A. Weisenburger

Results of steel corrosion tests in flowing liquid Pb/Bi at 420–600 °C after 2000 h

Abstract Corrosion tests were carried out on austenitic AISI 316L and 1.4970 steels and on MANET steel up to 2000 h of exposure to flowing (up to 2 m/s) Pb/Bi. The concentration of oxygen in the liquid alloy was controlled at 10 −6 wt%. Specimens consisted of tube and rod sections in original state and after alloying of Al into the surface. After 2000 h of exposure at 420 and 550 °C the specimen surfaces were covered with an intact oxide layer which provided a good protection against corrosion attack of the liquid Pb/Bi alloy. After the same time corrosion attack at 600 °C was severe at the original AISI 316L steel specimens. The alloyed specimens containing FeAl on the surface of the alloyed layer still maintained an intact oxide layer with good corrosion protection up to 600 °C.

Corrosion Behavior of FBR Candidate Materials in Stagnant Pb-Bi at Elevated Temperature

Three materials proposed for use in fast breeder reactors—FBR grade type 316ss, high chromium type martensitic steel and oxide dispersion strengthened martensitic type steel—were subjected to corrosion tests in stagnant lead-bismuth eutectic (LBE) containing 10-6 wt% of oxygen at 500–650—C for up to 5,000 h. After each test, the specimens were analyzed metallurgically using a scanning electron microscopy with energy dispersive X-ray analyzer and an X- ray diffractometer. It proved that, at temperatures not higher than 550°C, martensitic steels containing chromium to more than 9wt% presented good resistance against corrosion by the formation of spinel layer on the base metals, though outer porous oxide formed on the steels was detached and/or dissolved into LBE. At temperatures above 600°C, the oxide layer thickness diminished. This alteration in corrosion behavior seems to be ascribable to a change taking place at 570°C in the stable form of iron oxide from magnetite to wustite. At the temperatures, dissolution attack was observed at some portions. It was estimated that the oxide became to lose its adhesion to the base metal.

The Issue of Accelerator Beam Trips for Efficient ADS Operation

Abstract The development of accelerator-driven systems (ADSs) is motivated by the potential of these machines to reduce the volume and the radiotoxicity of accumulated nuclear waste, more particularly that of minor actinides currently generated by the operation of existing pressurized water reactors. The reduction of both volume and radiotoxicity of nuclear waste is achieved by transmutation and fission of minor actinides into less-active isotopes or shorter-lived by-products. Various technical challenges exist regarding designing reliable and efficient ADSs. The key points are very much linked to the design of the spallation module, the assurance that reactivity remains below criticality under any circumstances, and the accelerator reliability. This paper addresses the latter two challenges imposed on the accelerator in order to assure safe and reliable ADS operation. It discusses the possibility of performing online absolute reactivity measurements and the limits in the number of allowable accelerator beam trips, which might impede plant integrity and/or plant efficiency.

Surface Layer Dynamics During E-Beam Treatment

The interaction of a pulsed intense electron beam with a metal target leads to rapid heating and subsequent cooling of the surface layer, accompanied by a series of phase transitions among the solid, liquid, vapor, and plasma phase. As a consequence of the treatment, depending on the beam parameters, the metal target is eroded and a topographical pattern (waviness, craters, etc.,) evolves on its surface. Surface roughening, a major drawback of pulsed intense electron beam treatment, is well known but lacks comprehensive understanding. In this paper, the process of pulsed intense electron beam interaction with metal targets is studied with special attention to the dynamics of the target surface layer and the development of surface roughness. The pulsed electron beam facility GESA generates electron beams with power density 0.5-2 MW/cm2, electron energy up to 120 keV, and pulse duration up to 200 μs. Different fast in situ diagnostics are applied to study the various processes occurring at the target surface: 1) melting and resolidification are visualized by time and space resolved imaging of the surface specular reflectivity; 2) spectroscopy is used to characterize the plasma phase adjacent to the target surface; 3) the evolution of irregularities and bubbles at the surface is studied by high-resolution microscopy; and 4) a stroboscopic imaging technique is applied to catch the evolution of the surface topography. The experimental data are compared with numerical simulations of heat transfer. All results and processes involved in pulsed intense electron beam treatment are discussed with respect to the target surface layer dynamics.

The consequences of a sharp temperature change in the fuel pins of an accelerator-driven subcritical system

Abstract The effect of temperature changes and in particular those that are accompanied by strong gradients was extensively investigated for fast reactors. Subcritical systems designed for their transmutation ability are to some extent similar to critical power reactors in their subassembly structure. However, they differ in two main aspects. First, the coolant in a subcritical system is lead or lead-bismuth eutectic (LBE) and not sodium, and second, the main cause for steep temperature gradients in a fast power reactor is sudden control rod insertion, or scram, whereas in subcritical systems shutdown of the accelerator and its proton beam is the main cause for temperature gradients. Furthermore, the increased probability of operational interruptions in an accelerator-driven system is largely due to the instability of the accelerator generating the proton beam. This study uses the knowledge gained from fast reactors as a preliminary reference and concentrates further on the unique features of the proposed subcritical systems. In particular, the effect of beam trips on the fuel pin integrity is evaluated as a function of the temperature gradients and the duration of the beam trips. It seems, however, that the largest hazard to the fuel pin integrity is due to the lead (or LBE) coolant. In particular, the stability of the protective oxide layer built on the clad surface with the lead coolant appears quite sensitive to sudden temperature changes. In the second part of this study, several available experimental results show that even very moderate temperature changes are sufficient to cause crack formation in the oxide layer thereby exposing the clad surface to enhanced LBE corrosion. In the worst case, complete exfoliation of the magnetite outer layer is observed. As a consequence, clad failure probability due to corrosion is considerably increased.

Application of pulsed electron beams for improvement of material surface properties

The pulsed electron beam facilities GESA I and GESA II were developed in cooperation between Efremov Institute St. Petersburg, Russia and the Research Center Karlsruhe (FZK), Germany for large area surface treatment with beam diameter of 4 – 10 cm. It melts the material surface down to a depth of 10–100 µm. Technological applications are improvement of high temperature oxidation resistance of MCrAlY and of its bonding properties to TBCs, prevention of liquid metal attack on steels, surface hardening and increase of wear resistance and lowering of sea water corrosion and high cycle fatigue of aircraft engine blades. An overview is given on the status of the current applications.

Influence of composition and heating schedules on compatibility of FeCrAl alloys with high-temperature steam

Abstract FeCrAl alloys are proposed and being intensively investigated as alternative accident tolerant fuel (ATF) cladding for nuclear fission application. Herein, the influence of major alloy elements (Cr and Al), reactive element effect and heating schedules on the oxidation behavior of FeCrAl alloys in steam up to 1500 °C was examined. In case of transient ramp tests, catastrophic oxidation, i.e. rapid and complete consumption of the alloy, occurred during temperature ramp up to above 1200 °C for specific alloys. The maximum compatible temperature of FeCrAl alloys in steam increases with raising Cr and Al content, decreasing heating rates during ramp period and doping of yttrium. Isothermal oxidation resulted in catastrophic oxidation at 1400 °C for all examined alloys. However, formation of a protective alumina scale at 1500 °C was ascertained despite partial melting. The occurrence of catastrophic oxidation seems to be controlled by dynamic competitive mechanisms between mass transfer of Al from the substrate and transport of oxidizing gas through the scale both toward the metal/oxide scale interface.

Development of high temperature liquid metal test facilities for qualification of materials and investigations of thermoelectrical modules

Three classes of experimental liquid metal facilities have been completed during the LIMTECH project aiming the qualification of materials, investigation of thermoelectrical modules, investigation of sodium transitional regimes and fundamental thermo-dynamical flows in concentrating solar power (CSP) relevant geometries. ATEFA facility is dedicated to basic science investigation focussed on the alkali metal thermal-to-electric converter (AMTEC) technology. Three SOLTEC facilities are aimed to be used in different laboratories for long term material investigation sodium environment up to a 1000 K temperature and for long term tests of AMTEC modules. The medium scale integral facility KASOLA is planned as the backbone for CSP development and demonstration.

Design and construction of the ATEFA facility for experimental investigations of AMTEC test modules

The Alkali Metal Thermal-to-Electric Converter (AMTEC) is an electrochemical cell that requires a high temperature heat source to generate electricity. At KIT the AMTEC technology is being investigated focusing on the use of concentrating solar energy as heat source. First a review on AMTEC technology is given. Further, the design and realization phases of the AMTEC Test Facility (ATEFA) and AMTEC test cell are presented, including the data acquisition and control system and two key technology developments: a ceramic to metal joint for high temperatures (800 – 1000 °C) and the magnetron sputtering of cathode layers on the ceramic electrolyte. The sheet resistance of several electrode samples has been analyzed using the 4-point probe technique and the microstructure of the cathode layer has been examined using the scanning electron microscopy (SEM).

Numerical investigations of radially converging electron beam generated in cylindrical GESA IV facility

The cylindrical triode-type electron accelerator GESA IV was developed for treatment of metallic rods, specifically cladding tubes for nuclear reactors. The target (anode) diameter is therefore fixed at about 10 mm by the application, which leads to problems of homogeneity and stability of the radially converging beam. Due to the large difference between cathode diameter (about 150 mm) and anode diameter, a virtual cathode may form between grid and anode, electrons may miss the target and start to circulate around the anode, and the self-induced magnetic field may lead to large distortion of the electron trajectories. In this study, we investigate the influence of various crucial effects on the beam performance by PIC code simulations using the software package MAGIC. In particular, we consider monopolar and bipolar flow (i.e., the influence of ions generated at the target and moving towards the cathode), the effects of scattering at the grid and of backscattering at the target, the angular velocity spread of the electrons at emission, and the influence of the grid potential. The numerical results are compared with experiments performed at the GESA IV facility, where the influence of the target material and of the self-induced magnetic field on the beam performance are investigated.