In-situ investigation of hydrogen diffusion in Zircaloy-4 by means of neutron radiography

Abstract

Abstract. The hydrogen diffusion from the gas phase into Zircaloy-4 solid cylinders was investigated at various temperatures between 823 and 1473 K. Diffusion coefficients were fitted using axial hydrogen distributions for all temperatures investigated. The activation energy of the hydrogen diffusion was calculated for temperatures at which the bcc β phase is stable. The determined value of 48.3 kJ mol -1 K -1 is significantly higher than values published by other authors. At lower temperatures the hydrogen uptake is connected with a phase transformation from the hcp α-to the bcc β-phase. The spatial phase distribution and the hydrogen concentrations in the two phases were analysed. IntroductionThe progression of accidents in nuclear power reactors are studied experimentally at the Karlsruhe Institute of Technology (KIT, the former Research Centre Karlsruhe -FZK) in the framework of the QUENCH program. The investigations comprise loss of coolant accidents (LOCA) and severe accidents (accidents beyond LOCA). The processes occurring during such accidents are simulated on fuel rod bundle scale in the large scale QUENCH tests. The very expensive tests were supplemented by separate-effect tests on laboratory scale. In the framework of this program, the hydrogen diffusion in the β-phase of the classic nuclear fuel rod cladding material Zircaloy-4 (Zry-4) was studied in-situ by means of neutron radiography. During loss of coolant accidents, free hydrogen is produced by the reaction between the hot zirconium alloy cladding material covering the fuel rods and steam. Very simplified the reaction can be described by:(1)Part of the produced hydrogen is released as gaseous H 2 into the reactor environment. The release can result in hydrogen detonations as happened in the Fukushima accident. The other part of the produced hydrogen is absorbed by the remaining metallic zirconium alloy. It degrades the mechanical properties and in particular the thermo-shock stability. An emergency cooling to terminate the accident can then result in a destruction of the cladding tube connected with release and redistribution of fuel and fission products. Fig. 1

Kinetic Model of Incipient Hydride Formation in Zr Clad under Dynamic Oxide Growth Conditions

“…Earlier literature on these measurements [25,84,85] reveals pre-factors ranging from 7.00 × 10 −8 to 4.15 × 10 −7 m·s −1 and migration energies between 0.3 and 0.5 eV, all in a similar temperature range. More recent experiments and molecular dynamics simulations are also consistent with these values [86,87].…”

H2 PERMEATION BEHAVIOR OF Cr2AlC AND Ti2AlC MAX PHASE COATED ZIRCALOY-4 BY NEUTRON RADIOGRAPHY

“…In order to suppress hydrogen uptake of the Zry-4, the substrate specimens were pre-oxidized at 1173 K and normal pressure for 30 min in Ar/O 2 atmosphere to produce a dense ZrO 2 oxide layer of around 30 µm thickness [9]. The pre-oxidized layer can effectively suppress the hydrogen uptake besides the uncoated area during hydrogen permeation experiments [9]. The oxide layer was removed at one base plane of the cylinders by mechanical grinding and polishing allowing hydrogen uptake through this plane.…”

Quantitative analysis of hydrogen uptake, diffusion and distribution in nuclear fuel rod claddings made of zirconium alloys

“…It can be fi tted with a solution of Ficks' law to determine the diffusion coeffi cient. Details of the experiments and discussion of the results are given in [6,11,13] …”