C-14 release behavior and thermal decomposition characteristics of crud particles
collected from the coolant filter of commercial PWR
Shintaro Tsujia, Toshikazu Wakia, Akira Sakashitab, Kenichiro Kinoc
aThe Kansai Electric Power Co. Inc.
b
Mitsubishi Heavy Industries, LTD.
c
Nuclear Development Corporation
a3-6-16 Nakanoshima Kita-ku Osaka-shi, Osaka 530-8270 Japanb
16-5 Konan 2-chome, Minato-ku, Tokyo 106-8215 Japan
c622-12, Funaishikawa, Tokaimura Ibaraki 319-1111 JapanKey words; Waste Disposal, Radionuclide migration, Safety assessment, C-14, TG/MSAbstractIn the safety assessment of radioactive waste buried repository, it is extremely important to evaluate themigration behavior of long half-life radionuclides in the disposal environment. C-14 is a key radionuclide inwaste disposal because of the long half-life (5,730years) and the considerable effect on public exposure due tothe less capability of capturing C-14 by artificial barrier material. In addition, C-14 is a radioactive isotope ofcarbon which forms various chemical forms in the process of its formation, and the migration behavior in theburied environment greatly differs depending on its chemical form.

Some previous studies have reported the generation of insoluble C-14 in pressurized water reactor(PWR), and suggested that ion-exchange resin, which is used for purifying primary coolant, could be its
origin. However, this hypothesis has not been well evidenced yet, because the information about thegeneration mechanism and the chemical form of insoluble C-14 is still limited.
In this study, in order to investigate the chemical form of C-14 contained in radioactive particles generated by
PWR, radioactive particles collected from the coolant filter of PWR were analyzed by TG/MS, and the gas
released by TG and residual C-14 in the compounds was also analyzed at the same time.
As a result, organic molecules presumed to originate from ion exchange resin were detected from the releasedgas components, and it was confirmed that 90% or more of C-14 was detected in residual components of heatingin the inert gas.
From the results of this study, the chemical form of insoluble C-14 generated by PWR could not be clarified,
but it was revealed that the majority of C-14 may exist in amorphous and thermally stable compounds.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan1. Introduction
When radioactive waste is buried, safety assessment
based on the retardation of migration of radionuclides
contained in radioactive waste is conducted. Radioactive
waste generated at PWRs contains many radionuclides.
Among these radionuclides, C-14, which is a radioisotopeof carbon, is a nuclide that may have various forms. The
migration retardation effect for C-14 in the burial
environment largely depends on its chemical form.
However, only part of the chemical forms of particulate
C-14 generated at PWRs has been clarified.
In Japan, concerning C-14 contained in solid radioactive
waste generated at PWRs, the presence of C-14 in theforms of inorganic matter that is decomposed by an acidinto CO2, and chemical matter that is not easily
decomposed unless an oxidant (e.g., potassiumpermanganate) is activated at high temperature, is known.
It has been confirmed that in these forms (of C-14), C-14
that is present as a solid (particle), not in an inorganic form,
is a “hardly soluble component” that is hardly dissolved in
an organic solvent or the like.(2)In that report, an observation result that showed thepresence of crud, which is a metal oxide, in addition toparticles centering on carbon is reported. Higuchi et alconsidered that concerning the observed particles, theparticulate carbon generated at the PWR was generated
from a degraded product of the ion exchange resin.
Meanwhile, Research on the crud of a primary system
is advanced in some PWR plants with aim of reducing
exposure. At the Callaway Plant in the United States, theresult of observing flaked crud that contained carbon isreported. (1)The crud observed at the Callaway Plant and the crudthat Higuchi et al reported have many similarities inappearance, and both contain much carbon. We
considered that the origins of particles formed from flaked
carbon were common to each other.
Therefore, for this paper, we first analyzed a solid
specimen with C-14 collected from the commercial PWR
plant that has not experienced zinc injection, and thenexamined the chemical form of C-14 contained in aparticulate component generated at the PWR, and its
generation mechanism.
2. Material and methods
2.1 Specimens
The major contamination source of solid radioactive waste
generated at the PWR is considered as crud contained in
the reactor coolant. Therefore, for this paper, a coolant
filter that captured much crud of the primary system of thePWR was collected, and the components of the crud
captured at the filter were used as a specimen for theexperiment.
In the PWR, the reactor coolant, which cools nuclear fuelsin the reactor, is pressurized to 15.4 MPa, and circulates in
non-boiling state at approximately 325C. Part of thisreactor coolant is constantly cleaned by a coolant filter anda mixed bed demineralizer filled with an ion exchange
resin after being cooled/depressurized to 80C or below ata chemical and volume control system (CVCS) (Fig. 1).
In a mixed bed demineralizer inlet filter (hereafter called“inlet filter”) installed in the CVCS, particulatecomponents (crud) that are generated due to corrosion or
the like of the material constituting the primary coolingsystem are captured, and the particles captured in the filter
can be considered to represent radioactive particles
generated at the PWR.
For the commercial plant specimen, the inlet filter
collected from Ikata Unit 3 of Shikoku Electric Power Co.,
Inc. was used. The plant, from which this specimen wascollected, does not adopt zinc injection.
In this study, we prepared mock crud by autoclaving theion exchange resin used at the PWR under conditions
close to the water quality of the primary system of thePWR, and obtained data through an experiment describedlater, with the aim of comparing the mock crud with thecommercial crud specimen.

Fig. 1: Schematic diagram of the primary coolingsystem and the chemical and volume control system ofthe PWR
2.2 Preparation of an experimental specimen
The inlet filter collected from the plant was partiallycut (Fig. 2) and cleaned by ultrasonic washing todisperse the particles captured in the filter into ultrapure
water, and crud captured on a milli-pore membranefilter (hereafter called “MF”) of 0.45 μm was collected(Fig. 3).

2.3 Analysis of the particulate components using
SEM/EDSTo check similarity between the specimen used foran experiment in this study and flaked crud containing
carbon, which has been reported in Japan and theUnited States in the past, we conducted a scanningelectron microscopic observation of the experimental
specimen.
Using scanning electron microscopes (SEM:S-3400N by Hitachi High-Technologies, Tokyo, Japan;
EDX: 350xt/Wave500, Oxford inst Energy, Oxford,UK), we observed the shapes of the particles captured
on the 0.45 μm MF, and conducted element
composition analysis using an energy dispersive X-ray
micro analyzer for the respective visual field.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

2.4 Molecular structure analysis using FT-IR
To check part of the chemical structure of the carbon
component in the crud specimen used in this study, we
conducted an FT-IR analysis.
The FT-IR spectra (transmission spectra in awavenumber range of 4,000 to 400 cm-1) of theparticulate components collected on the 0.45 μm MF
were measured using the KBr method. For the FT-IR
analyzer, FTIR-8400 of Shimadzu Corporation (Kyoto,Japan) was used.
2.5 Analysis of the thermal decomposition behaviorof C-14 with TG/MS
The thermal decomposition behavior of the crud waschecked using a thermo-balance. In addition, an off-gas
released by heating with the thermo-balance wasdivided, and part of the off-gas was induced to a mass
spectrometer, and the rest to a tubular furnace filledwith an oxidative catalyst. The mass spectra of theoff-gas released were analyzed with the mass
spectrometer, and the C-14 radioactivity of thecomponents oxidized in the tubular furnace wasanalyzed.
(1) Experiment method
For the thermo-balance, STA449F3 (manufactured byNETZSCH) was used. For the mass spectrometer,
JMS-Q1500GC (manufactured by Japan Electron Optics
Laboratory) was used. The gas released from thethermo-balance was mixed with a supporting gas (pureair) via a stainless steel piping kept heated at 200C,induced to a tubular furnace (furnace tubing: quartz;
oxidative catalyst: Co3O4 [2 mm mesh] 50 g) heated to900C, and the released gas was oxidized to CO2. The
components containing C-14 oxidized to CO2 wascollected using a bubbler filled with a 1 mol sodium
hydride solution (Fig. 6). C-14 collected in the sodium
hydride solution was re-condensed to a carbon dioxide
absorbent (Carb-Sorb E by Perkin Elmer) for liquid
scintillation counter measurement, then mixed with aliquid scintillator (Permafluor E+ by Perkin Elmer), a
beta ray emitted from C-14 was quantified with theexternal standard source channel ratio method (ESCR)
using a liquid scintillation counter (LSC-5000 by Aloka,Japan).
(2) Experiment conditions
A mock specimen irradiated with a gamma ray after
heat treatment, and the components captured in the inlet
filter collected from the commercial equipment, wereplaced on a thermo-balance, and the heating of them wasstarted under the following conditions. The C-14 release
behavior was checked only for the commercialequipment specimen. The heating of an inert gas using
the TG was performed after “in an inert gas (He)” andpure air were selected. In the heating in the inert gas, asupporting gas was added to the gas components
released from the TG to combust the gas, and C-14 wascollected as CO2 in an alkaline solution (Fig. 7).

◎TG treatment temperature◎Heating rate
◎Carrier gas◎Range of mass
spectrometer scanning

r.t. to 900C20C/min
He or airm/z = up to 1000

In addition, the specimen, for which the released
components and mass change were checked byapplying each carrier gas, was cooled to the room
temperature, and then heated again to 900C in an airstream. Subsequently, the released gas was heated to950C and oxidized into CO2 using an oxidative
catalyst to produce C-14, which was then collected, andthe portion of C-14 remaining in the particles wasobtained.

Fig. 2: The mixed bed demineralizer inlet filter after
being cut

Fig. 3: Particulate components captured on the 0.45 μm
MF

3. Results
Fig. 4 shows the result of a property observation using
an SEM. In the SEM observation, flaked crud wasobserved. On this flaked crud, small-grain particles were
further observed.
Table 1 shows the result of an element composition
analysis in the observation visual field. It was found that
58.6% of the element composition in this visual field wascarbon (Table 1). Fig. 5 shows the FT-IR spectra of crud
whose main component is this carbon. In the FT-IRspectra, transmission peaks that characterize a carboxylgroup and a hydroxyl group were observed (Fig. 5).
Fig. 8 shows the result of TG/MS analysis in the He
stream. In the thermal decomposition behavior of thecommercial equipment specimen with TG using He as thecarrier gas, the mass change was minimal in a range fromthe start of heating to 200C. However, a mass reduction

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

was observed around 400C. Beyond 400C, nocharacteristic mass change was observed from 500C to900C. In addition, around 430C where a major mass
reduction was observed, peaks were observed at m/z = 44,
77 and 105 on the mass spectrometer. The signals detected
for the respective mass-electrolysis ratios are consideredas the following analytical fragments (Fig. 8)
m/z = 44: CO2m/z = 77: C6H5m/z = 105: C6H5CHCH2Fig. 9 shows the result of TG-DSC analysis in which
He and He+O2 are used as the carrier gases.
In the TG analysis in the carrier gas containing oxygen,since the organic components in the specimen wereoxidized, the minimum value is observed in the DSC
voltage at temperature where a mass reduction became
obvious (around 430C), and the mass reduction ratio waslarger than the TG test in a helium stream (Fig. 9). Basedon this result, the particulate carbon contained in thecommercial equipment specimen is considered as aflammable organic component that ignites around 430C.
In the TG analysis in the He + O2 carrier gas, most of
the gas components released from the thermo-balance isreleased as H2O (m/z = 18) and CO2 (m/z = 44).
Table 2 shows the result of analyzing radioactivity of
C-14 released in an off-gas during the thermal
decomposition test of the hardly soluble particles.
It was confirmed that C-14 contained in the hardly
soluble particles collected from the commercial equipment
was little contained in CO2 released to the gas phasethrough thermal decomposition in a helium stream, or in
the organic components of various aromatic ring
structures as observed in Fig. 4, but released to the gas
phase only under conditions where oxygen was containedin the carrier gas.
In the TG-DSC analysis, the mass reduction ratio inhelium was approximately 40%, while the mass reduction
ratio the He + O2 carrier gas was approximately 70%.
Based on this result, the component remaining in thehelium stream is considered to be a stable carbon
component.

Fig. 4: A SEM image of the particulate components ofthe commercial equipment

Table 1: Result of analyzing the element compositionof the particulate components of the commercialequipment

Fig. 5: FT-IR spectra of the mock specimen and thecommercial equipment specimen

Fig. 6: Experiment apparatus for checking the thermal
decomposition behavior of hardly soluble particulate
C-14

Fig. 7: Experiment flow for checking the thermal
decomposition behavior of hardly soluble particulate
C-14

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Fig. 8: Result of TG/MS measurement when thecommercial equipment specimen is heated in a Hestream

Fig. 9: Result of TG-DSC measurement when thecommercial equipment specimen is heated in a He stream
and an air stream

Table 2: Released proportion of C-14 during thethermal decomposition test of the hardly solubleparticles

4. Discussion

Based on the result of the SEM/EDX analysis, it was
found that a metal oxide produced through corrosion of
the plant’s components, and flaked crud whose main
component is carbon, were captured in the coolant filterof the PWR plant. Such flaked crud has also beenreported in past studies conducted at the Callaway Plant
in the United States and PWR plants in Japan.(2)Therefore, this flaked crud whose main component iscarbon is considered to have originated in a common
source that could be produced also at PWR plants thathave not experienced zinc injection.
In the analysis with FT-IR, absorption peaks thatcharacterize a carboxyl group and a hydroxyl groupwere observed. For the specimen analyzed as areference in the analysis, hardly soluble particles thatwere adjusted from the ion exchange resin while
simulating the RCS environment of the PWR to theextent possible (the actual in-core temperature andradiation environment were not reproduced) were used.
The FT-IR spectra of both specimens have common
characteristics and this result indicates that part of adeteriorated product of the ion exchange resin iscaptured in the coolant filter of the PWR plant.
In the analysis with TG/MS, an organic component
having an aromatic ring structure was observed in thecomponent released through heating in inert gases, andthis result supports that the particles captured in thecoolant filter of the PWR plant originate in the ion
exchange resin.
In the meantime, only little C-14 radioactivity wasdetected in the gas components with aromatic ringorganic matter released with TG/MS, and therefore, it isconsidered that C-14 was not induced in molecules that
constitute the framework of the ion exchange resin. It isconsidered that much C-14 is contained in thermallystable molecular structures, which are not contained inthermally decomposed components that are notassociated with oxidation reaction through heating ininert gases.
It has been confirmed in a TG analysis using acarrier gas containing oxygen that this C-14 is oxidized
to CO2 at temperature of 400 to 500C, and it has been
confirmed that C-14 is contained in flammableparticles.
With these results, it is considered likely that crudcontaining a carbon component that is contained inhardly soluble particles produced at the PWR is formed
when the iron exchange resin leaking from ademineralizer installed in the CVCS is altered in ahigh-radiation environment with high temperature,high pressure and reducibility in the reactor. C-14
produced in the reactor is taken in these hardly soluble
particles, and considered as being contained in particles
composed of more thermally stable components. In themeantime, if C-14 originating in impurity nitrogen isreleased into the reactor coolant in the form of crud in
addition to corrosion of metal material, crud containing
C-14 is also considered as a thermally stable molecule.
5. Conclusion

The result of a simulation using an ion exchange
resin indicates that the more exposed to the primaryenvironment of a PWR, the more amorphous form ofcarbon the resin is altered to. Since the hardly soluble
particles collected from the commercial equipment
include particles having aromatic ring structures that
constitute the framework of the ion exchange resin,
analysis data that support the result of the simulation
were obtained. In the meantime, a result indicating that
C-14 contained in hardly soluble particles produced at
the commercial PWR is not contained in molecules thathave aromatic ring structures forming the framework of
the iron exchange resin and are thermally decomposed

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanrelatively easily, but much C-14 is contained inamorphous carbon components exposed to the reactor
environment for a long period, was obtained. An
experiment result indicating that these molecules arethermally stable in a reducing atmosphere, and C-14 is
not easily released from their structures, was obtained.
To assess the migration behavior of C-14 in the burial
environment, our future study tasks will be to analyze
the surface characteristics of hardly soluble particles
containing C-14, and its stability, etc. in the actual
burial environment.
6. Acknowledgement
The authors are grateful to prof. K. Ishigure and prof. K.
Takao for insightful advices for our research.
7. References(1) J. A. Sawicki et al. Characterization of Corrosion
Products on the Callaway Cycle 9 PWR Core, EPRI,
Palo Alto, CA: 2001. 1003129.
(2) N, Higuchi et al. Characterization of C-14 in PWRRadioactive Waste. Journal of Power and Energy
Systems, Vol.2, No.1 2008. 215-220.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan