The first year of operation at 9500C in the AVR power station

Before the increase from 850 to 9500C of the hot gas temperature the bizonal core of the AVR reactor was optimized in such a way as to keep maximum fuel element temperatures as low as possible. In doing so it was achieved that the fuel elements were not stressed unduly and that no impermissibly high water gas concentrations would be produced in cases of accident caused by water leaking in and reacting with the hot fuel element graphite. The calculated concentrations remain far below the ignition limit at anytime and anyplace. No further preparations for the temperature increase were necessary. The concentrations of inactive cooling gas impurities have grown slightly with the temperature increase, the CO level having increased to 3-5 times its former value. As had been expected, post-irradiation examinations of separated fuel elements did not show any damages or other changes attributable to the higher operation temperature. The rare gas activities released from the fuel elements increased only by 30-50%. The increase is reversible and well within the range expected from earlier measurements. The concentrations of solid fission and activation products are determined in the hot and the cold sections of the primary circuit. The major part of the concentrations is temperature dependent, reaching values in the hot gas at 9500C operation which do not imply any safety risks in loss of coolant accidents. However, the activities may plate-out on hot section components and accumulate in such a way that their radiation must be taken into account in maintenance work. In the AVR reactor, the isotopes generating the most intense γ radiation are 110Agsup(m), 137Cs and 134Cs. The occurrence of very intense 65Zn activities in the hot gas is particularly surprising. Their origin is unknown. This may possibly be a specific AVR phenomenon which must not be expected to occur in other HTR installations. All data and conclusions based on the measurements of solid fission and activation products are provisional and require further confirmation. (author)