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Radiation Hazards to Consider During the Decommissioning of NPP for Radiation Protection

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2019 ⦽ǎႊᔍᖒ⠱ʑྜྷ⦺⫭ ⇹ĥ⦺ᚁݡ⫭ םྙ᫵᧞Ḳ

385

Radiation Hazards to Consider During the Decommissioning of NPP for Radiation

Protection

Gi-Lim Kim and Hyung-Woo Seo

KHNP Central Research Institute, 70, Yuseong-daero 1312beon-gil, Yuseong-gu, Daejeon, Republic of Korea *

sehywo1727@khnp.co.kr

1. Introduction

The final goal of decommissioning of nuclear facilities is to safely eliminate the exposure risk, and release facilities and sites form regulation applied for operation. The Dismantling and Decontamination (D&D) works involve all the elements of the site from WKH IDFLOLW\¶V PDLQ FRPSRQHQWV DQG V\VWHPV WR WKH buildings, and it is necessary to secure the safety of workers, general public and the environment from the radiation hazards. To do this, the decommissioning plan for various activities should be well prepared, and the radiation protection plan for the workers should be appropriately (As Low As Reasonably Achievable) performed at the stage of executing the D&D activities.

However, the radiation hazards during the operation of nuclear facilities are different from that at the time of decommissioning. Therefore, from this understanding, the radiation hazards for decommissioning could be adequately addressed to minimize its effect. The purpose of this study is to review the contents that need to be described in decommissioning plan in terms of radiation protection and to investigate the radiation KD]DUGV EDVHG RQ -RVH &DEUHUD QXFOHDU SRZHU SODQW¶V decommissioning characteristic, which is a basic point for radiation protection plan for the safety of workers.

2. Methods

2.1 Contents of Radiation Protection in Final Decommissioning Plan

In Korea, according to Nuclear Safety and Security Commission (NSSC) Notice No. 2018-10 (Regulation on creation of nuclear power plants decommissioning plans, etc.) [1], the contents to be described in the Final Decommissioning Plan (FDP), which is a licensing document that must be approved for the decommissioning of nuclear facilities, are specified.

The FDP composed of 13 chapters, of which the radiation protection is included in chapter 7. There are four items to be described: ALARA application, radiation protection design characteristics, dose

assessment, and radiation protection plan. In order to properly cover these contents, it is necessary to understand the radiation hazard from the aspect of radiation protection in decommissioning as mentioned in the introduction. Therefore, this study refers to technical report of EPRI [2]. In this report, radiation exposure data is based on Jose Cabrera nuclear power plant in Spain, which is currently toward the end of decommissioning phase. As a risk that can occur during decommissioning, the two aspects of hazards from the building and from the contamination are largely discussed.

3. Results & discussion

3.1 Radiation Hazard From Buildings During Decommissioning

Considerations for radiation protection during decommissioning include D&D plans for large components and radioactively contaminated equipment, handling of radioactive wastes and spent nuclear fuels, level and classification of risks, standards and measures, controls, decontamination of structures or facilities, area classification, etc. In summary, when taking into account the effects of radiation on major components, equipment and systems during decommissioning period, the buildings arising hazard are as follows.

- Reactor building - Auxiliary building

- Waste treatment facility (Evaporator) - Radioactive waste storage building - Dismantling auxiliary building

- Cutting and decontamination workshop (turbine building if it is renovated)

- Spent nuclear fuel storage

In addition, the main materials of radiation that should be noted are as follows.

- Systems, structures and radioactive materials or contaminants

- Radioactive waste

- Radioactive material in case of accidents - Spent nuclear fuels

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2019 ⦽ǎႊᔍᖒ⠱ʑྜྷ⦺⫭⇹ĥ⦺ᚁݡ⫭םྙ᫵᧞Ḳ

3.2 Exposure Activities in Each Building

Among major exposure cases in various areas of facility, in case of reactor building, the most important ones in terms of hazard of external exposure are:

- RVI disassembly, cutting and treatment of RV and large components

- Dismantling of radioactive equipment

- Bioshield and demolition of contaminated concrete

- In-situ decontamination of parts and walls The turbine building is basically a non-radioactive area, but if it is renovated to use for storage, cutting and decontamination, the radiation exposure is related to the treatment of radioactive materials. In the auxiliary building, the radiation exposure is caused by contaminated walls inside or outside the building, or by internal dismantling equipment. In the waste treatment building, there is exposure risks from waste treatment system such as concentration tanks or pumps, drainage tanks or pumps, recirculation pumps, analyzers, separators, heaters, condensers, coolers and steam and ventilation. In the radioactive waste storage, its function of storage is maintained during decommissioning and the exposure will be from the operation of radioactive materials. In the spent nuclear fuel storage, when decommissioning is initiated, all spent nuclear fuels will be transported to the spent nuclear fuel storage and the activated part of the reactor vessel and its internal parts above the waste classification of intermediate level will also be stored. The risk of exposure to the spent nuclear fuel storage will be caused by facility operation.

3.3 Radioactive Contamination of Nuclear Facilities

For most nuclear power plants, mostly radiation controlled area, the main radiological hazard is from beta or gamma radionuclides; Co-60, Fe-55, Cs-137, Ni-63, and so on. Alpha radionuclides (Am-241 and Pu-238) are also present in the facility. But among them, the most contributions are attributed to Co-60 and most of them are similar in each buildings.

The contamination caused by decommissioning activities, full system chemical decontamination before dismantling, metal or concrete segmentation, etc., are different in each type of building. Most of the contaminated Structure, System, and Components (SSC) are located inside the reactor building, and the proportion of the contamination of the reactor building is the highest among the dismantling facilities. It should be noted that contamination occurs mainly from activities like cutting, dismantling, and managing contaminants.

3.4 Categorize of Decommissioning Tasks for Radiation Protection

During the decommissioning, the exposure activities could be classified into 13 contents:

a. Transfer of spent nuclear fuel to dry storage b. Defueling, modifications and disposal of

operational waste

c. Full system chemical decontamination d. Maintenance, Engineering and management e. Radiation protection

f. Operations and inspections

g. Segmentation/Removal of large components (reactor internals, reactor vessel, steam generator, pressurizer, reactor coolant pump, reactor coolant piping, etc.) and metal components

h. Biological shield removal i. Spent fuel pool decontamination

j. Dismantling work in containment and other buildings

k. Other concrete decontamination l. Radioactive waste management m. Miscellaneous

4. Conclusion

On the basis information of the 13 contents in section 3.4, the common considerations in term of presence of radiation hazards for all of building are related with items of d, e, f, j, k, and l. Except these 5 items, the reactor building contains most of radiological hazards; b, c, g, h and i. However, the hazardous items of other buildings are rather simple. The turbine building, if renovated, contains only g. Also, the spent nuclear fuel storage includes a. The auxiliary building contains c and g. And then, both of the waste treatment facility and the radioactive waste storage building are connected to l. By identifying the radiation hazard from each building and its activities during the decommissioning, we expect that it will contribute to the prediction and reduction of the risk to the radiation.

REFERENCES

[1] Nuclear Safety and Security Commission, Regulation on creation of nuclear power plants decommissioning plans, etc., Notice No. 2018-10, 2018 (In Korean).

[2] Analysis of Dose Reduction Technology During Decommissioning, EPRI, Palo Alto, CA: 2018, 3002013095.

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