Evaluation of Radiological Protection in Hot Cell Facility during Processing Decommissioning Radioactive Waste

When decommissioning a nuclear facility, radioactively contaminated metals and concrete will make up most of the radioactive decommissioning waste. To guarantee such safety, the analysis of the characteristics of the radioactive decommissioning waste must be carried out in a facility with hot cells. Also, the behavior of the air source term that occurs during radioactive waste processing when considering ventilation in the hot cell facility was examined.

The radioactive waste to be handled in the hot cell facility is composed of carbon steel and stainless steel as the reactor pressure vessel and reactor internal structure extracted from the reactor. More than 99% of the radioactivity inventory of the reactor pressure vessel and core vessel of the Pressurized Water Reactor (PWR) was composed of six nuclides; 60 Co. In addition, the Monte Carlo N-Particle (MCNP) transport code was used to evaluate the workers' external exposure dose in the event of an accidental ventilation failure, a broken window, and normal operations at the hot cell facility during the processing of radioactive waste .

Consequently, a high efficiency particulate air filter (HEPA) with an efficiency of 99.97% was used to detect the emission concentration of the dominant nuclides (3H, 14C, 55Fe, 59Ni, 60Co and 63Ni) from the air source term, which complies with the emission management standard of Nuclear Safety Commission Notice No. Based on these results, it is possible to evaluate the emission management in the air source term while the radioactive waste is processed in a hot cell facility and radiation protection technology is carried out.

Introduction

This process requires samples to be extracted from nuclear reactors, which are key components of nuclear power plants, and then processed directly in hot cell facilities to understand the characteristics of the extracted radioactive waste. Since metallic radioactive waste is processed in a hot cell, evaluation of hot cell radiation safety technology is an essential process. The hot cell facility handles radioactive waste and consists of a shield wall and leaded glass for worker safety.

In such a thermal cell facility, radiation safety assessment by workers and environmental impact assessment by filter systems are essential. Waste waste treated in hot cell facilities is metal waste, which consists of nuclear strip line extracted from the reactor pressure vessel, core weld and core vessel in the reactor's internal structure. The core belt line and the core weld are made of carbon steel, and the core barrel is made of SUS304 stainless steel.

When the sample of dismantling waste collected in this way is processed with a circular saw or a band saw, the volume of cutting loss from the saw is generated, some of which is generated in the form of an aerosol. Since this aerosol can cause internal exposure by human inhalation, the radiation safety screening of radiation workers based on the ventilation and leakage rate of hot cell facilities was evaluated through the MCNP code to determine if worker doses meet the standards regulatory.

Figure 2. Sampling and processing of decommissioning waste

Literature Study

• Decommissioning
• Decommissioning strategy
• Status of decommissioning at domestic and abroad
• Decommissioning process according to regulation
• Generation and types of decommissioning radioactive waste
• Regulation measure of radioactive waste
• Physicochemical properties of decommissioning waste
• Hot cell facility
• Post irradiation examination facility
• Radioactive waste handling process in hot cell facility
• Radiation work and dose regulation measures

Since the basic specifications for decommissioning have a major influence on the economics and safety of the decommissioning process, the process of establishing a decommissioning strategy is a very important process. The basic specifications for decommissioning are the extent of decommissioning, the time and method of decommissioning, the final status of the decommissioning site, evaluation and determination of decommissioning costs, and the final waste disposal plan. Investigating the radioactive properties of nuclear facilities is the most important task to be carried out before decommissioning and is the process carried out in this study.

This preliminary decommissioning plan presents the nuclear decommissioning strategy, schedule, measures for the prevention of radiation disasters and technical standards in connection with decontamination with radioactive materials in Article 85, para. 7, of the Nuclear Safety Act. In addition, Articles 28 (Decommissioning of Nuclear Power Reactors and Related Facilities) and 42 (Decommissioning of Nuclear Fuel Cycle Facilities) of the Nuclear Safety Act propose that the final decommissioning plan be approved, submitted, the decommissioning report checked and inspected. In addition, it is proposed in Article 28, paragraph 2, of the Act on Nuclear Safety to present a quality assurance plan, including a final decommissioning plan, which describes the approval requirements, the residents' opinion on the draft decommissioning plan and the results of the public consultation [2].

Since some of the radioactive waste generated after the decommissioning of nuclear power facilities can be disposed of on its own through regulatory cleanup, it is possible to effectively treat the radioactive waste generated during the decommissioning process of nuclear power plants through sampling, analysis of nuclides and radioactivity measurement processes. [2]. The amount of decommissioning waste is not easy to predict with a generalized method, and the amount of decommissioning waste is determined by the operation history, decommissioning strategy of the nuclear power plant operator or national radioactive waste management regulations. Furthermore, for contamination management in the radiation controlled area, the design dose rate of the hot cell facility must be managed within the dose limit.

In the general oxide layer appearance of nuclear fuel rods investigated in commercial reactors, a black protective oxide layer could be observed up to a certain distance from the bottom of the fuel rod [8].

Table 1. Decommissioning completion status [2]

Materials and Methods

• Basic design of hot cell facility
• Decommissioning radioactive waste from PWR
• Nuclide inventory evaluation of decommissioning radioactive waste
• Aerosol production during cutting process
• Aerosol mass
• Main routes of exposure for radiation workers
• Airborne source term concentration
• Indoor particle concentration
• MCNP evaluation methodology

The metallic decommissioning waste generated during the decommissioning of Kori Unit 1, a PWR-type NPP, is radioactive waste from the pressure vessel and reactor internal construction [13]. It is possible that the composition of the reactor was changed by high-capacity neutrons during the NPP's lifetime of 38 years. It is therefore intended to provide important information for estimating the radiological characteristics of the PWR's reactor pressure vessel.

The top and bottom of the vertical and girth welds were cut using the core barrel metal material cutting technique [13]. Using the report as a resource, such as NUREG/CR. The specifications and aspects of the radio source used for the treatment of dismantled PWR waste at KSHF were determined. The cutting procedure of dismantled radioactive metal waste was performed for both mild steel and stainless steel, because most of the large metal structures that had to be processed in AHCF were constructed of SUS304 stainless steel and mild steel [14].

The ratio of these six main nuclide radioactivity concentrations in the core band line of reactor pressure vessel is shown in Table 1. It is possible to investigate the actual reactor pressure vessel's radiation fragility using the processed core band line metal, as well as to obtain crucial information for preserving the integrity of the reactor's long-term operation [14]. The circular saw's thickness is double that of the band saw, as can be seen in Table 9, which results in a doubling of the volume cutting loss.

Calculations were made to determine the volume of the sample and the cutting loss volume after the core band line metal was uniformly cut three times to a length of 22 cm in the reactor pressure vessel. Particles are suspended due to the saw blade's movement, which causes heat energy to be produced from the friction between the cutting material and the saw blade. Physical elements and the speed of the saw blade such as the length of the contact surface and the horizontal drag When the saw blade touches the material, what determines these variables are the frictional force and thermal energy.

In addition, 0.0671% of ARF is produced when using a reciprocating saw to cut a SUS304 stainless steel sample due to the loss mass of 1936 g/m per unit cutting length and the amount of aerosol formation of 1.30 g/m. The air source term concentration in the AHCF is calculated using Eq. 2) using the aerosol mass in Eq. The weekly exposure to external radiation of the radiation controlled area is 400 µSv, as stated in Article 3 of the Technical Guidelines for Radiation Safety Management.

Figure 10. Typical Pressurized Reactor

Results and discussions

Environment radiation emission evaluation

The average value was calculated by setting the treatment time per week to 40 hours to calculate the average released aerosol concentration in accordance with radiation protection regulations. Calculation of the equilibrium concentration in AHCF under ventilation requires an understanding of the indoor generation rate. Therefore, it was assumed that a sample would be treated for one week (40 hours) and that the indoor generation concentration would be constant while calculating the indoor particle concentration.

The weekly amputated samples were chosen because, as stated in Table 14, according to article 6 of the radiation protection regulations notification, the average concentration of emissions per week will be taken into account for the determination of emission management limits during the discharge. In accordance with radiation safety regulations, the concentration of nuclides was set to 1 for two or more different types of nuclides. It was possible to estimate the sum of discharge concentrations in accordance with discharge management standards, which were, respectively, and 6.03.

10-3, when cutting the belt core metal with a circular saw and then with a band saw, as shown in Tables 9 and 10, respectively. When the weld core metal was cut with a circular saw and a band saw, the aerosol generated was calculated to be 7.22E- 03 and 3.61E-03, respectively, as shown in Tables 11 and 12. According to Tables 13 and 14, when the core tube was amputated with a circular and band saw, the aerosol released was 1.10E-02 and 5.47, respectively E-03.

Thus, it was confirmed from Table 15 that radioactivity from the emission of nuclides in the exhaust met the emission control standards.

Table 8.Data of core beltline metal aerosol with circular saw

Dose evaluation of MCNP simulation

Radioactive concentration evaluation of working area

Internal dose evaluation of normal operation

Internal dose evaluation of accident scenarios

Conclusion

Comprehensive review of decommissioning of nuclear power plants - status of closure/permanent decommissioning of nuclear facilities and technical standards for key safety regulations. A study on the decontamination and dismantling of the DUPIC experimental equipment in the hot cell PIEF 9405 (No. KAERI/TR. State of the art of the declassification method for spent nuclear fuel rods.

Preliminary Safety Analysis Report for the Auxiliary Hot Cell Facility, Sandia National Laboratories, Albuquerque, New Mexico (No. SAND99-3005). Sandia National Lab.(SNL-NM), Albuquerque, NM (USA); Sandia National Lab.(SNL-CA), Livermore, CA (USA). Aerodynamic diameter and radioactivity distributions of radioactive aerosols from activated metal cutting to decommissioning nuclear power plants.

Internal dose assessment of workers of radioactive aerosol generating during mechanical cutting of radioactive concrete.