Development of protective layers on high-temperature alloys against hydrogen permeation

Abstract

For TEXTOR a multiple pellet injector system is in preparation which consists of a) nine freezing cells for the pellet formation accommodated in a common cryostat and nine gas guns being built at CEA, Service des Basses Temperatures in Grenoble, b) a system for the propellant-gas handling together with the pellet diagnostics being built at KFA Julich. The pellets have a cylindrical shape with a size of 1.3 mm in diameter and a length of 1.5 mm. The guns are operated with a propellant gas pressure of up to 200 bar and the pellets are accelerated up to velocities of 1.4 km/s. The propellant gas valve of each gun is optimized to release as little gas as possible; nevertheless the gas release amounts to 0.3 bar·l. This amount of gas (typically hydrogen) has a mass more than a factor of hundred larger than that of one pellet, but only an amount of one percent of the pellet mass can be tolerated. The conventional way of handling the propellant gas is either to feed the pellets through a capillary guiding tube or by using two or three differential pumping chambers. The first scheme may cause pellet ruptures at velocities higher than 1 km/s and the second one requires large expansion volumes and strong pumping. In the TEXTOR pellet injector we are using a combination of a muzzle brake, three small expansion volumes and two fast closing valves for each gun. The muzzle brake diffuses the stream of the propellant gas immediately after the end of the barrel. Most of the gas is directed into two expansion chambers which have volumes of only 0.5 l each. Within 1 ms after the passage of the pellet, a newly developed valve closes the first two expansion volumes from TEXTOR, thus reducing the residual propellant gas to less than 0.5 mbar·l. A second combination of an expansion volume of 1 l and a fast valve reduces the propellant gas further down to about 10−2 mbar·l, which is less than 1 % of the pellet mass (typically 4 mbar·l). Three IR-light barriers are integrated in each pellet path; their photodiodes provide signals for the pellet speed determination and trigger signals for the fast closing valves. They also give the open/closed status of the valves. The mass of the individual pellet is measured by a microwave system consisting of a μW-cavity, a wave guide, a gunnplexer oscillator and a detection circuit operating at a frequency of 10.3 GHz.