Tank Closure Research Articles

Sodium strontium phosphate nonahydrate (NaSrPO4·9H2O) found in Hanford nuclear waste

The Hanford site, near Richland, WA, in the U.S.A., has legacy radioactive waste left over from plutonium production. Strontium-90 and its short-lived daughter Y-90 account for about half of the radioactivity of the waste, yet there has been limited understanding of the speciation of strontium in the waste. The speciation of strontium will drive its behavior during any waste separation or treatment options, and may determine the Sr-90 source term to the environment after tank closure. A strontium-bearing phase containing sodium, phosphorus, and oxygen, was found in Hanford waste using energy dispersive spectroscopy (EDS). Polarized light microscopy determined that this phase was isotropic so had to have a cubic structure. NaSrPO4·9H2O is the only known cubic phase containing these EDS observable elements. These results confirm that NaSrPO4·9H2O is present, so the solubility of this species may drive the behavior of strontium in the waste.

A multiple lines of evidence approach for identifying geologic heterogeneities in conceptual site models for performance assessments.

Model-based decision making is commonly used in performance assessments to assure water resource protection for both human health and the environment for hundreds of years into the future. To make decisions regarding aquifer protection against potential contamination, a conceptual site model (CSM) describing the hydrodynamic behavior needs to account for subsurface heterogeneities in sufficient detail. When site-specific data are sparse, larger-scale geologic descriptions are adopted with the consequence of losing small-scale features (at the cm scale) that can control contaminant transport. In this study, a multiple lines of evidence approach is used to construct vadose zone CSMs based on an evaluation of several types of data, including geologic logs, borehole moisture content and concentration data, geophysical spectral gamma logging data, and groundwater concentration data for a tank farm at the Hanford Site in southeastern Washington State. The resulting CSMs of the unsaturated zone represent a synthesis of what is known about flow and transport processes at the site-scale and maintain consistency with knowledge that has been accumulated at the regional scale. Through a process of extensive data analyses, a systematic approach is described to create an evidence base that supports the evaluation and development of CSMs. Numerical models are then used to evaluate the impact that smaller-scale heterogeneities have on contaminant transport through the vadose zone for a performance assessment on waste tank closure. Together, the field data and the numerical experiments suggest that although small-scale features close to source releases can have an impact on horizontal spreading, overall there is a relatively minor impact on transport for the site under study as evaluated by differences in peak fluxes and arrival times for historical leak events, and for potential releases resulting from waste tank closure. Use of alternative CSMs, developed through careful examination of available characterization and monitoring data, provides confidence that geologic heterogeneities do not impact contaminant transport behavior significantly enough to alter the assessment of risk for closure at this site.

Effects of hydrated lime on radionuclides stabilization of Hanford tank residual waste

Chemical stabilization of tank residual waste is part of a Hanford Site tank closure strategy to reduce overall risk levels to human health and the environment. In this study, a set of column leaching experiments using tank C-104 residual waste were conducted to evaluate the leachability of uranium (U) and technetium (Tc) where grout and hydrated lime were applied as chemical stabilizing agents. The experiments were designed to simulate future scenarios where meteoric water infiltrates through the vadose zones into the interior of the tank filled with layers of grout or hydrated lime, and then contacts the residual waste. Effluent concentrations of U and Tc were monitored and compared among three different packing columns (waste only, waste + grout, and waste + grout + hydrated lime). Geochemical modeling of the effluent compositions was conducted to determine saturation indices of uranium solid phases that could control the solubility of uranium. The results indicate that addition of hydrated lime strongly stabilized the uranium through transforming uranium to a highly insoluble calcium uranate (CaUO4) or similar phase, whereas no significant stabilization effect of grout or hydrated lime was observed on Tc leachability. The result implies that hydrated lime could be a great candidate for stabilizing Hanford tank residual wastes where uranium is one of the main concerns.

Modeling Carbonation of High-Level Waste Tank Integrity and Closure

The Cementitious Barriers Partnership (CBP) is focused on reducinguncertainties in current methodologies for assessing cementitious barrier performanceand increasing the consistency and transparency in the assessment process. Oneimportant set of US Department of Energy challenges is assessing the integrity andclosure of the high-level waste (HLW) tanks that currently store millions of gallons ofhighly radioactive wastes. Many of these tanks are decades past their design lives, haveleaked or been overfilled, and must be emptied and closed to satisfy regulatoryagreements. Carbonation-induced corrosion has been identified as a primary degradationand possible failure mechanism for the HLW tanks prior to closure. After closure theimpact of carbonation (and concurrent oxidation) may be to increase the release andshort-range transport of contaminants of concern. HLW tanks may be significantlyempty for many years (and possibly decades) prior to closure; the performance of theclosed tank over centuries, if not millennia, must be assessed to evaluate the potentialrelease of residual radionuclides to the environment.CBP is developing models to evaluate a representative HLW tank closure scenarioincluding the potential impacts of carbonation on waste tanks prior to and post closure.CBP modeling tools, including LeachXS™/ORCHESTRA, are being used to simulatewaste tank carbonation, major constituent leaching, and contaminant releases to evaluatethe source term and near-field conditions. Simulations presented here include sensitivityanalysis for uncracked concrete to varying input parameters including composition,effective diffusivities, and thermodynamic parameters.

Radiometric detector options to aid in DOE high activity waste tank in situ characterization efforts

Radioactive waste cleanup and subsequent closure of waste storage tanks is currently underway at the Savannah river site, prompting the need to characterize the residual contents (heels) of the tanks. Occasionally, results from laboratory analyses indicate alternative sub-sampling strategies are needed, resulting in repetitive efforts to sample and analyze tank bottoms. The development of a system for in situ tank analyses using a radiometric probe, which could be lowered into a waste tank, could aid in identifying waste structures on tank bottoms requiring further sampling and characterization. Ideally, the probe would provide information for determining which structures were higher in concentrations of actinides and fission products characteristic of DOE high level waste (HLW) heels. Although only a limited set of isotopes can be measured directly without extensive radiochemical separations, the low-energy photon spectra of HLW do offer some intriguing possibilities for characterization using a radiometric probe. One possibility for obtaining a low-energy photon spectrum in the presence of high levels of interfering radiation would be to design a probe primarily based upon recently developed technology from Amptek Inc. Such a detector would be relatively insensitive to the high photon background, which would paralyze conventional gamma probes (i.e. sodium iodide) subjected to the same radiological conditions. The prototype detector is capable of successfully obtaining high resolution measurements at very high count rates (in excess of 500,000 counts per second). An overview of measurements obtained from various HLW samples using the prototype Amptek detector, as well as some additional detector technologies, which could enhance this prototype, will be discussed.

Analysis of 36Cl in Savannah River Site radioactive waste

Radioactive waste cleanup and subsequent closure of waste storage tanks is currently underway at the Savannah River Site, prompting the need to characterize the residual contents (heels) of the tanks. Each tank heel has variable chemical constituents and radiological distributions that pose unique analyte characterization and quantification challenges. For example, similar target detection limits for each tank heel are often difficult to meet as a result of complex matrices and methods must be tailored to characterize each new tank heel to the level of detection required. Furthermore, continuous programmatic evaluations of allowable closed-tank radiological inventories result in steadily decreasing detection limit targets for many of the isotopes required for characterization. Some of the target isotopes required for characterization need to be quantified to levels approaching 10 orders of magnitude below the radioactive concentrations of interfering isotopes. One particularly troublesome target analyte is 36Cl. As a pure beta-emitting radionuclide, 36Cl (T1/2 = 3.01 × 105 ± 3,000 years) must be isolated from interfering beta-emitting radionuclides such as 90Sr, 90Y, 137Cs, 99Tc, and 14C, as well as actinides and lanthanides prior to quantification. Due to its half-life, the target detection limit for 36Cl is extremely low compared to the concentrations of other beta-emitting radionuclides typically present in residual tank solids. A method of analysis for 36Cl was developed which utilizes a combination of traced acid digestion of radioactive waste tank samples followed by aggressive decontamination of interfering beta-emitting radionuclides via repeated resin-strikes, distillation, precipitation as AgCl, and analysis by gas flow proportional counting for the 36Cl—containing precipitate. The separation is traced by neutron activation analysis of stable chloride imparted during the initial digestion, and results are corrected for the separation yield. Recent campaigns have successfully achieved interference-free AgCl precipitates and, ultimately, detection limits of 0.37 Bq/g from matrices containing ~1E7 Bq/g of other beta-emitting radionuclides.

Hanford tank residual waste – Contaminant source terms and release models

Residual waste is expected to be left in 177 underground storage tanks after closure at the U.S. Department of Energy’s Hanford Site in Washington State (USA). In the long term, the residual wastes represent a potential source of contamination to the subsurface environment. Residual materials that cannot be completely removed during the tank closure process are being studied to identify and characterize the solid phases and estimate the release of contaminants from these solids to water that might enter the closed tanks in the future. As of the end of 2009, residual waste from five tanks has been evaluated. Residual wastes from adjacent tanks C-202 and C-203 have high U concentrations of 24 and 59 wt%, respectively, while residual wastes from nearby tanks C-103 and C-106 have low U concentrations of 0.4 and 0.03 wt%, respectively. Aluminum concentrations are high (8.2 to 29.1 wt%) in some tanks (C-103, C-106, and S-112) and relatively low (<1.5 wt%) in other tanks (C-202 and C-203). Gibbsite is a common mineral in tanks with high Al concentrations, while non-crystalline U-Na-C-O-P±H phases are common in the U-rich residual wastes from tanks C-202 and C-203. Iron oxides/hydroxides have been identified in all residual waste samples studiedmore » to date. Contaminant release from the residual wastes was studied by conducting batch leach tests using distilled deionized water, a Ca(OH)2-saturated solution, or a CaCO3-saturated water. Uranium release concentrations are highly dependent on waste and leachant compositions with dissolved U concentrations one or two orders of magnitude higher in the tests with high U residual wastes, and also higher when leached with the CaCO3-saturated solution than with the Ca(OH)2-saturated solution. Technetium leachability is not as strongly dependent on the concentration of Tc in the waste, and it appears to be slightly more leachable by the Ca(OH)2-saturated solution than by the CaCO3-saturated solution. In general, Tc is much less leachable (<10 wt% of the available mass in the waste) than previously predicted. This may be due to the coprecipitation of trace concentrations of Tc in relatively insoluble phases such as Fe oxide/hydroxide solids.« less

Thermodynamic Model for Uranium Release from Hanford Site Tank Residual Waste

A thermodynamic model of U solid-phase solubility and paragenesis was developed for Hanford Site tank residual waste that will remain in place after tank closure. The model was developed using a combination of waste composition data, waste leach test data, and thermodynamic modeling of the leach test data. The testing and analyses were conducted using actual Hanford Site tank residual waste. Positive identification of U phases by X-ray diffraction was generally not possible either because solids in the waste were amorphous or their concentrations were not detectable by XRD for both as-received and leached residual waste. Three leachant solutions were used in the studies: deionized water, CaCO3 saturated solution, and Ca(OH)2 saturated solution. Analysis of calculated saturation indices indicate that NaUO2PO4·xH2O and Na2U2O7(am) are present in the residual wastes initially. Leaching of the residual wastes with deionized water or CaCO3 saturated solution results in preferential dissolution Na2U2O7(am) and formation of schoepite. Leaching of the residual wastes with Ca(OH)2 saturated solution appears to result in transformation of both NaUO2PO4·xH2O and Na2U2O7(am) to CaUO4. Upon the basis of these results, the paragenetic sequence of secondary phases expected to occur as leaching of residual waste progresses for two tank closure scenarios was identified.

Benzene contamination at a metal plating facility

A metal plating facility in central Kentucky was required to complete a RCRA Facility Investigation to address a number of Solid Waste Management Units at the site. Twenty monitoring wells were installed at the facility. Ground water from the wells was sampled for total and dissolved metals, polychlorinated biphenyls, acid extractable compounds, base neutral compounds, and volatile organic compounds. Unexpectedly, relatively large concentrations of benzene, up to 120 μg/l, were detected in samples from some of the wells, including wells that should have been hydraulically upgradient from the facility. As a result of the detection of benzene, the facility completed an investigation to identify the source. A nearby facility had completed a gasoline underground storage tank (UST) closure at about the time of the installation of the 20 wells. Reportedly the UST had small holes when removed. Three potential pathways of migration (a ditch, sanitary sewer, and a sink hole) from the nearby facility to the metal-plating facility and residual soils with very large concentrations of benzene, toluene, ethylbenzene, and xylenes have been identified.

High Level Waste Tank Closure Project: ALARA Applications at the Idaho National Engineering and Environmental Laboratory

Bechtel BWXT Idaho, Maintenance and Operating Contractor for the Department of Energy at the Idaho National Engineering and Environmental Laboratory, has emptied, cleaned, and sampled six of the eleven 1.135 x 10(6) L high level waste underground storage tanks at the Idaho Nuclear Technology and Engineering Center, well ahead of the State of Idaho Consent Order cleaning schedule. Cleaning of a seventh tank is expected to be complete by the end of calendar year 2004. The tanks, with associated vaults, valve boxes, and distribution systems, are being closed to meet Resource Conservation and Recovery Act regulations and Department of Energy orders. The use of remotely operated equipment placed in the tanks through existing tank riser access points, sampling methods and application of as-low-as-reasonably-achievable (ALARA) principles have proven effective in keeping personnel dose low during equipment removal, tank, vault, and valve box cleaning, and sampling activities, currently at 0.03 Sv.

Risks and Uncertainties Associated With High-Level Waste Tank Closure

ABSTRACTClosure of tanks containing high-level radioactive waste (HLW) is a challenging problem involving potentially competing influences from economic, societal, and technological considerations. The U.S. Department of Energy (DOE) is faced with protecting public health and the environment while making economically responsible decisions. Risk (i.e., annual dose) is becoming more prominent as DOE's metric to evaluate the economic consequences of its decisions. Risks are assessed through modeling and calculations commonly known as performance assessment (PA). In the process of tank closure, the U.S. Nuclear Regulatory Commission (NRC) is typically consulted to perform an independent review of DOE's PAs.The NRC staff developed a generic PA model, applicable to HLW tank closure, which NRC utilizes to complete its independent review. The model was developed using the generic simulation software, GoldSim, because of its probabilistic capabilities and its adaptability to different problems [1]. The NRC staff uses the resultant risk from the generic models to evaluate the reasonableness of performance assessment models submitted by DOE. Large differences in the estimates of risk between the generic PA model and the DOE PA would likely indicate a need for stronger technical basis for processes significantly contributing to annual dose (risk) reduction.

Elastic-plastic Buckling of the Top End Closure of Cylindrical LNG Storage Tanks under Internal Pressure.

In the top end closure of cylindrical storage tanks for refrigerated liquefied natural gases (LNG)subjected to internal pressure, one failure mode is bifurcation buckling. The top end closure is torispherical, and buckling may occur elastic-plastically. This paper describes the buckling characteristic and the buckling pressure of a top end closure of LNG tanks, which is designed according to the current design code, API Standard 620. The axisymmetric shell finite-element method considering both geometrical and material nonlinearity in the prebuckling state is carried out. Results show that the current design code estimates the buckling pressure fairly conservatively.

Buckling Analysis of the Top End Closure of Oil Storage Tanks under Internal Pressure.

Bifurcation buckling will occur under internal pressure in the top end closure of cylindrical oil storage tanks with fixed roofs. In the design of the tanks, the shell-to-roof joint is considered frangible and is expected to fail before failure occurs in the shell-to-bottom joint. This paper describes the buckling analysis of the top end closure of the tank. The influence of both the tank diameter and the slope angle of the roof on the buckling internal pressure is investigated by the axisymmetric finite-element method. Large-deflection elastic analysis in the prebuckling state is performed. The values of the buckling pressure obtained by FEM are compared with those calculated by a formula of API standard 650, which is a design code for the oil storage tanks. Results show that the API formula gives smaller buckling internal pressure than FEM in the range of the small tank diameter, and seems to underestimate the pressure from the viewpoint of the frangible joint.