In-situ ex-core monitoring using optical fibers with scintillators

In-situ radiation monitoring in the outside region of a reactor core has been experimentally demonstrated using radiation-resistant optical fibers with scintillators. To avoid influences of luminescence (400-450 nm) from the fibers by radiation, Y₃A₁₅O₁₂:Ce scintillators were adopted because the luminescence has its peak around 550 nm. In this method, low radiation field was monitored with a pulse counting method, while high field with an integral method. The demonstration experiments shows that radiation intensity outside a reactor core can be well monitored with the system from the reactor power of 1 kW to 3.5 MW. The vertical radiation intensity distribution around the reactor core was also measured at 3.5 MW.