SANS examination of irradiated RPV steel welds during in-situ annealing

doi:10.1016/j.jnucmat.2015.02.036

Journal of Nuclear Materials

Volume 461, June 2015, Pages 45-50

https://doi.org/10.1016/j.jnucmat.2015.02.036 Get rights and content

Abstract

An in-situ annealing experiment was performed using SANS measurements to examine the distribution and thermal stability of irradiation-induced solute clusters in RPV steel welds. Samples were sequentially annealed for 30 min at ∼50 °C intervals in the temperature range 295–497 °C. A methodology was developed to correct the observed data to allow for increased thermal diffuse scattering during annealing which enabled analysis of the changes in coherent scattering in isolation.

Results for a low-Ni weld irradiated at low temperature showed apparent decreases in the volume fraction of solute clusters during annealing. However the cluster size was unaffected and these results could have arisen from reduced scattering contrast due to compositional changes, rather than cluster dissolution. A similarly irradiated high-Ni weld exhibited cluster coarsening at high annealing temperatures. Samples of both welds irradiated at a higher temperature were relatively unaffected by annealing except at high temperatures where some shrinkage, indicative of cluster dissolution, occurred.

Introduction

Irradiation hardening and embrittlement of reactor pressure vessel (RPV) steels is generally attributed to the formation of matrix defects and solute clusters (often enriched in Cu, Mn, Ni and Si or just Mn, Ni and Si). A number of experimental techniques may be used to examine irradiation damage in RPV steels, although no single technique is currently able to provide a complete description of the damage structures. Small angle neutron scattering (SANS) examination is a useful technique for investigating the distribution of irradiation-induced features (including solute clusters, precipitates, voids and gas bubbles) which give rise to changes in scattering contrast. However detailed analysis of the data is not straightforward and additional information regarding the composition of the irradiation-induced features, for example from atom probe tomography (APT), is required to enable a fuller interpretation of the SANS measurements to be made [1]. Even so, some aspects of the structure of irradiation-induced solute clusters, such as the distribution of solutes within them, their vacancy content, and the nature of cluster–matrix interfaces, remain uncertain.

Additional information about the structure and stability of the defects formed in RPV steels during irradiation may be obtained from post-irradiation annealing experiments. Although post-irradiation annealed RPV steels have subsequently been examined at room temperature using SANS by, for example, Ulbricht et al. [2] and Wagner et al. [3], SANS measurements during in-situ annealing have not previously been attempted. Scarcity of material is a limiting factor for many studies of irradiated materials, and the ability to gather more information from a single specimen is a major advantage of an in-situ annealing experiment compared to the examination of multiple specimens given different annealing treatments.

The results obtained from SANS measurements made at the Institut Laue-Langevin (ILL) during in-situ annealing of irradiated RPV steel weld samples are reported here.

Section snippets

Experimental

Samples from two high-Cu ferritic steel welds, one with a low level of Ni and the other with a high Ni content, were examined in this experiment. Material compositions are given in Table 1. The samples, of dimensions 10 × 10 × 2 mm³, had been irradiated in a test reactor at a dose rate of ∼6 × 10⁻⁹ dpa s⁻¹ to doses in the range 8–24 mdpa at temperatures ranging from 220 °C to 310 °C, (Table 2).

In-situ annealing experiments on the irradiated samples were carried out using a vacuum furnace provided by ILL.

Assessment of thermal diffuse scattering

Increases in the absolute differential scattering cross-section, I(Q) (barns/steradian–Fe atom), at high Q were generally evident in the irradiated samples during high temperature annealing relative to room temperature measurements. Examples of this effect are shown in Fig. 1(a) for the low-Ni sample SH533 and in Fig. 1(b) for the high-Ni sample WV388. Increases in scattering intensity at high Q are evident in these samples at all annealing temperatures, even though I(Q) decreases at low Q at

Results

A maximum entropy algorithm [8], [9] was used to fit the coherent scattering cross-section data as a function of Q to obtain volume-weighted particle size distributions. Since the composition of the (assumed to be spherical) irradiation-induced/annealed particles is not known precisely, a nominal contrast factor of 1 × 10²⁸ m⁻⁴ was used in the fitting process and the resulting particle volume fractions are therefore relative rather than absolute. Relative particle number densities were also

Discussion

Although SANS examination of irradiated RPV steels during in-situ annealing offers advantages over the examination of separately annealed samples (in particular in economy of material usage and continuous monitoring of the annealing process as a function of time/temperature) data assessment is further complicated by the need to correct for increases in the intensity of incoherent thermal diffuse scattering. For this reason it is important to utilise as high a Q range as possible for the SANS

Summary and conclusions

An in-situ annealing experiment has been performed using the SANS instrument D11 at ILL to examine the distribution of irradiation-induced solute clusters in RPV steel welds. Samples from two high-Cu ferritic steel welds, one with low Ni and the other with high Ni content, were examined. Samples were irradiated to doses in the range 8–24 mdpa at temperatures from 220 °C to 310 °C. The irradiated samples were initially examined at room temperature, then sequentially annealed at 295, 347, 397, 447

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SANS examination of irradiated RPV steel welds during in-situ annealing

Abstract

Introduction

Section snippets

Experimental

Assessment of thermal diffuse scattering

Results

Discussion

Summary and conclusions

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Solid State Phys.

Proc. Phys. Soc. A