Development of a Dynamic Food Chain Model DYNACON and Its Application to Korean Agricultural Conditions

A dynamic food chain model DYNACON was developed to simulate the radionuclide transfer on agricultural ecosystems. DYNACON estimates the radioactivity in each compartment of food chains for three radionuclides, nine plant species and five animal products as a function of the deposition date. A number of the parameter values used in this study are representative of Korean agricultural conditions. The model was expressed by coupled differential equations and the radioactivity in each compartment was solved as a function of time following an acute deposition. Although DYNACON is structurally based on existing models, it was designed in order to simulate more realistic radionuclide behavior in Korean agricultural conditions and to save computation time. It was found that the radioactivity in foodstuffs depends strongly on the date of deposition. A comparative study between DYNACON and an equilibrium model showed good agreement for depositions that occur during the growing season of plants. DYNACON is going to be implemented in a Korean real-time dose assessment system FADAS.     

Computer Code System DSOCEAN for Assessing the Collective Dose of Japanese due to Radionuclides Released to the Ocean from a Reprocessing Plant

A computer code system DSOCEAN has been developed for assessing the collective dose of Japanese due to radionuclides released to the ocean from a spent nuclear fuel reprocessing plant. This computer code system uses a box model which represents the transfer of radionuclides between boxes of seawater into which the ocean around Japan is divided. The code system consists of a series of three interlinked main computer codes, which estimates the exchange rates of radionuclides between the boxes, the radionuclide concentrations in each box, and the collective dose from various exposure pathways, respectively.

By using DSOCEAN, two calculations were carried out to estimate the collective dose from a liquid effluent. One is associated with a routine release of radionuclides from a hypothetical reprocessing plant. The other is an application of the code system to disposal of liquid radioactive waste to the surface water of the ocean. The calculated results identified important radionuclides and exposure pathways.     

Development of an Atmospheric Dispersion Model for Accidental Discharge of Radionuclides with the Function of Simultaneous Prediction for Multiple Domains and its Evaluation by Application to the Chernobyl Nuclear Accident

A particle random-walk model GEARN for nuclear emergency response system, Worldwide version of System for Prediction of Environmental Emergency Dose Information (WSPEEDI), was improved to predict the atmospheric dispersion of radionuclides in detail around a release point as well as on a regional scale for a transboundary nuclear accident. The main improvement is simultaneous atmospheric dispersion calculations of two nested domains, local and regional areas, achieved by exchanging particle information between the domains. In the application of the model to the Chernobyl accident, the distribution of surface deposition of ¹³⁷Cs was predicted well in the local area around Chernobyl and the European regional area. The improvements were mainly due to the consideration of the reentry of particles from the regional area to the local one and the enhancement of prediction accuracy for precipitation by the nesting calculation in the meteorological model MM5 combined with GEARN. It is concluded that the nesting model developed in this paper is appropriate for nuclear emergencies in which the prediction of both local and regional scale dispersions are required.     

Development of a hybrid method to improve the sensitivity and uncertainty analysis for homogenized few-group cross sections

In the framework of two-step method of reactor core calculation, few-group homogenized cross sections generated by lattice-physics calculations are key input parameters for the three-dimensional full-core calculation. Conventional method for few-group cross-sections sensitivity and uncertainty (S&U) analysis related to the nuclear data was performed based on the effective self-shielding cross sections instead of the continuous-energy cross sections, which means resonance self-shielding effect (implicit effect) is neglected. Furthermore, the multi-group covariance data is generated from the continuous-energy cross sections. Therefore, in order to perform S&U analysis with respect to the continuous-energy cross sections for both accuracy and consistency, a hybrid method is proposed in this paper. The subgroup-parameter sensitivity-coefficients are calculated based on the direct perturbation (DP) method. The sensitivity-coefficients of the effective self-shielding cross sections and the responses (k_eff and few-group homogenized cross sections) are calculated based on the generalized perturbation theory (GPT). A boiling water reactor (BWR) pin-cell problem under different power conditions is calculated and analyzed. The numerical results reveal that the proposed hybrid method improves the sensitivity-coefficients of eigenvalue and few-group homogenized cross sections. The temperature effects on the sensitivity-coefficients are demonstrated and the uncertainties are analyzed.     

Development of a Non-conservative Radionuclides Dispersion Model in the Ocean and its Application to Surface Cesium-137 Dispersion in the Irish Sea

A numerical simulation model system that consists of an ocean current model, Princeton Ocean Model (POM), and a particle random-walk model, SEA-GEARN, has been developed to describe the migration behavior of non-conservative radionuclides in a shallow water region. Radionuclides in the ocean are modeled in three phases, i.e., the dissolved phase in seawater, the adsorbed with large particulate matter (LPM) and the adsorbed with active bottom sediment. The adsorption and desorption processes between the dissolved and solid phases are solved by the stochastic method with the kinetic transfer coefficients. Deposition of the LPM and re-suspension from bottom sediment are also considered. The system was applied to simulate the long-term (24-year) dispersion of 137Cs actually released from the BNFL spent nuclear fuel reprocessing plant at Sellafield in United Kingdom. The calculation well reproduced the main characteristics of migration of dissolved ¹³⁷Cs concentration in the Irish Sea.     

Development of a data acquisition and control system for the International Thermonuclear Experimental Reactor neutron flux monitor

To satisfy high-precision, wide-range, and real-time neutron flux measurement requirements by the International Thermonuclear Experimental Reactor(ITER), a data acquisition and control system based on fission chamber detectors and fast controller technology, has been developed for neutron flux monitor in ITER Equatorial Port #7. The signal processing units which are based on a field programmable gate array and the PXI Express platform are designed to realize the neutron flux measurement with 1 ms time resolution and a fast response less than 0.2 ms,together with real-time timestamps provided by a timing board. The application of the widerange algorithm allows the system to measure up to 10~(10) cps with a relative error of less than 5%.Furthermore, the system is managed and controlled by a software based on the Experimental Physics and Industrial Control System, compliant with COntrol, Data Access and Communication architecture.     

Development of a Fast Valve for Disruption Mitigation and its Preliminary Application to EAST and HT-7

In large tokamaks, disruption of high current plasma would damage plasma facing component surfaces (PFCs) or other inner components due to high heat load, electromagnetic force load and runaway electrons. It would also influence the subsequent plasma discharge due to production of impurities during disruptions. So the avoidance and mitigation of disruptions is essential for the next generation of tokamaks, such as ITER. Massive gas injection (MGI) is a promising method of disruption mitigation. A new fast valve has been developed successfully on EAST. The valve can be opened in 0.5 ms, and the duration of open state is largely dependent on the gas pressure and capacitor voltage. The throughput of the valve can be adjusted from 0 mbar·L to 700 mbar·L by changing the capacitor voltage and gas pressure. The response time and throughput of the fast valve can meet the requirement of disruption mitigation on EAST. In the last round campaign of EAST and HT-7 in 2010, the fast valve has operated successfully. He and Ar was used for the disruption mitigation on HT-7. By injecting the proper amount of gas, the current quench rate could be slowed down, and the impurities radiation would be greatly improved. In elongated plasmas of EAST discharges, the experimental data is opposite to that which is expected.     

Development and validation of a heuristic model for evaluation of the team performance of operators in nuclear power plants

The global concerns about safety in the digital technology of the main control room (MCR) are growing as domestic and foreign nuclear power plants are developed with computerized control facilities and human–system interfaces. In a narrow space, the digital technology contributes to a control room environment, which can facilitate the acquisition of all the information needed for operation. Thus, although an individual performance of the advanced MCR can be further improved; there is a limit in expecting an improvement in team performance. The team performance depends on organic coherence as a whole team rather than on the knowledge and skill of an individual operator. Moreover, a good team performance improves communication between and within teams in an efficient manner, and then it can be conducive to addressing unsafe conditions. Respecting this, it is important and necessary to develop methodological technology for the evaluation of operators’ teamwork or collaboration, thus enhancing operational performance in nuclear power plant at the MCR.     

Development of Three-Dimensional Numerical Model for 222Rn and Its Decay Products Coupled with a Mesoscale Meteorological Model II. Numerical Analysis on the Increase in Gamma Dose Rate Observed in the Coastal Area of Fukushima Prefecture

A three-dimensional Eulerian numerical model for ²²²Rn and its decay products coupled with a mesoscale meteorological model, MM5-TMNR, is applied to investigate the mechanism of the naturally induced increase in gamma dose rate observed at the coastal area of Fukushima prefecture in October 2002. The results obtained by MM5-TMNR are verified by comparisons with observed wind, precipitation and gamma dose rate to be adequate for examining the mechanism. The unusual increase in gamma dose rate is observed by the combination of the synoptic-scale (about 2000 to 5000 km horizontally) transport of natural radionuclides due to the inflow of cold air mass from the Asian Continent and the meso-_-scale (about 20 to 200 km horizontally) precipitation process within the coastal area of Fukushima prefecture. The contribution rate of natural radionuclides from the Asian Continent to the increase in gamma dose rateis estimated to be more than 60%.