Waste Form Specimens Research Articles

Utilization of recycled cement powder as a solidifying agent for radioactive waste immobilization

Due to the uniquely high disposal cost for disposal of Intermediate/Low level radioactive waste in Republic of Korea, reduction of concrete waste by separating activated/contaminated portion of the concrete needs to be accomplished to minimize cost and environmental burden. Since the activated or contaminated portion of the concrete is on the hydrated cement paste, a 2 h heat treatment at 600 °C and 22 h milling process were applied to mortar specimens to separate recycled cement powder from fine aggregate. Separated recycled cement powder that passed 150 μm sieve was evaluated for the utilization as a solidifying agent for liquid radioactive waste immobilization. The 3 M CoCl2 and 3 M CsCl solutions were used as waste simulants containing Co and Cs, respectively. It was found that the wasteform made of 3 M CoCl2 and 3 M CsCl with recycled cement powder met the strength requirement of 3.45 MPa that was required for disposal of solid wasteform. Wasteform specimens also passed thermal cycling and water immersion requirements. Leachability indices of Co and Cs containing wasteform specimens were in the range of 17.2 ~ 18.7 and 6.7 ~ 15.3, respectively. It was found that recycled cement powder can be used for immobilization of liquid radioactive waste.

Chemical stability of seven years aged cement–PET composite waste form containing radioactive borate waste simulates

Different samples of radioactive borate waste simulate [originating from pressurized water reactors (PWR)] have been prepared and solidified after mixing with cement–water extended polyester composite (CPC). The polymer–cement composite samples were prepared from recycled poly (ethylene terephthalate) (PET) waste and cement paste (water/cement ratio of 40%). The prepared samples were left to set at room temperature (25 °C ± 5) under humid conditions. After 28 days curing time the obtained specimens were kept in their molds to age for 7 years under ambient conditions. Cement–polymer composite waste form specimens (CPCW) have been subjected to leach tests for both 137Cs and 60Co radionuclides according to the method proposed by the International Atomic Energy Agency (IAEA). Leaching tests were justified under various factors that may exist within the disposal site (e.g. type of leachant, surrounding temperature, leachant behavior, the leachant volume to CPCW surface area…). The obtained data after 260 days of leaching revealed that after 7 years of aging the candidate cement–polymer composite (CPC) containing radioactive borate waste samples are characterized by adequate chemical stability required for the long-term disposal process.

A Procedure to Evaluate the Potential for Microbially Influenced Degradation of Cement-Solidified Low-Level Radioactive Waste Forms

AbstractBecause of its apparent structural integrity, cement has been widely used in the United States as a binder to solidify Class B and C low-level radioactive waste (LLW). However, the resulting cement preparations are susceptible to failure due to the actions of stress and environment. An environmentally mediated process that could affect cement stability is the action of naturally occurring microorganisms. The U.S. Nuclear Regulatory Commission (NRC), recognizing this eventuality, stated in their Technical Position on Waste Form, Revision 1, that the effects of microbial action on waste form integrity must be addressed. This paper provides recent results from a program that examined the effects of microbially influenced degradation (MID) on cement-solidified LLW. Data are provided which were obtained during the development of an evaluation method using acid-producing bacteria. Results presented here are from work with one type of these bacteria, the sulfur-oxidizingThiobacillus. Commercially prepared, cement-solidified, low-level radioactive waste form samples made from power reactor wastes were evaluated using a new biodegradation test developed for the NRC. Testing demonstrated that MID has the potential to severely compromise the structural integrity and nuclide retentiveness of ion-exchange resin and evaporator-bottoms wastes that have been solidified with cement. It was found that the waste form specimens physically deteriorated after 60 days of exposure to the thiobacilli. Also, the data show that significant amounts of Cs-137, Cs-134, Co-60, C-14, Tc-99, and Sr-90 contained in the waste forms were leached in the presence ofThiobacillus.

Comparison of Ceramic Waste forms Produced by hot Uniaxial Pressing and by Cold Pressing and Sintering

AbstractSynroc C waste form specimens prepared using the Australian-developed technology are uniaxially pressed in stainless steel bellows at 1200°C and 20MPa. This produces a material with high chemical and physical durability and with the radioactivity enclosed inside both the waste form and the bellows. An alternative method of producing the ceramic product is to use cold pressing of pellets followed by reactive sintering to provide densification and mineralization. Depending on the scale of waste form preparation required and on the activity level and nature of the waste streams, the cold press and sinter method may have advantages. To evaluate the effects of production method on waste form characteristics, especially resistance to dissolution or leaching of waste elements, we have prepared two simulated waste samples for evaluation. Both samples were prepared from liquid precursor materials (alkoxides, nitrates, and colloidal silica) and then doped with waste elements. The precursor material in each case corresponded to a basic phase assemblage of 60% zirconolite, 15% nepheline, 10% spinel, 10% perovskite, and 5% rutile. One sample was doped with 25% by weight of U; the other with 10% by weight each of U and Gd. Each sample was calcined at 750°C for 1 hr. in a 3.5% H2 in N2 atmosphere. Then one portion of each sample was hot pressed at temperatures ranging from 1120 to 1250°C and 20MPa pressure in steel bellows. A separate portion of each sample was formed into pellets, cold pressed, and sintered in various atmospheres at 1200°C to produce final products about 2/3 cm in diameter. Samples were then examined to determine density of the product, grain sizes of the phases, phase assemblage, and the location of the U and Gd in the final phases. Density data indicate that sintering gives good results provided that the samples are held at 200°C for long enough to allow trapped gases to escape.

Leachability of chelated ion-exchange resins solidified in cement or cement and fly ash

Leach tests were conducted at the Idaho National Engineering Laboratory ∗ on six small-scale specimens of cement-solidified ion-exchange resin wastes. The ion-exchange resins had been used to process reagent solutions following chemical decontaminations of primary coolant systems at five commercial light water reactors. The decontaminations were performed using the AP/Citrox, Can-Decon, Dow NS-1, and Low Oxidation-State Transition-Metal ION (LOMI) processes. The ion-exchange resin wastes were loaded with radionuclides, transition metals, and organic chelating agents, and were solidified in either unmodified Portland Type I cement or in a mixture of Portland Type I cement and fly ash. Wasteform specimens were leach-tested in deionized water at 23°C using the American National Standards Institute/American Nuclear Society (ANSI/ANS) Standard 16.1 procedure. Release rates, effective diffusivities, and leachability indexes of radionuclides, chelating agents, and stable metals were determined using ANS-16.1 diffusion release models. Releases of radionuclides, chelating agents, and metals from waste forms that degraded during leaching were similar to releases from waste forms that maintained their physical integrity during leaching. The presence of chelating agents in the waste forms did not adversely affect the leachability of the waste forms.