Leaching characteristics of paraffin waste forms generated from Korean nuclear power plants

Leaching behaviors and mechanisms of vitrified forms for the low-level radioactive solid wastes

Leaching behaviors and mechanisms of commercialized glass wasteforms to sequester low-level solid-wastes were investigated: SG glass for resin waste and DG-2 glass for dry active waste. After ANS 16.1 leaching test, leachabilities of the nuclides, Co, Cs, and Sr, were all lager than 14, which met the requirement of the US-Nuclear Regulatory Commission. Holes of diameters 5-10 μm remained on the surface of the SG and crevices of lengths 10-50 μm were observed on the surface of the DG-2. We analyzed elemental compositions of the SG and the DG-2 with depths. For the SG, Si, Al, Ca, and Mg were accumulated and Na was depleted up to nearly 1.5 μm compared to an internal glass. For the DG-2, concentrations of B, Na, Al, Ca and Sr started to decrease from 2.5 μm even though other minor elements are still remained their concentrations. We suggested leaching mechanisms: alkali elements including H would diffuse through the holes on the SG, while most of the elements including Si and Al would diffuse through the crevices on the DG-2.

Cesium retention and release from sulfur polymer concrete matrix under normal and accidental conditions

This paper proposes an efficient two-stage process for stabilization and solidification of the Cs-137 isotope in a sulfur polymer concrete (SPC) matrix. Lignite slag (SL) and fly ash (FA) were applied as active fillers for cesium immobilization. To study the release of Cs-137 isotope and determine the tracer activity in the leachates, we applied a slightly modified ANSI/ANS 16.1 protocol and the gamma spectrometry technique. The measured effective diffusion coefficients for the Cs-137 isotope were between 0.84·10^-9 and 3.10·10^-9 cm²·s^-1. Normalized leaching rates were within the range of 1.74·10^-5 - 3.85·10^-5 g·cm^-2·d^-1, fulfilling acceptance criteria for radioactive wasteforms. As well as standard leaching under static conditions, we also studied dynamic leaching of SPC samples at increased temperatures and leaching in an aggressive environment. The Cs-137 effective diffusion coefficients were found to increase by 3 - 4 orders of magnitude (10^-6 - 10^-5 cm²·s^-1), while the normalized leaching rate reached values of up to 2.36·10^-3 g·cm^-2·d^-1 after 28 days of leaching. The proposed cesium immobilization mechanism is based on the formation of cesium silicate and aluminosilicate phases, together with effective matrix sealing during the SPC manufacturing process.

Kinetic analysis of data obtained from studies on microbial degradation of cement waste forms, using shrinking core models

Model equations based on analytical solutions of two shrinking core models (acid dissolution or shrinking unreacted core (SUC) model, and bulk diffusion model), were used to analyze the kinetics of microbial degradation of cement waste forms. Two current approaches of waste form microbial stability evaluation (Nuclear Regulatory Commission (NRC) method and refined biofilm formation) were used to generate the data. Good linear correlations with R(2)>0.95 were obtained for the leaching data from both the NRC and biofilm approaches, using the model equation based on the bulk diffusion concept. Analyses using the model equation based on the acid dissolution model generally gave poor correlations except when data obtained from biofilm formation method was normalized.

Modeling of nuclide release from low-level radioactive paraffin waste: a comparison of simulated and real waste

Nuclide leaching models based on mass transfer theory are reviewed and evaluated to analyze the leaching test results of simulated and real paraffin waste from Korean nuclear power plants (NPPs). An empirical model (EM), bulk diffusion model (BDM), coupled diffusion/dissolution model (CDDM), shrinking core model (SCM), modified SCM (MSCM), and uniform reaction model (URM) are selected for comparison. In case of simulated paraffin waste form, the experimental results are satisfactorily explained by the SCM which is based on a diffusion-controlled dissolution reaction. Leaching behavior of real paraffin waste form is well predicted by URM that considers inter-aggregated porous medium and intra-aggregated porous medium separately. If real paraffin waste forms are manufactured with relatively uniform composition, their leaching behaviors are expected to be similar to those of simulated paraffin waste forms.

Improvement of nuclide leaching resistance of paraffin waste form with low density polyethylene

Low-level liquid borate wastes have been immobilized with paraffin wax using a concentrate waste drying system (CWDS) in Korean nuclear power plants. The possibility for improving chemical durability of paraffin waste form was suggested in this study. A small amount of low density polyethylene (LDPE) was added to increase the leaching resistance of the existing paraffin waste form. The influence of LDPE on the leaching behavior of waste form was investigated by performing leaching test according to ANSI/ANS-16.1 procedure during 325 days. It was observed that the leaching of nuclides immobilized within paraffin waste form made a marked reduction although little content of LDPE was added to waste form. The acceptance criteria of paraffin waste form associated with leachability index (LI) and compressive strength after the leaching test were fully satisfied with the help of LDPE.