G. Breitbach

Performance of beryllium, carbon, and tungsten under intense thermal fluxes

Abstract Transient heat loads on a millisecond timescale with deposited energy densities beyond 1 MJ m −2 have been simulated in a plasma accelerator facility (VIKA) and in two high power electron beam teststands (JUDITH, JEBIS). Main objective of these experiments was to study and to compare the behaviour of different plasma facing materials (Be, CFC, W) under heat loads which occur during disruptions in future thermonuclear fusion reactors such as ITER. In these tests special attention was paid to the thermal shock resistance, the processes during melt layer formation, and the resulting material erosion. To perform these tests specific loading techniques and diagnostics have been developed and applied. Among these are high heat flux loading experiments at elevated temperatures ( T > DBTT ) of the test coupons, fast surface pyrometry, and reliable techniques for the quantification of the absorbed energy.

Structural design criteria for HTR — a summary report —

Abstract The results of the German research and development project ‘HTR Design Criteria’ has been presented during the workshop on ‘Structural Design Criteria for HTR’. The work is organized in four areas: (1) technical safety boundary conditions, (2) metallic structural components, (3) prestressed concrete pressure vessel, and (4) graphitic structural components. In this summary report some essentials are discussed for all four areas. On basis of these results, the formulation of structural design (KTA) rules are in preparation.

Investigation of the ratcheting phenomenon for dominating bending loads

For components subjected to strong mechanical and cyclically thermal loads, proof of a limitation of permanent strain is of great significance due to the danger of deformation progressing from cycle to cycle (ratcheting). Within the framework of investigations concerning the load situation of the first wall of a fusion reactor, experimental and theoretical studies on ratcheting were carried out. Progressive deflections were identified under suitable conditions on internally cooled beam-shaped components subjected to cyclic heating and a bending load. A simple beam model can be used for systematic theoretical analysis in this case. Its treatment leads to the development of so-called Bree diagrams with which the structural behaviour can be characterized as a function of the applied primary and secondary stresses. The peculiarities arising under predominant bending loads will be discussed.

Aspects of design codes for metallic high temperature components

Abstract High temperature component design is very important in different industrial fields. This paper discusses work done in Germany on the design of metallic components for the Helium Cooled High Temperature Reactor. Candidate materials for application at the different temperature regions are presented. Criteria are developed to specify the allowable strength values of the materials (S m , S t ) for use in design codes. The following typical relevant phenomena are treated and discussed: • —creep under uniaxial and multiaxial conditions up to temperatures of 1000° C; • —creep fatigue interaction, where holding times during the stress strain cycles have a great influence on the life time; • —creep buckling for pressure loaded shells with initial imperfections; • —creep ratcheting as a phenomenon, where cyclic secondary stresses produce enhanced deformations; • —fracture mechanics at high temperatures with the aspects of creep crack growth and fatigue crack growth.

High heat flux tests on brazed divertor components in electron and ion beam test facilities

Abstract Two different designs of single-tube metallic heat sinks with carbon armour tiles, designed by the Next European Torus (NET) team and manufactured by Metallwerk Plansee, were tested under thermal loading conditions simulating the normal operation parameters expected for the NET/ITER (International Thermonuclear Experimental Reactor) divertor. The experiments were performed in the Material Research Ion Beam Facility (MARION) and the Julich Divertor Test Facility/Hot Cells (JUDITH), both at the Research Centre Julich, and in the JAERI Electron Beam Irradiation Stand (JEBIS) at the Japan Atomic Energy Research Institute. The thermal response of the components was determined by stepwise increase of the heat flux, and the thermal fatigue behaviour was studied by cyclic loading at different heat flux levels. Additionall, finite elements analysis of the temperature and stress fields resulting from thermal loading was performed. No failure of the brazes resulted from exposing the component to a peak heat flux of 20·7 MW/m2 in the screening test. In the thermal fatigue experiments, 300 cycles at 15 MW/m2 were sustained in JEBIS, whereas in MARION, failure occurred after 376 cycles at 10 MW/m2 surface heat flux. The results of the numerical analysis showed good agreement with the values measured in the experiments.

Experimental and theoretical investigations of creep buckling on NiCr 22 Co 12 Mo tubes at 950°C

Abstract The postulated secondary-side depressurization accident in an HTR plant raises the question of whether creep buckling can occur under external pressure in the tubes of the heat exchangers whose cross sections may be slightly eccentric due to manufactural tolerances. Experimental and theoretical investigations were carried out on the problem of creep buckling. Within the scope of these experiments, tubes made of NiCr 22 Co 12 Mo with different adjusted initial ovalities were exposed to pressures of 35–45 bar at 950°C. The buckling times were measured. Theoretical analyses were carried out using a model conceived by Hoff as well as the Finite Element Method, allowing for the scattering of relevant test parameters. Both methods of computation are suitable for investigating the phenomenon of creep buckling. It was found that the calculated buckling times are affected by great uncertainties taking into account the scatter of influential test parameters. In connection with the accidental stressing in an HTR plant it was confirmed that creep buckling is not a problem of high safety-related relevance.

Investigations of creep ratcheting on thick-walled tubes

Abstract The superposition of load-controlled stresses and cyclic thermal stresses at very high temperatures can produce remarkably enhanced creep deformations (creep ratcheting). During an experimental project thick-walled tubes made from ALLOY 800 were loaded by an axial force at temperatures around 900°C. The inductive heating facility was cyclically operated in such a manner that in the tube walls temperature differences of 40°C occurred. Thermal stresses were so induced. The axial elongation was measured. For comparison at the same specimen the elongation was also measured under constant temperatures (at the mean temperature of the cycle). Under cyclic conditions a larger elongation rate was observed. The ratcheting effect was clearly seen. Inelastic stress analysis using simple constitutive creep equations predicted a lower effect.