API PUBL 7103 : 1997

Description / Abstract:

API PUBL 7103, 1997 Edition, November 1997 - Management and Disposal Alternatives for Naturally Occurring Radioactive Material (NORM) Wastes in Oil Production and Gas Plant Equipment

Natural radioactivity occurring at trace concentrations in oil and gas production streams occasionally accumulates as scale or sludge in tubing and in surface equipment to exceed background levels. Since the radioactivity is generally low and of natural origin, its accumulation and significance were not noted and studied until recently. The American Petroleum Institute (API) has subsequently sponsored studies to characterize accumulations of naturally-occurring radioactivity in oil-field equipment, and to determine safe methods for their disposal. This report presents the analyses of disposal methods for naturally-occurring radioactive materials (NORM) from oil and gas production. It builds on results of a previous safety analysis of disposal methods for NORM wastes in Texas, including a broader range of petroleum industry wastes, more detailed characterization, and covering a broader range of disposai alternatives.

Understanding the radiological safety of NORM waste disposal alternatives is vital to waste management and disposal decisions. Priorities in these decisions are to protect against harmful radiation exposures and to accomplish the disposal in a practical manner proportionate to any hazards posed by the NORM. Since radiation exposures depend on both the quantity of NORM and on its isolation, disposal safety depends on both the waste characteristics and the disposal method.

NORM concentrations vary from background levels to levels exceeding those of some uranium mill tailings, suggesting a similarly broad range of suitable disposal alternatives. Disposal of wastes containing NORM clearly does not require precautions for common cases in which NORM occurs at background levels. When elevated occurrences are found, their disposal should be handled in a way that protects against significant radiation exposure. The disposal problem is compounded by the lack of StandardDetails for pertinent alternative disposal methods or for defining the precautions needed for different kinds of NORM. Although detailed regulations provide for disposal of radioactive wastes that clearly pose health risks, there is less guidance on the disposal of wastes containing NORM with elevated radionuclide concentrations. As a result, some wastes containing extremely small amounts of NORM are occasionally sent to elaborate disposal sites at extremely high costs, wasting money, manpower and resources.

This report addresses the problem of what can be done with residues and equipment containing elevated NORM. It systematically identifies the maximum quantities or concentrations of NORM that can utilize various disposal alternatives, implemented at either arid or humid sites. It considers NORM that occurs in sludges from surface equipment, in pipe and tube scales, in cleaned equipment containing residual scales, and on surfaces of gas plant equipment. The waste disposal alternatives analyzed for sludges and scales include landspreading, landspreading with dilution, surface pipe non-retrieval, burial at unrestricted sites, disposal at commercial oil-field waste sites, disposal at licensed NORM disposal sites, disposal at licensed low-level radioactive waste sites, burial in surface mines, placement into wells being plugged and abandoned, injection into inactive wells, hydraulic fracturing into unused formations, and injection into salt domes. Disposal alternatives analyzed for equipment containing NORM residues include release for general use, release for re-use within the petroleum industry, storage in an oil-field equipment yard, release for smelting, and burial with NORM scales and sludges. For each waste disposal alternative, radiation exposures are considered from radon gas inhalation, external gamma-ray exposure, groundwater consumption, surface water consumption, dust inhalation, and food consumption. Using the NORM concentration limits for each disposal alternative, NORM wastes can be reliably managed in the most cost-effective manner while still protecting public radiological safety.

ORIGIN AND NATURE OF NORM

Naturally-occurring radioactive materials are ubiquitous in the environment, and commonly occur in soils, water, food and air. The NORM that accumulates in surface petroleum production equipment is predominantly radium-226 and radium-228 and their progeny, which come from the uranium-238 and thorium-232 decay chains, respectively (Figure 1-1). Both uranium and thorium occur naturally in underground formations and remain mostly in place. However their radium decay products are slightly soluble, and under

some conditions they become mobilized by liquid phases in the formation. When brought to the surface with liquid production streams these nuclides may remain dissolved at dilute levels, or may precipitate because of chemical changes and reduced pressure and temperature as the fluids are separated and processed. Since radium concentrations in the original formations are highly variable, production fluids also are highly variable and occasionally may exhibit elevated radioactivity. Varied formation and surface chemistries cause additional variations in radioactivity brought to the surface. Fluids injected into formations also affect the mobilization of natural radioactivity, and surface processes further vary the accumulation of any radioactivity in scales, sludges, and waste products. Scales and sludges accumulated in surface equipment thus may vary from background concentrations of NORM to elevated levels depending on formation radioactivity and chemistry and process characteristics. As used in this report, the tem NORM refers only to the radionuclides of the uranium and thorium decay chains, ignoring naturally-occurring potassium-40 and other nuclides that occur naturally throughout the environment but have not been known to accumulate in residues from oil and gas production.

The NORM accumulated in production equipment scales typically contains radium coprecipitated in barium sulfate. Sludges are dominated by silicates or carbonates, but also incorporate trace radium by coprecipitation. Typically, radium-226 is in equilibrium with its decay products but radium-228 has sub-equilibrium decay products. Reduced concentrations of radium-228 daughters result from the occurrence in the thorium-232 decay chain of two radium nuclides separated by the 1.9-year half-life thorium-228 (Figure 1-1). Thus radium mobilized from the formation initially becomes depleted in radium-224 (3.6 days) until more is generated by radium-228 decay through the thorium-228 intermediate. Long-term radiological concern in waste disposal is dominated by the uranium chain due to the long half-life (1,600 years) of radium-226. Both are usually considered together in waste disposal decisions, however, since they are not distinguished by simple field measurements.

NORM deposits also may accumulate in gas-plant equipment from radon-222 (radon) gas progeny, even though the gas is removed from its radium-226 parent. The more mobile radon gas mostly originates in underground formations and becomes dissolved in the organic petroleum fractions in the gas plant. Once in surface equipment, it is partitioned mainly into the propane and ethane fractions by its solubility. Gas-plant deposits differ from oil production scales and sludges by having very low mass, typically consisting of an invisible plate-out of radon daughters on the interior surfaces of pipes, valves and other gas-plant equipment. These deposits accumulate from radon daughters at natural levels from the very large volumes of gas passing through the system. Since radon decays with a 3.8-day half-life, the only nuclide remaining in gas-plant equipment that affects its disposal is lead-210, which has a 22-year half life. Lead-210 decays by beta emission, with only low-intensity, low-energy gamma rays. It thus poses less disposal hazard than other NORM deposits in most cases.

PRECEDENT FOR UNREGULATED DISPOSAL OPTIONS

Current legislation and regulations have acknowledged the gap between background levels of radioactivity and levels that require regulation. On an international level, the International Atomic Energy Agency has developed a method to determine de minimis levels for radioactive waste disposal that is consistent with the methods used here. In the United States, the Atomic Industrial Forum has sponsored studies of de minimis disposal of nuclear power reactor wastes that consider some of the same disposal alternatives and exposure pathways analyzed in this report. The half-lives of the reactor wastes have similar longevity to the NORM nuclides considered here. In addition with U.S. Low-Level Radioactive Waste Policy Amendments Act directs the Nuclear Regulatory Commission (NRC) to promulgate regulations to exempt the disposal of waste that is "below regulatory concern" (BRC) from license control, and to develop StandardDetails and procedures for considering and acting upon petitions for de minimis disposal. The nuclear power industry has responded to the congressional mandate to NRC by preparing a petition for NRC to allow disposal of wastes containing very low levels of radioactivity at facilities other than those licensed under 10 CFR 61. Proposed disposal alternatives include municipal sanitary landfills and burial at the facility. A separate petition for NRC to allow BRC disposal by non-utility industrial and institutional radioactive waste generators also is being prepared.

In other national actions, NRC has examined the consequences of disposing of standard low-level radioactive waste streams in sanitary landfills, and the Environmental Protection Agency (EPA) is developing a general de minimis regulation for sanitary landfill disposal as part of their low-level radioactive waste disposal rulemaking. Both of these activities have used methods similar to those used here. The NRC and the state of Texas both have developed de minimis biomedical waste disposal rules for tritium (H-3) and carbon-14 that define alternative concentration limits for waste treatment and disposal. The Texas Department of Health also has approved regulations permitting disposal of wastes containing only short-lived radionuclides (half-lives less than 1 year) in non-radiological facilities such as sanitary landfills, and permitting local disposal of low-level cesium-137 contaminated soils based on similar safety analyses.

In the foregoing safety and disposal analyses of de minimis disposal of radioactive wastes, the analysis methods are similar to those used in this report. As shown in Figure 1-2, radiation exposure limits first are defined, followed by conservative calculations of modeled radiation exposures via all possible pathways for the proposed disposal alternative. In the calculations the source concentration is adjusted until the calculated radiation doses are equal to the defined exposure limits. This procedure defines NORM nuclide concentration limits for each disposal alternative and for each pathway. The final nuclide concentration limit for each disposal alternative and geohydrologic setting is the lowest source concentration limit from the limiting pathway. The present methods thus estimate maximum disposable quantities objectively, and systematically define the best alternatives in the intermediate range between background and regulated levels. For completeness and consistency, NORM disposal concentrations up to 100,000 pCi/gram are presented as calculated in the analyses, even though regulated levels overlap much of the reported range.