Deuterium Pickup Research Articles

Towards quantitative analysis of deuterium absorption in ferrite and austenite during electrochemical charging by comparing cyclic voltammetry and cryogenic transfer atom probe tomography

Hydrogen embrittlement mechanisms of steels have been studied for several decades. Understanding hydrogen diffusion behavior in steels is crucial towards both developing predictive models for hydrogen embrittlement and identifying mitigation strategies. However, because hydrogen has a low atomic mass, it is extremely challenging to detect by most analytical methods. In recent years, cryogenic-transfer atom probe tomography (APT) of electrochemically-deuterium-charged steels has provided invaluable qualitative analysis of nanoscale deuterium traps such as carbides, dislocations, grain boundaries and interfaces between ferrite and cementite. Independently, cyclic voltammetry (CV) has provided valuable analysis of bulk hydrogen diffusion in steels. In this work, we use a combination of CV and cryogenic-transfer APT for quantitative analysis of deuterium pickup in electrolytically charged pure Fe (ferrite) and a model austenitic Fe18Cr14Ni alloy without any second phase. The high solubility and low diffusivity of hydrogen in austenite versus ferrite and potential influence of oxide layer are highlighted to result in clear observable signatures in CV and cryogenic-transfer APT results. The remaining challenges and pathway for enabling quantitative analysis of hydrogen pick up in steels is also discussed.

Corrosion and deuterium uptake of Zr-based alloys in supercritical water

To increase the thermodynamic efficiency above 40% in nuclear power plants, the use of supercritical water as the heat transport fluid has been suggested. Zircaloy-2, -4, Zr–Cr–Fe, Zr–1Nb and Zr–2.5Nb were tested as prospective fuel cladding materials in 30MPa D2O at 500°C. Zircaloy-2 showed the highest rates of corrosion and hydriding. Although Zr–Cr–Fe initially showed a very low corrosion rate, it displayed breakaway corrosion kinetics after 50h exposure. The best-behaved material both from a corrosion and hydrogen uptake point of view was Zr–2.5Nb. However, the Zr–2.5Nb oxide growth rate was still excessive and beyond the current CANDU®11CANDU – CANada Deuterium Uranium is a registered trademark of Atomic Energy of Canada Ltd. design allowance. Similar coupons, coated with Cr, were also tested. The coated layer effectively prevented oxidation of the coupons except on the edges, where the coating was thinner and had some flaws. In addition, the Cr-coated Zr–2.5Nb coupons had the lowest deuterium pickup of all the alloys tested and showed no signs of accelerated or non-uniform corrosion.

Determination of Deuterium Pick-Up in Zr-Nb Alloy by Hot Vacuum Extraction-Quadrupole Mass Spectrometry

Zirconium alloys are used extensively by the nuclear industry as reactor core components. In pressurized heavy water reactors (PHWRs) zirconium-niobium alloy pressure tubes (also called coolant channels) are used with D2O as the primary heat transport fluid. Due to continuous usage of the pressure tubes inside the reactor, heavy ingress of deuterium into the pressure tubes due to corrosion reactions could occur resulting in hydrogen (or deuterium) embrittlement. Aging management therefore requires that this deuterium pick-up rate should be quantified for the remedial measures to be taken for the smooth operation of the reactor. For this purpose, a hot vacuum extraction-quadrupole mass spectrometry (HVE-QMS) technique has been developed in our laboratory. Experimental conditions like temperature, time of extraction of liberated gases, and measurement methodology have been optimized. Sensitivity factors have been derived for quantification of analytical results. Precision of the methodology was found to be 10%. Also, detailed studies have been carried out on the formation of hydrogen (or deuterium) molecular ion () in QMS as a function of pressure in the measurement system and its possible effect on the results.

In-reactor performance of pressure tubes in CANDU reactors

The pressure tubes in CANDU reactors have been operating for times up to about 25 years. The in-reactor performance of Zr–2.5Nb pressure tubes has been evaluated by sampling and periodic inspection. This paper describes the behaviour and discusses the factors controlling the behaviour of these components in currently operating CANDU reactors. The mechanical properties (such as ultimate tensile strength, UTS, and fracture toughness), and delayed-hydride-cracking properties (crack growth rate V c, and threshold stress intensity factor, K IH) change with irradiation; the former reach a limiting value at a fluence of <1 × 10 25 n m −2, while V c and K IH reach a steady-state condition after a fluence of about 3 × 10 25 n m −2 and 3 × 10 24 n m −2, respectively. At saturation the UTS is raised by about 200 MPa, toughness is reduced to about 40% of its initial value, V c increases by about a factor of ten while K IH is only slightly reduced. The role of microstructure and trace elements in these behaviours is described. Pressure tubes exhibit elongation and diametral expansion. The deformation behaviour is a function of operating conditions and material properties that vary from tube-to-tube and as a function of axial location. Semi-empirical predictive models have been developed to describe the deformation response of average tubes as a function of operating conditions. For corrosion and, more importantly deuterium pickup, semi-empirical predictive models have also been developed to represent the behaviour of an average tube. The effect of material variability on corrosion behaviour is less well defined compared with other properties. Improvements in manufacturing have increased fracture resistance by minimising trace elements, especially H and Cl, and reduced variability by tightening controls on forming parameters, especially hot-working temperatures.

Role of hydrides on the oxidation and deuterium pickup of Zr–2.5Nb in D 2O at 573 K

This study was initiated to determine the role of hydrides on the oxidation and D-pickup rates of Zr–2.5Nb pressure tubes used in CANDU ® 1reactors. Coupons of Zr–2.5Nb pressure tube material were charged to H concentrations ranging from 5 to 800 mg H/kg metal. A continuous layer of hydride was also introduced on the surface of some coupons. Tests were conducted in lithiated D 2O at 573 K for about 4 years. The coupons were removed at certain intervals for oxide thickness, D-pickup and various other evaluations such as metallography. All the coupons, except those with a surface hydride layer, showed comparable oxidation and D-pickup rates. The coupons with a surface hydride layer showed oxidation and D-pickup rates of about a factor of two higher than the other coupons. These findings, as well as results from the other examinations are reported in this paper.

Residual carbon impurities in Zr–2.5Nb and their effect on deuterium pickup

The presence of residual carbon in Zr–2.5Nb at parts per million (ppm) levels affects the alloy's rates of oxide film growth and deuterium absorption (in 573 K lithiated heavy-water). Pickup rates increase with increasing carbon concentrations between 40 and 300 ppm with a possible inflection in the kinetic curves in the vicinity of 120 ppm. Increased rates are likely due to enhanced development of an inter-connected, large-scale, pore network within the oxide film. Oxides on Zr–2.5Nb containing 145 ppm carbon appear to be fully stoichiometric with a well developed network of large pores, while the oxide on the 85 ppm C alloy contains a high density of fine pores and is likely sub-stoichiometric.

The influence of impurities in Zr-2.5Nb on oxygen and deuterium pickup at 573 K in D2O

Short term corrosion tests in D2O at 573 K (pH = 10.5) of Zr-2.5wt% Nb containing small, deliberate additions of impurity elements have shown that elements such as Sn, Fe, Cr, V and Si lead to total weight gains lower than a set of Zr-2.5Nb controls. Low total weight gains did not always convert to low deuterium pickups. Low Cr and Fe additions resulted in relatively low deuterium uptakes but high Fe and Sn, in any concentration, caused high deuterium uptake values. Ni additions resulted in high specimen to specimen variability and high percent theoretical deuterium uptakes.

Hydrogen Pickup and Degradation of Heat-Treated Zr-2.5 wt%Nb Pressure Tube.

Heat-treat (HT) Zr-2.5 wt%Nb pressure tubes which have high-temperature strength are used for the Advanced Thermal Reactor Fugen (boiling-light-water-cooled heavy-water-moderated pressure-tube-type reactor) in Japan. In general, zirconium alloys are considered to be degraded in the fracture toughness by picking up hydrogens from reactor coolant during reactor operation. It is reported in CANDU (Canada Deuterium Uranium) reactors that the deuterium pickup rate of cold-worked (CW) Zircaloy-2 pressure tubes is very high but that the deuterium pickup rate of CW Zr-2.5 wt%Nb pressure tubes is very low

Analysis of ZrNb fuel channel surfaces for hydrogen and other elements using Secondary Ion Mass Spectrometry (SIMS)

Understanding the mechanisms of hydrogen ingress into CANDU pressure tubes fabricated from zirconium-2.5 wt% niobium alloy requires knowledge of the hydrogen concentration in the surface oxide, the oxide/metal interface and in the alloy phase beneath the interface. Secondary Ion Mass Spectrometry (SIMS) has been shown to have attractive capabilities for detecting hydrogen isotopes in such surface films. The depth concentration profiles of deuterium, lithium, fluorine and iron have been measured on specimens taken from several locations on two pressure tubes from Unit 3 of the Pickering Nuclear Generating Station. These tubes were known to have widely differing deuterium concentrations in their bulk phases. SIMS profiles were obtained from both the outside (OD) and inside (ID) tube surfaces. The OD studies show that for regions where bulk deuterium pickup is small, the deuterium concentration within the surface oxide decreases steadily with depth and is 50–100 times lower near the oxide/alloy interface. For regions of high pickup, the profiles showed only a small concentration gradient of deuterium across the oxide thickness. This infers the importance of the surface oxide in suppressing hydrogen ingress. The ID studies also show that regions with high deuterium pickup have anomalous deuterium depth profile structure near the oxide metal interface. This suggests that the integrity of the ID oxide film may also bear some responsibility for enhanced deuterium ingress at these locations. Variations in iron, lithium or fluorine concentrations within the ID or OD oxide scales do not appear to have a significant effect on the deuterium profiles measured.

Analysis of the Pressure Tube Failure at Pickering NGS “A” Unit 2 Nuclear Systems Department

The failure of a Zircaloy-2 pressure tube in Pickering Unit 2 in August 1983 has been found to have been caused by an accelerated pickup rate of deuterium, contact between the pressure tube and its surrounding calandria tube, and rapid growth of zirconium hydride blisters.The pressure tubes in all later CANDU reactors are made from a zirconium-niobium alloy. Examination of several Zircaloy-2 and zirconium niobium pressure tubes from different reactors has clearly shown that the deuterium pickup rate of the two materials is significantly different. The zirconium niobium pressure tubes have absorbed very little deuterium and they do not appear to be susceptible to the type of failure experienced at Pickering Unit 2.

Exchange of HD with boron phosphate

The reaction of HD on boron phosphate produces H2 and D2 in approximately 4/1 proportions. The concentrations of HD, H2 and D2 present in the gas phase at various times were employed to calculate rates of reaction and the loss of deuterium atoms from the gas phase as a function of time. This latter data, when combined with a suitable model for the rate of chemisorption, permitted activation energies for deuterium pickup by the surface as a function of surface coverage to be calculated. The two samples of boron phosphate employed yield data which suggest the existence of both empty and exchangeable sites for deuterium, and a distribution of exchange or hydrogen binding energies over the surface of each catalyst.

THE MECHANISM OF THE CARBONYL ELIMINATION REACTION OF BENZYL NITRATE KINETIC ISOTOPE EFFECTS AND DEUTERIUM EXCHANGE

The carbonyl elimination reaction (ECO2) of benzyl nitrate has been investigated with the object of distinguishing between the concerted and carbanion mechanisms. A deuterium exchange experiment resulted in a very small amount of deuterium pickup. The nitrogen isotope effect, k14/k15, associated with formation of the nitrite ion was found to be 1.0196 ± 0.0007 at 30 °C. The two results taken together exclude the formation of a carbanion intermediate but are consistent with a concerted mechanism.Benzyl-α-d2 nitrate has been prepared and the rate of its carbonyl elimination reaction compared with that of the undeuterated compound. The deuterium isotope effect was 5.04 ± 0.25 at 60 °C. The significance of the magnitude of the nitrogen and deuterium isotope effects and of their interrelationship with the Hammett reaction constant rho is discussed in terms of the nature of the transition state and a comparison is made with other E2 elimination reactions.