Permeation Reduction Factor Research Articles

Hot-dip aluminium deposit as a permeation barrier for MANET steel

A hot-dip process developed in Forschungzentrum Karlsruhe (FzK), was used to produce a hydrogen permeation barrier on MANET II steel. The hot-dip aluminising method produced an intermetallic layer of Al 3Fe by immersing the specimens in molten aluminium at 1023 K for either 5 or 2 min. Subsequent heat treatment of the samples at 1023 K for 15 and 30 h, respectively, in air transformed the brittle Al 3Fe layer into a softer phase and additionally produced alumina (Al 2O 3) on the surface. All processes were carried out at temperatures below 1053 K (above Ac 1b=1053 K the austenitisation of MANET II begins) in order to avoid the repetition of the whole MANET II heat treatment (1348 K, 30 min fast cool; 1023 K, 2 h). The maximum obtained permeation reduction factor (PRF), compared with the uncoated MANEA II steel, has been obtained for the specimens oxidised for 15 h in air: yielding PRF’s of 260 at 743 K and 1000 at 573 K.

LIBRETTO-3: modelling tritium extraction/permeation and evaluation of permeation barriers under irradiation

Permeation barriers are required in order to limit the size and cost of the detritiation plants for future fusion reactor blankets of the water-cooled Pb 17Li type. The LIBRETTO irradiations were performed to evaluate the efficiency of permeation barriers under high flux reactor (HFR) conditions. Tritium extraction and permeation characteristics from Pb 17Li under variable temperatures 553–723 K, H 2 doping (0–1 vol%) and purge gas flow rates 20–100 scc/min were tested in LIBRETTO-3. An external TiC coating, an internal (TiC + Al 2O 3), both produced by chemical vapour deposition (CVD), and an internal Al 2O 3 produced by pack cementation (PC) on AISI 316L steel were tested as permeation barriers. The release mechanisms, experimental uncertainties and method for permeation barriers qualification are presented. As a result permeation reduction factors (PRF) at 0.1 dpa of 17 and 34 were obtained for the CVD-Al 2O 3 at 498 K and for the PC-Al 2O 3 at 508 K, respectively. These values were confirmed by a residence time analysis and are higher than in a preliminary analysis [1].

Hydrocarbon barrier performance of plasma-surface-modified polyethylene

Plasma modifications were applied on the inner surfaces of high-density polyethylene bottles. The methods applied include Ar gas plasma treatment, plasma polymerization with tetrafluoroethylene (TFE), trimethylsilane (TMS) + O2 (1:4), CH4, and C2H2 monomers, plasmainduced acrylic acid grafting polymerization, and C2H2 plasma polymerization plus acrylic acid plasma polymerization. Solvent weight-loss data are reported primarily for the n-hexane/HDPE bottle system. The best permeation reduction factor of 0.03 was obtained with C2H2 plasma polymerization at a high energy level followed by acrylic acid plasma polymerization at a low energy level. C2H2 plasma polymerization at an energy level of 1010 J/kg and C2H2 plasma polymerization followed by acrylic acid grafting polymerization offer a similar permeation reduction factor of 0.07. A combination of improved surface polarity and tightness of the surface is responsible for remarkable reductions in permeation rates. © 1996 John Wiley & Sons, Inc.

Tritium Confinement Requirements for the Water-Cooled Pb-17Li Blanket for DEMO

In the water-cooled liquid Pb-17Li blanket concept for DEMO the limitation of tritium permeation from the breeder material into the cooling water will be required. In order to find out on what conditions this tritium permeation remains within reasonable limits, a 1-d FEM code was developed which evaluates the tritium partial pressure in Pb-17Li, the tritium inventory in the blanket material, and the tritium permeation from the Pb-17Li into the cooling water as a function of the permeation reduction factor of a barrier and the efficiency of the tritium extraction from Pb-17Li. With a parametric study the conditions were identified which allow a permeation rate of as little as 1 g.d -1 without pushing the requirements for permeation barriers and extraction efficiencies excessively far. An example is a barrier with a permeation reduction factor of 75 together with an extractor efficiency of approximately 83%. In these conditions the expected tritium inventory is attributed to approximately one third to the martensitic blanket structure (some ten grams) while two thirds will be found in the Pb-17Li. These inventory values are two orders of magnitude lower than in solid breeder blankets and are thus not considered a critical issue.

Tritium/hydrogen barrier development

A review of the hydrogen permeation barriers which can be applied to the structural metals used in fusion power plants is presented. Both implanted and chemically available hydrogen isotopes must be controlled in fusion plants. The need for permeation barriers appears strongest in Pb-17Li blanket designs, although barriers are also necessary for other blanket and coolant systems. Barriers which provide greater than a 1000- fold reduction in the permeation of structural metals are desired. In laboratory experiments, aluminide and titanium ceramic coatings provide permeation reduction factors (PRFs) of 1000 to over 100 000 with a wide range of scatter. The rate-controlling mechanism for hydrogen permeation through these barriers may be related to the number and type of defects in the barriers. Although these barriers appear robust and resistant to liquid metal corrosion, irradiation tests which simulate blanket environments result in very low PRFs in comparison with laboratory experiments, i.e. less than 150. It is anticipated from fundamental research activities that the radiation- and electric-field-induced enhancement of hydrogen diffusion in oxides may contribute to the lower PRFs during in-reactor experiments.

LIBRETTO-3: Performance of tritium permeation barriers under irradiation at the HFR Petten

The LIBRETTO-3 irradiation was performed at the HFR Petten during 77 full power days in three cycles to compare the efficiency of three different tritium permeation barriers in presence of Pb17Li to uncoated AISI 316L steel. For this purpose four steel capsules ( ⊘ 0 = 10 mm, ⊘ i , l = 300 m ) were filled with 28 g Pb17Li. The coatings included CVD TiC (outside), CVD TiC + Al 2O 3 (inside), and pack cementation aluminisation (inside). The generated tritium was partly extracted by bubbling, partly it permeated through the capsules. Permeated and extracted tritium were measured as a function of temperature (280–450°C), H 2 doping (0–1 vol%) and purge gas flow rate. The driving partial pressure in the coated capsules were from an extraction model calibrated by the uncoated capsule for which tritium partial pressure could be calculated. In LIBRETTO-3 conditions, the best barrier was pack cementation aluminisation. The first interpretation of the experiment could, however, not confirm permeation reduction factors of 100 or more expected from this barrier.

Mathematical Model Applied to Permeation Problems of Process Designs for Tritium Recovery from Liquid Blankets

A mathematical model has been developed to determine the amount of tritium that permeates the cooling circuit of a tritium breeding blanket containing the liquid eutectic alloy 17Li-83Pb. This model, which has been applied to phase 2A of the International Tokamak Reactor/Next European Torus project, is used to predict the effect of the operating conditions of the blanket, as well as those of a spray tower employed as a tritium recovery unit, and the kinetic parameters for the permeation and desorption processes. The results of this theoretical study indicate that the amount of permeated tritium proved to be not very different for the maximum (10.82 kPa/sup 1/2/ . m/sup 3/(mol . T)/sup -1/) and minimum (0.7 kPa/sup 1/2/ . m/sup 3/(mol . T)/sup -1/) values of Sievert's constant (K/sub s/) existing in literature. This amount, moreover, can be reduced to 0.1 to 0.01 g/day of tritium by the presence of small oxide barriers (a permeation reduction factor of ..cap alpha..approx. =100) on the cooling tubes and by the easy operating conditions of the spray tower, which include a droplet diameter of 0.5 mm; a tritium pressure of 0.13 kPa at 673K; and a residence time of 0.5 s.