Volume and External Dose during Kori unit 1 Nuclear Power Plant Decommissioning

Specifically, as the Kori 1 unit would be the first commercial nuclear power plant for the D&D project, proper preparation beforehand is more important. To avoid such an error, specific modeling of the Kori Unit 1 radioactivity inventory was performed. Prior to commencing modeling at Kori Unit 1, a literature review and case study on radioactivity inventory assessment at similar foreign nuclear power plants were conducted to identify variables and information to be identified in the modeling.

Based on the case study, the research aims to investigate the radioactivity inventory of the bioshield on Kori Unit 1 and estimate the potential quantity and cost of radioactive waste management and guide workers in external dose analysis. The Trojan nuclear power plant was used as a benchmark for the validation of the calculation model to ensure its reliability. After verification was completed, a specific Kori unit 1 bioshield assessment with MCNP 6 based on Monte Carlo probability theory was adopted with Boltzmann neutron transport.

Based on the analysis of the radioactivity inventory, the regulation for the level of cleanup has been adopted and the amount of potential LLW has been estimated with an additional change from 5 to 30 years after the closure of Kori unit 1. As a result, the biodefense in Kori Unit 1 showed an average contamination of 812 Bq/g with the main radioactive nuclide as 60Co.

Introduction

Decommissioning: The Emerging Challenges

The Importance of the Radioactivity Inventory: Safety and economics

• Radioactivity Assessment on Kori unit 1 Bioshield
• Computer Modeling on Spatial Radioactivity

As bioshield is highly activated by the neutron flux originating from the reactor core, it poses a high potential risk for workers to access the site without prior risk assessment. However, computer modeling with a reasonable error condition can enable the reduction of uncertainty about coring to targeted spot samples, which closes the assurance of worker safety from the radioactive hazards, encourages the optimization of concrete demolition and decontamination methods, and waste generated from the process, to the limited to a minimum. which ultimately enables the reduction of the management budget for radioactive waste handling [6]. The handling of waste management budget is a crucial part in the whole D&D process, which can be found in the case for the Connecticut Yankee nuclear power plant in the United States of America which was decommissioned.

The Connecticut Yankee D&D project's pre-estimation of the amount of radioactive waste, especially low-level waste (LLW), has included a large amount of forecast error. Later, an additional 163,954 200 liter barrels of LLW were shown, causing 228% of the total additional waste management budget, delaying the entire D&D project.

Figure 2. Major activation on the primary circuit of the nuclear power plant [5]

Literature Review

Case of Radioactivity Estimation for Bioshield

• United States of America
• United Kingdom
• Italy

Each EFPY case was divided into 3 sections, which are 0 years, 10 years and 100 years after the closure of the facility. Cases of 9 EFPY in 0 years after shutdown were evaluated as on-site monitoring of the samples and others were evaluated with computer coding scheme. In the case of the United Kingdom, statutory and economic value dominate the characterization program for decommissioning.

D&D project on Italy is mainly focused on safe containment of the facility which is different from the United States of America which focuses on safe license termination of the site. Its analysis on neutron flux was based on two-dimensional computer code DOT 3.5 and the activity of the contaminated site was evaluated with Origen-S computer code. As shown in Table 6, the total radioactivity of the waste was evaluated with various radioactive nuclides.

Similar to other activity analyzes of the United States and the United Kingdom, no spatial analysis was performed, which did not lead to the identification of a clearance boundary or amount of waste volume [15]. As shown in Tables 8 and 9, the average radioactivity of major nuclides was assessed under the condition of decades after the shutdown of the nuclear power plant [16].

Table 3. Radioactive inventory of materials in Trojan nuclear power plant (Bq) [10]

Improvements Needed: Connection to Safety and Waste

Research Design

Research Objectives and Approaches

Mathematical Equation and Numerical Methods

The level input is the radioactivity of the target nuclides that we identified in the previous phase using MS-EXCEL.

Defining of Source Term

Method Validation against Trojan Nuclear Power Plant

Trojan Modeling Condition

Radioactivity Assessment Validation

Although the difference ratio with IAEA modeling could not be compared, the average difference with MCNP 6 versus on-site monitoring showed +29.2 % which was lower than that of. Overall, MCNP 6 modeling showed higher sensitivity in all radionuclides as IAEA ORIGEN-2 showed relatively conservative results in radioactivity compared to MCNP 6. Compared to target radionuclides, MCNP 6 showed a reasonable margin of variation in so that it could be converted to modeling in Kori unit 1.

Table 16. 60 Co radioactivity concentration

Radioactivity Inventory of Bioshield in Kori unit 1

Kori unit 1 Modeling Conditions

Radioactivity Assessment of bioshield in Kori unit 1

The trend of the radioactivity showed corresponding reduction along the radial distance in the same way as the neutron flux distribution. The range from 357.75 cm to 463.75 cm showed the same increasing trend in difference ration that was also confirmed on 60Co. As the maximum radioactivity was lower than 0.1 Bq/g, 134Cs alone showed complete clearance on bioshield.

Figure 14. A: 60 Co radioactivity concentration B: 3D 60 Co radioactivity

Safety, Economics and Waste Management for Bioshield

External Dose for Workers on Bioshield in Kori unit 1

Since current regulation on professional radiological workers in the field allows 20 mSv/year, the possible working hours in a year must be limited as indicated in Table 29. Since most of the high-cost process and the high-risk dose at the bottom of the structure, sensitivity test with additional regional space analysis should be made from 330 cm height to 165 cm area for further concrete reliability. From the allowable working hours limitation data it is possible to explain which method can be used on decontamination of the bioshield.

In addition, a large amount of secondary waste is generated from the solution that is used in the process. In order for both heads to match the 3 mm depth decontamination in 1 hour, stripping requires an average of 50 people and shaving requires an average of 1 person in the biological protection of the Kori 1 unit. Since the clearance limit is 425 cm from the reactor core, it requires 363 hours of working time.

As shown in Figure 22, scabbing requires a steady supply of 3522 people and shaving requires a total of 70 people in a year.

Table 29. External dose rate on major nuclides [mSv/h]

Spatial Distribution for Radioactivity Waste

Cost Reduction by Waste Volume Reduction

Conclusion

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