Investigation of the radioactivity inside the KSTAR vacuum vessel after shutdown by using gamma-ray spectrometry

Activation products from structural materials inside the Korea Superconducting Tokamak Advanced Research (KSTAR) vacuum vessel at 1 month after shutdown were investigated via gamma-ray spectroscopy using a high-purity germanium (HPGe) detector. To carry out this work, the HPGe detector system was placed on the midplane of port P inside the vacuum vessel, and counts were accumulated for a total time of 220,000 seconds (∼2.5 days). Many gamma activities of structural materials with half lives of a minimum of a week were measured. Some of the detected isotopes were from photonuclear reactions via processes such as (γ, n), (γ, 2n), (γ, p), (γ,np), (γ,3He), etc. due to high energy x-rays produced during the operation of KSTAR. Some of them resulted in the same final radioactive isotope. Activation products generated by photonuclear reactions in the structural materials were determined although each reaction could not always be uniquely identified by means of the gamma-ray spectroscopy technique. This investigation has shown that runaway electrons with energies up to ∼ 20 MeV were produced in the2010 KSTAR operation.