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Journal Articles

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, 10 Pages, 2023/00

 Times Cited Count:0 Percentile:0.05(Nuclear Science & Technology)

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.

Journal Articles

Clearance measurement for concrete waste generated by the decommissioning of uranium processing facilities

Yokoyama, Kaoru; Ohashi, Yusuke

Annals of Nuclear Energy, 175, p.109240_1 - 109240_7, 2022/09

 Times Cited Count:0 Percentile:0.01(Nuclear Science & Technology)

Decommissioning is planned at nuclear facilities that have been discontinued. We examined the evaluation method of uranium radioactivity for concrete waste generated by the decommissioning of nuclear facilities. Since the peaks of Ac-228, Tl-208, and K- 40 are derived from concrete waste, it is difficult to distinguish the 1001 keV peak emitted from the uranium source. We have derived a formula to correct gamma rays from concrete and the environment, and the amount of uranium was quantified. When the weight of concrete waste is about 300 kg, if the weight of uranium is 3 g or more, it can be quantified within a relative error of about 30%. Measurement tests were performed using homogeneous simulated concrete waste. Since uranium contamination is on the concrete surface at the uranium processing facility and small chunks generated by scraping the concrete surface will be stored in a drum and measured, it seems that the test of homogeneous concrete reflects the actual waste.

Journal Articles

Clearance measurement for general steel waste

Yokoyama, Kaoru; Ohashi, Yusuke

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

 Times Cited Count:4 Percentile:55.75(Nuclear Science & Technology)

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).

Journal Articles

Development of clearance verification equipment for uranium-bearing waste

Yokoyama, Kaoru; Ohashi, Yusuke

Applied Radiation and Isotopes, 145, p.19 - 23, 2019/03

 Times Cited Count:5 Percentile:57.06(Chemistry, Inorganic & Nuclear)

Dismantled materials generated from nuclear facilities are reused or directed to repository sites. If scrap metals with complicated shapes can be cleared, the amounts of radioactive waste can be reduced. A clearance verification system is constructed to determine the amount of uranium in decontaminated metals in a drum using the 1.001 MeV gamma rays of $$^{rm 234m}$$Pa, produced in the decay of $$^{238}$$U. The experimental study with simulated waste drums demonstrated that the quantification errors of uranium fall within 25% for 0.5g of uranium.

Journal Articles

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$$^2$$ system have been adopted to evaluate uranium contents in the wastes. About 10 g or more of uranium in a 200 $$ell$$ drum can be evaluated by these systems. The equivalent model developed to correct the evaluation results with Q$$^2$$ 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 $$^{235}$$U, it was demonstrated that the shielding factor, $$X_{geometry}$$ for evaluation of 1001 keV $$gamma$$-ray is also possible to use for evaluation of 186 keV $$gamma$$-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 $$gamma$$-ray spectrum.

Journal Articles

Verification of the quantitative method of caesium radioactivity concentration of decontamination wastes

Yokoyama, Kaoru; Suzuki, Atsuo*; Ishimori, Yuu

Radioisotopes, 66(3), p.117 - 125, 2017/03

Decontamination wastes contaminated with radioactive cesium ($$^{134}$$Cs, $$^{137}$$Cs) are housed in the flexible container bags. Canberra manufactured flexible container concentration measurement car has been developed for the radioactivity measurement of decontamination waste. For reduction of background, the flexible container bags is mounted to the frame, it is measured from the lower side. In the flexible container bag, if there is uneven distribution of the source, it is estimated that quantification error increases. We proposed the method quantifying the shielding situation of $$gamma$$ rays. Then, it demonstrated that quantification accuracy of radioactivity can be improved.

Journal Articles

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

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 $$gamma$$ 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 $$gamma$$ 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.

Journal Articles

Verification of a quantitative method of uranium 238 in the radioactive waste using photon occurred by Compton effect

Yokoyama, Kaoru; Hata, Haruhi; Naganuma, Masaki; Ohara, Yoshiyuki; Ishimori, Yuu

Radioisotopes, 64(11), p.687 - 696, 2015/11

Authors developed the new analysis technique (hereinafter referred to as the equivalent model) which calculates the amount of uranium by correcting the influence of uneven distribution of the uranium. Two $$gamma$$ rays different in the shelter effect are used in the equivalent model. The $$gamma$$ rays (766 keV, 1001 keV) released from $$^{rm 234m}$$Pa are used for uranium quantitative determination. The quantity error is decided by the $$gamma$$ ray with the small calculation rate. The way to get the high calculation rate is considered to reduce the quantity error. Many $$gamma$$ rays are scattered by the Compton effect in radioactive waste, and scattered photons occur. We applied the scattered photon with the big count rate to equivalent model. It was effective to apply the count rate of the scattered photon by the Compton effect to equivalent model.

Journal Articles

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

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

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

 Times Cited Count:7 Percentile:54.35(Chemistry, Inorganic & Nuclear)

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 $$gamma$$-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.

Journal Articles

Electron and ion coincidence momentum imaging of multichannel dissociative ionization of ethanol in intense laser fields

Itakura, Ryuji; Hosaka, Koichi*; Yokoyama, Atsushi; Ikuta, Tomoya*; Kannari, Fumihiko*; Yamanouchi, Kaoru*

Progress in Ultrafast Intense Laser Science XI; Springer Series in Chemical Physics, Vol.109, p.23 - 42, 2015/00

We investigate the multichannel dissociative ionization of ethanol in intense laser fields by the photoelectron-photoion coincidence momentum imaging and identify separately the ionization and subsequent electronic excitation in ethanol. From the energy correlation between a photoelectron and a fragment ion, we reveal the amount of the internal energy gained by ethanol cations from the laser field varies depending on the respective ionization and electronic excitation pathways.

Journal Articles

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

The amount of $$^{238}$$U in uranium-contaminated waste drums generated in the decommissioning of nuclear facilities is evaluated using $$gamma$$ ray measurement. We used the $$gamma$$-ray measurement system made from CANBERRA: Qualitative and Quantitative (Q$$^{2}$$) 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 $$^{238}$$U quantitative value of this equipment has been evaluated.

Journal Articles

The Burning process simulation of incineration ashes

Yokoyama, Kaoru; Rong, D.*; Ohara, Yoshiyuki

Kagaku Kogaku, 78(11), p.818 - 821, 2014/11

By the thermal disposal of the city garbage polluted with radioactive cesium, Cs adheres to incineration ashes and remains in municipal waste incineration facility. In order to acquire the distribution information in the waste incineration plant of the incineration ashes to which Cs adhered from viewpoints of radiation protection, burning analysis is conducted with the existing incinerator and the action of incineration ashes is evaluated. This approach is introduced by this report.

Journal Articles

Quantitative test for radioactivity of uranium filled in waste drums using Q$$^{2}$$ low level waste assay systems

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

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

We have proposed a new theory on passive $$gamma$$ assay for $$^{238}$$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 $$^{238}$$U contained in drums nondestructively using count rates of rays of two energies (1001 keV and 766 keV) emitted from $$^{238}$$U progeny nuclide $$^{rm 234m}$$Pa. This technique is aimed final at introducing into the existing $$gamma$$ ray measuring device. In this paper, we have verified the theory using the existing $$gamma$$ ray measuring device by tests under various simulated waste drums conditions. We used the $$gamma$$-ray measurement system made from CANBERRA: Qualitative and Quantitative (Q$$^{2}$$) Low Level Waste Assay Systems. The application validity of this evaluation technique was shown.

Journal Articles

Correlation between a photoelectron and a fragment ion in dissociative ionization of ethanol in intense near-infrared laser fields

Hosaka, Koichi*; Yokoyama, Atsushi; Yamanouchi, Kaoru*; Itakura, Ryuji

Journal of Chemical Physics, 138(20), p.204301_1 - 204301_9, 2013/05

 Times Cited Count:13 Percentile:47.01(Chemistry, Physical)

Dissociative ionization of ethanol (C$$_{2}$$H$$_{5}$$OH) induced by an intense near-infrared laser pulse are investigated using photoelectron-photoion coincidence method. It is shown that both the electronic ground state and the first electronically excited state of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ are produced at the moment of photoelectron emission. From the observed correlation between the electronic states of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared at the moment of photoelectron emission and the kinetic energy release of the fragment ions, it is revealed that C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared in the electronic ground state at the photoelectron emission gains larger internal energy in the end than that prepared in the electronically excited state. The averaged internal energy of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ just before the dissociation is found to increase when the laser field intensity increases from 9 to 23 TW/cm$$^{2}$$. And when the laser pulse duration increases from 35 to 800 fs.

Journal Articles

Verification tests on nondestructive assay for $$^{238}$$U content in uranium-contaminated waste drums using $$gamma$$ method

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

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

Radioactive waste generated from nuclear fuel facilities must have the acceptance criterion of disposal. Although the $$gamma$$ 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 $$gamma$$ ray of two energies emitted from $$^{238}$$U progeny nuclide ($$^{rm 234m}$$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.

Journal Articles

Chemical decontamination using IF$$_7$$ gas

Hata, Haruhi; Yokoyama, Kaoru; Sugitsue, Noritake

Proceedings of 20th International Conference on Nuclear Engineering and the ASME 2012 Power Conference (ICONE-20 & POWER 2012) (DVD-ROM), 7 Pages, 2012/07

The uranium enrichment facilities, located at Ningyo-toge in Japan, are contaminated by uranium fluorides. There is the risk of exposing the HF gas when the equipments will be dismantled. Therefore, we performed the systematic chemical decontamination by using IF$$_7$$ gas. This paper reports the actual results of the decontamination by using IF$$_7$$ gas. The result shows that the decontamination factor is approximately 100 or more. In addition, we performed the simulation of the reactions with IF$$_7$$ gas and uranium fluorides.

JAEA Reports

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


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$$_{7}$$) 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.

Journal Articles

Separation of ionization and subsequent electronic excitation for formation of electronically excited ethanol cation in intense laser fields

Ikuta, Tomoya*; Hosaka, Koichi*; Akagi, Hiroshi; Yokoyama, Atsushi; Yamanouchi, Kaoru*; Kannari, Fumihiko*; Itakura, Ryuji

Journal of Physics B; Atomic, Molecular and Optical Physics, 44(19), p.191002_1 - 191002_5, 2011/10

 Times Cited Count:10 Percentile:48.25(Optics)

Ionization and subsequent electronic excitation occurring within the same laser pulse (400 nm, 96 fs, 1.3$$sim$$18 TW/cm$$^{-2}$$) are separately investigated by measuring in coincidence an electron and a product ion produced from C$$_{2}$$H$$_{5}$$OH. We reveal that the nascent population in the electronically excited C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared by the ionization decreases as the laser intensity increases, while the subsequent electronic excitation is enhanced through the resonant electronic transitions. Ionization and electronic excitation mechanisms are described based on the electronic state distributions of C$$_{2}$$H$$_{5}$$OH$$^{+}$$.

Journal Articles

$$gamma$$ 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

For determining radioactive of waste drums using passive $$gamma$$ ray, we showed a new analysis method deriving the linear relation between radiation source distribution and $$gamma$$ 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 $$gamma$$ 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 $$frac{1}{(ln(k/bar{R}))^{2}}$$ and $$bar{n}$$, where "$$bar{R}$$" is the geometric mean of two $$gamma$$ ray (766 keV and 1001 keV) count rates ratios. "${it k}$" is the ratio of emission rates of the two $$gamma$$ rays. "$$bar{n}$$" are the geometric mean of $$gamma$$ ray (1001 keV) count rates.

JAEA Reports

Preliminary test for elution of uranium waste; Concept and application for elution test

Hata, Haruhi; Yokoyama, Kaoru; Tsunashima, Yasumichi; Ohashi, Yusuke; Koga, Osamu; Sugitsue, Noritake

JAEA-Research 2011-022, 35 Pages, 2011/09


For disposal of Very low-level radioactive Waste (VLLW) from nuclear related facilities, one of important factors for safety assessment is the characteristics of elution. As for VLLW from the nuclear power plant, concrete pit and trench disposals have been performed and the evaluation methods for the characteristics have been established. On the other hand, as for the uranium waste, the concept on how to test the elution characteristics is not shown yet. Based on these circumstances, preliminary tests have been conducted to study elution characteristics of uranium waste. The results show that the important factors for the uranium elution are how uranium exists in waste. In addition, the elution characteristics also depend on the precipitation amount on the disposal site. Therefore, to assess the elution rate from uranium waste, these factors must be considered.

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