Jongtae Jeong

THE CUPID CODE DEVELOPMENT AND ASSESSMENT STRATEGY

A thermal-hydraulic code, named CUPID, has been being developed for the realistic analysis of transient two-phase flows in nuclear reactor components. The CUPID code development was motivated from very practical needs, including the analyses of a downcomer boiling, a two-phase flow mixing in a pool, and a two-phase flow in a direct vessel injection system. The CUPID code adopts a two-fluid, three-field model for two-phase flows, and the governing equations are solved over unstructured grids with a semi-implicit two-step method. This paper presents an overview of the CUPID code development and assessment strategy. It also presents the code couplings with a system code, MARS, and, a three-dimensional reactor kinetics code, MASTER.

PRELIMINARY MODELING FOR SOLUTE TRANSPORT IN A FRACTURED ZONE AT THE KOREA UNDERGROUND RESEARCH TUNNEL (KURT)

Migration tests were performed with conservative tracers in a fractured zone that had a single fracture of about 2.5 m distance at the KURT. To interpret the migration of the tracers in the fractured rock, a solute transport model was developed. A two dimensional variable aperture channel model was adopted to describe the fractured path and hydrology, and a particle tracking method was used for solute transport. The simulation tried not only to develop a migration model of solutes for open flow environments but also to produce ideas for a better understanding of solute behaviours in indefinable fracture zones by comparing them to experimental results. The results of our simulations and experiments are described as elution and breakthrough curves, and are quantified by momentum analysis. The main retardation mechanism of nonsorbing tracers, including matrixdiffusion, was investigated.

A comparative study for the determination of uranium and uranium isotopes in granitic groundwater

In this study, the activities of uranium isotopes for granitic groundwater samples were determined using alpha (α)-particle spectrometry and liquid scintillation counting (LSC). The activity ratios of 234U/238U and the total mass concentration of uranium were also investigated for groundwater samples from different depths. The uranium isotopes were out of secular equilibrium owing to an increased water–rock interaction and to different origins of the groundwater samples. The concentration of 238U in the groundwater samples determined by ICP-MS, showed relatively good consistency with those by α-particle spectrometry, and less so for those determined by LSC.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions.

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

A visual environment for system analysis codes

Abstract A visual environment for system analysis codes (hereinafter called “ViSA”) has been developed to support code users in their input preparations, code executions, and output interpretations. ViSA provides a more convenient way for base input data generation and modification on a user-friendly basis. It also provides on-line graphical displays to give an in-depth understanding of transient thermal-hydraulic behaviors in nuclear power plants. This paper presents the main features of ViSA.

Development of a natural analogue database to support the safety case of the Korean radioactive waste disposal program

In this study, the status of natural analogue studies in Korea is briefly summarized and applicability of existing natural analogue information to the Korean safety case has been evaluated. To enable effective application of natural analogue information to the overall evaluation of long-term safety (the “safety case”) for the geological disposal of radioactive wastes, a natural analogue database has been developed by collecting, classifying, and evaluating relevant data. The natural analogue data collected were classified into categories based on site information, components/processes of the disposal system, properties/phenomena, reference, safety case application, application method, and suitability to a safety case. Suitability of the natural analogue data to a specific safety case was evaluated based upon the importance and the applicability to the Korean safety case. As a result, 75 natural analogue datasets were selected as important for the Korean safety case. The database developed can now be utilized in the RD&D (Research, Development, and Demonstration) program development for natural analogue studies. In addition, the methodology developed and the database compiled in this study may assist in the development of safety case including safety assessment for high-level radioactive waste disposal in Korea as well as in other countries.

Modeling in-situ transport of uranine and colloids in the fracture network in KURT.

An in-situ dipole migration experiment was conducted using the conservative tracer uranine and latex colloids in KAERI (Korea Atomic Energy Research Institute) Underground Research Tunnel (KURT). The location and dimensions of the fractures between the two boreholes were estimated using the results of a borehole image processing system (BIPS) investigation, and the connectivity of the fractures was evaluated by a packer test. To investigate the flow and transport of uranine and colloids through an in-situ fracture network, a fracture network transport model was newly developed. The model consists of a series of one-dimensional advection-dispersion-matrix diffusion equations for each channel of the fracture network. Using the fracture network transport model, the most probable representation and the hydrologic parameters of the fracture network can be estimated by fitting the breakthrough of uranine. While the fracture network might not be unique, the representation chosen was adequate to describe the breakthrough of uranine and it represents a reasonable approach to modeling transport in the fracture network. An additional evaluation showed that the colloid transport in this study was influenced by filtration on the fracture surface rather than the enhancement of the colloid velocity. Overall, the model can explain successfully the in-situ experimental results of uranine and colloid transports through the fracture network.

Model Development for Risk-Based Safety Assessment of a Geological Disposal System of Radioactive Wastes Generated by Pyroprocessing of Pressurized Water Reactor Spent Fuel in Korea

Abstract New methodology for a risk-based safety assessment of a geological disposal system of nuclear waste was implemented using the numerical Korea Atomic Energy Research Institute (KAERI) Performance Assessment Model (K-PAM). K-PAM was applied to a conceptual geological disposal system for pyroprocessed radioactive wastes based on the KAERI Underground Research Tunnel (KURT) site. The methodology was systematically organized for model development considering two types of external events: earthquakes and well intrusion. Following description of its conceptual models and submodules, K-PAM was partially verified by comparing the consequences of two major modules of K-PAM—engineered barrier system and natural barrier system—with those by a well-known, comparable process model using COMSOL. In addition, K-PAM was demonstrated using three scenarios: (1) the reference scenario, in which the normal consequences of the disposal system without external events could be predicted; (2) the deterministic complex scenario, in which the impacts of individual external events on the disposal system could be estimated separately; and (3) the probabilistic complex scenario, in which the efficiency of the new methodology for a risk-based safety assessment could be confirmed numerically by showing the probable maximum dose rate according to any single scenario, the convergence of risk, the dominant impacts contributing to the maximum dose rate, and the probability of occurrence of the scenario groups.

Reactive transport of uranium with bacteria in fractured rock: Model development and sensitivity analysis

A numerical model for the reactive transport of uranium and bacteria in fractured rock was newly developed. The conceptual model consists of four phases (fracture, fracture surface, matrix pore, and matrix solid) and eight constituents (solutes in the fracture, on the fracture surface, on mobile bacteria, on immobile bacteria, in the rock matrix pores and on the rock matrix solids, and bacteria in the fracture and on the fracture surface). In addition to the kinetic sorption/desorption of uranium and bacteria, uranium reduction reaction accompanying with bacteria growth was considered in the reactive transport. The non-linear reactive transport equations were numerically solved using the symmetric sequential iterative scheme of the operator-splitting method. The transport and kinetic reaction modules in the developed model were separately verified, and the results were reasonably acceptable. From the sensitivity analysis, the uranium transport was generally more sensitive to the sorption rate rather than desorption rate of U(VI). Considering a uranium reduction reaction, bacteria could considerably retard the uranium transport no matter the uranium sorption/desorption rates. As the affinity of U(VI) onto the bacteria becomes higher than that onto a rock fracture surface, a biofilm effect, rather than a colloidal effect, of the bacteria becomes more influential on the uranium transport.

ESTIMATION OF EXPOSURE DOSES FOR THE SAFE MANAGEMENT OF NORM WASTE DISPOSAL

Naturally occurring radioactive materials (NORM) wastes with different radiological characteristics are generated in several industries. The appropriate options for NORM waste management including disposal options should be discussed and established based on the act and regulation guidelines. Several studies calculated the exposure dose and mass of NORM waste to be disposed in landfill site by considering the activity concentration level and exposure dose. In 2012, the Korean government promulgated an act on the safety control of NORM around living environments to protect human health and the environment. For the successful implementation of this act, we suggest a reference design for a landfill for the disposal of NORM waste. Based on this reference landfill, we estimate the maximum exposure doses and the relative impact of each pathway to exposure dose for three scenarios: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. Also, we estimate the possible quantity of NORM waste disposal into a landfill as a function of the activity concentration level of U series, Th series and 40K and two kinds of exposure dose levels, 1 and 0.3 mSv/y. The results of this study can be used to support the establishment of technical bases of the management strategy for the safe disposal of NORM waste.