Research on environment monitoring of radiation emergency

In a case of a nuclear accident at nuclear facilities, strong radiation such as g-rays and neutrons might radiate at a burst in the initial stage. For the establishment of dose estimation system for such accidents, the experiments were done using the He+2 beam accelerated by Tandem in the W-MAST. The following results were obtained. 1) Neutron measurements using a rem counter yielded that dose equivalent was about 9.4 mSv/h at a position 100 cm from the Be target when the beam current of 15 MeV He2+ was 0.8 μA. Neutron measurement by means of Au and In foil activation method and by use of TLD element revealed that dose equivalents were to be 16-27 μSv/h for thermal neutron, and 30-41 μSv/h for sub-fast neutron (20 keV). Therefore, it was concluded that neutron field was mainly composed by fast neutron. 2) Linearity of the rem-counter out put vs neutron flux was valid under the condition that the count rate of the rem-counter was less than 10 kcps. 3) Computer simulation using NRESP code indicated that upper limit of the count rate might be 3 kcps. In an accident occurring in nuclear facility, radiation monitoring using an unmanned helicopter might be great helpful technique. We developed the dust sampler system and demonstrated its performance using an unmanned helicopter. Sources of variation in environment radiation measurements near nuclear power plants can be classified into two main sources; artificial one and natural one. It was shown that one can distinguish these two sources in the existing monitoring systems and also that one can know the natural source of the variation quantitatively with continuum measurements. Cosmic rays are known to be one of the biggest factors in the monitoring variation. Ion chambers are quite sensitive to detect charged cosmic rays, while the response to these particles are out of the energy range at measurements in NaI (Tl) scintillation detectors. In the meantime, the detector sensitivity does not change between these two types of detectors at measurements of the cosmic neutrons. As a conclusion, dose rate monitored by ion chambers are shown to grow up following to the increase of the flux of the charged cosmic rays. (author)