3FTFBSDI

Development Study of Fast Reactor Core withHydride Neutron Absorber
We are now developing hafnium hydride control rods6,7
for Fast Breeder Reactor (FBR) in order to drasticallyprolong its in-core life accounting its superb characteristicsof helium and swelling free features.A new hydride neutron
absorber of nuclear reactor has proposed based on theresults of irradiation experiments in Japan Materials Testing
Reactor (JMTR) of Japan Atomic Energy Agency (JAEA). A
metal-hydride has very high hydrogen atom density, whichis equivalent to that of liquid water. Fast neutrons in nuclear
reactors are efficiently moderated and are absorbed in themetal-hydride. The Hafnium hydride and Gd hydride areconsidered as neutron absorber in FBR.

is larger than that of B10 40% enriched B4C.
One of the most important R&D items to establish theidea of Hf-hydride control rod is the development of Hfhydride pellet, which can be safely irradiated for long time.
The idea has proposed based on the results of irradiation
experiments in JMTR of JAEA. Fig. 2 shows the structure ofcapsule for irradiation experiments, where the actinide
hydride pellet were loaded. After irradiations, non-destructive
examinations (X-ray photograph, gamma scanning and soon) and destructive examinations (electron microprobe
analysis, physical property measurements and so on) were
performed and the integrity of the hydride pellet during
irradiation was assured.

Fast breeder reactor is considered as the major nuclear
power source for the future. The B4C has been mainly used
for control and shut down material in FBRs. But the life of
B4C control rods is restricted by Pellet-Cladding MechanicalInteraction (PCMI) failure due to the He gas swelling of B4C
pellet which is caused by the following nuclear reaction,
B10 (n, α) Li7.

(1)

Fig. 2 Schematic of HfHx application concept to FBR control rod.

The concept how to prolong control rod life of FBR byusing HfHx is illustrated in Fig. 1. In order to prolong thecontrol rod life–time we propose to use HfHx as absorbermaterial for FBR by the reasons that He gas is not generated
in nuclear reaction of HfHx, and that HfHx can be expected toabsorb neutron for more than 40 years owing to the fact that
Hf178 and Hf179 which are generated by neutron captures ofHf177 and Hf178 respectively, also have large neutron capture
cross sections which are expressed by the following
reaction,.

The development program of hydride neutron absorberhas been started, which is accepted as an innovative nuclear
research and development program of Ministry of Education,Culture, Sports, Science and Technology of Japan.

Hf177 (n, γ) Hf178 (n, γ) Hf179 (n, γ) Hf180.

(2)

References[1] K. Konashi et al., “Development of advanced control rodof hydride hafnium for fast reactor”, Proc. of the ICAPP2006,
Reno, USA (2006).
[2] K. Konashi et al., “Study on application of hafnium hydride
control rods to fast reactors”, Proc. of the 15th Pacific Basin
Nuclear Conference, Sydney, Australia (2006).
[3] K. Konashi and M. Yamawaki, “The development of thorium
hydride fuel”, Characterization and quality control of nuclear
fuels edited by Ganguly C. and Jayaraj R.N., 92, Allied
Publishers Pvt. Ltd. (2004).
Contact toKenji Konashi (International Research Center for Nuclear
Materials Science)e-mail: konashi@imr.tohoku.ac.jp

Fig. 1 Schematic of HfHx application concept to FBR control rod.
The control rod assembly, which contains Hf-hydride, isconsidered to enhance neutron absorption in fast reactors.
In general, a metal-hydride has very high hydrogen atom
density, which is equivalent to that of liquid water. Fastneutrons in nuclear reactors are efficiently moderated andare absorbed in the metal-hydride. As the ratio of H to Hfincreases, the Hf-control rod can more efficiently absorb
neutrons in fast reactor core. For example, the worth of HfH1.0

Advanced Nuclear Energy Materials

KINKEN Research Highlights 2007 IMR39