J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Evaluation Study of Source Term for
Severe Accident Analysis of Molten Salt Reactors
Michio YAMAWAKI*a, Yuji ARITAa, Ken-ichi FUKUMOTOa, Masami TAIRAb andRyota FUJIMURAb
Research Institute of Nuclear Engineering, Univ. of Fukui, Tsuruga, Fukui, 914-0055, Japan
Graduate School of Nuclear Power and Energy Safety Engineering, Univ. of Fukui, Fukui, 914-0055, Japanab(Received: 24 September 2014 / Accepted: 19 January 2015)

Source term for severe accident analysis of molten salt reactors (MSRs) has been investigated as part ofpreliminary studies to develop MSRs. As a severe accident of MSRs, exposure of heated fluoride fuel molten salt to
atmosphere was assumed to take place. Vaporization of fluoride molten salt was studied by means of the Knudsen
effusion mass spectrometry for pseudo-binary fluoride systems to clarify the behaviors of cesium and iodine in the
fluoride molten salt. Besides, corrosion of nickel-based Hastelloy-N, candidate alloy for the structural material of
MSRs, was investigated metallographically. In these studies, emphasis was laid on the interactions among molten
fluoride, fission products and atmosphere. From the studies, a lot of useful information related to the source termfor MSRs have been obtained. This work suggests how to tackle the problem to establish the source term for severe
accident analysis of MSRs.
Keywords: molten salt reactor(MSR), thorium, severe accident, source term, FLiNaK, fluoride, vapor pressure
measurement, Knudsen effusion mass spectrometry, Hastelloy-N alloy, corrosion

There have been considerable interest in molten saltbecause of their uses as coolant [1], as heat transfer media,
and as thermal storage media in many energy-related
applications, which are due to their high volumetric heat
capacity. MSRs use fluoride salts as nuclear fuel as wellas coolant, while High-temperature reactors cooled withfluoride salt (FHRs) use molten fluoride salts as coolant.
These salts have a high boiling temperature and highsolubility of many FPs. MSRs, original one wasdeveloped at Oak Ridge National Laboratory during
1950–1970, are intended for operation at 650°C or higher
temperatures[2, 3]. Under the program of Molten Salt
Reactor Experiment (MSRE), a 7.5 MW molten salt
reactor with uranium fuel dissolved in a molten fluoride
salt, was built and was operated at 650°C. Corrosion of
structural materials in molten fluoride salt is an important
issue in MSR and FHR operation [4]. Hastelloy-N, arelatively low Cr content alloy with Mo addition, wasdeveloped for the MSRE program, exclusively for
applications using molten fluoride salt. Its corrosion in
molten salt was strongly suppressed: corrosion depth at
560/700°C reached only ~100μm within nine years [3, 4].
This alloy must be code-certified for use at the hightemperatures required for MSRs and FHRs. Accordingly,
corrosion testing and evaluation of all materials in molten
salt at temperatures of MSRs and FHRs are required andare conducted as part of FHR development programs all

1. Introduction

It is an important subject to carry out theprocessing and disposal of the fuel debris takenout of the damaged Fukushima Daiichi Reactors aswell as the high level radioactive wastes
originating from the nuclear power plants.
Together with fast reactors and accelerator-driven
subcritical systems, which the Japanese
Government is supporting financially, molten salt
reactors (MSRs) should be also supported by theGovernment for their development. This isbecause MSRs are inherent safe reactors and canbe used to substantially decrease the amount ofhigh level radioactive waste including the fueldebris from the damaged Fukushima Reactors aswell or in better ways. Relevant fission products
such as cesium and iodine can be kept
thermodynamically stable in the fluoride molten
salt so that the severe accidents such as occurredin light water reactors will not take place in MSRs.
In this work, a hypothetical severe accident has
been assumed to occur in MSRs and thevaporization behavior of fluoride molten salt
containing fission product (FP), cesium or iodine,
was investigated as to the vapor species and vapor
pressures. Further, the corrosion of structural
material candidate, Hastelloy-N, by molten salt
has been investigated.

author’s e-mail: yamawaki@u-fukui.ac.jp

113©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors

equationover the world.
Resistance of structural materials against
deterioration due to a beyond-design-basis accident in a
nuclear power plant is a consideration in the developmentof nuclear reactor materials and in the design of advanced
nuclear reactors such as the molten salt breeder reactor in
the GEN-IV program. This property ensures safety andprevents severe accidents. For example, the cladding
material of a PWR should be requested to be ideallystrengthened to increase their melting point and to
suppress hydrogen gas formation during core melting due
to a LOCA incident. In the case of 㹋MSRs or FHRs,
emission of fuels and volatile FPs due to leakage ofmolten salt in a LOCA incident poses hazards. The
engineering design of a FHR should prevent extensive
damage caused by heavy corrosive attack during a severe
accident and prevent deterioration of the interaction
between structural materials and molten salt coolant in air.
In the present work, the effect of ambient conditions on
the corrosion behavior of structural material in molten
salt coolant containing FP elements was investigated toevaluate the safety of Hastelloy-N, a structural material
for MSR and FHR cores. The molten salt FLiNaK wasused instead of FLiBe in this work.

(1)
where V is the ionization cross section, J is the isotope
abundance ratio, S is the relative multiplier gain, 'E isthe electron beam energy minus the appearance potential,
T is the absolute temperature, and subscript i denotes themeasured gas species, respectively. The ionization
cross-section of Li, F, I and Cs atomic vapor species each
was taken from the report given by Mann [7]. The
relative multiplier gain was calculated by assuming theinverse proportionality to the square root of the mass ofthe vaporizing species [8].
2.2 Results and discussion
Gaseous species and ionization efficiency curves
Li+, LiF+ Li2F+, LiI+, I+ and I2+ were identified as the
predominant vapor species over LiF-LiI systems, while
Li+, LiF+ Li2F+ and Cs+ were detected over LiF-CsF
systems. The results are shown in Figs. 1 and 2. The
ionization efficiency curves for each species wereobtained for ionization energies from 10 to 30 eV. By
linear extrapolation of the ionization efficiency curves tozero intensity, the appearance potentials of the above
species were determined as shown in Table 1. To provide
a sufficiently high ion current, an electron ionizationenergy of 22 eV was chosen for the measurements.
From a thermodynamic calculation using MALT
database and "gem" program [9], the intrinsic gas species
are LiF and (LiF)2 for LiF, and LiI, (LiI)2 for LiI and CsF
and (CsF)2 for CsF. Therefore, Li+ (or Cs+) and Li2I+ wereconsidered to be created as follows:
LiI + e- → Li+ + I + 2e(LiI)2 + e- → Li2I+ + I + 2e-

2. Vaporization study of fluoride molten salt
Release of radioactive material to atmosphere incase of a hypothetical severe accident of MSRs such as
breaking of a main fuel pipe was studied from theviewpoint of vaporization behavior of molten salt mixture.
Identification of the vapor species and measurement of
the vapor pressure of molten salt mixture have been
performed with the high temperature mass spectrometry,
2.1 Experimental
Anhydrous salts of LiF, LiI and CsF with 99.99%
purity each were purchased from Kojundo Chemical
Laboratory Co. LTD. LiF-LiI (0, 25, 50, 70 and 100
mol% LiI) mixed salts and LiF-CsF (0, 25, 50, 75 and100 mol% CsF) mixed salts were prepared by mixing
each salt in a tungsten crucible and then heating at 973K
in vacuum.
The vapor pressures of the mixed salts were measured
using a quadrupole mass spectrometer (Pfeiffer Vacuum
QMG700) with a Knudsen cell [5, 6]. The Knudsen cell
was made of tungsten, and had an internal diameter of 8mm, and a height of 10 mm. The diameter of the effusion
orifice was 0.3 mm. The electron energy used to ionize
the gaseous species was set to 22 eV. The absolute vapor
pressure of each species was determined by comparing
the intensity of its ionic current (I ) with that of LiF vapor
over pure LiF. The correlation factor is denoted k ineq.(1). The partial pressure was calculated using the

Fig. 1

114

Vapor mass spectrum of LiF-LiI system

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors

Fig. 2

Fig. 3 Temperature dependencies of the vapor pressurescorresponding to Li+, LiF+, Li2F+, I+, LiI+, Li2I+ and I2+ over
LiF-70mol% LiI.

Vapor mass spectrum of LiF-CsF system

Table 1 Gaseous species and its appearance potentials.
Ion species
Li+
I+
LiF+
Li2F+
LiI+
I2+
Cs+
Cs2F+

appearance potential
(eV)
15.2
14.7
14.8
16.3
19.4
18.0
14.4
14.0

Vapor pressures
The temperature dependences of vapor pressures for
LiF-LiI and LiF-CsF systems are shown in Figs. 3 and 4,
respectively. The vapor pressures were increased withtemperature below 800 K. While, above 800 K, they weresaturated because of the limit of the ion current detector.
Li(g), LiF(g), (LiF)2(g), LiI(g) and (LiI)2(g) were themajor volatile species in the LiF-LiI molten salts. It
turned out that the fission product iodine evaporates
easily together with lithium. For LiF-CsF pseudo-binary
molten salt, Cs(g) and Li(g) were the major volatile
species. Cesium evaporates independently unlike iodine.
Total vapor pressure of iodine species, LiI(g), at 800
K was larger than that of cesium species, Cs(g), in twoorders of magnitude. Therefore, fission product iodine
may be released easily from fuel salt.

Fig. 4 Temperature dependencies of the vapor pressurescorresponding to Li+, LiF+, Li2F+, Cs+ and Cs2F+ over
LiF-75mol%CsF.
(3) Iodine was more volatile than cesium.
3. Corrosion study of hastelloy-n alloy in molten salt
Corrosion tests were performed on Hastelloy-N
specimen in FLiNaK molten salt at temperatures of 773
to 973K for 100 hours. After corrosion, cross sections of
the specimens were observed and measured withscanning
electronmicroscope
(SEM)
and
energy-dispersive X-ray spectrometry (EDS). Effects of
FPs and atmosphere were tested by adding CsI to molten
salt and by exposure of molten salt to air, respectively.

2.3 Conclusion of Section 2
From measuring the vapor pressure over LiF-LiIand LiF-CsF molten salts using mass spectrometry, thefollowing conclusions were obtained:(1) The predominant vapor species over LiF-LiI molten
salts were identified to be Li(g), LiF(g), (LiF)2(g), LiI(g)and (LiI)2(g).
(2) The predominant vapor species over LiF-CsF molten
salts were identified to be Cs(g) and Li(g).

3.1 Experimental
A rod specimen (1 × 1 × 20 mm) of nickel-based
Hastelloy-N alloy (Ni-7Cr-16Mo-5Fe (wt%)) was115

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors

microstructure of the sample were observed. Electron
diffraction method was performed in order to characterize
corrosion products observed with TEM.

prepared. The specimen surface was polished with sandpaper (~#15000). Granules of FLINAK (LiF/NaF/KF
ratio of 46.5:11.5:42 (wt%)) were also prepared. Figure 5
shows an illustration of the testing appaatus for molten

3.2 Results
Metallographic examination of Hastelloy-N tested
at 773 K showed little evidence of corrosive attack otherthan shallow surface roughening (Fig. 6). There was nodifference in the extent of corrosive attack on the surfacesof specimens in the alumina crucible and of those in thegraphite crucible. The relative depth of attack wasmeasured in the test done at 873 K. Figure 7 includes a
SEM image and EDS mapping images for the specimens
tested at 873 K. Chromium and molybdenum depletion
down to a 10 μm depth were observed in the EDS
mapping images. Subsurface voids were seen in the
microstructure of the specimens. The grain size of the
Hastelloy-N specimen was 20–30 μm, and there was no
typical intergranular corrosion of the metallic surface.
The depth of corrosion attack increased with increasing
test temperature up to 923 K. Figure 8 shows SEM and
EDS mapping images of a specimen immersed in molten
salt and those of a specimen in molten salt–cesium iodide
mixture at 923 K. The depth of corrosion attack in these
specimens reached 50 μm, with the depth in the specimen
in the graphite crucible being larger than that in thealumina crucible. There was no difference in the depths
of corrosive attack between the specimen immersed inpure molten salt and that immersed in the molten
salt–cesium iodide mixture. The average depth ofcorrosive attack was measured by metallographic
examination. Figure 9 shows the temperature dependenceof the depth of corrosive attack on the specimen surfaces
exposed to various conditions. In order to compare thecorrosion behavior of specimens in the molten salt
solution against that of specimens reported in previous
works, the corrosion rate was estimated from themeasured depth of attack. When corrosion proceeds
through an interfacial reaction, and when its reaction rate
is proportional to the corrosion time, ~1.1 μm depth ofcorrosive attack is equivalent to ~1 mm/yr of uniform
corrosion rate. TEM observation and EDS were
performed on a section of corroded area by extracting a
TEM sample from the deepest part of the site undercorrosion attack. Some grains of molten fluoride salt suchas those of the K3(Cr, Mo)F3 type with 0.81 ± 0.01 nm
lattice parameter could be identified in the section of
corroded area. These grains suggest that the solution ofchromium-enriched or molybdenum-enriched molten
fluoride salt accumulated on the corroded surface during
a long period and that chromium and molybdenum did
not quickly diffuse to the free surface during intergranularsalt corrosion. A 99.9% alumina crucible or a graphite
crucible and a quartz tube were annealed at 1173 K withdegassing in vacuum. All loading operations of theapparatus with a quartz tube, as shown in Fig. 5, werecarried out in an argon-filled glove bag. The specimen
and granules of molten salt were stacked in the alumina
crucible or graphite crucible (4–5 mm I × 27 mm). Someof the molten salt was stacked with cesium iodide at amolten salt/cesium iodide weight ratio of 10:1 toinvestigate the effect of FPs, which are to be generated inmolten salt containing nuclear fuel or waste fuel, on the
corrosion behavior of Hastelloy-N. A check valve was
attached on top of the quartz tube after the crucible was
loaded in the quartz tube. The closed quartz tube, which
was filled with argon gas, was taken from the argon-filled
glove bag. Some quartz tubes were filled with air toinvestigate the effect of air on corrosive attack of molten
salt on Hastelloy-N in conditions simulating severe
accidents. Afterward, the quartz tube was subjected to a
corrosion test at high temperature by heating it in anelectric furnace at 773 to 973 K for 100 h. After the test,
the specimen was embedded in conductive resin, which
was then cut with a cutting machine to observe the depth
profile of corroded areas in cross sections. The cut
surfaces were polished with sand paper (~#2000) andabrasive diamond grains (~0.25 μm) to prepare a milledsurface for cross-sectional observation. The surface crosssections were observed with SEM, and surface chemical
analysis was performed with EDS to obtain the depth
profile of the corroded area. Thin-film samples for TEM
observation were obtained from the corroded area by
focused-ion-beam method. Corrosion products in the
Valve
Rubber Tube
Specimen

Crucible

Quartz Tube
Thermocouple

Electric
Furnace

FLiNaK

Fig.5: Illustration of the corrosion test apparatus

116

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors

corrosion.
Figure 10 shows the depth of corrosive attack on
the surface of specimens tested in argon gas and in air at
823 K. Apparently, corrosive attack in air was more
extensive than that in argon gas. The depth of corrosion
attack on the specimen immersed in pure molten salt wasalmost the same as that on the specimen in the molten
salt–cesium iodide mixture (Fig. 10). This similarity
suggests that corrosive attack in air significantly affected
corrosion of Hastelloy-N in molten salt and that thesynergistic effect of added FPs on corrosion in molten
salt was negligible under these conditions.

(a)

SEM

(a)

(b)

Ni

Cr

(b)
K

30 μm

30 μm

Cs

Fig. 6: Cross-sectional images of Hastelloy-N specimen
immersed in molten salt at 773 K: (a) immersion test in
an alumina crucible and (b) immersion test in graphite
crucible.

Fig .8: SEM and EDS mapping images of Hastelloy-N
immersed in (a) molten salt and in (b) a molten
salt–cesium iodide mixture tested at 923 K.

SEM

Depth of corrosive attack(μm)

Immersion test in an alumina crucible at 873K

K

60

Alumina Crucible
Graphite Crucible

40

Alumina Cruciblewith CsI

20

0

773K

823K

873K

923K

Fig. 9: Plot of the depth of corrosive attack versus test
temperature.

Cr

Depth of corrosive
attack (μm)

Ni
30 μm

Fig. 7: Cross-sectional images of Hastelloy-N immersedin molten salt at 873 K. (a) SEM image and (b)–(d)element mapping images for K, Ni, and Cr, respectively,from EDS.

15823K
Graphite Crucible

10

5
0

Ar atmosphere
FLiNaKAr atmosphere
FLiNaK䠇CsI

Air atmosphere
FLiNaK

Fig. 10: Depth of corrosive attack on Hastelloy-N in puremolten salt under air (a) and under Ar gas (b) and on
Hastelloy-N in a molten salt–cesium iodide-mixture
under Ar gas (c).

117

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors

3.3 Discussion15]. The additional effect of tellurium in molten salt on

Fluoride salts constituting the molten salt are morecorrosive attack on Hastelloy-N should thus be studied inthermodynamically stable than are fluorides of the metals

the future.
Corrosion in molten salts includes dissimilarmaterial corrosion in which differences in elemental
activity between various materials in the molten salt drive
deterioration. Corrosion of stainless steel in molten
fluoride salt in graphite crucibles is significantly greaterthan that in stainless-steel crucibles [4]. In this work,
there was no evidence of surface roughening of thealumina crucible after the immersion test, and thespecimen was stable against corrosion in molten salt. The
extent of surface corrosion of the specimen in molten salt
in the graphite crucible was greater than that of thespecimen in the alumina crucible, similar to theobservation in a previous report [4, 15]. Graphite acts as a
reductant for moisture and gas impurities in the molten
salt solution and promotes the chemical interaction
between the molten salt and specimen surface. Previous
studies [16, 17] showed the acceleration of corrosion
within corrosion capsules in a graphite crucible. This
acceleration is due to the tendency of FLiNaK to cause
chromium to take on multiple valence states, as well as
the difference in chemical activities between theelectronegative metal chromium in the alloy and thegraphite crucible and components. When adding FPs in
simulations of corrosion behavior in molten salt fuelcontaining nuclear waste, the effect of the crucible must
be considered to ensure that the essential chemicalinteraction between molten salt and materials is not
disrupted.

constituting the structural alloys; thus, most structural
alloys are stable in molten fluoride salts. Nickel, inparticular, is relatively resistant to corrosion in molten
fluoride salts. Previous works showed that impurities
such as moisture could drastically accelerate corrosion
via formation of hydrogen fluoride [4]. The effect of
impurities in the molten fluoride salt on corrosion could
be expressed in the following chemical reactions:
2NaF + H2O → Na2O + 2HF
2LiF + H2O → Li2O + 2HF
2HF + Cr → CrF2 + H2
NiF2 + Cr → CrF2 + Ni
In the case of FLiNaK, water could react withsodium fluoride and lithium fluoride in the salt andproduce HF gas, which could leach chromium fluorides
in the alloy into the salt. Impurities such as molybdenum
or iron in salts for nickel-based matrixes can also promotedissolution of chromium into the salt. The present study
differs from previous works in terms of the molten
fluoride

salt

used

(e.g.,

LiF–BeF2

systemvs.

LiF–NaF–KF system) and the experimental procedure(e.g., static immersion test vs. thermal-convection looptest). However, the same tendency of corrosion behavior
involving thermal interaction between molten fluoride
salt and Hastelloy-N could be reproduced in this work.
The extent of corrosion of Hastelloy-N in molten
salt under air was significantly greater than that in argon
gas. Oxidation of molten fluoride salt probably caused

3.4 Conclusion of section 3
Effects of air on the corrosion of Hastelloy-N alloy
in molten salt containing FP elements were investigatedto evaluate the behavior of structural materials on asevere accident of MSR or FHR. Corrosion tests on
Hastelloy-N in the molten fluoride salt FLiNaK were
performed. The tests were performed at 773 to 923 K for
100 h in alumina crucible or graphite crucible underargon gas or under air. Metallographic examination of
Hastelloy-N tested at 773 K showed little evidence ofcorrosive attack other than shallow surface roughening.
The depth of corrosive attack, as well as the extent ofchromium and molybdenum depletion, increased withincreasing temperature. The extent of Hastelloy-N
corrosion in molten salt under air was significantly
greater than that in argon gas. The effect of cesium iodide
added to molten salt on corrosion in a simulated test on
the molten salt containing nuclear fuel or waste fuel was
negligible. When adding FPs in corrosion test on molten

loss of electrical neutrality and enhancement of thechemical activity of the surface reaction. In severe
accidents such as leakage of molten salt into the primarycoolant pipe, these lead to extensive, beyond-design-basis
corrosion of the pipe surface. Addition of the FP cesium
iodide to molten salt did not influence the corrosion
attack on the microstructure of the specimen surface bymolten salt. Some cesium remained in the molten salt
phase at microstructural examination; only cesium or
cesium iodide did not attack the surface directly. The
synergistic effect of FPs such as cesium iodide on
corrosion of the specimen surface in molten salt was notapparent; thus, the effect of the interaction between FPs
and molten salt on corrosion of Hastelloy-N was
negligible. However, tellurium or cesium telluride wasreported to heavily attack the surface of stainless steel
and of nickel-based alloy used in MFSR operation [14,118

M. Yamawaki et al., Evaluation Study of Source Term for Severe Accident Analysis of Molten Salt Reactors[12] L. C. Olsen, J. W. Ambrosek, K. Sridharan, M. H.
Andersen and T. R. Allen, “Materials corrosion in molten
LiF–NaF–KF salt”, J. Fluorine Chem., 130 67-73 (2009)[13] I. Ozeryanaya, “Corrosion of metals by molten salts
in heat treatment process”, Met. Sci. Heat Treat. 3 (1985)
184–188.

salt fuel, the effect of the crucible must be considered so
as not to disrupt the essential chemical interaction
between the molten salt and material.
4. Overall summary
Source term for severe accident of MSRs was
tackled by performing a vaporization study and corrosion
study of fluoride molten salt. From the vaporization study,
vapor species and vapor pressures of Cs- and I-containing
species were identified and measured. From the corrosion
study, effect of temperature, atmosphere (air, crucible)
and FPs on corrosion attack of Hastelloy-N alloy byfluoride molten salt were determined. Results of thisstudy suggest how to evaluate the source term for severe
accident analysis of MSRs.
Acknowledgements
The study was partially supported by Chubu ElectricPower Co.,Inc.
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