Assessment of the Radiation Dose Level of the PAL-XFEL Hard X-ray Beamlines under Accident Conditions

A PAL-XFEL is a free electron laser using 10-GeV, 0.2-nC electron beams. The failure of the main dump magnet can cause a serious radiation risk because electron beams go straight to the experimental area of the hard X-ray beamlines and high-energy radiation is produced in the forward direction. This type of accident should be prevented through a suitable shielding design and a Personal Safety and Interlock System. A proper safety design based on a reliable method for estimating the radiation dose level is as important as anything else. In this study, the leakage dose rate was estimated under predictable accident conditions, and the suitable radiation shielding of the hard X-ray beamlines was designed using a well-known Monte Carlo code, FLUKA, in preparation for an accident. The result was verified by measurements of the radiation dose rate during commissioning period and was compared with the result obtained by using a semi-empirical code, SHIELD11. The measured radiation dose rates in the lateral direction under failure of the main dump magnet were agreed to the radiation dose rates calculated by using the SHIELD11 well, and the FLUKA results were relatively overestimated. In addition, the assessment error of the neutron dose rate depending on the type of neutron rem-counter was verified for high-energy neutrons. The effect of an accidental loss is negligible compared to the annual dose received by beamline users.