Current status of decommissioning and waste management at the Ningyo-Toge Environmental Engineering Center

Ohashi, Yusuke; Shimaike, Masamitsu; Matsumoto, Takashi; Takahashi, Nobuo; Yokoyama, Kaoru; Morimoto, Yasuyuki

Nuclear Technology, 209(5), p.777 - 786, 2023/05

https://doi.org/10.1080/00295450.2022.2145136

At the Ningyo-Toge Center, technical development related to uranium refining conversion and enrichment has been completed, and decommissioning of these facilities has begun. The error between the quantity of dismantled materials estimated from the facility design drawings and the actual quantity of dismantled materials was minimal when averaging over the entire Uranium Refining and Conversion Plant and Uranium Enrichment Engineering Facility, which results indicated that the preliminary estimate of the quantity of dismantled materials for decommissioning was reasonable. Most of the dismantled materials, which have no contamination history and are properly managed were able to be carried out to recyclers as non-radioactive waste (NR). In addition, the possibility of evaluating the uranium concentration of clearance level in dismantled objects was confirmed through gamma-ray measurement tests using mock-up waste.

Clearance measurement for general steel waste

Yokoyama, Kaoru; Ohashi, Yusuke

Annals of Nuclear Energy, 141, p.107299_1 - 107299_5, 2020/06

https://doi.org/10.1016/j.anucene.2019.107299

A large amount of general steel waste is generated during decommissioning and dismantling of nuclear facilities. Very low-contaminated radioactive waste, whose radioactivity is below clearance level, generated from the demolition process may be reused for general use. We examined the feasibility of the clearance verification system for uranium waste. The relative error of uranium determination was within 30% for 1 g of uranium when measuring steel materials (angle bar, channel steel, pipe steel, square steel tube, fragments of metal tube).

Current status of uranium measurements and their related techniques at JAEA Ningyo-toge

Ishimori, Yuu; Yokoyama, Kaoru*; Hayakawa, Tomoya; Hata, Haruhi; Sakoda, Akihiro; Naganuma, Masaki

Dekomisshoningu Giho, (55), p.36 - 44, 2017/03

This paper gives an outline of the current status of uranium measurements and their related techniques at the Ningyo-toge Environmental Engineering Center of Japan Atomic Energy Agency. The JAWAS-N and the Q system have been adopted to evaluate uranium contents in the wastes. About 10 g or more of uranium in a 200 drum can be evaluated by these systems. The equivalent model developed to correct the evaluation results with Q system is not available to less than dozens of grams of uranium in a 200 L drum. The paper illustrates the advantage of use of the improved equivalent model which evaluates uranium content from full energy peak of 1001 keV and its Compton spectrum in order to correct the inhomogeneous distribution of uranium in measuring objects. The use of model achieved the limit of uranium quantitative determination under one tenth of those of previous evaluation methods. To determine U, it was demonstrated that the shielding factor, for evaluation of 1001 keV -ray is also possible to use for evaluation of 186 keV -ray. The measurement systems adopting the model have been introduced to other nuclear operators in Japan. In addition, it is also examined to use for clearance. As a related technique, feasibility studies on machine learning algorithms have been performed to classify the waste drums depending on their -ray spectrum.

Verification of the quantitative method to measure enrichment of uranium-235 in radioactive waste

Yokoyama, Kaoru; Sato, Katsunori*; Yamanaka, Takashi*; Ishimori, Yuu

Radioisotopes, 65(11), p.441 - 450, 2016/11

https://doi.org/10.3769/radioisotopes.65.441

It is important for the processing manufacturers of the uranium fuels to determine the quantity of U-235 and the enrichment. This study shows that the U-235 content evaluated from measurement of 186 keV rays emitted from U-235 can be corrected by a shielding factor, Xgeometry which quantified uneven distribution of U-238. The Xgeometry is evaluated from the direct and the scattered rays from the 1001 keV emitted from the Pa-234m. The Xgeometry was originally introduced for U-238 measurements. Because U-235 coexists with U-238, the Xgeometry is also possible to apply to the U-235 measurements. The experimental study with simulated waste drums demonstrated that the quantification errors of the U-235 content and the enrichment are reduced considering the factor.

Application of support vector machine to rapid classification of uranium waste drums using low-resolution -ray spectra

Hata, Haruhi; Yokoyama, Kaoru; Ishimori, Yuu; Ohara, Yoshiyuki; Tanaka, Yoshio; Sugitsue, Noritake

Applied Radiation and Isotopes, 104, p.143 - 146, 2015/10

https://doi.org/10.1016/j.apradiso.2015.06.030

We investigated the feasibility of using support vector machine (SVM), a computer learning method, to classify uranium waste drums as natural uranium or reprocessed uranium based on their origins. The method was trained using 12 training datasets were used and tested on 955 datasets of -ray spectra obtained with NaI(Tl) scintillation detectors. The results showed that only 4 out of 955 test datasets were different from the original labels-one of them was mislabeled and the other three were misclassified by SVM. These findings suggest that SVM is an effective method to classify a large quantity of data within a short period of time. Consequently, SVM is a feasible method for supporting the scaling factor method and as a supplemental tool to check original labels.

Verification of uranium 238 quantity calculated using waste assay systems

Yokoyama, Kaoru; Hata, Haruhi; Ohara, Yoshiyuki; Naganuma, Masaki; Sugitsue, Noritake

Radioisotopes, 63(12), p.559 - 566, 2014/12

https://doi.org/10.3769/radioisotopes.63.559

The amount of U in uranium-contaminated waste drums generated in the decommissioning of nuclear facilities is evaluated using ray measurement. We used the -ray measurement system made from CANBERRA: Qualitative and Quantitative (Q) Low Level Waste Assay Systems and measured the waste drum. This equipment assumes uniform distribution of the uranium radiation source. But, homogeneity is not checked with a real waste drum. Authors developed the new analysis technique which calculates the amount of uranium by correcting the influence of uneven distribution of the uranium radiation source. As a result of evaluating using the new analysis technique, the error which the radiation source uneven distribution in a drum gives to U quantitative value of this equipment has been evaluated.

Quantitative test for radioactivity of uranium filled in waste drums using Q low level waste assay systems

Yokoyama, Kaoru; Hata, Haruhi; Ohara, Yoshiyuki; Sugitsue, Noritake

Radioisotopes, 62(11), p.833 - 840, 2013/11

https://doi.org/10.3769/radioisotopes.62.833

We have proposed a new theory on passive assay for U determination of uranium-contaminated waste drums. According to this theory, regardless of the inhomogeneity of waste matrix density or uranium source distribution, we can accurately determine the amount of U contained in drums nondestructively using count rates of rays of two energies (1001 keV and 766 keV) emitted from U progeny nuclide Pa. This technique is aimed final at introducing into the existing ray measuring device. In this paper, we have verified the theory using the existing ray measuring device by tests under various simulated waste drums conditions. We used the -ray measurement system made from CANBERRA: Qualitative and Quantitative (Q) Low Level Waste Assay Systems. The application validity of this evaluation technique was shown.

Verification tests on nondestructive assay for U content in uranium-contaminated waste drums using method

Yokoyama, Kaoru; Sugitsue, Noritake; Muroi, Masayuki*; Suzuki, Yasuo*

Radioisotopes, 62(1), p.1 - 17, 2013/01

https://doi.org/10.3769/radioisotopes.62.1

Radioactive waste generated from nuclear fuel facilities must have the acceptance criterion of disposal. Although the ray measurement technique is expected as the measurement technique, the correction for the influences of the heterogeneous waste distribution was needed. Therefore, the computational technique using the ray of two energies emitted from U progeny nuclide (Pa) was developed, and verified the validity with simulated waste. As a result, the relative error was less than 20%, and detection limit was 1.2 Bq/g.

Study on efficiency of dry decontamination technique by numerical method, 3 (Joint research)

Hyakutake, Toru*; Muto, Akinori*; Sasakura, Mariko*; Minowa, Hirotsugu*; Suzuki, Kazuhiko*; Yokoyama, Kaoru; Takahashi, Nobuo; Hata, Haruhi; Sugitsue, Noritake

JAEA-Research 2011-035, 53 Pages, 2012/01

JAEA-Research-2011-035.pdf:3.13MB

System decontamination has been generally carried out with the aim of reducing the amount of radioactive waste generated and minimizing exposure to radiation released from nuclear fuel facilities. At the Ningyo-Toge Environmental Engineering Center, metal surfaces that are contaminated by uranium are dry decontaminated by using iodine heptafluoride (IF) as a system decontaminator. Only a few studies have been carried out on the decontamination efficiency, mechanism, level, etc. of dry decontamination techniques that use a decontamination gas. Therefore, the generalization of dry decontamination techniques is required. In the present study, the efficiency of a dry decontamination technique was assessed by a numerical method using decontamination data obtained at the Ningyo-Toge Environmental Engineering Center. A concrete analytical content is a depositing of uranium hexafluoride.

ray measuring method for a simple determination of radioactivity of uranium in waste drums

Yokoyama, Kaoru; Sugitsue, Noritake

Radioisotopes, 60(10), p.409 - 416, 2011/10

https://doi.org/10.3769/radioisotopes.60.409

For determining radioactive of waste drums using passive ray, we showed a new analysis method deriving the linear relation between radiation source distribution and ray count rate under the homogeneous waste density. On the other hand, there were problem applying the linear relation to the case of inhomogeneous waste density. We proposed to measure the ray count rate at the positions on spherical surface surrounding the waste drum. Intending to create the approximative homogeneous waste density state, we proposed to calculate the geometric mean of measurements. As a result, we showed a relation under the inhomogeneous waste density. The relation is represented by and , where "" is the geometric mean of two ray (766 keV and 1001 keV) count rates ratios. "" is the ratio of emission rates of the two rays. "" are the geometric mean of ray (1001 keV) count rates.