Development of Monitoring System for Gaseous Fission Products in the Primary Coolant Helium of HTGR´s

The experiments were carried out to develop a monitoring method of fission products in the primary coolant helium of the experimental VHTR(Very High Temperature Reactor). In the experiments, the high-temperature in-pile gas loop OGL-1 (Oarai Gas Loop-l)was used to simulate a primary circuit condition of the VHTR. The monitoring system consists of a conventional wire-driving precipitator and a Ge gamma-ray spectrometer. The gross counts due to fission products in the primary circuit are measured by the precipitator, and the gamma-ray spectra of the fission products are monitored by the Ge spectrometer for identification of the fission product nuclides. It was observed that gross counts due to fission products released from coated particle fuel samples change significantly depending on the reactor power and the irradiation temperature in the manner of 10cps(12MW, 350°C), 150cps(50MW, 1100°C) and 325cps(50MW, 1350°C). Individual contributions of each fission-product nuclide to the gross count were monitored in detail and were Kr-88:Kr-85M:Kr-87:Xe-133:Xe-135:Xe-135M:Xe-137: Xe-138 = 1.00:0.40:1.70:0.181.17:1.30:1.59:2.51. The concentration of Kr-88 was 7.36 Bq/ml(1.99×10-4 ¿Ci/ml) at that time. It was also determined that the gross inventory of the fission-product activities was 3.4× 1011 Bq(9.2 Ci) refered to the primary coolant of the experimental VHTR and the sensitivity of the experimental monitoring system was 9.6×10-10cps/Bq (35cps/Ci) as the equivalent activity to Kr-88.