Decrease in air dose rate after decontamination relating to difference in the ground surface properties

Nakama, Shigeo; Yoshimura, Kazuya; Fujiwara, Kenso; Ishikawa, Hiroyasu; Iijima, Kazuki

KEK Proceedings 2018-7, p.154 - 158, 2018/11

https://lib-extopc.kek.jp/preprints/PDF/2018/1825/1825007.pdf

Decrease in air dose rate in decontaminated area is essential information to estimate external exposure and to facilitate return of local residents, but the factors to control the decrease rate have not been cleared wholly. To clarify the effect of ground surface type (i.e. paved and soil surfaces) on the decrease in air dose rate at 1 m above the ground, surface dose rate at 1 cm above the ground and the air dose rate were monitored for four years since decontamination in 2011, and their decrease rates were compared relating to the ground surface type. Decrease in the air dose rate and the surface dose rate on the asphalt pavement showed faster rates than those on the soil surface. Ratio of decrease in the air dose rate and surface dose rate (decrease rate ratio) was distributed between 0.8 and 1.2 on open place not affected by surrounding environment. Therefore, decrease in the air dose rate was in agreement with the decrease of the surface dose rate, which is greatly affected by the ground surface. It became clear that the decrease rate constant of the air dose rate differs depending on the difference in the ground surface. Furthermore, it was also confirmed that the local soil erosion and sedimentation of the ground surface does not affect the decrease rate of the air dose rate.

Experimental investigation of reaction behavior between carbon dioxide and liquid sodium

Miyahara, Shinya; Ishikawa, Hiroyasu; Yoshizawa, Yoshio*

Nuclear Engineering and Design, 241(5), p.1319 - 1328, 2011/05

https://doi.org/10.1016/j.nucengdes.2010.08.006

Reaction behavior of carbon dioxide (CO) with a liquid sodium pool was experimentally investigated to understand the consequences of boundary tube failure in a sodium-CO heat exchanger (HX). In this study, two kinds of experiments, namely fundamental experiment and demonstration experiment which simulate the incident of CO leakage in HX, were carried out to investigate the reaction behavior. From these experiments, it became clear that the exothermic reaction occurred above a threshold temperature, and useful and indispensable information such as the resulting temperature and pressure rise and the behavior of solid reaction products in the pool was obtained to evaluate the consequences of boundary tube failure incident in the sodium- CO heat exchanger.

Experimental study of sodium/carbon dioxide reaction

Ishikawa, Hiroyasu; Miyahara, Shinya; Yoshizawa, Yoshio*

Nihon Genshiryoku Gakkai Wabun Rombunshi, 7(4), p.452 - 461, 2008/12

https://doi.org/10.3327/taesj.J07.054

Supercritical CO is being investigated as a material for a secondary cooling system of Na-cooled fast reactor in to avoid Na/water reaction. In this type of reactor, however, it is necessary to consider the consequences of Na/CO reaction, which might occur in the case of tube rupture in a heat exchanger between primary and secondary systems. Experiments were carried out with test equipment for the Na/CO reaction, which can handle 1-5 g order of Na. The solid products of the Na/CO reaction sampled from the equipment were analyzed by XRD and chemical analysis. From these experimental results, we proved that the reaction proceeded between liquid Na and CO. The Na/CO reaction stopped only the pool surface reaction with a small quantity of aerosol emission when the temperature was lower than 570C. On the other hand, the reaction continuously proceeded with on orange-colored flame and aerosol release when the temperature was higher than 580C.

Falling sodium droplet experiment (FD-3)

Doda, Norihiro; Ishikawa, Hiroyasu; Ohno, Shuji; Miyahara, Shinya

JNC TN9400 2005-048, 52 Pages, 2005/04

JNC-TN9400-2005-048.pdf:9.54MB

The burning behavior of a single sodium droplet has been studied for understanding of spray combustion which is one of the combustion forms in sodium leakage. This study serves as the basis of the mechanistic sodium fire analysis method. A burning experiment with initial droplet size of 3.34mm and 5.85mm different from FD-2 experiment condition (4.75mm), an inert gas condition experiment and a polypropylene sphere experiment were performed to investigate the relation between initial droplet size and burnt mass, and the effect of burning phenomena on the droplet motion. (1)Drag coefficient of a burning sodiuum droplet is 1.42.2 times greater than that of a solid sphere of the same size. (2)The increase in drag force of a burning sodium droplet is mainly due to the increase in gaseous viscosity around the droplet with heat of combustion. Sodium droplet has the ellipsoidal shape with aspect ratio 0.89 when falling by about 8 meters, but the effect of droplet deformation is negligible small. In addition, evaporation and buoyancy have also little effect on the increase in drag force. (3)The burning of sodium droplet follows the D-law when the initial droplet diameter changes. In the calculation which assumes that the sodium combustion quantity agrees with the measurement and that combustion of sodium droplet obeys that law, the NaO ratio of reaction products becomes 0.51-0.75.

Conclusive Experimental Study of Prevention Measures Against Sodium Combustion Residuum Reignition; Run-F9-1, Run-F9-2

Ishikawa, Hiroyasu; Ohno, Shuji; Miyahara, Shinya

JNC TN9400 2004-038, 84 Pages, 2004/04

JNC-TN9400-2004-038.pdf:10.3MB

Nitrogen gas can be an extinguisher or a mitigating material in the case of sodium leak and fire accident in an air atmosphere, which may occur at a liquid metal cooled nuclear power plant. However, sodium combustion residuum sometimes reignites in the air atmosphere even at room temperature when it was produced by nitrogen gas injection to the burning sodium. Then, in this study we executed conclusive experiments of prevention measures against sodium combustion residuum reignition by a mixture of carbon-dioxide (C0 sub2) gas, humidity and nitrogen gas. The experiments were carried out with the FRAT-1 test equipment; the humidity conditions were changed in air which were used to sodium combustion atmosphere and exposure air for confirmation of prevented combustion residue reignition. First of all, the sodium of about 2.5kg was leaked in air atmosphere, and next, the sodium combustion was stopped by nitrogen gas injection. Next, the combustion residuum was cooled in the nitrogen atmosphere, and then the combustion residuum was exposed to atmosphere of carbon-dioxide (4%); humidity (6000vppm); oxygen (3%)-nitrogen (based gas) mixture. It was confirmed that the combustion residuum was not reignition even if exposed to the air atmosphere again at the end of experiment. We had confirmed that the prevention measures against sodium combustion residuum reignition to establish by this research were effective.

Experimental study of prevention measures against sodium combustion residuum reignition

Ishikawa, Hiroyasu; Ohno, Shuji; Miyahara, Shinya

JNC TN9400 2002-081, 46 Pages, 2003/01

JNC-TN9400-2002-081.pdf:2.21MB

Nitrogen gas can be an extinguisher or a mitigating material in the case of sodium leak and fire accident in an air atmosphere, which may occur at a liquid metal cooled nuclear power plant. However sodium combustion residuum sometimes reignites in the air atmosphere even at room temperature when it was produced by nitrogen gas injection to the burning sodium. In this study we have been investigating the cause of reignition and prevention measures. Experiments were carried out with small type test equipment, which can handle 1g order sodium fire and extinguishment. Sodium combustion residua, which were made by our equipment and sampled, were analyzed by X-ray diffraction and chemical analysis. The chemical analysis of reignitable residua showed that the residuum contained metallic sodium of about 40wt-% (61 mol-%) to 60-wt % (76mol-%) and most of the rest was sodium-monoxide (NaO). Sodium-peroxide (NaO) was also included in less than 1wt-% of the residuum. Sodium or NaO cannot ignite by itself in the air atmosphere at room temperature in a few minutes. Therefore the reignition seems to be due to increase in the local temperature that is caused by oxidizing heat of Na and by adiabatic effect of NaO. It is important to deactivate this dispersed sodium on oxygen for prevention of the residuum reignition, hence it is considered as a rational measure to change the sodium to sodium-carbonate. Our experiments showed that the dispersed sodium on the exterior of residuum could be changed to carbonate by a mixture of carbon-dioxide (CO) gas (2 to 8vol-%), humidity (0.6 to 3vol-%) and nitrogen gas. The deactivated residuum did not reignite in the air atmosphere below 473K.

Experimental Study of Prevention Measures against Sodium Combustion Residuum Reignition

Ishikawa, Hiroyasu; Ohno, Shuji; Miyahara, Shinya

Nihon Nensho Gakkai-Shi, 45(134), 248 Pages, 2003/00

Nitrogen gas can be an extinguisher or a mitigating in the case of sodium leak and fire accident in an air atmonphere, which may occur at a liquid metal cooled nuclear power plant. However sodium combustion residuum sometimes reignites in the air atmosphere even at room temperature when it was produced by nitrogen gas injection to the burning sodium. In this study we have been investigating the cause of reignition and prevention measures. Experiments were carried out with small type test equipment, which can handle 1g order sodium fire and extinguishment. Sodium combustion residua, which were made by our equipment and sampled, were analyzed by X-ray diffraction and chemical analyses. The chemical analysis of reignitable residua showed that the residuum contained metallic sodium of about 40Wt-%(61mol-%)to 60-wt %(76mol-%) and most of the rest was sodium-monoxide (Na2O).Sodium-peroxide (Na2O2) was also included in less than 1wt-% of the residuu. Sodium or Na2O cannot ignite by itself in the air atmosphere at room temperature in a few minutes. Therefore the reignition seems to be due to increase in the local temperature that is caused by oxidizing heat of Na and by adiabatic effect of Na2O. It is important to deactivate this dispersed sodium on oxygen for prevention of the residuum reigntion, hence it is considered as a rational measure to change the sodium to sodium-carbonate. Our experments showed that the dispersed sodium on the exterior of residuum could be changed to carbonate by a mixture by a mixture of carbon-dioxide(CO2) gas (2 to 8vol-%),humidity (0.6 to 3vol-%) and mitrogen gas. The deactivated residuum did not reignite in the air atmospherebe bwlow 473K.

None

; Hayafune, Hiroki; ; ; ; Kawasaki, Nobuchika;

JNC TY9400 2000-024, 706 Pages, 2000/06

JNC-TY9400-2000-024.pdf:28.02MB

no abstracts in English

Studies on sodium cooled fast breeder reactor

Nibe, Nobuaki; Shimakawa, Yoshio; ; Hayafune, Hiroki; ; ;

JNC TN9400 2000-074, 388 Pages, 2000/06

JNC-TN9400-2000-074.pdf:13.32MB

Large sized sodium-cooled fast breeder reactors of large-size are being studied and have been operated in Japan and many countries. ln this feasibility study, evaluation was made on technical feasibinty for design concepts or 1 loop type and 3 pool types, specially from the viewpoint of improvement of economical competence. The design concepts include the ideas of cost reduction measures such as large-scaled components, reduction of loop number and integration of components on the basic of utilization of sodium characteristics. From the results of the evaluation, it may be possible for all the concepts to attain the economical target of 200 thousands yen per kilowatt, though further confirmation should be made for technical feasibility of those concepts. ln addition, the following items were listed up as further cost-reduction measures. (1)Higher temperature cooling system and steam cycle efficiency (2)Shortening of construction term (3)Reduction of safety systems by using measuring instruments with high performmce (4)Adoption of SG-ACS

Experimental study of sodium reignition behavior

Ohno, Shuji; ; *; ;

JNC TN9400 99-081, 74 Pages, 1999/10

JNC-TN9400-99-081.pdf:1.82MB

Reaction residue of sodium combustion has possibility of reignition in an air atmosphere even at 50C when the sodium pool fire was extinguished by nitrogen (N) gas. Thus we carried out an experimental study of this behavior for the purpose to clarify the cause of the reignition and to make a suggestion for a preventive measure. The sodium burning and extinguishing experiments were performed changing fraction of oxygen (O), carbon-dioxide (CO) and humidity in the atmosphere, using 1g sodium and a glass-made chamber of 70mm inner diameter and 300mm length. By the observation of phenomena, measurement of reignition temperature, and analyses of reaction residue, the following results and conclusions were obtained. (1)When the burning was distinguished by pure N gas, reignition did not occur at 59C. However, reignition occurred at 50C when the distinguishing N gas contained over 1% O. (2)Reignition temperature became higher by supplying humidity and CO after cooling down the residue, even when the distinguishing N gas contained O. (3)The reaction residue consisted mainly of NaO and Na, partly of NaOH, NaCO, and NaO depending on the experimental condition. No peculiar compound was detected. The reignition might be attributed to the sharply projecting-shaped gray-white-colored reaction products which contains NaO.