Operating experience with an UHV system in the vicinity of a fission-reactor core

For an experiment on neutron decay, an UHV system has been developed for a tangential beam-tube insert of the ASTRA reactor (peak dose and flux density in the beam tube at 8 MW reactor power: 44 Mrad/h and 4x1012 n/sub th//cm2 s, respectively). The central section of the beam-tube insert is evacuated mainly by means of a Zr--Al getter pump, giving an effective pumping speed of 40 l/s (N2 equivalent) for the irradiated section. The main vacuum system is situated outside the reactor shielding and is evacuated by a pump combination (orbitron pump, titanium-sublimation pump, and sputter-ion pump); preevacuation is achieved by cryosorption pumps. During reactor operation the pressure can be kept <10-5 Pa in the irradiated section of the beam-tube insert (<10-7 Pa for the reactor switched off). The radiation-induced desorption (RID) yields mainly CO2 and CO for the residual gas atmosphere. Experience from a three-year operation of this vacuum system is discussed in detail