Hydrogen Trapping Studies in Fusion First Wall Materials
Hydrogen trapping in the first wall of fusion reactors is one of the most serious materials problems facing the designers of magnetic confinement devices. The release of this trapped hydrogen from the near surface region of the first wall during a tokamak discharge has been shown to play a major role in the hydrogen fueling of today's plasmas, while trapped hydrogen isotopes will directly influence the on-site tritium inventory and the structural properties of future D-T reactors. This paper summarizes the laboratory ion implantation sttidies on the hydrogen trapping characteristics of first wall materials. Emphasis is placed on the near term materials problems associated with austenitic stainless steels that are currently in use in magnetic confinement devices. Hydrogen trapping experiments involving more exotic first wall structural and coating materials are briefly reviewed.
Thermally activated processes in hydrogen recycling
Abstract The thermally activated processes of hydrogen diffusion in wall materials such as stainless steels and inconels and of its surface release (including desorption) play an essential role in the hydrogen recycling in many fusion devices. Non thermally activated processes such as deposition, implantation and direct reflection (backscattering) of atomic particles, particle- and photon-induced desorption and trapping effects must also be considered. The thermal release of dissolved and adsorbed hydrogen occurs in the form of molecules at the wall temperatures of present devices and of plasma systems of the next generations. This leads one to expect that 1. (a) a large hydrogen concentration exists in the subsurface layers during the discharge and even a considerable time after its termination, 2. (b) pronounced memory effects should exist as evidenced during isotope changeover experiments; the behaviour should differ markedly when the second isotope is fed into the system at the start or when it is injected during the discharge. 3. (c) the hydrogen inventory in and its permeation through the wall should be larger than the value evaluated from its solubility at the filling pressure. These effects — and in particular their variation with the wall temperature and with the parameters of the impinging particles — are discussed and compared with available data obtained in tokamaks and in simulation devices. The domains of applicability of relevant computer codes are briefly addressed. The need for additional information on specific material data and existing methods to acquire them are indicated.
Ignition and burning rates of segregated waste combustion in packed beds.
Recent developments in national recycling and re-use programmes for municipal waste have led to segregation of an increasing proportion of waste to enhance material recovery. Several of the segregated streams contain materials that can not viably be re-used or recycled but can be used for energy recovery. In this study, the combustion of cardboard and waste wood was investigated in a small-scale packed bed reactor in order to provide fundamental data for the design/operation of moving bed furnaces. Key parameters of combustion including the ignition and burning rates were evaluated for various air flowrates and compared to the modelling results. Two successive stages of combustion were identified for both samples: the propagation of ignition front into the bed and combustion of the fuel above the ignition front. The burning rate of cardboard reached a peak of about 300 kg/m(2)h at the air flowrate of 936 kg/m(2)h and decreased at higher air flowrates. For waste wood, both the ignition and burning rates increased in the tested range of the air flowrate up to 702 kg/m(2)h, of which the values were very close to those for the cardboard. The model prediction was in good agreement with the test results for waste wood. However, the burning rate for cardboard was under-predicted due to strongly irregular shapes of the fuel.