Copper canister is a central technical barrier for radioactive release from high level nuclear waste in the so called KBS-3 concept planned to be used in Finland and Sweden for disposal of spent nuclear fuel. Canisters will be placed in the granitic bedrock at about 400-500 m depth and surrounded by a layer of bentonite clay planned to protect the canister from any chemical and mechanical damage, and especially acting as a diffusion barrier. While researching for the possibility of stress corrosion cracking in phosphorus microalloyed copper in presence of sulphides in the groundwater, indications were found for a new potential degradation mechanism involving internal diffusion of sulphide. This paper describes the evidence for the new mechanism and discusses the scenarios involving diffusion of sulphide onto the copper canister surface.
In Finland and Sweden nuclear waste disposal is planned to be executed according to the KBS-3-concept. In the concept copper canisters containing the used nuclear fuel are buried in 400-500 m deep in the bedrock. Eventually the surface of the copper canister will come in contact with bentonite pore water which contains corrosive species such as sulphide. In this paper the effect of exposure to sulphide containing bentonite pore water at room temperature on the mechanical properties of oxygen free phosphorous doped copper (CuOFP) was studied with standard ex situ tensile and creep tests. Stress corrosion cracking (SCC) susceptibility of CuOFP was studied with in situ slow strain rate testing (SSRT). Tensile tests performed at room temperature showed a slight trend of degradation in mechanical properties with increasing sulphide concentration. Creep tests were accelerated by performing them at an elevated temperature of T5215uC, but still within the power law creep regime. In creep tests the same kind of trend as in the tensile tests could be observed but when the results were compared to the publicly available CuOFP creep data the variations between the exposed and reference samples lay within the data scatter.
The update of the ASME III design fatigue curve for stainless steel in conjunction with the Fen model described in the NUREG/CR-6909 report has been criticized since publication. Data used to develop curves and models raises more questions than it answers. Material testing in a simulated light water reactor environment is difficult due to the temperature and pressure involved. The experimental challenge makes it tempting to take shortcuts where they should least be taken. Facing and overcoming the challenges, direct strain-controlled fatigue testing has been performed at VTT using a unique tailored-for-purpose EAF facility. The applicable ASTM standards E 606 and E1012 are followed to provide results that are directly compatible with ASME Code Section III. Several earlier PVP papers (PVP2016-63291, PVP2017-65374) report lower than calculated experimental Fen factors for stabilized stainless steels. In this paper new results, in line with the previous years’ conclusions, are presented for nonstabilized AISI 304L tested with dual strain rate waveforms. To model environmental effects more accurately, an approach accounting for the damaging effect of plastic strain is proposed. A draft Fen model, similar in structure to the NUREG model but with additional parameters, is shown to significantly improve the accuracy of Fen prediction.
A tailored-for-purpose environmental fatigue testing facility was previously developed to perform direct strain-controlled tests on stainless steel in simulated PWR water. Strain in specimen mid-section is generated by the use of pneumatic bellows, and eddy current measurement is used as a feedback signal. The procedure conforms with the ASTM E 606 practice for low cycle fatigue, giving results which are directly compatible with the major NPP design codes. Past studies were compiled in the NUREG/CR-6909 report and environmental reduction factors Fen were proposed to account for fatigue life reduction in hot water as compared to a reference value in air. This database exclusively contained non-stabilized stainless steels, mainly tested under stroke control. The applicability of the stainless steel Fen factor for stabilized alloys was already challenged in past papers (PVP2013-97500, PVP2014-28465). The results presented in this paper follow the same overall trend of lower experimental values (4.12–11.46) compared to those expected according to the NUREG report (9.49–10.37). In this paper results of a dual strain rate test programme on niobium stabilized AISI 347 type stainless steel are presented and discussed in the context of the NUREG/CR-6909 Fen methodology. Special attention is paid to the effect of strain signal on fatigue life, which according to current prediction methods does not affect the value of Fen.
Environmental effects of LWR coolant need to be factored in when defining cumulative fatigue usage of primary circuit components. The basis is a set of codified design rules and fatigue design curves, based on experimental data. To accurately quantify environmental effects, the reference curve in air to which fatigue life in water is compared shall be as reliable as possible. Literature studies and accumulated data at VTT reveal that the use of common reference curves for a wide range of austenitic stainless steel alloys and temperatures is unreliable. Some design codes already include measures to consider this but ASME III is not yet among them. The ASME III design curve is adopted from NUREG/CR-6909 and contains no consideration for dependence of temperature or stainless steel grade. Two different stainless steel grades, AISI 304L and 347, have previously been used in environmentally-assisted fatigue experiments at VTT. In this paper, reference curves for the AISI 304L heat are presented at room temperature and 325 °C to complement the curves already available for AISI 347. Demonstration of realistic environmental effect quantification is done using these reference curves as an alternative to the NUREG methodology.
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