A. Jianu

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.

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.

Corrosion, Corrosion Barrier Development and Mechanical Properties of Steels Foreseen as Structural Materials in Liquid Lead Alloy Cooled Nuclear Systems

A key problem in development of heavy liquid metal cooled nuclear energy and transmutation reactors is the corrosion of structural and fuel cladding materials in contact with the liquid metal. Lead and lead bismuth attack unprotected steel surfaces by dissolution of the metallic components into the liquid metal. It is common understanding that oxide scales on the surface provide the best protection against dissolution attack. However, at temperatures above 500°C austenitic steels suffer from severe dissolution attack, while martensitic steels form thick oxide scales, which hinder heat transfer from the fuel pins and which may break off and eventually lead to a blocking of the coolant channel. Above 500°C steels have to be protected by stable, thin oxide scales. A well understood measure is alloying of stable oxide formers into the surface. Al has shown its ability to form such oxide scales. In the range of 4–10 wt% Al on the surface a stable thin alumina scale is formed by Al diffusion to the surface and selective oxidation. The alumina scale grows only very slowly and prevents migration of oxygen into the steel as well as migration of steel components onto the surface. A number of corrosion experiments showed the good protective behaviour of Al scales in LBE with 10−6 wt% oxygen up to 650°C and for exposure times up to 10000 h. Alloying Al into the surface was done by diffusion processes and also by pulsed electron beam (GESA) melting of a thin surface layer on which Al or an Al containing alloy was precipitated before. This presentation gives an overview on investigations of the steel behaviour in HLM environment carried out to explore their suitability for systems with Pb/LBE coolants. Results of experiments with static and flowing LBE are discussed. The behaviour of steels examined and their respective application ranges are described. Part of the presentation deals with protective barrier development on the steel surface by alloying of Al and its effect on the corrosion resistance. Furthermore the influence of parameters like stresses in the cladding wall, creep behaviour, different flow velocities of the LBE and changing temperatures and oxygen concentrations in LBE is discussed.Copyright

Overview on applications of materials modifed by intense pulsed electron beams

Summary form only given. Surface modification of materials using intense pulsed electron beams result in an improvement of several properties like wear, corrosion and oxidation resistance. Three different treatment modes can be distinguished: rapid melting and solidification, surface alloying of coatings into the bulk and surface-fusing of coatings to the bulk. All three versions of surface treatment were performed using the GESA facilities having following parameters: accelerating voltage 80-400 kV; power-density 2-6 MW/cm2; beam-diameter 4-10 cm; pulse-duration 4-250 mus. Such pulses applied on material surfaces lead due to the rapid re-solidification to a change in microstructure and in the case of surface alloying also to a different chemical composition. The change in microstructure of 16MnCr5 steel after the GESA treatment increases the hardness by 60 to 80%. Gears of such material treated by GESA and tested under realistic conditions showed an increase of wear resistance by a factor of 6 to 8. Other applications are stationary gas turbine blades. There the thermal cycle stability of thermal barrier coatings deposited on HVOF sprayed MCrAlY coatings could be increased by a factor of two compared to none treated HVOF sprayed coatings. Here the reduced surface roughness in combination with an almost theoretical density of the treated layer is the major distinction of the GESA process in comparison to the non treated case. Beside rapid solidification surface alloying is an other subject of recent investigations. For example, surface alloying of Zr into MCrAlY reduces the growth rate of alumina scales under oxidizing atmosphere at 950degC. However the Zr content after treatment must below 1 wt% to minimize the brittle cracking of the surface layer after cooling down. Surface alloying of Al into various steels using an intense pulsed electron beam increase its oxidation resistance in liquid lead alloys by the selective formation of an alumina scale. More than 1000 h of exposure without significant scale growth was already observed. Surface-fusing of FeCrAlY coatings is investigated as a corrosion protection layer in liquid lead alloys too. Such layers show the same positive oxidation behaviour like the Al-surface alloyed materials. The influence of surface fused coating on the mechanical properties is investigated performing low-cycle-fatigue tests and pressurized tubes. Both tests do not show any negative response of the surface modified layer onto the mechanical properties.