I.

Project ResearchProject 8

PR8Production Mechanism of Radioactive Aerosols
Released from Fukushima Daiichi Nuclear Power Plant

K. Takamiya
Research Reactor Institute, Kyoto University
OBJECTIVE: The chemical and physical characteristics of radioactive materials released from Fukushima
Daiichi Nuclear Power Plant to the atmosphere has been
investigated by various methods to elucidate the environmental dynamics of the radioactive materials. A radioactive aerosol is one of the most important released materials,
and the production mechanism and measurement technique has been studied in this project. The generation
method for solution radioactive aerosols had been developed in the previous studies in this project, and the newsubject in which the generated solution aerosol had beenapplied was added in this year period in order to understand the environmental dynamics of the radioactive aerosols.
RESEARCH SUBJECTS:
The project research iscomposed by five individual subjects in this year period.
Two are developments of production method of radioactive aerosols for simulation experiments, one is development of measurement method of radioactive aerosols, andone is elucidation of the production mechanism of aerosol
under the specific situation. And one subject about interaction of radioactive cesium in the environment was addedin this year period. The respective subjects of the researchgroups of this project are described as follows;
P8-1: Development of production method of radioactive
aerosols by attaching radioisotopes to aerosol particles
P8-2: Development of measurement method of radioactive
aerosols under severity conditions
P8-3: Development of production method of radioactive
aerosols by heating radioisotopes
P8-4: Study for production mechanism of radioactive aerosols in the presence of sea water
P8-5: Interaction of radioactive cesium and suspended particles in river water
In this year period, experiments of subjects P8-3 and P84 were not carried out because of no operation of KUR.
RESULTS: In the subject P8-1, the solution concentration dependence of production behavior for radioactive solution aerosols has been investigated. Primary aerosols
were generated from sodium chloride solution of various
concentration (0.005, 0.01 and 0.02 M). The production
ratio of radioactive aerosols was estimated as attachment
ratio of fission products to aerosol particles in a chamberin which 252Cf spontaneous fission source was placed. The
attachment ratio of 104Tc can be obtained only with enoughstatistics as a function of total surface area of aerosol particles in this experiment. It was found that there are differences among aerosols of three different concentrations in
the relationship. The result might suggest the solute con-28P8

centration of solution aerosols affects the production process of radioactive aerosols. The relationships were fitted
by an equilibrium function on the basis of an assumptionof the adsorption-desorption equilibrium between fissionproducts and surface of aerosol particles. The order ofmagnitude of the estimated equilibrium constants is 0.02
M > 0.01 M > 0.005 M. This trend might be caused bydensity of the anions in the solution aerosol particles.
For the development of measurement method of radioactive aerosols under severity conditions (P8-2), time variation of particle size of the radiation-induced aerosols wastried to be measured in an electron linac facility by usinga scanning mobility particle sizer (SMPS) or a wire screen
method. According to the SMPS measurement, no aerosol
particles were observed in the chamber immediately after
the filtration of the irradiated air; however, rapid formationof the aerosol occurred in the chamber. It was found thatthe particle size increased to approximately 100 nm (geometric mean) with a decrease in the number concentration,and time to maximum size from the end of air sampling
was 9 h. The size and concentration are expressed as functions of the rates of formation, coagulation, evaporation,
dilution and disappearance due to collision to the insidewall of the chamber.
For the research about interaction of radioactive cesiumin the environment（P8-5）, the effect of cesium adsorption
to suspended particles on the desorption behavior was investigated. The suspended particles were sampled from
Abukuma river in Date, Fukushima. For the adsorption ofcesium, 100 mg of the particle sample was exposed by thecesium solution aerosol generated by an atomizer containing 0.4 wt% of CsCl, for 2.5 hours or 90 seconds. The
amount of loaded cesium on the particles was determined
by ICP-MS using a part of particle samples. And the particle samples exposed by the aerosol were mixed with various desorption agents as the cesium desorption treatment.
The supernatant was filtered by a 0.45 µm pore size membrane filter, and the cesium concentration in it was measured by ICP-MS to estimate the desorption ratio. The results of desorption experiment indicated that all desorption
agents (470 mM NaCl, seawater, 1 M KCl) except ultrapure water desorbed cesium more than 90 % from thehigh cesium loading sample. In contrast, only 30 % cesium
was desorbed from the low cesium loading sample by seawater. As a result, the cesium desorption percentage by
seawater was 30 %, where the cesium loading on the suspended particles was about 1µg/g which was close to theloading amount of 90 seconds aerosol atomized sample (=6.9µg/g). The observed agreement of the desorption percentages from different way of cesium loading indicatesthat the desorption behavior is simply governed by the cesium loading amount on the particles rather than the wayof loading.
In the near future, experiments with a neutron irradiation using KUR will be performed, and further progress isexpected in this project.

PR8-1

Solution Concentration Dependence of Production Ratio ofRadioactive NaCl Solution Aerosols

K. Takamiya, Y. Nishizawa, S. Sekimoto, Y. Okiand T. Ohtsuki
Research Reactor Institute, Kyoto University
INTRODUCTION: Large amount of radioactive materials released into the atmosphere from Fukushima Daiichi
Nuclear Power Plant have been transported by aerosols. Itwas suggested that the potential transport medium for radioactive cesium were sulfate aerosols [1, 2]. On the otherhand, the initial production process of radioactive aerosolsin the reactor building is not clear because there is not direct measurement data. Attachment behavior of fission
products to solution aerosol particles have bee studied in
our previous works [3, 4] to elucidate the production
mechanism of radioactive solution aerosol in the initial
production process. It was found that there seems to be
an adsorption equilibrium process in the radioactive aerosol production and the equilibrium constants depends on akind of solution. In the present work, the solution concentration dependence of production behavior for radioactive
solution aerosols has been investigated.
EXPERIMENTS: The production method of radioactive aerosols using a spontaneous fission source of 252Cf
was explained in the previous reports [3, 4]. Primary aerosols were generated from sodium chloride solution of various concentration (0.005, 0.01 and 0.02 M). In order toexamine the relationship between surface area of aerosol
particles and the production ratio of radioactive aerosol,
the primary solution aerosol was classified by size using a
differential mobility analyzer in the region of diametersmaller than about 100 µm. In the larger diameter region,
the surface of the aerosols was controlled by changing theaerosol number concentrations. The radioactive aerosol
produced by attaching fission products to aerosol particles
were collected by a polycarbonate filter. On the other hand,
all fission products emitted from the 252Cf source werecollected by a grease-applied cellulose filter placed directly on the source. Gamma-ray spectrometry using a Gedetector was performed for the filters to estimate the production ratio of radioactive aerosols which were produced
by attaching fission products to aerosol particles.
RESULTS: The production ratio of radioactive aerosols
was estimated as attachment ratio of fission products toaerosol particles from the ratio of photopeak areas for eachfission product. The attachment ratio of 104Tc can be obtained only with enough statistics and shown in Fig. 1 as a

28P8-1function of total surface area of aerosol particles. The symbols of circle, square and triangle show the attachment ratio for sodium chloride solution of 0.005, 0.01 and 0.02 M
solution concentrations, respectively. The difference arefound among aerosols of three different concentrations,and this result means the solute concentration of solution
aerosols affects the production process of radioactive aerosols. The relationships were fitted by an equilibrium
function on the basis of an assumption of the adsorptiondesorption equilibrium between fission products and surface of aerosol particles. The results of the fitting areshown by lines in Fig. 1. The order of magnitude of theestimated equilibrium constants is 0.02 M > 0.01 M >0.005 M. This trend might be caused by density of the anions in the solution aerosol particles.

Fig. 1. Relationship between the attachment ratio of104
Tc to NaCl solution aerosol and the total surface
area of the aerosol particles. Circles, triangles andsquares show the attachment ratio to NaCl solution
aerosols of 0.005, 0.01 and 0.02 M concentrations,
respectively.
REFERENCES:[1] N. Kaneyasu et al., Environ Sci Technol 46 (2012)
5720–5726.
[2] Y. Miyamoto et al., J. Environ. Radioact. 132 (2014)
1–7.
[3] K. Takamiya, et al., J. Radiat. Prot. Res. 41 (2016) 350353.
[4] K. Takamiya, et al., J. Radioanal. Nucl. Chem. 307(2016) 2227-2230.

PR8-2Growth of Aerosol Particles in Irradiated Air of an Accelerator RoomY. Oki, K. Takamiya, N. Osada1, S. Sekimoto andT. Ohtsuki
Research Reactor Institute, Kyoto University
Advanced Science Research Center, Okayama University

yed for size measurement of the aerosol. The particle size
distribution was calculated from penetration ratios for thescreens obtained with condensation particle counter(CPC).

1

INTRODUCTION:
Radioactive aerosols were released to the environment in both accidents of the Fukushima Daiichi Nuclear Power Plant (FDNPP) and Japan
Proton Accelerator Research Complex (J-PARC). The
release occurred due to melting of the highly radioactive
nuclear fuel or the metallic target. Radionuclides evaporated from the molten fuel or metals are incorporated into
ambient aerosol particles to form the radioactive aerosolsof various radionuclides. The radiation-induced aerosol isa principal ambient aerosol in intense radiation fields;
however, most of its nature concerning formation andgrowth in radiation fields remain unexplained.
In this work, time variation of particle size of the radiation-induced aerosols was studied using a scanningmobility particle sizer (SMPS) from the time of theirbirth in an electron linac facility. In addition, a wire
screen method was also applied for the size measurement[1].
EXPERIMENTS:
An air irradiation experiment was
carried out in the 46-MeV electron linear accelerator of
the Research Reactor Institute (KURRI). A tantalum target was installed at the end of the beam line, and was
bombarded with a 30-MeV electron beam to produce
bremsstrahlung and neutrons. The bremsstrahlung ionizes
air and produces radiation-induced aerosol. The beam
current was ca. 100 m. During the irradiation, the air
ventilation in the target room was stopped not to disturb
air. Air was collected from the target room to a 200-L
vacuum aerosol chamber through a sampling tube penetrating a 3-m thick shielding wall. The target room air
was introduced to the vacuum chamber after filtration
with a HEPA filter. A 1.5-L ionization chamber wasused for monitoring concentration of radioactive gases.
The particle size distribution and number concentration of the aerosols were measured continuously withSMPS. The irradiated air was sampled from the chamberto the SMPS at the flow rate of 0.3 L/min. A filtered aerosol-free air was supplied to the chamber at the same
flow rate to keep pressure in the chamber.
A screen device consisting of a stack of 20 pieces of
500-mesh stainless steel wire screens was also employ

28P8-2

Fig. 1 Experimental Setup for Sampling of the irradiatedair from the target room
RESULTS: According to the SMPS measurement, noaerosol particles were observed in the chamber
imediately after the filtration of the irradiated air;
however, rapid formation of the aerosol occurred in thechamber.
Preliminary results showed that the particle sizeincereased to approximately 100 nm (geometric mean)
with a decrease in the number concentration. It took 9 h
to reach the maximun size from the end of air sampling.
The size and concentration are expressed as functions of
the rates of formation, coaguration, evaporation, dilution
and dissapearance due to collision to the inside wall of
the chamber.
Oxygen-15 (half life: 2 min), 13N (9.965 min) and41
Ar are principal radionuclides formed in the targetroom air. Nitrogen-13 was a dominant gaseous nuclide in
the chamber. The radioactivity in the chamber isconsidered to influence the formation rate. The formation
rate decreased as 13N decayed.
The screen device was not applied to radioactivity-based particle size measurement because conversionof radioactive 13N gas to aerosols was not observed in thechamber. In the number-based particle size measurement,
the number concentration of the aerosols was measured
by varying the sampling flow rate to the device, not bychanging number of the screens, in order to minimize thesampling volume from the chamber.
REFERENCES:[1] Y. Oki, et al., J. Radiat. Prot. Res., 41 (2016) 216-221.

PR8-3

Interaction of Radioactive Cesium and Suspended Particles in River Water

A. Kirishima, M. Onodera, K. Takamiya1, T. Ohtsuki1, Y.
Oki1, S. Sekimoto1 and N. Sato
Institute of Multidisciplinary Research for Advanced
Materials, Tohoku University
1
Kyoto University Research Reactor Institute
INTRODUCTION: Radioactive Cs deposited in river
due to the nuclear accident of Fukushima in 2011 isthought to be adsorbed in suspended particles (SP) andmigrate along the river flow. Because SP are mainlycomposed by clay minerals, they can retain Cs on thesurface and inter-layers of clay minerals by cation-exchange reaction. When retained radioactive Cs arrives at brackish water area, they could be desorbed fromSP in response to solution condition change. The understanding of this Cs speciation change mechanism is very
important for the prediction of Cs migration from mountainy area to coast area. Therefore, this study carried out
cesium adsorption and desorption experiment for SP bybatch method. To investigate the effect of the Cs adsorption method to the particle on the desorption behavior,cesium desorption experiment from the suspended particles that adsorbed cesium by atomizing cesium aerosol
was carried out in this study.
EXPERIMENTS: SP was sampled from Date placelocated in about 60 km northwest of the 1F NPS (Yanagawa-Ohashi, Yanagawa-cho, Date-shi, Fukushima prefecture, Japan) of Abukuma river and named as DateSample. The river water containing suspended particles
were collected about 20 L by a Teflon container. The
suspended particles were then collected on a filter having
0.45 µm pore, and these particles were freeze dried. Finally, black brown color powdered suspended particles
were obtained. Solution aerosols were generated using acombined system of an atomizer 3076 (TSI, Shoreview,MN) containing 0.4 wt % of CsCl, a electric furnace, anda diffusion dryer. Nitrogen gas was supplied at pressureof 2.5 kg/cm2 to the atomizer to discharge CsCl solution
aerosols [1]. The aerosols were passed through the furnace, which was kept constant at 80 ˚C and the diffusion
dryer with an infill of silica gel sequentially. For the adsorption of cesium, 100 mg of the Date sample was exposed by the cesium aerosol for 2.5 hours or 90 seconds
with mixing the particles every 30 minutes and 10 second,respectively by a spatula. To determine the loaded cesium
amount on the suspended particles, about 4 mg of theaerosol exposed Date samples were dissolved in themixed solution of 3 ml of conc. HNO3 and 300 µl of 46
wt % Hydrofluoric acid in a PTFE beaker (Heatablebeaker, Sanplatec Co.) with heating at 160 ℃. After thecomplete dissolution of the particles, the solution wasdried up, then about 5 ml of 1 M HNO3 solution was28P8-5

added to dissolve remained salts. The cesium concentration in this solution was then measured by an induced
coupled plasma – mass spectrometry spectrometer(ICP-MS, ELEMENT II, Thermo Fisher Scientific Co.)
using tellurium ion as the internal standard. These dissolution and measurement procedure were repeated 4 times
for one sample of the suspended particles to get an average value of the cesium concentration. The loaded cesium
concentration of the Date sample from the cesium aerosol
was thus calculated from the measured cesium concentration by ICP-MS. Subsequently, the Date samples exposedby the aerosol were mixed with desorption agents in asolid-to-liquid ratio of 0.17 g/l for 6 days at 25 ℃ as thecesium desorption treatment. The supernatant was then,filtered by the 0.45 µm pore size membrane filter, and thecesium concentration in it was measured by ICP-MS.
Finally the desorption percentages were calculated byfollowing equation:
Desorption [%]=

(C/C0) ×(S/L)×100,(1)

where C0 is initial cesium concentration (g/g) in the Datesample exposed by the aerosol, and C is cesium concentration (g/l) in the supernatant solution after the desorption treatment, (S/L) is solid-to-liquid ratio (g/l) at thedesorption treatment.
RESULTS: The loaded cesium amount on the Datesamples atomized by the CsCl aerosol were determined
to be 113 µg/g for 2.5 hours of the atomization sample
and 6.9 µg/g for 90 seconds atomization by ICP-MS. The
results of desorption experiment indicated that all desorption agents (470 mM NaCl, seawater, 1 M KCl) except
ultrapure water desorbed cesium more than 90 % fromthe high cesium loading sample (2.5 hours atomization).
In contrast, only 30 % cesium was desorbed from the low
cesium loading sample (90 seconds atomization) by seawater. In the desorption experiment using cesium adsorbed particles from the solution, the cesium desorption
percentage by seawater was 30 %, where the cesium
loading on the suspended particles was about 1µg/g
which was close to the loading amount of 90 seconds
aerosol atomized sample (= 6.9µg/g). The observed
agreement of the desorption percentages from different
way of cesium loading indicates that the desorption behavior is simply governed by the cesium loading amounton the particles rather than the way of loading.
REFERENCES:[1] K. Takamiya, T. Tanaka, S. Nitta, S. Itosu, S.
Sekimoto, Y. Oki, T. Ohtsuki, Attachment Behavior ofFission Products to Solution Aerosol. J Radiat Prot Res,
(2016) 41, 350–353.