Investigation of thermally sensitised stainless steels as analogues for spent AGR fuel cladding to test a corrosion inhibitor for intergranular stress corrosion cracking

Whillock, G.O.H.; Hands, B. J.; Majchrowski, Tom; Hambley, David

doi:10.1016/j.jnucmat.2017.10.017

Cited by 15 publications

(15 citation statements)

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“…Although the stabilising effect of Nb has been studied in the past [16], the effect of these NbC precipitates on corrosion behaviour is not clear. Recent research on this material in open literature has focussed on irradiation effects [17] [18] [19], thermal processing [20] [21] [22], the use of novel corrosion techniques [23], evaluating the alloy in conditions relevant to a pond storage environment [24] [25] [26] [27] [28], dry storage [29] [30], and final geological disposal [31].…”

Section: Introductionmentioning

confidence: 99%

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

et al. 2020

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Section: Introductionmentioning

confidence: 99%

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

et al. 2020

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“…Figure 1a shows a 3D HS-AFM topographic map collected from a sample of AISI Type 304 stainless steel heat treated at 600°C for 70 h to produce a thermally sensitised microstructure 33 . The material and sensitisation conditions were selected to produce material that may act as a proxy to specific nuclear applications 33 , whilst also being relevant within other industries. The microstructure of this material was measured by electron backscattered diffraction (EBSD) as shown in Supplementary Fig.…”

Section: Introductionmentioning

confidence: 99%

Observation of stress corrosion cracking using real-time in situ high-speed atomic force microscopy and correlative techniques

et al. 2021

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Contact-mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in situ stress corrosion cracking (SCC) with nanometre resolution on AISI Type 304 stainless steel in an aggressive salt solution. SCC is an important failure mode in many metal systems but has a complicated mechanism that makes failure difficult to predict. Prior to the in situ experiments, the contributions of microstructure, environment and stress to SCC were independently studied using HS-AFM. During SCC measurements, uplift of grain boundaries before cracking was observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam milling revealed a network of intergranular cracks below the surface lined with a thin oxide, indicating that the SCC process is dominated by local stress at oxide-weakened boundaries. Subsequent analysis by atom probe tomography of a crack tip showed a layered oxide composition at the surface of the crack walls. Oxide formation is posited to be mechanistically linked to grain boundary uplift. This study shows how in situ HS-AFM observations in combination with complementary techniques can give important insights into the mechanisms of SCC.

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“…Moreover, the formation of Nb carbonitrides prior to the cladding lifetime in the reactor is meant to control the austenite grain size, increase the material's resistance to thermal creep in the oxidising AGR core environment (Knowles, 1977), and also trap carbon and nitrogen so as to prevent the formation of chromiumcontaining carbides and nitrides at the grain boundaries of the matrix (Ramaswamy & West, 1970;Sourmail, 2001). The discharged fuel and the cladding are planned to be stored upon removal from the reactor core in caustic-dosed water ponds for at least 25 years (Whillock et al, 2018). A reduction in the local Cr content below 12wt.% would increase the susceptibility of this steel to intergranular corrosion attacks during medium-to-long term storage in aqueous environments ( Al-Shater et al, 2017;Whillock et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…The discharged fuel and the cladding are planned to be stored upon removal from the reactor core in caustic-dosed water ponds for at least 25 years (Whillock et al, 2018). A reduction in the local Cr content below 12wt.% would increase the susceptibility of this steel to intergranular corrosion attacks during medium-to-long term storage in aqueous environments ( Al-Shater et al, 2017;Whillock et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…Despite the formation of Nb(C, N) particles during the steel processing, yielding a reduction in free interstitials in the matrix available to bind with Cr under reactor service conditions, significant Cr depletions have been detected in specific fuel cladding pins (i) at the (sub)surface due to the C intake from the CO 2 -based coolant leading to the formation of Cr carbides, and (ii) through the cladding wall thickness triggered by the evolution in time of the microstructural damage caused by the neutron bombardment in the reactor core (Taylor, 1986). As a consequence of the latter process, the steel cladding can become susceptible to intergranular corrosion attacks and stress corrosion cracking in storage water ponds containing a concentration of ࣙ0.2 mg L -1 Cl − (Whillock et al, 2018). This phenomenon, termed radiation-induced sensitisation (Kenik et al, 1991), is understood to be a consequence of the evolution of self-interstitials and vacancies after the heat spike of the radiation-induced displacement cascade, together with the coupling of point defect diffusion with a biased flux of Cr atoms away from grain boundaries, and concomitantly of Ni, Si and P towards grain boundaries (Was, 2007;Ardell & Bellon, 2016).…”

Section: Introductionmentioning

confidence: 99%

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Recrystallisation behaviour of a fully austenitic Nb‐stabilised stainless steel

Barcellini

Dumbill

Jimenez-Melero

2018

Journal of Microscopy

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Summary We have performed an in‐depth characterisation of the microstructure evolution of 20Cr‐25Ni Nb‐stabilised austenitic stainless steel during 1 h isochronal annealing up to 1100°C using scanning electron microscopy. This steel grade is used as cladding material in Advanced Gas‐cooled fission reactors, due to its resistance to thermal creep and oxidation. The initial deformed microstructure undergoes recrystallisation via a strain‐induced boundary migration mechanism, attaining a fully recrystallised microstructure at 850°C. A number of twins are observed in the vicinity of deformation bands prior to the start of recrystallisation. New Nb(C, N) particles form gradually in the microstructure, and the particle dispersion presents a maximum volume fraction of 2.7% at 930°C. At higher temperatures, the smaller particles become unstable and gradually dissolve in the matrix. Consequently, the Zener pinning pressure exerted on the grain boundaries is progressively released, triggering the growth of the austenite grains up to an average size of ∼47 μm at 1100°C. The observed temperature window for recrystallisation and grain growth can be predicted by a unified model based primarily on the migration of high‐ and low‐angle grain boundaries. Lay Description Austenitic stainless steel containing high percentage of chromium and nickel is currently used as fuel cladding material in the British Advanced Gas‐cooled Reactors (AGR). This material has been chosen because of its high resistance to thermal creep and corrosion, both enhanced by the presence of a fine dispersion of carbonitrides precipitated during the cladding thermomechanical processing. During the time spent in the reactor core, few fuel cladding elements can become susceptible to local chromium depletion at grain boundaries, which is ascribed to the time evolution of the microstructural damage caused by the neutron bombardment in the reactor core. This depletion might increase the susceptibility of this steel to intergranular corrosion attacks during medium‐to‐long term storage of spent fuel elements in water ponds. The severity of the local chromium depletion depends not only on the irradiation conditions, but also on the grain boundary geometry. We have investigated the recovery, recrystallisation and grain growth of AGR stainless steel during 1 h annealing at selected temperatures relevant for the thermomechanical processing of the steel claddings, focusing on the formation and evolution of grain boundaries and second phases. These two features play a key role in the progression of the neutron damage and the subsequent development of local chromium depletion during reactor service operations. A deep understanding of the mechanisms and conditions behind their formation during the thermomechanical processing of the cladding material and their interaction with each other constitutes the foundation to evaluate, and potentially mitigate, the effect of irradiation on the cladding material.

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Investigation of thermally sensitised stainless steels as analogues for spent AGR fuel cladding to test a corrosion inhibitor for intergranular stress corrosion cracking

Cited by 15 publications

References 15 publications

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

Observation of stress corrosion cracking using real-time in situ high-speed atomic force microscopy and correlative techniques

Recrystallisation behaviour of a fully austenitic Nb‐stabilised stainless steel

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