Fiber optic-type compact dosimeter using Nd:YAG for basement investigation inside primary containment vessel

For decommissioning the Fukushima Dai-ichi Nuclear Power Station, information on the radiation environment such as dose rate and radionuclides inside the primary containment vessel (PCV) is crucial. The authors have developed a fiber optic-type compact dosimeter and demonstrated it for a basement investigation inside the PCV of Unit 1. The dosimeter consisted of a Nd:YAG crystal that emitted infrared photons of 1064 nm by gamma-ray irradiation, an optical fiber as signal transmitter, and a measurement instrument for pulse height analysis. The dose rate linearity of the dosimeter was evaluated in the dose rate range from 0.01 to 8760 Gy/h by using gamma rays emitted from Co-60. The Nd:YAG crystal installed in a sensor unit was placed in a shape changing robot, PMORPH2, and dose rate distributions were obtained at 10 measurement points. The dose rate increased gradually when the sensor unit was close to the surface of some sediment. A comparison of measured dose rate attenuations obtained at two measurement points and calculated results showed that Cs-137 might stay on the surface or inside the sediment as the dominate radionuclide.