Use of moderator materials in a fast breeder reactor is effective for transmutation technology, and 11B4C is one of the candidates. Up to now, the behavior of 10B4C as the control rod material is well known, but that of 11B4C has hardly been investigated. In this paper, irradiation effects of 11B4C pellet irradiated in the experimental fast reactor JOYO were studied. From the experimental results, it was observed that no macro-cracks were recognized but bubble nucleation was found in the grain and along the grain boundaries of 11B4C. Furthermore, it was shown that the thermal conductivity of 11B4C was higher than that of 10B4C. During the annealing from room temperature to 1400°C, two recovery stages were found on thermal conductivity. It was suggested that the recovery of B4C was related to the volume and distribution of helium bubbles. Since the 11B4C pellet was mechanically more stable compared with the 10B4C pellet under irradiation, it was shown that 11B4C had high applicability for a moderator material.
