Jae Hyun KIM,Myeong Hyeon WOO,Chang Ho SHIN,Jong Kyung KIM

(Department of Nuclear Engineering,Hanyang University,222,Wangsimni-ro,Seongdong,Seoul,Korea)

Abstract:The content analysis of radioactive waste and radiation dose evaluation is considered as one of the important factors in the reactor facility design.This kind of buildings consists of the concrete for the most part and uses it as the structure and shield of the building.Generally,the concrete has impurities such as cobalt,europium,nickel,and cesium with specific content depending on the production method or manufacturing company.Dominant radioactive nuclides generated from the fundamental components of concrete are considered that it is less contributed to the radiation dose because they are beta decay nuclides in general.Thus,impurities of irradiated concrete in the reactor facilities,are considered occasionally an important evaluation factor for induced activity.In this study,the influence on the activation of impurities in concrete was evaluated from the radiation dose and induced activity calculations.The calculation was evaluated at the bio-shield which is one of the areas with the highest neutron irradiation among the concrete structure in the reactor facility.The results show that radioactive nuclides with gamma decay were produced in these impurities.Moreover,the radiation dose of concrete with impurities was higher than concrete without impurities.The increased radiation dose was quantified through the content of impurities.

Key words:radiation dose;concrete;impurity;activation analysis

1 Introduction

Nowadays in Korea,how to decontaminate and disband the aging reactor such as Kori NPP Unit 1 becomes one of the biggest concerns in the nuclear industry.Only some nuclear-advanced countries have dismantling experience,and technology is not yet available in Korea.Because the number of reactors required to dismantle will further increase in Korea,continuous study for the development of reactor dismantling technology is required.Reactor dismantling consists of the steps of shutdown,planning,removal of radioactive contaminants,demolishment,disposal of radioactive waste,and environmental restoration.Because structures and facilities in the reactor have been activated by neutron irradiation for a long period,for the disbandment of the reactor,it is not easy to decontaminate,and the cost is not cheap.For such a reason,in the step of planning on dismantling of the reactor,the assessment of radionuclides inventory and radiation dose evaluation is essentially considered.

The majority of structures in the reactor are concrete.The concrete is used as the structure and shield of the building.Generally,the concrete has impurities such as cobalt,europium,nickel,and cesium with specific content depending on the production method or manufacturing company.Dominant radioactive nuclides generated from the fundamental components of concrete are beta decay nuclides in general.So,these are less contributed to the radiation dose and are small numbers.Also,the content of impurity nuclides compared with main nuclides is low,but some impurity nuclides with a high neutron absorption cross-section can have an influence on radiation dose.

In this study,the influence on the activation of impurities in concrete was evaluated from the radiation dose and induced activity calculations.The calculation is evaluated at the bio-shield which is one of the areas with the highest neutron irradiation among the concrete structure in the reactor facility.

2 Model and method

2.1 Composition and impurities of concrete

The concrete used in this study is the bio-shield of the Kori NPP unit 1.The density of concrete is 2.229 4 g/cm3and the material composition is shown in Table 1[1].

As mentioned earlier,it is difficult to quantify the composition and contents of concrete impurity,which depends on the production procedure of concrete,the production place,and so on.Therefore,throughout the literature review,seven major nuclides which contribute most to the activation of concrete impurities were selected in the activation calculation.For conservative evaluation,the highest value for each of the isotopes in the references was applied to the calculation of this study.The details are described in Table 2.

Tab.1 Composition of concrete without impurity

Tab.2 Impurities of concrete

2.2 Calculation model and irradiation scenario

The main purpose of this study is to evaluate the activation influence depending on the content of impurities in the concrete.Therefore,for effective comparison,the source term was defined by assuming that the point source is located at the center of the effective fuel area in the corner,and MCNP modeling was done throughout 3D geometry simplification as shown in Figure 1 and Figure 2.Also,the irradiation scenario used in this study was assumed as a total of 30 years of continuous operation.

2.3 Codes and library

In this study,MCNP5 ver.1.6 simulation code[6]was used in the neutron transport calculation,and average neutron flux from the F4 tally was derived in the region of interest.FISPACT-2010 with the EAF-2010 library[7]was used in the activation calculation.Also,the neutron spectrum was calculated by the 175 groups of vitamin-j.

Fig.1 Kori Unit 1 reactor 3D geometry Simplification[8]

Fig.2 MCNP modeling in Kori unit 1 reactor vessel

3 Results

3.1 Neutron spectra in bio-shield

In this study,the neutron flux and energy spectrum used as the inputs to the FISPACT activation calculation code were calculated by the MCNP5 computer code.The calculated total neutron flux was calculated as 1.05×106neutrons/(sec·cm2),and the energy of each case is shown in Figure 3.Figure 3 shows that the energy spectra of each case have a similar trend.

Fig.3 175-groups neutron energy spectra of each case at the concrete of the bio-shield

3.2 Comparison of induced activity of concrete

Figure 4 shows the comparison of total activity of concrete.After shutdown,though the contents of impurities are relatively lower than the contents of the major nuclides of concrete,total activity with impurity was evaluated a little higher than total activity without impurity.Total activity without impurity decreased sharply within 10 years.However,total activity with impurity gradually decreased during 100 years of cooling time.Since the activity of impurity including long-lived radionuclides such as152Eu,63Ni,and60Co,and they are the most dominant radionuclides,it makes the difference of total activity between with and without impurity.

Fig.4 Comparison of total activity of concrete in the bio-shield of Kori unit 1

In the concrete without impurities of the bio-shield during operation,54Mn,56Mn,37Ar,47Sc,49Sc,55Fe,59Fe,28Al,41Ca,and45Ca are generated as the dominant radioactive nuclides.After shutdown,39Ar,55Fe,and51Ca,which are long half-life,remain as the dominant radionuclides.Figure 5 shows the activities of dominant radionuclides in the concrete without impurity.

In the concrete with impurity,3H,152Eu,154Eu,60Co,58Co,63Ni,and134Cs were produced from impurities.After shutdown,3H,152Eu,63Ni,and60Co are dominant radionuclides.These radionuclides remain relatively higher values than dominant radionuclides without impurity during 100 years of cooling time because of the long half-life.Figure 6 shows the activities of radionuclides from impurity.

Fig.5 Activities of dominants radionuclides in the concrete without impurity

Fig.6 Activities of dominants radionuclides in the concrete with impurity

The decay gamma-ray spectra were evaluated by using activities produced in each radionuclide throughout the activation calculation.The difference of the decay gamma-ray spectra with and without impurities of concrete in bio-shield depending on cooling time after the shutdown is shown in Figure 7.The intensity of gamma-ray from concrete with impurity was calculated bigger than without impurity.Due to the influence of long-lived radioactive nuclides,the difference of the decay gamma-ray spectra of below the 1 day after shutdown is calculated lowly,while the decay gamma-ray spectra of above the 1 year after shutdown show a high difference.For a more reliable comparison,the dose rate distribution of the concrete in the bio-shield was calculated as shown in Figure 8.The result was shown as a similar trend to total gamma activity.Due to the activation of the impurities,the dose rate was calculated higher compared to without impurities for a long time.

Fig.7 Difference with and without impurities of decay gamma-ray spectra generated from activated concrete

4 Conclusion

In this study,the influence of the impurity in the concrete was evaluated from the activation calculation at the concrete within the bio-shield in the Kori NPP unit 1 reactor.The result shows that even a small amount of impurities may have a considerable effect on the view of the activation.Especially,activities of certain impurities such as Europium,Cobalt,and Lithium were relatively higher than dominant radionuclides of concrete without impurity.Therefore,for the design of nuclear facilities using concrete as a structure,the impurities influenced the activity should be considered in the activation analysis.Finally,these results will be able to be utilized to establish further detailed and effective design and decommissioning schemes of a reactor facility.

Acknowledgements:This work was supported by the R&D program throughout the National Fusion Research Institute (NFRI) funded by the Ministry of Science of the Republic of Korea (Project No.:201800000002652),and Innovative Technology Center for Radiation Safety(ITRS).