Project ResearchProject 10

PR10

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

K. Takamiya
Research Reactor Institute, Kyoto University
OBJECTIVE: Environmental distribution and transition of radioactive materials released from Fukushima
Daiichi Nuclear Power Plant has been investigated bymeans of air dose rate measurements and radioactivity
measurements for environmental samples. On the other
hand, chemical forms of the radioactive materials released from the power plant and transition mechanisms inthe environment has not been revealed. The chemicalform of the radioactive materials just behind the release isimportant to evaluate the internal exposure by radioactive
aerosols and the transition of the materials in soils andplants after the fall out. A radioactive aerosol is one of themost important transport medium of the radioactive materials. In the present project research, the production
mechanism of radioactive aerosols has been studied toelucidate the chemical form of the radioactive materials
released from the power plant.
RESEARCH SUBJECTS: The project research iscomposed by four individual subjects in this year. Twoare developments of production method of radioactiveaerosols for simulation experiments, one is developmentof measurement method of radioactive aerosols, and one
is elucidation of the production mechanism of aerosol
under the specific situation. The respective subjects of theresearch groups of this project are described as follows;
P10-1: Development of production method of radioactive
aerosols by attaching radioisotopes to aerosol particles
P10-2: Development of measurement method of radioactive aerosols under severity conditions
P10-3: Development of production method of radioactive
aerosols by heating radioisotopes
P10-4: Study for production mechanism of radioactiveaerosols in the presence of sea water
In this year period, experiments of subjects P10-3 andP10-4 were not carried out because of no operation ofKUR.
RESULTS: In the subject P10-1, an attachment ratio offission products to aerosol particles was estimated using asolution aerosol production apparatus and a spontaneous
fission source of 252Cf in order to elucidate the attachment mechanism of fission products to aerosol particles.

The solution aerosols were generated from alkali halide(NaCl, NaBr, NaI, CsCl) and ammonium sulfate solutionof 0.1 wt% concentration using an atomizer. The attachment ratio can be considered by associating with surface
area of aerosol particles definitely because aerosols classified by size were used in the experiments. The relationships between the surface area and the estimated attachment ratio for a fission product of 104Tc were fitted bya function on the basis of the assumption of the adsorption-desorption equilibrium between fission products andsurface of aerosol particles. From results of the fitting,equilibrium constants and saturated attachment ratios
were obtained for the combination of 104Tc and each solution aerosol. In the comparison among the estimatedequilibrium constants of 104Tc for solution aerosols, it
was found that the equilibrium constant for sodium chloride solution aerosol is smaller than those for other solution aerosols. This difference might be caused by density
profiles of the anions in the fine solution particles that isexplained by molecular dynamics simulations.
For the development of measurement method of radioactive aerosols under severity conditions, an air irradiation experiment was performed using the FFAG (FixedField Alternating Gradient) proton accelerator, and a
combination method of imaging plate (IP) measurement
and the wire screen technique was tried to estimate particle size of 11C-bearing aerosols which were produced inthe air irradiation. The 11C activities collected by eachscreen and the backup filter were estimated from the decay analysis. The activity ratio of particles penetrating thescreens to whole particles before penetrating the screens
is governed by their diffusion coefficient expressed by afunction of particle size. The size distribution was calculated by fitting the curve of the penetrating ratios vs. thenumber of screens to the theoretical equation.
Log-normal distributions are assumed as the size distribution function in the calculation because the number-based size distribution was confirmed to be
log-normal by another measurement method. The geometric mean diameter for the 11C-bearing aerosol particles was successfully obtained using the combination
method of the wire screen collection and the IP measurement. The activity-based geometric mean diameters
were larger than the number-based one by ca. 20 nm.
This shows that attachment process of radioactive atomsto the surface of aerosol particles is included in the radioactive particle formation.

27P10- 55 -Adsorption-desorption Equilibrium in Attachment ofFission Product to Solution Aerosol

K. Takamiya, T. Tanaka1, S. Nitta1, S. Sekimoto Y. Okiand T. Ohtsuki
Research Reactor Institute, Kyoto University
1
Graduate School of Engineering, Kyoto University
INTRODUCTION: Almost radioactive materials released in the air from Fukushima Daiichi Nuclear Power
Plant have been transported by aerosols. Measurement
results about size distributions and activity size distributions of the radioactive aerosols have been reported, andit was suggested that sulfate compounds were the potential transport medium for radioactive cesium isotopes [1,2]. However, the reason for the selectivity to sulfate
compounds in production mechanism of radioactive aerosols is not clear. Attachment behavior of fission products
to solution aerosol particles has been investigated in order to elucidate the production mechanism of radioactiveaerosols. The aerosols which contains a radioactive fission product were generated and attachment ratio of fission product aerosol was measured to reveal the production mechanism of the radioactive solution aerosols in thepresent work.
EXPERIMENTS: The fission-product aerosols wereproduced by passing a primary solution aerosol through achamber in which 252Cf source was placed. The 252Cfsource was covered by two Havar foils which thickness is2.5 µm each to reduce kinetic energy of fission products
emitted from the source. Primary aerosols were generated
from alkali halide (NaCl, NaBr, NaI, CsCl) and ammonium sulfate solution of 0.1 wt% concentration using anatomizer. The primary solution aerosol was classified bysize using a differential mobility analyzer before the injection to the 252Cf chamber. The radioactive aerosol wasproduced in the chamber by attaching fission products toaerosol particles and collected on a polycarbonate filter.
Radioactivity of fission products attaching to aerosol particles was measured by gamma-ray spectrometry using aGe detector. Fission products emitted in the chamberfrom the 252Cf source were also collected on a cellulose
filter placed directly on the source and measured by theGe detector.
RESULTS: The attachment ratio of fission products toaerosol particles was estimated from the ratio of photopeak areas for each fission product and shown in Fig. 1
as a function of total surface area of aerosol particles.
The attachment ratio seems to be proportionate to thesurface area of aerosol particles in the small surface area
region. The increasing rate of the relationship varies withmaterials of the aerosol particles. The proportional relation and the variation of increasing rates suggest that theattachment ratio of fission products to aerosol particles

depends on both geometric collision rates and chemicaleffects. On the other hand, the increasing rates decrease
and the attachment ratios were saturated in the larger
region of the surface area of aerosol particles. This trend
indicates that the attachment behavior of fission products
to aerosol particles could be held in an adsorption-desorption equilibrium.
The relationships between the surface area and theattachment ratio for 104Tc were fitted by a function on thebasis of the assumption of the adsorption-desorption
equilibrium between fission products and surface of aerosol particles. As results of the fitting which are shownby solid lines in Fig. 1, equilibrium constants and saturated attachment ratios were obtained for the combination of 104Tc and each solution aerosol. In the comparison
among the estimated equilibrium constants of 104Tc forsolution aerosols, it was found that the equilibrium constant for sodium chloride solution aerosol is smaller thanthose for other solution aerosols. This difference might
be caused by density profiles of the anions in the finesolution particles that is explained by molecular dynamics simulations [3, 4].

80

Attachment ratio [%]PR10-160

NaCl
NaBr
NaI
CsCl
(NH4)2SO440200 -2
1010-1
1102Total surface area [cm ]102Fig. 1 Attachment ratio of 104Tc to various solution aerosol as a function of total surface area of aerosol particles.
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] P. Jungwirth et al., Chem. Rev. 106 (2006)
1259–1281.
[4] Wei Hua et al., J. Phys. Chem. B. 118 (2014)
8433–8440.

27P10-1- 56 -PR10-2

An Application of Imaging Plates to Size Measurenent of Radioactive Aerosol
Particles Produced in Accelerator Room Air

Y. Oki, K. Takamiya, N. Osada1, T. Tanaka2, S. Nitta2,Y. Ishi, T. Uesugi, Y. Kuriyama, M. Sakamoto andT. Ohtsuki
Research Reactor Institute, Kyoto University
1
Advanced Science Research Center, Okayama University
2
Grad. School of Eng., Kyoto University
INTRODUCTION: Very fine radiation-induced aerosol particles are often observed in air of acceleratorrooms during machine operation. The particles are produced by an air ionization process by primary beams orby charged particles emitted from beam loss points. Theparticles incorporate radioactive atoms to form radioactive particles. The size for radioactive particles was often
measured using a wire screen technique in accelerator
facilities. Convenient size measurement techniques areneeded from the viewpoint of radiation protection in accelerator facilities.
When aerosol particles pass through a stack of wire
screens, a part of the particles are trapped on the screens
according to their particle size. As the loss by the screensis expressed as a function of particle size, coarseness ofscreens, and flow rate of particles, the particle diametercan be calculated by measuring the loss by each screen.
In this work, air irradiation experiment was performed in the FFAG (Fixed Field Alternating Gradient)
proton accelerator in the Research Reactor Institute(KURRI), and a combination method of imaging plate(IP) measurement and the wire screen technique was applied to estimate particle size of 11C-bearing aerosols.
EXPERIMENTS: A screen device consisting of a
stack of stainless steel wire screens and a PTFE backup
filter was used for size measurement of the radioactiveaerosol. The stacking order and number of the screens are
single 100, 200, 300-mesh screens and 40 pieces of
500-mesh screens in a direction toward the downstream.
An air-irradiation chamber was installed at the upstream of the beam dump of the beam extraction line of
the FFAG main ring (Fig. 1). The chamber was irradiatedfor 1 h with a stable beam of nominal 150-MeV protons.
The beam current was 1.8 nA. During the irradiation aerosol-free air was continuously introduced at the flow rateof 8.1 L/min to the chamber, and the irradiated air wassampled with the screen device at the rate of 7.5 L/min. A
part of the irradiated air was introduced to an SMPS
(Scanning Mobility Particle Sizer) to monitor the number-based aerosol size simultaneously with the screencollection.
In the aerosol the radionuclides affecting the IP exposure are virtually only positron emitters such as 15O(half life: 2 min), 13N (9.965 min) and 11C (20.39 min).
After the short-lived 15O was decayed, a radiation imageof selected screens and the backup filter was obtained by

30-min exposure of one single IP (Size 35 x 43 cm). To
evaluate contribution of 11C and 13N to the intensity ofphotostimulated luminescence (PSL), a separate air irradiation experiment was performed under the same irradiation and sampling conditions. After the irradiation thedecay of activity for the screen and the backup filter wasanalyzed using GM detectors.
In comparison with 11C, the 478-keV photopeak for7
Be was analyzed using Ge detectors to obtain size distribution for 7Be-bearing aerosol particles.

Fig. 1 Irradiation Chamber installed in the beam extraction lineRESULTS: The 11C activities for each screen and thebackup filter were estimated from the decay analysis
mentioned above. The particle size of 13N-bearing
particles was not analyzed in this work because 13N
gaseous species like nitric acid interferes with collectionof the 13N-bearing particles [1]. The activity ratio of particles penetrating the screens to whole particles before
penetrating the screens is governed by their diffusion
coefficient expressed by a function of particle size. The
size distribution was calculated by fitting the curve of thepenetrating ratios vs. the number of screens to the theoretical equation [2]. Log-normal distributions are assumed as the size distribution function in the calculation
because the number-based size distribution was confirmed to be log-normal by the SMPS measurement.
The geometric mean diameter, dg, for the 11C-bearing
aerosol particles was successfully obtained using thecombination way of the wire screen collection and the IP
measurement. The diameter was found to be 68.0 nm forthe 11C-bearing particles, while that for 7Be-bearing
particles obtained by the Ge detector measurement was63.4 nm. The activity-based geometric mean diameters
were larger than the number-based one by ca. 20 nm.
This shows that attachment process of radioactive atoms
to the surface of aerosol particles is included in theradioactive particle formation.
REFERENCES:[1] N. Osada, Y. Oki, et al., Prog. Nucl. Sci. Tech., 1(2011) 483-486.
[2] Y.S. Cheng and H.C. Yeh, J. Aerosol Sci., 11 (1980)
313-320.

27P10-2- 57 -