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Enhancement of Transmutation of Minor Actinides by Hydride Target

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

Introduction

High-level wastes generated after reprocessing spent nuclear fuels include longlived 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:

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 ZrH1.6 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.

Design of MA-Hydride Target

The TRIGA fuel consists of a U-metal phase and a Zr-hydride phase at high temperature in the reactor. The MA-hydrides are stable at high temperature [8]. The phase relationship of the U-Th-Zr hydride has been studied, considering Th as a surrogate of MA. Figure 16.1 shows the microstructure of UTh4Zr10H24: black areas are Zr hydride, gray region is ThZr2Hx, and white areas are uranium metal. The thermodynamic analysis shows that the MA-hydride consists of MA-hydride, MA-Zr-hydride, and Zr-hydride (Fig. 16.1).

Fig. 16.1 Pellet of (U,Th,Zr)Hx and microstructure

Fig. 16.2 MA-hydride target pin

For the application of Hf-hydride to neutron absorber material in FBR [3], the Hf-hydride pin has been developed. The fabrication of sodium-bonded Hf-hydride pins has been demonstrated. The pins were successfully irradiated in BOR-60 for 1 year [7]. The MA-target pin was designed based on the foregoing experiences. Figure 16.2 shows a target pin that includes MA-hydride ((MA, Zr)H1.6) pellets. The gap of the MA hydride pellet-stainless steel cladding was filled with liquid sodium to keep the temperature of the pellets low. The results of irradiation experiments show that the sodium also reduces loss of hydrogen from the hydride pin.

 
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