) without degradation of core safety characters. Fast neutrons generated in the core region of the FBR are moderated in the MA-hydride target assemblies and then efficiently absorbed by MA. The MA-hydride target pin has been designed in the light of recent research of hydride materials. This chapter shows the feasibility of MA transmutation by an existing reactor, Monju.

Keywords Fast breeder reactor • Hydride • Minor actinide

16.1 Introduction

High-level wastes generated after reprocessing spent nuclear fuels include long — lived radioactive nuclides of minor actinides (MA), such as 237Np, 241Am, 243Am, and 244Cm. A currently available method for the final disposal of the high-level wastes is to vitrify them under rigid control, to store them in monitored locations until the radiation decays to allowable levels, and then to dispose them underground.

The transmutation of MA by the fast breeder reactor (FBR) has been intensively studied to reduce radioactivity of the wastes [1]. Transmutation rate, which is one of the most important factors for transmutation methods, is determined by values of neutron flux and nuclear reaction cross section as follows:

K. Konashi (*)

Institute for Materials Research, Tohoku University, Oarai, Ibaraki-ken 311-1313, Japan e-mail: konashi@imr. tohoku. ac. jp

T. Yokoyama

Toshiba Nuclear Engineering Services Corporation, 8 Shinsugita, Isogo-ku, Yokohama 235-8523,Japan

© The Author(s) 2015 169

K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste Disposal, DOI 10.1007/978-4-431-55111-9_16

FBRs provide high fast neutron flux, wherein the neutron reaction cross sections are small compared with those in the thermal energy region. Moderation of fast neutrons by hydride materials was considered to increase the transmutation rate [2—5]. In this chapter, enhancement of transmutation of MA by an MA-hydride target is studied. Target assemblies containing MA-hydrides are placed in the radial blanket region. Fast neutrons generated in the core region are moderated in the hydride target assembly and then produce high flux of thermal neutrons, which have large nuclear reaction cross sections to actinides. The MA-hydride target also has another advantage to load MA to limited space. The target of (MA, Zr)Hx increases mass of MA and hydrogen density in the blanket region compared with MA and ZrH16 loaded separately [4, 5].

Hydride fuels have been used in TRIGA reactors of General Atomics (GA) for many years [6]. On the other hand, hydride materials do not have much history of use in FBRs. Recently, a control rod of FBR with hafnium (Hf)-hydride has been studied [7]. In this chapter, the MA-hydride target pin was designed using exper­imental data of Hf-hydride.