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Report No.
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Irradiation behavior of the B$$_{4}$$C neutron absorber materials; Results of the JOYO AMIR irradiation experiments

Kimura, Yoshio; Kaito, Takeji ; Onose, Shoji ; Sato, Yoshinori; Shibahara, Itaru

The B$$_{4}$$C neutron absorber material has great advantage of high neutron absorbing capability, although the He release and the swelling limit the life time of control rod. So it is important for the life time extension and reliability improvement to investigate the irradiation behavior of the B$$_{4}$$C. This report discuss the important parameters which may control the absorber pin behavior based on the irradiation test results obtained recently by the JOYO AMIR experiments, and re-evaluate the current technical issues and the way of improvements. The He release from B$$_{4}$$C pellet increase rapidly after a burnup of 100$$times$$10$$^{20}$$ cap/cm$$^{3}$$ and exceed the value predicted by the equation which has been applied to the MONJU design. The data is still required when we adopt the seal type absorber pin to use at high burnup. The He release found to depend on the $$^{10}$$B enrichment as well as the grain size. The He release could be reduced by the optimization of these parameters although the swelling behavior should also be considered since the swelling generally increases with decreasing the He release. The absorber cladding mechanical interaction (ACMI) is accelerated due to pellet cracking and relocation and essentially limits the life time of control rod. The relocation becomes generally extensive as the pellet-to-cladding gap is increased, but this behavior consider to be stochastic and difficult to make quantitative prediction. Improvement could be attained by using a shroud type absorber pin. Reduction of thermal conductivity during irradiation is considered to be caused by micro-defects including the micro-cracking. The degradation may be improved by introducing B$$_{4}$$C cermet.

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