Liquid Nuclear Waste Research Articles

Determination of the secondary phases at the acidic LNW disposal

ABSTRACTThe migration behavior of the long-lived actinides was studied under the conditions of the deep disposal of the acidic liquid nuclear waste (LNW). Composition of LNW varies significantly including acidic technological wastes (pH ∼2.4), which consist of sodium nitrate, acetic acid, corrosion products (Fe, Cr, Mn, Ni, Al), fission products and actinides. Corrosion products tend to precipitate under the LNW disposal conditions that favor forming of the phases with high sorption capacity towards actinides. Sands of reservoir bed have their own initial sorbent surfaces besides new secondary phases that have formed as a result of interaction with acidic LNW. The nearest to the injection well conditions are gradually changing from pH ∼2.4 till neutral values due to the dilution by groundwater with formation of new precipitated phases of corrosion products. The solid phases characterization is a necessary step on the path of knowledge of migration behavior of actinides. The secondary phases of both corrosion products and sands of reservoir bed under LNW disposal conditions were characterized using XRD, SEM and Mössbauer spectroscopy. The recent results of the analyses of the behavior of actinides (Pu. U, Np, Am) under the conditions of the injection of the acid LNW are presented in the paper.

CFD simulations on the dynamics of liquid sloshing and its control in a storage tank for spent fuel applications

Spent nuclear liquid waste is often kept in partially filled storage tanks. When such storage tanks are subjected to wind and/or earthquake induced excitations, this could lead to detrimental conditions. Therefore, storage tank designers should ensure safe design margins and develop methodologies to overcome a wide range of possible scenarios. In the present study, systematic numerical simulations are carried out to investigate the sloshing dynamics of liquid in a storage tank, subjected to seismic excitation. As a precursor, the influence of resonant harmonic excitation on the free surface displacement, pressure distribution, slosh forces etc. is studied. To suppress the free surface fluctuations and the associated slosh force, two types of baffles viz., ring and vertical baffle are examined. Based on the response to an imposed harmonic excitation, the vertical baffle plate in the middle of the tank, was found to be effective and its dimensions are systematically optimized. This baffle geometry was tested for a well known seismic excitation (El Centro) and it was observed to effectively suppress free surface fluctuations and the slosh forces.

Introducing a New Concept of De-Radioactivity

De-radioactivity is characteristically opposed to the concept of radioactivity. The possible mechanism behind the concept of de-radioactivity involves the intra-trapping, intra-embedding and intra-fixing of the metallic radioisotope or stable isotope within the framework of the novel molecules loaded with the potential transfer of excess electrons to the metal nucleus to suppress the decay of the instable neutron to the proton and electron with the release of energy, stabilising the stable isotopes or metal radioisotopes so strongly as to result in zero dissociation of metal (stable isotope or metal radioisotope) and thus depriving metal radioisotope of its free status - an essential condition of metal radioisotope to decay. The whole study has been based on metal stable isotopes extendable to metal radioisotopes. The novel molecules (SSS-101, SSS-102, SSS-103 and SSS-104) which bring about intra-trapping and intra-fixing of the metal radioisotopes are called Radiostabilisers. Humanity may find a silver lining in this new concept for future safety with its possible wide spectrum practical applications. Radiostabilisers may help in the purification and reclaiming of metal radioisotope(s) contaminated sea-water or water, atmospherics and the nuclear liquid waste materials. The radiostabilisers fail to intra-trap the stable or radioisotopes of hydrogen, helium, carbon, boron, phosphorus, iodine, neon, argon, krypton, xenon, oxygen, fluorine, sulphur and chlorine.

Chemical stabilization of chromate in blast furnace slag mixed cementitious materials

Cement waste form (CWF) technology is among the leading approaches to disposing of metals and liquid low-level nuclear waste in the United States. One such material, saltstone, includes slag, fly ash and Portland cement to enhance the immobilization of contaminants (e.g., Cr, 99Tc) in alkaline liquid wastes. To evaluate the stability of such redox sensitive contaminants in saltstone, the effects of slag as a source of reductant on Cr immobilization was evaluated in aged (<300d) saltstone monoliths. Specifically, we investigated the effects of artificial cement pore waters on the Cr release and the spatially resolved Cr chemical state analysis using synchrotron based microfocused X-ray microprobe analysis. The microprobe analysis indicated the heterogeneous distribution of insoluble Cr(III)-species in saltstone. Although at most of 20% Crtotal was leached at the top few (2–3) millimeter depth, the release of Cr(VI) was small (<5%) at 5–30mm with slight changes, indirectly suggesting that Cr is likely present as insoluble Cr(III) species throughout the depths. The study suggests that this saltstone formulation can effectively retain/immobilize Cr under the oxic field condition after ⩽300d of aging time.

Partitioning and speciation of Pu in the sedimentary rocks aquifer from the deep liquid nuclear waste disposal

Abstract Plutonium partitioning and speciation was studied under the conditions of a liquid nuclear waste repository site. The dominating effect of corrosion products (Fe and Cr) oxides on plutonium partitioning was established in batch sorption tests at 150 ℃ and by using sequential extraction. Various techniques including Mössbauer spectrometry, XRD and TEM were used to characterize Fe/Cr oxide phases that form both precipitates and surface coatings which sorb plutonium. The sorption and speciation of plutonium was modeled thermodynamically using Pourbaix diagram that takes into account both surface complexation reaction and precipitation of insoluble PuO2.

Materials in Nuclear Waste Disposition

Commercial nuclear energy has been used for over 6 decades; however, to date, none of the 30+ countries with nuclear power has opened a repository for high-level waste (HLW). All countries with nuclear waste plan to dispose of it in metallic containers located in underground geologically stable repositories. Some countries also have liquid nuclear waste that needs to be reduced and vitrified before disposition. The five articles included in this topic offer a cross section of the importance of alloy selection to handle nuclear waste at the different stages of waste processing and disposal.

Evaluation of Nitrate and Nitrite Reduction Kinetics Related to Liquid-Air-Interface Corrosion

Liquid-air interface (LAI) corrosion has been a concern for causing leaks in the carbon steel tanks used for holding high-level radioactive liquid waste. To assist in understanding the mechanism of LAI corrosion, the kinetics of nitrate and nitrite reduction reactions were investigated electrochemically. Cyclic voltammetry and cathodic polarization measurements indicated that the nitrite reduction reaction exhibited faster kinetics than the nitrate reduction reaction at higher cathodic overpotential. However, the primary reduction reaction at the open circuit potential under aerated conditions was the oxygen reduction reaction. The reduction of residual oxygen was also the dominant cathodic reaction at open circuit potential in deaerated conditions. Moreover, the kinetics of oxygen reduction on steel electrodes were significantly influenced by the sample immersion conditions (partial vs. full) for aerated liquid nuclear waste simulants, but not for deaerated conditions. Lastly, the gaseous products formed during LAI corrosion were analyzed using the gas detector tube method and gas chromatography-mass spectrometry and found to contain NH3, NO2 and NO. However, the results suggested that these products were caused by the local acidification generated by the hydrolysis of cations after LAI corrosion underwent extensive propagation, instead of being directly reduced in alkaline conditions. Thus, the results in this work showed that the kinetics of nitrate and nitrite reduction could not generate a salt concentration cell in the meniscus region to cause LAI corrosion.

Radiation-modified natural zeolites for cleaning liquid nuclear waste (irradiation against radioactivity)

There have been comparatively few investigations reported of radiation effects in zeolites, although it is known that these materials may be modified substantially by exposure to ionizing radiation. Thus, by exposure to γ-rays or high-energy particles, the charge states of atoms may be changed so to create, and accumulate, lattice point defects, and to form structurally disordered regions. Such a technique may permit the creation, in a controlled fashion, of additionally useful properties of the material while preserving its essential stoichiometry and structure. Accordingly, we present an application, in which the cation-exchange capacity of a natural zeolite (clinoptilolite) is substantially enhanced, for the treatment/decontamination of water contaminated with radionuclides e.g. 134Cs, 137Cs and 90Sr, by its exposure to high-energy (8 MeV) electrons, and to different total doses.

Examination of Mechanisms for Liquid-Air-Interface Corrosion of Steel in High Level Radioactive Waste Simulants

Several hypothesized mechanisms of liquid-air-interface (LAI) corrosion in simplified simulants of high level liquid nuclear waste were investigated. The salt concentration cell theory is based on the possible development of an enhanced nitrate concentration or a depleted nitrite concentration in the meniscus solution compared to the bulk solution. In situ Raman spectroscopy was employed to monitor the time-dependent changes in nitrate concentration, nitrite concentration, and the nitrate-to-nitrite concentration ratio in the meniscus region during LAI corrosion. However, there was no evidence to support the proposed nitrate- or nitrite- salt concentration cell theory. The IR drop theory was ruled out as being the cause of LAI corrosion based on local potential measurements using customized micro-reference electrodes. A theory of CO2-induced local hydroxide depletion was tested with a small pH electrode. However, the meniscus and bulk solutions exhibited no difference in pH before the initiation of LAI corrosion under anodic potentiostatic polarization. A local acidification mechanism, which results from borderline passive dissolution of steel in the meniscus and subsequent hydrolysis similar to the phenomenon of crevice corrosion, remains possible.

Determination of ethylenediaminetetraacetic acid in nuclear waste by high-performance liquid chromatography coupled with electrospray mass spectrometry

EDTA is a chelating agent that has been used in decontamination processes. Its quantification is required for nuclear waste management because it affects the mobility of radionuclides and metals in environment and, thus, can harm the safety of the storage. Ion-pair chromatography coupled with electrospray mass spectrometry detection is a convenient method for quantitative analysis of EDTA but EDTA should be present as a single anionic chelate form. However, radioactive liquid wastes contain high concentrations of heavy metals and salts and consequently, EDTA is present as several chelates. Speciation studies were carried out to choose a metal cation to be added in excess to the solution to obtain a major chelate form. Fe is the predominant cation and Fe(III)–EDTA is thermodynamically favored but these speciation studies showed that ferric hydroxide precipitated above pH 2. Consequently, it was not possible to quantify EDTA as Fe(III)–EDTA complex. Therefore, Ni2+ was chosen but its use implied pretreatment with a base of the solution to eliminate Fe. Deuterated EDTA was used as tracer in order to validate the whole procedure, from the treatment with a base to the final analysis by HPLC–ESI-MS. This analytical method was successfully applied for EDTA quantification in two real effluents resulting from a nuclear liquid waste process. A recovery rate between 60 and 80% was obtained. The limit of detection of this method was determined at 34×10−9molL−1.

Radiation Effects on Zeolite Nanomaterials - Some Potential Implications for Cleaning Liquid Nuclear Waste and for Enhanced Radioactive Decontamination

A series of group 1 and group 2 element cation-exchanged samples of zeolite X were prepared and exposed to a beam of spin-polarised, positive muons (of energy 28MeV/c) from a particle accelerator. Longitudinal field repolarisation measurements were made which identified, in each case, both diamagnetic and paramagnetic muon fractions. One of the paramagnetic components revealed a hyperfine field compatible with values reported previously for free muonium atoms (1585 G), while a secondary component was detected showing roughly half the hyperfine coupling measured for atomic muonium, leading to the conclusion that the muonium had become chemically-bound within the zeolite structure, probably via a single-electron bond to a coordinatively unsaturated aluminium atom. A significant “lost fraction” was also indicated along with superhyperfine and anisotropic components that repolarised in weaker magnetic fields (0-100G). As a general trend, the muonium atom yield was found to increase in order of the increasing radius/charge (riz) ratio, though it is constant within the experimental errors, while the chemically bound muonium was formed in similar proportion in all samples (14.1 + 1.9% of the total muonium polarisation), with the exception of HX and the parent NaX (from which the other zeolite samples were prepared by cation-exchange), in which it was absent. Along with data from zero-field experiments which were also undertaken, this is interpreted in terms of an entrapment or effective “scavenging” of radiolytic electrons by the cations (in the order Na+&gt;K+; Mg2+&gt;Ca2+&gt;Sr2+&gt;Ba2+) whose combination with positive muons is therefore impeded, thus reducing the yield of muonium atoms. Results for LiX are anomalous since the muonium yield is greater than expected, possibly due to a greater degree of covalency in the Li-0 bonding than pertains for the other cations. These results may have some implications for zeolites that suffer longterm radiation exposure/damage when they are used to clean radioactive cations (e.g. Cs +, Sr2+) from the waters of nuclear power stations and as materials whose cation-exchange capacity has been deliberately enhanced by prior exposure to ionising radiation.

Thermally Grown Oxide Layer on Aluminized Superalloy 690 Substrate and its Stability in Nitrate-Based Environment

Aluminides were formed on Ni-Cr-Fe based superalloy 690 substrates using pack aluminization process at 1273 K in controlled atmosphere. Thermal oxidation of aluminized specimens was carried out at 1273 K for a total period of 4 hours in air. The thermally grown oxide layer was examined using X-ray diffraction (XRD) studies on top surface and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis along the cross-section of the sample. The oxide layer developed on aluminized superalloy 690 substrate consisted of Al2O3 layer with a thickness of about 2 μm. The oxidized specimens were exposed in nitrate-based environment (simulated high-level nuclear liquid waste) at 373 K for a total period of 216 hours. A good adherence of aluminide coatings was noticed even after prolonged exposure in nitrate-based solution with a little amount of material dissolution from the edges of the specimens. XRD studies on exposed specimen indicated existence of Al2O3 layer on the top surface, which is believed to have resulted in good adherence of aluminide coatings.

Sorption of Nuclear Waste Components by Layered Hydrazinium Titanate: a Straightforward Route to Durable Ceramic Forms

ABSTRACTLayered hydrazinium titanate LHT-9, (N2H5)1/2Ti1.87O4 is a new nanohybrid material related to lepidocrocite-type titanates. Unique combination of ion exchange, reductive properties, surface activity due to Brønsted acid sites and occurrence of surface titanyl groups allows exploring LHT-9 for simultaneous uptake of almost all components of liquid nuclear wastes. LHT-9 irreversibly removes technetium, molybdenum, palladium and selenium from their aqueous solutions by specific mechanism of reductive adsorption. For removal of cesium, strontium, transition elements, actinides and lanthanides LHT-9 provides mechanisms of ion exchange and surface complexation. Products of adsorption are nanocrystalline and homogeneous powders loaded with 5 to 15 wt. % of radionuclides and non-radioactive elements. LHT-9 can be applied as ready-to-use precursor for one-step synthesis of durable titanate ceramic waste forms similar to SYNROC. An essential advantage of LHT-9 in comparison with other titanate sorbents (monosodium titanate and peroxo-titanate materials) is the absence of Na in its composition that permits arbitrary tailoring of sorbent properties by simple pre-treatment with the desired elements. Results on sorption of americium, cesium, strontium and lanthanides by LHT-9 are discussed.

ChemInform Abstract: Properties and Applications of Zeolites

AbstractReview: 207 refs.

Removal of Cesium from Aqueous Solutions Using Crosslinked Amphoteric Starch Prepared by Microwave Treatment

Crosslinked amphoteric starches (CAS) with quaternary ammonium cationic and carboxymethyl anionic groups were prepared by a microwave heating and traditional heating method, respectively . The Structural of the two CAS particles were characterized with powder X-ray diffraction (XRD)and SEM. Sorption capacity of two CAS were investigated for cesium (Cs) in batch equilibrium studies. Kinetic studies showed that the sorption process were rapid and saturation were attained within 40 min. The two different equilibrium sorption isotherm models applied to the experimental data and the model parameters were obtained from these non linear regressions. The studies revealed that CAS prepared using microwave seems to be one of a promising biosorbents for recovery of cesium from liquid nuclear wastes.

Arresting the Accidental Discharge of Nuclear Wastes by Modeling the Fuel and Clad

The paper deals with the method employed to arrest the temperature of vertical nuclear fuel-clad so as to keep clad fairly below the softening temperature. Objective of the paper is to control the generation of solid, liquid and gaseous nuclear waste as a result of split or rupture in clad. The fuel and clad have been modeled. The governing equations with adequate boundary conditions were solved using marching technique. The computer program was developed to predict the rewetting velocity. The paper mainly covers the generation of rewetting transients which were used to predict the rewetting velocities. Method of calculating acceptable rate of rise of clad temperature such that complete clad is saved from attaining softening temperature is discussed in detail in the paper.

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO 3 in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO 3 concentration and higher temperature.

Properties and applications of zeolites.

Zeolites are aluminosilicate solids bearing a negatively charged honeycomb framework of micropores into which molecules may be adsorbed for environmental decontamination, and to catalyse chemical reactions. They are central to green-chemistry since the necessity for organic solvents is minimised. Proton-exchanged (H) zeolites are extensively employed in the petrochemical industry for cracking crude oil fractions into fuels and chemical feedstocks for other industrial processes. Due to their ability to perform cation-exchange, in which the cations that are originally present to counterbalance the framework negative charge may be exchanged out of the zeolite by cations present in aqueous solution, zeolites are useful as industrial water-softeners, in the removal of radioactive Cs+ and Sr2+ cations from liquid nuclear waste and in the removal of toxic heavy metal cations from groundwaters and run-off waters. Surfactant-modified zeolites (SMZ) find particular application in the co-removal of both toxic anions and organic pollutants. Toxic anions such as arsenite, arsenate, chromate, cyanide and radioactive iodide can also be removed by adsorption into zeolites that have been previously loaded with co-precipitating metal cations such as Ag+ and Pb2+ which form practically insoluble complexes that are contained within the zeolite matrix.

Removal of Cesium from Savannah River Site Waste with Spherical Resorcinol Formaldehyde Ion Exchange Resin: Experimental Tests

A principal goal at the Savannah River Site (SRS) is to safely dispose of the large volume of liquid nuclear waste held in many storage tanks. In-tank ion exchange (IX) columns are being considered for cesium removal. The spherical form of resorcinol formaldehyde ion exchange resin (sRF) is being evaluated for decontamination of dissolved saltcake waste at SRS, which is generally lower in potassium and organic components than the Hanford waste. The sRF performance with SRS waste was evaluated in two phases: resin batch contacts and IX column testing with both simulated and actual dissolved salt waste. The tests, equipment, and results are discussed.

A new experimental procedure for determination of photoelectric efficiency of a NaI(Tl) detector used for nuclear medicine liquid waste monitoring with traceability to a reference standard radionuclide calibrator

To determine photopeak efficiency for 99mTc of the NaI(Tl) detector used for liquid waste monitoring at the Nuclear Medicine Unit of IRCCS Paediatric Hospital Bambino Gesù in Rome, a specific experimental procedure, with traceability to primary standards, was developed. Working with the Italian National Institute for Occupational Prevention and Safety, two different calibration source geometries were employed and the detector response dependence on geometry was investigated. The large percentage difference (almost 40%) between the two efficiency values obtained showed that geometrical effects cannot be neglected.