Development of Dilatometer for in situ Measurement of Small Radiation-Induced Length Change

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The present study has been planned to perform measurements of small volume change induced by 14 MeV neutron irradiation generated by RTNSII at Lawrence Livermore National Laboratory, University of California. Fusion reactors, development of which is carried out in many countries, generate 14 MeV neutrons, while neutrons generated in existing fission reactors have energies below 1 MeV. Thus material test by 14 MeV neutrons is one of the most crucial problem for development of fusion reactors. The purpose of the present work is to develop a dilatometer to determine the length change induced by 14 MeV neutrons from RTNSII in ceramics, for which the length change is the most useful measure of radiation damage, and to carry out length change measurements of <Al_2> <O_3> and Mg <Al_2> <O_4> .The dilatometer, developed in the present study, measures the difference in the length of two specimens, one placed in the proximity of the neutron source and the other placed away by 2 cm, by converting the difference to electrostatic capacitance. Specimens were placed in a chamber shielded by vacuum from the outside. A capacitance bridge was used to record the length difference during irradiation every 0.1 second.The length changes of <Al_2> <O_3> both parallel and perpendicular to the c-axis have been measured at 50゜C in situ for a three weeks run. The main results obtained are as follows:1) A dilatometer has been developed capable to obtain a length change as small as <10^(-6)> in situ under neutron irradiation with a high stability. 2) The length change of Mg <Al_2> <O_4> induced by neutron irradiation is about a half of that of <Al_2> <O_3> at 50゜C. 3) The length change along the c-axis of <Al_2> <O_3> is larger than that along the a-axis.The dilatometer developed is capable to be used to determine the length change by electron and ion irradiation.