Dismantling work of the JAERI electron linac and measurement of induced radionuclides

This paper described the outline of dismantling work of JAERI electron linac (electron energy 20 MeV, beam current 40 mA) and measurement of induced radionuclides. The machine was constructed in 1960 and used for over 30 years for the major purpose of measuring the neutron cross section for designating reactors. The work started at 4 months after ceasing the operation of the accelerator for reduction of the ambient dose rate due to the decrease of short half life radionuclides. The equivalent dose rate was found only 2.5 μSv/hr on the magnet surface which had conceivably undergone the most significant radioactivation. The external exposure was monitored with the film-badge and pocket dosimeter to find out that there was no exposure exceeding the detection limit of the badge (0.2 mSv). Radionuclides in the concrete shielding were measured on samples obtained by boring the wall with NaI(Tl) scintillation counter and their γ-spectra were analyzed with Ge-semiconductor detector. Radioactivity was the highest in the region 3-6 cm deep from the surface and decreased exponentially from there. Nuclides such as 60Co, 134Cs, 152Eu, 154Eu, 22Na and 54Mn were found. When the radionuclides in the Vth accelerating tube where the electron energy had been the highest, were examined, 54Mn, 57Co, 58Co and 60Co were found remained at 7 months after ceasing the operation. These data would be useful for dismantling such accelerators. (K.H.)