Structural Material Anomaly Detection System Using Water Chemistry Data, (II)

A calculation model was developed to predict the dose rate, caused by ⁵⁸CO which is formed by the activation of ⁵⁸Ni, around the recirculation pipes m boiling water reactors (BWRs). The model is characterized by considering direct deposition of Ni ion on the nuclear fuel cladding surface and the geometrical contact probability for the ferrite formation reaction between deposited Ni(Co) and Fe₂O₃ on fuel cladding surface.

This model showed the important role of the amount of Fe crud on the surface to reduce ⁵⁸CO ion concentration in the reactor water. And the necessary Fe concentration in the feedwater for reducing the dose rate in the primary system was estimated as a function of the operating time. This model also enables the quantitative predictions of the effect of prefilming treatment of the feedwater heater tubes or another methods to reduce dose rate in an Fe crud suppressed BWR.     

Deposition of Nickel and Cobalt Ions with Iron Crud on Heated Stainless Steel Surface under Nucleate Boiling Conditions

Deposition behaviors of Ni (II) and Co (II) ions on a heated surface using simulated Fe crud which was mainly composed of amorphous Fe(III) hydroxide have been studied under nucleate boiling conditions at 553 K and 70 atm. The deposition process of Ni (II) and Co (II) ions is divided into two stages. The first stage is the deposition of hydroxide precipitate on the heated surface by microlayer evaporation and drying out in the nucleate boiling bubble. The second is settlement by conversion of hydroxide into oxides such as NiO, NiFe₂O₄, CoO and CoFe₂O₄. The effective deposition coefficients of Ni(II) and Co (II) ions, without supplying Fe crud, are smaller than that of α-Fe₂O₃ because of the solubility of those hydroxides at a low concentration condition. Their effective deposition coefficients increase with the simulated Fe crud concentration in feedwater and saturate at a value of 0.3 which is calculated theoretically, because the Ni and Co hydroxides react with the simulated Fe crud to produce insoluble NiFe₂O₄ and CoFe₂O₄ on the heated surface. The reaction of Ni deposit with α-Fe₂O₃ does not proceed, but NiO is produced. The reaction rate of Ni deposit with the simulated Fe crud on the heated surface is higher than that of Co deposit.     

Crud Behaviors in High Temperature Water, (II)

Sample specimens of type 304 stainless steel and Zircaloy-2 were exposed to the OWL-1 loop water at 280°C for 23d. After the exposure, a number of crystallites are distributed either separately or sometimes in agglomerate on the surfaces. The X-ray diffraction analysis and XMA analysis of the type 304 stainless steel surface revealed that more than 95% of the crystallites have a M₂O₃-type of structure, and some are enriched in Cr. In the sub-sequent radiochemical analysis of the fractions detached by the various cleaning processes (ultrasonic, electrostripping and electropolishing), it is noticed that the ⁶⁰Co/Co ratio decreases with the depth of the corrosion layer from the value close to that of insoluble crud in the water (crud-P) to that of soluble crud (crud-F).     

UO2 corrosion in an iron waste package

In order to investigate the interactions between spent nuclear fuel, corroding iron waste packages, and water under conditions likely to be relevant at the proposed repository at Yucca Mountain, six small-scale waste packages were constructed. Each package differed with respect to water input, exposure to the atmosphere and temperature. Two of the packages contained 0.1 g UO₂. Simulated Yucca Mountain process water (YMPW) was injected into five of the packages at a rate of 200 μl per day for up to 2 years, at which point the solids were characterized with X-ray powder diffraction, scanning electron microscopy, wet chemistry and electron microprobe analysis. Fe(II) is abundant in the corrosion products that form, and the dominant crystalline product in all cases according to X-ray diffraction is magnetite or the structurally similar maghemite. Minor phases included akaganeite (β-FeOOH) and possibly also hematite (Fe₂O₃), lepidocrocite (γ-FeOOH) and green rust (Fe(II)_1−xFe(III)_x(OH)₂Y_x/n). Under these conditions, UO₂ is expected to alter to the uranyl silicate uranophane (Ca[(UO₂)SiO₃(OH)]₂·5H₂O). Neither oxidation of the UO₂ nor any oxidized (uranyl) solid was observed, suggesting that conditions were sufficiently reducing to kinetically hinder U(IV) oxidation.     

Magnetic Measurements of NiFe2O4 Formation from Iron Hydroxides and Oxide in High Temperature Water

Formation of NiFe₂O₄ from iron hydroxides and an oxide has been studied in high temperature water by in situ measurements. The five kinds of Fe compounds used in the experiments are known as components of Fe crud in BWR condensate water. Among the reactants and products, only NiFe₂O₄ shows ferromagnetism. An in situ method, using a magnetic balance, was developed to measure the mass of NiFe₂O₄ in high temperature water. The lower detection limit of this method is 0.01 mg of NiFe₂O₄ with a maximum detection error of ±2%. The iron hydroxides react with Ni(OH)₂ to form NiFe₂O₄ and α-Fe₂O₃ through a dehydration reaction in 553 K water. Amorphous iron hydroxide and γ-FeOOH react rapidly, reaching saturated values of the conversion to NiFe₂O₄ within a few hours at 553 K. The dissolved oxygen has no influence on the NiFe₂O₄ formation. The amount of generated NiFe₂O₄ decreases with the reduction of the pH value, due to dissolution of the reactant. The rate of NiFe₂O₄ formation increases with reaction temperature, while the saturated value of formed NiFe₂O₄ decreases.     

Mitigation of Caesium and Cobalt Contamination on the Surfaces of Ram Packages

Abstract

Techniques for mitigating the adsorption of ¹³⁷Cs and ⁶⁰Co on metal surfaces (e.g. RAM packages) exposed to contaminated water (e.g. spent-fuel pools) have been developed and experimentally verified. The techniques are also effective in removing some of the ⁶⁰Co and ¹³⁷Cs that may have been adsorbed on the surfaces after removal from the contaminated water. The principle for the ¹³⁷Cs mitigation technique is based upon ion-exchange processes. In contrast, ⁶⁰Co contamination primarily resides in minute particles of crud that become lodged on cask surfaces. Crud is an insoluble Fe–Ni–Cr oxide that forms colloidal-sized particles as reactor cooling systems corrode. Because of the similarity between Ni²⁺ and Co²⁺, crud is able to scavenge and retain traces of cobalt as it forms. A number of organic compounds have a great specificity for combining with nickel and cobalt. Ongoing research is investigating the effectiveness of chemical complexing agent EDTA with regard to its ability to dissolve the host phase (crud) thereby liberating the entrained ⁶⁰Co into a solution where it can be rinsed away.     

Characterization of solidified melt among materials of UO2 fuel and B4C control blade

To predict the fundamental phase relationships in the solidified core melt of the Fukushima Daiichi Nuclear Power Plant, solidified melt samples of the various core materials [B₄C, stainless steel, Zr, ZrO₂, (U,Zr)O₂] were prepared by arc melting. Phases and compositions in the samples were determined by means of X-ray diffraction, microscopy, and elemental analysis. With various compositions, the only oxide phase formed was (U,Zr)O₂. After annealing, the stable metallic phases were an Fe-Cr-Ni alloy and an Fe₂Zr-type (Fe,Cr,Ni)₂(Zr,U) intermetallic compound. The borides, ZrB₂ and Fe₂B-type (Fe,Cr,Ni)₂B, were solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere (Ar-0.1%O₂) resulted in the oxidation of U and Zr in the alloy and in ZrB₂, and consequently the (Fe,Cr,Ni)₂B and Fe-Cr-Ni alloy became dominant in the metallic part. The experimental phase relationships in the metallic part agreed reasonably with the thermodynamic evaluation of equilibrium phases in a simplified B₄C–Fe–Zr system. The metallic Zr content in the melt was found to be a key factor in determining the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides, especially ZrB₂, showed notably higher hardness than any other oxide or metallic phases.     

Characterization of phases in ‘crud’ from boiling-water reactors by transmission electron microscopy

This paper reports phases identified in samples of crud (activated corrosion products) from two commercial boiling-water reactors using transmission and analytical electron microscopy and selected-area electron diffraction. Franklinite (ZnFe₂O₄) was observed in both samples. Hematite (α-Fe₂O₃), crystalline silica (SiO₂), a fine-grained mixture of iron oxides probably including magnetite (Fe₃O₄), hematite (α-Fe₂O₃), and goethite (α-FeOOH), and an unidentified high-Ba, high-S phase were observed in one of the samples. Willemite (Zn₂SiO₄), amorphous silica, and an unidentified iron-chromium phase were observed in the other. Chloride-bearing phases were found in both samples, and are assumed to represent sample contaminants. Because of the small sample volumes and numbers of particles studied and the possibility of contamination, it is not clear whether the differences between the phases observed in the two crud samples represent actual differences in the assemblages formed in the reactors.     

Compression and Decompression Waves in Steam-Water System

The ratio of the γactivities per fission from fission products of ²³⁹Pu and ²³⁵U, and its time dependence were measured by double fission chamber technique. The γ-activity from the fission products of ²³⁹Pu fission was lower than the corresponding activity relevant to ²³⁵U fissions. The ratio varied with the cooling time allowed after irradition.

This ratio was applied to power distribution measurements by γ-scanning method in multi-region cores composed of PuO₂-UO₂ and UO₂ fuels. To obtain the relative power, the measured γ-activities from the fission products in the fuel rods were corrected for the difference between the γ-activities per fission from the fission products.     

Structural and magnetic characterization of plasma ion nitrided layer on 316L stainless steel alloy

In this study, an FeCrNi alloy (316L stainless steel disc) was nitrided in a low-pressure R.F. plasma at 430 °C for 72 min under a gas mixture of 60% N₂–40% H₂. Structural, compositional and magnetic properties of the plasma nitrided layer was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and magnetic force microscopy (MFM). The magnetic behaviour of the nitrided layer was also investigated with a vibrating sample magnetometer (VSM). Combined X-ray diffraction, cross-sectional SEM, AFM and MFM, as well as VSM analyses provide strong evidence for the formation of the γ_N phase, [γ_N-(Fe, Cr, Ni)], with mainly ferromagnetic characteristics. The uniform nature of the γ_N layer is clearly demonstrated by the XRD, cross-sectional SEM and AFM analyses. Based on the AFM and SEM data, the thickness of the γ_N layer is found to be ～6 μm. According to the MFM and VSM analyses, ferromagnetism in the γ_N layer is revealed by the observation of stripe domain structures and the hysteresis loops. The cross-sectional MFM results demonstrate the ferromagnetic γ_N phase distributed across the plasma nitrided layer. The MFM images show variation in the size and form of the magnetic domains from one grain to another.     

Characterization of the reaction layer in U-7wt%Mo/Al diffusion couples

The reaction layer in chemical diffusion couples U-7wt%Mo/Al was investigated using optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction (XRD) techniques. When the U-7wt%Mo alloy was previously homogenized and the γ(U, Mo) phase was retained, the formation of (U, Mo)Al₃ and (U, Mo)Al₄ was observed at 580 °C. Also a very thin band was detected close to the Al side, the structure of the ternary compound Al₂₀UMo₂ might be assigned to it. When the decomposition of the γ(U, Mo) took place, a drastic change in the diffusion behavior was observed. In this case, XRD indicated the presence of phases with the structures of (U, Mo)Al₃, Al₄₃U₆Mo₄, γ(U, Mo) and α(U) in the reaction layer.     

Chemical analysis of fuel crud obtained from Korean nuclear power plants

Crud samples were obtained from two different kinds of used fuels in PWRs. The constituent elements were analysed according to the shapes of crud particles by SEM and EPMA. The principal elements of octahedral crystal particles were identified as Ni and Fe, where the ratio (Fe/Ni) was approximately 2. The major element of the observed needle-like structures was determined to be Ni. In addition, Zr composed the main element of particles shaped like broken fragments sized at 10-50 μm in diameter. The round particles less than 20 μm in diameter were identified as Si-containing compounds. We also found the Zn element in a series of fuel crud samples obtained from a plant. Zn was mainly detected in the hard crud, inner layer of crud, but not on the surface of crud. It was observed that there is an inverse relationship between the Ni and Zn contents in the hard crud.     

Separation of Uranium from Fission Products

A simple procedure has been developed for separating U from F.P. and neutron-irradiated Th. The separation is performed with tri-n-butyl phosphate in a system of dodecane-mixture of sulfuric acid and aluminium nitrate.

Uranium dioxide was irradiated with 20 MeV bremsstrahlung, which produced both ²³⁷U and F.P. The target was dissolved in dilute nitric acid and U was extracted into the organic phase of the above mentioned system. Finally, U in the organic phase was back-extracted into an aqueous phase. The γ-ray spectrum and decay curve of the separated U fraction show no radioactive nuclides other than U isotopes and its decay products.

This method can also be applied to the preliminary separation of ²³⁸U from neutron- irradiated Th.

The distribution ratios (Kd) for U, Th and some other elements in the extraction system are also given.     

Determination of carbon contents in the iron meteorites Toluca and Algarrobo

The nuclear microprobe at the cyclotron of the Vrije Universiteit in Amsterdam, The Netherlands, was employed for a study of the iron meteorites Toluca and Algarrobo. Samples were irradiated with ~ 0.4 nA of 1.4 MeV deuterons. Carbon was detected by measuring the proton spectrum of the ¹²C(d, p)¹³C nuclear reaction. The X-ray spectrum of Fe and Ni was measured simultaneously. Area scans of squares approximately 1 × 1 mm² were made with beam size approximately 20 × 20 μm². Spots chosen for the analysis contained areas of taenite, plessite and martensite surrounded by kamacite. Our area scans show clearly that regions of martensite are more C-rich than those of plessite and taenite with lowest C intensity in kamacite. At this point it is still impossible to determine the C-concentrations absolutely with good precision due to possible C-contamination during the target preparation and/or the C-deposition on the irradiated spot during the measurement.However, minimal concentrations of carbon in plessitic and martensitic areas were calculated assuming that all the C counts in kamacite areas come from contamination and using a Fe-C standard with 0.8% C.Further improvements of the technique are planned, including measuring immediately after polishing samples with Al₂O₃, as well as employing a cold trap inside of the irradiation chamber to reduce carbon deposition during measurement.     

Ion Exchange Separation for Decontamination of Centrifuge Enrichment Plant

Ion exchange separation of uranyl ion (UO²⁺ ₂) from metal cations has been carried out by the columnar operation using ion exchange resins in 0.1 mol/dm³ sulfuric acid medium. Uranyl ion was adsorbed in an anion exchange resin and the rest metal cations, Fe(III), Al(III), Cr(III), Ni(II) and Cu(II) ions, were adsorbed in a cation exchange resin in this system. Desorption of uranyl ion and metal cations adsorbed in the resins were tested by 2 mol/dm³ sulfuric acid solution. Desorbed elements were confirmed to be precipitated by appropriate alkaline solutions. On the basis of the results obtained, a concept was made on a decontamination system for uranium-contaminated waste solution from centrifuge enrichment plant.     

Characterization of the interaction layer grown by interdiffusion between U-7wt%Mo and Al A356 alloy at 550 and 340 °C

U(Mo) alloys are under study to get a low-enriched U fuel for research and test reactors. Qualification experiments of dispersion fuel elements have shown that the interaction layer between the U(Mo) particles and the Al matrix behaves unsatisfactorily. The addition of Si to Al seems to be a good solution. The goal of this work is to identify the phases constituting the interaction layer for out-of-pile interdiffusion couples U(Mo)/Al(Si). Samples γU-7wt%Mo/Al A356 alloy (7.1 wt%Si) made by Friction Stir Welding were annealed at 550 and 340 °C. Results from metallography, microanalysis and X-ray diffraction, indicate that the interaction layer at 550 °C is formed by the phases U(Al,Si)₃, U₃Si₅ and Al₂₀Mo₂ U, while at 340 °C it is formed by U(Al,Si)₃ and U₃Si₅. X-ray diffraction with synchrotron radiation showed that the Si-rich phase, previously reported in the interaction layer at 550 °C near U(Mo) alloy, is U₃Si₅.     

K-shell X-ray fluorescence cross-sections and intensity ratios for some pure metals at 59.5 and 123.6 keV

K-shell X-ray fluorescence cross-sections for some pure metals such as Cr, Fe, Co, Cu, Zn, Ga, Se, Y, Mo, Cd, In, Sn, Te, Ba, Ta, W and Bi have been theoretically and experimentally determined. The Cr, Fe, Co, Cu, Zn, Ga, Se, Y, Mo, Cd, In, Sn, Te and Ba metals were excited by 59.5 keV γ-ray from 50 mCi ²⁴¹Am radioactive source and the Ta, W and Bi targets were excited by 123.6 keV γ-ray from 25 mCi ⁵⁷Co radioactive source. The characteristic K X-rays emitted by samples were detected by using a super Si(Li) detector having a resolution of 150 eV at 5.9 keV. In addition, the I_Kβ/I_Kα intensity ratios for these metals were studied. The obtained experimental values of the K-shell X-ray fluorescence cross-sections and the I_Kβ/I_Kα intensity ratios have been compared with theoretical values. The measured values were in good agreement with theoretical values.     

A method of examining iron oxides speciation and transport to steam generators during nuclear power reactor startups

Secondary side corrosion products (sludge) collected during one of CANDU1¹ reactor startups from wet layup have been examined by X-ray fluorescence and Mössbauer spectroscopy. The transport and chemical form of iron oxides and oxyhydroxides were determined in condensate, feedwater and preheater outlet as a function of temperature and time. The sludge burst and oxidation states of iron oxides were correlated with the rise of reactor power and corresponding changes in temperature, condensate vacuum and water flow rate. In particular, a sharp γ-FeOOH to Fe₃O₄ switch was observed that coincided in time with the onset of condensate vacuum. Also, it was found that the startup after wet layup is characterized by only brief and fairly small sludge burst at about 30% reactor power and which contributes only a small amount of undesirable α-Fe₂O₃ to total iron transport to steam generator. Thus, sludge burden to steam generators can be minimized with proper layup and startup practices.     

Measurement of keV-neutron capture cross-sections and capture γ-ray spectra of Fe and Fe

The neutron capture cross-sections and the capture γ-ray spectra of ⁵⁶Fe and ⁵⁷Fe have been measured in the neutron energy range from 10 to 90 keV. Pulsed keV-neutrons were produced from the ⁷Li(p,n)⁷Be reaction by bombarding a lithium target with a 1.5-ns bunched proton beam from a 3 MV Pelletron accelerator. The incident neutron spectrum on the capture sample was measured using a time-of-flight method with a ⁶Li-glass detector. The capture γ-rays emitted from an iron or standard gold sample were detected with a large anti-Compton NaI(Tl) spectrometer. The capture yield of the iron or gold sample was obtained by applying a pulse-height weighting technique to the corresponding capture γ-ray pulse-height spectrum. The capture cross-sections of ^56,57Fe were derived with errors less than 5% using the standard capture cross-sections of ¹⁹⁷Au. The capture γ-ray spectra were obtained by unfolding the observed capture γ-ray pulse-height spectra. The present results for the capture cross-sections were compared with the previous measurements and the evaluated values of ENDF/B-VII.0 and JENDL-3.3. The Maxwellian-averaged capture cross-sections of ⁵⁶Fe and ⁵⁷Fe at 30 keV are derived as 12.22 ± 2.06 mb and 44.48 ± 7.56 mb, respectively.     

Chemical State and Deposition Rate of Nickel Ion Deposit Produced on Heated Surface in High Pressure Boiling Water

Nickel ion deposit was produced on a heated rod surface in high pressure boiling water (150–285°C, 0.4–7.0 MPa). The deposit under the same temperature and pressure conditions as those for BWR reactor water (285°C, 7.0 MPa) was identified as NiO by spectrum profile analysis of the NiLα, NiLβ and 9th-order NiKα₁ lines. Deposition rate was obtained from in situ measurements of deposit thickness, by a photoacoustic method, and from chemical analysis of deposit amount. The deposition rate coefficients obtained in temperature and pressure ranges of 150–250°C and 0.4–4.0 MPa were 2 × 10^?3–5 × 10^?2, which were 0.15–0.45 times as large as those of iron crud. This was attributed to a dissolution effect of Ni ion from NiO. The deposition rate coefficient at 285°C, 7.0 MPa was estimated to be 4.4 × 10^?2–1.3 × 10^?1.