Clearance measurement for general steel waste
Introduction
A large amount of metal waste is generated during decommissioning and dismantling of nuclear facilities. Disposal of all these wastes as low-level radioactive waste has a large environmental impact. Radioactive waste generation can be minimized by increasing the reuse and the recycle of materials from decommissioning. Efforts towards minimizing the generation of radioactive waste should be encouraged as a means of reducing the costs of nuclear technology and saving resources (International Atomic Energy Agency, 2011). Hrncir et al. (2019) outlined preliminary cost-benefit analyses of the recycling of metals arising from the decommissioning of NPPs. Demonstration tests were conducted in Japan to reuse metals generated from nuclear reactors as disposal containers for radioactive waste.
Miscellaneous waste is generated by dismantling and decontamination of uranium enrichment and conversion facilities and stored in drums (Ohashi et al., 2016). The amount of uranium in these drums has been evaluated (Sakoda et al., 2018). In the case of uranium waste, decontaminated metal waste can be recycled if its uranium concentration is low enough to meet clearance criteria. In Japan, only uranium isotope concentration is required as a clearance standard for metal uranium waste (Standards Committee Atomic Energy Society of Japan, 2012). The clearance criterion is defined as 1 Bq/g for total alpha radioactivity concentration.
We investigated the types of steel products generated by the decommissioning of nuclear fuel fabrication manufacturers. Angle bar, channel steel, pipe steel, square steel tube, H steel, plate materials, tower, and vessel are frequently generated. These steels are cut and stored in drums. It is required to properly evaluate the radioactivity concentration of these drums for disposal and reuse. Chu et al. (2016) placed metal tubes in drums and measured the radioactivity of 137Cs and 60Co. When the radiation source is placed at the center of the drum, the amount of uncertainty is large due to the effect of waste shielding for gamma rays. Yeh and Yuan (2014) put metal slices in drums and used a large area source to evaluate the measurement efficiency of 137Cs and 60Co. When the metal slices are filled to 1/2 and 3/4 of drums, the efficiency changes depending on the filling height even if the average density per drum is the same. On the other hand, Yokoyama and Ohashi (2019) proposed Scattering Gamma Equivalent Model (SGE model) for quantifying the radioactivity regardless of the position of the radiation source in the drum. Tests were conducted using natural uranium as the radiation source and Raschig ring as the simulated waste. As a result, it was confirmed that the amount of uranium can be evaluated even with biased source arrangements.
In this study, we examined typical steel materials generated in large amounts by dismantling nuclear facilities. Consequently, it was verified that the SGE model was effective even in drums where general steel materials were cut and filled.
Section snippets
Testing
The representative metal wastes generated by dismantling of nuclear facilities were tested. A large amount of waste is generated from angle bar (Leg 75 mm, Thickness 12 mm), channel steel (Leg 75 mm, Base 150 mm, Thickness 12.5 mm), pipe steel (Outside diameter 60.5 mm, Inside diameter 43.1 mm, Wall thickness 8.7 mm) and square steel tube (Outer diameter 75 mm, Thickness 6 mm). The steel was cut and regularly placed in drums as shown in Fig. 1. Simulated wastes of channel steel, angle bar, and
Results and discussion
The measurement results were analyzed by SGE model. The SGE model used the 1.001 MeV gamma rays emitted from 234mPa and the scattered gamma rays generated by them. 234mPa is a member of the 238U decay series and reaches radiation equilibrium within about six months. The degree of gamma ray attenuation (represented by Xgeometry) is calculated from the background-subtracted 1.001 MeV gamma rays and the scattered gamma rays. Xgeometry is defined as:where na and nb are two
Conclusions
The amount of uranium in simulated drums (about 230 kg to 340 kg) in which general steel materials of various shapes were placed could be quantified within a relative error within 30 % using SGE model. Furthermore, it was found that the amount of uranium can be quantified within a relative error of 30 % by using a common calibration curve for drums of similar weight.
CRediT authorship contribution statement
Kaoru Yokoyama: Writing - original draft. Yusuke Ohashi: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was partially carried out by Electric power joint research program.
The authors are grateful to Mr N. Fujiki, Y. Shinmen and T Yamamoto (Ningyo-Toge Nuclear Industry) and M Naganuma (Japan Atomic Energy Agency) for their operation and management of the clearance verification system.
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