A technique for detection of WWER fuel failures by activity of Xe radionuclides during reactor operation
Nuclear Energy and Technology
A technique for detection of WWER fuel failures by activity of Xe radionuclides during reactor operation*
Peter M. Kalinichev 0
Igor A. Evdokimov 0
Vladimir V. Likhanskii 0
0 State Research Center of the Russian Federation ?Troitsk Institute for Innovation & Fusion Research? , bld. 12 Pushkovykh St., Troitsk, Moscow, 108840 , Russia
Fuel failures during operation of Nuclear Power Plants (NPPs) may lead to substantial economic losses. Negative effects of reactor operation with leaking fuel in the core may be reduced if fuel failures are detected in due time of the cycle. At present time, the ratio of the normalized release rates of 131I and 134I is used to detect fuel failures in WWERs during steady state operation. However, based on the activity of iodine radionuclides, it is not always possible to detect the fuel failure. This situation may occur in case of a small defect in cladding of a leaking fuel rod or for high burnup fuel if the defect is overlapped by the surface of the fuel pellet. If it is so, fuel deposits may be the dominant contributor to iodine activity, and the fuel failure may not be noticeable. In PWRs, fuel failures are detected by activity of radioactive noble gases. Noble gases are not adsorbed on cladding inner surface, as distinct from iodine radionuclides. Release of noble gases from the leaking fuel rod may be considerable even though defect in cladding is small. In this paper, a technique is proposed for detection of fuel failures at WWER reactors by activity of radioactive noble gases in the primary coolant. It is shown that radioactive noble gases may be a more sensitive indicator of fuel failures than iodine radionuclides. Detection of fuel failures is based on monitoring of the ratio between 133Xe and 135Xe activity. Some examples of practical applications are given.
Fuel failures may occur during operation of Nuclear
Power Plants (NPPs). This may lead to substantial
financial losses. Monitoring and analysis of primary coolant
activity is used to ensure the radiation safety of power
units and to mitigate the adverse effects of fuel failures
(Shumkova et al. 2003, Burman 1991, Beyer 1991, Parrat
The criterion of fuel failure detection using iodine
radionuclides under steady state operation conditions (see RD
(RD JeO 188.8.131.52.0521-2009 2016)
) is based on the
concept of the normalized release rate A*:
et al. 2003, El-Jaby et al. 2010, Likhanskiy et al. 2004, The procedure used to detect
Oliver et al. 2017, Slavyagin et al. 2003). The prime task fuel failures based in iodine
of coolant activity analysis is prompt identification of
fuel failures. radionuclides
According to the regulatory document (RD)
, if a spike event for
activity of the reference radionuclides is absent, fuel
failures should be detected by the ratio of the normalized
release rates between 131I and 134I during steady-state
reactor operation. However, fuel failures cannot be
always clearly identified based on activity of iodine ra- A* = A(? + ?)/(?Y),
dionuclides. Practice shows that this is possible when
the hydraulic resistance of the leaking fuel rod is high.
High resistance occurs in case of a small defect in fuel
cladding or when fuel burnup is high so that the defect
is overlapped by the surface of the fuel pellet. In this
case, the biggest contributor to the iodine activity may
be fuel deposits (tramp uranium contamination), and the
fuel failures may remain unnoticeable against the
background activity level.
Activities of radioactive noble gases (NGs) are used
for more reliable detection of fuel failures in PWR
reactors. Noble gases do not interact with the fuel cladding
(unlike surfactant iodine radionuclides). If much of the
iodine is adsorbed on the inner surface of the fuel cladding,
the release of the iodine radionuclides into the coolant,
with all other things being equal, turns out to be smaller
than the release of noble fission gases. The release of
noble gases from a leaking fuel rod is detectable even when
its hydraulic resistance is high.
In France, the fulfillment of one of the following
conditions is taken as the criterion of the fuel failure
during steady-state operation of PWR reactors: 1) the
absolute value of the 133Xe activity is higher than 106
Bq/kg, or 2) the ratio of the 133Xe and 135Xe activities is
higher than 0.9 with the absolute activity of 133Xe being dNI = RI + RI ? (?I + ?I )NI + ?TeNTe,
in excess of 1.85?105 Bq/kg
(The IAEA nuclear energy dt
The criterion given in
(The IAEA nuclear energy series dNTe = RTe + RTe ? ?TeNTe.
cannot be used unchanged for WWER reactors for dt
the following reasons. First, the absolute values and the
ratios of the NG activities depend on the rate of the gas
removal from the coolant (the rate of coolant degassing).
The coolant degassing parameters are different in PWR
and WWER reactors. Moreover, the rate of NG removal
from the coolant may differ markedly among WWER
units. Second, unlike PWR reactors, WWER power units
do not currently use in practice coolant sampling at the
reactor pressure. The gases are partially lost from the
coolant samples with the existing sampling procedures, and
the results of measuring the absolute NG activities may
fail to be representative.
Below, a technique is proposed which makes it possible
to detect fuel failures by NG activities during operation of
WWER power units. The advantages of this technique are
discussed against the criterion in
based on the iodine activities. Some practical
application examples are presented as well.
where A is the measured activity; ? is the nuclide decay
constant; ? is the rate of the radionuclide removal from
the coolant by the letdown filters; Y is the cumulative
radionuclide yield per fission which is composed of the
intrinsic yield y and the cumulative yield of the precursor
According to RD
(RD JeO 184.108.40.206.0521-2009 2016)
a conclusion concerning the fuel failure can be made
under the steady-state operation conditions in the event the
ratio of the normalized release rates for 131I and 134I starts
to exceed the threshold value. This value is equal to five
for WWER-1000 reactors.
The key concepts which make it possible to obtain the
criterion of type
(RD JeO 220.127.116.11.0521-2009 2016)
based on iodine activity are described below. After that, the
behavior of the noble fission gases in the coolant is
considered in the same approximations.
Behavior of iodine radionuclides in the primary
circuit. Balance equations
(Slavyagin et al. 2003a)
used to describe behavior of fission products (FPs) in the
primary coolant, which can be written for the iodine
radionuclides and their tellurium precursors as follows
Here, R and R are the rate of the FP release into the
coolant from fuel deposits and leaking fuel rod,
respectively; NI is the number of the iodine radionuclide atoms
in the primary coolant, and NTe is the number of tellurium
atoms in the primary circuit. Tellurium is more
superficially active than iodine, so it is assumed in equations (2),
(3) that tellurium is adsorbed on the structural surfaces
in the reactor core and is not removed from the primary
circuit as the coolant flows through the letdown filters.
The presence of the tellurium precursor is important to
take into account for 132I and 134I. It can be taken for 131I,
133I, and 135I that the precursor decays practically
If the reactor operates in a steady-state mode,
steady-state solution of equations (2) ? (3) can be used.
In these conditions, the normalized release rate (1)
defines the intensity of the FP source in the coolant. For each
iodine radionuclide, it is equal to the release rate into the
coolant divided by the probability of its formation
(cumulative yield) per one fission of heavy nucleus.
Release of iodine radionuclides from fuel deposits.
With no fuel failures in the reactor, the coolant activity is
defined by the FP release from fuel deposits. Fuel
deposits are uranium dust that builds up on the outer cladding
surface during fabrication and/or is accumulated inside
fuel assemblies when fuel is washed out from leaking fuel
rods during operation at NPPs.
One of the key mechanisms for the nuclide release from
fuel deposits is the direct escape of fission fragments into
the coolant. Another mechanism occurs if the fuel deposits
include particles with a size larger than the fission
fragment range in UO . In this case, fission fragments can stick
in fuel and escape further into the coolant by the radiation
transport mechanism. For the reference iodine
radionuclides, this mechanism is reduced to ?radiation-induced?
. The rate R of the FP release from
deposits is composed of the direct fission fragment escape
rate (recoil rate) Rr and the diffusive release rate RD:
R = Rr + RD.
exposed fuel surface; d? is the mean pore diameter in fuel
deposits; r? is the fission fragment range in the
pore-filling fluid; and k1 is the coefficient allowing for the pore
geometry. For a spherical pore with k1 = 2/3, the value
r? depends on what is filling the pore. If the pore is filled
with water with a pressure of 160 atm and a temperature
of about 600 K, the fission fragment range is about 3?10?5
m. If the pore is filled with gas, the fission fragment range
will be larger.
The closing term in the right-hand side of (7)
describes the diffusive release from fuel deposits. Here, k2 is the
coefficient of the order of unity taking into account the
geometry of the fuel particles; h = (D/l)1/2 is the
diffusion length; and D is the diffusion coefficient. According
(Turnbull and Friskney 1982)
, the diffusion length h
of the reference iodine radionuclides with a fuel
temperature of below 600 K is less than 10?7 m. The range rf of
heavy fission fragments (in particular, iodine) in fuel is
about 5?10?6 m
, which is much larger than
the diffusion length h. So the diffusion term in equation
(7) becomes important only with the developed surface of
the fuel particles in deposits (St >> Sg).
The normalized release rates of 131I, 133I, and 135I can be
Particles of fuel deposits may have a developed surface
(this is possible, e.g., for pieces of high burnup fuel
pellets that have entered the coolant) (White 2001, Olander A* = R/Y = Jf(h). (8)
1976). We shall assume that the developed surface has
been caused by the presence of open pores. Some fission In the case of ?even? iodine radionuclides, the
precursfragments can enter open pores near surface. This results or lifetime cannot be ignored and the release of 132Te and
in an increased rate of the radionuclide release into the 134Te needs to be considered separately from the iodine
recoolant. The expression for the recoil release rate was ob- lease. The release of tellurium is described by expression
. The model for the diffusive release (5). For 132I and 134I, the intrinsic yield y is used instead of
of nuclides with a short-lived precursor is described in the cumulative yield Y to describe the direct escape, and
. The values of the radiation-induced diffu- the diffusion release rate has the form
sion coefficients for radionuclides were determined in
experiments (Turnbull and Friskney 1982, Rossiter and RD = k2StFh(y + Yph/(h + hp)). (9)
With regard for the results in
(Wise 1988, Wise 1985)
, In this case, the function g(h, hp), similar to the function
the FP release from fuel deposits for nuclides with a short- f(h), may be introduced, and the rate of the radionuclide
lived precursor, such as 131I, 133I, and 135I, can be represen- release into the coolant can be written as
ted as follows
R = YJf(h),
J = erf SgF,
where rf is the fission fragment range in fuel; F is the
fission rate in fuel deposits in the core; e is the release
efficiency; and Sg is the ?geometrical? area of the fuel surface
(with no regard for open pores). The value e is defined
by the geometry and lies in a range of 0 < e ? 0.5 (Wise
1985). The function f in expression (5) has the form
f(h) = 1 + k1StDc/(SgRc) + k2Sth/(eSgrf).
The second term in the right-hand side of (7) describes
the increase in the release rate due to the fission fragment
?sticking? in open pores. Here, St is the total area of the
R = yJg(h, hp),
g(h, hp) = f(h) + k2hStYph/(erfSgy(h + hp)).
The normalized iodine release rate, with regard for (2),
(3), and (10), has the form
A*=(R+Rp)/(y+Yp) = Jf(h)+Ypk2Sthp2/(YeSgrf(h+hp)). (12)
In the event of a ?short-lived? precursor (hp >> 0),
equation (12) goes over to (8). If the diffusion length hp
cannot be neglected, as compared with h, then, as can be
seen from (12), the contribution of the precursor nuclide
can increase the normalized release rate markedly.
If fuel deposits consist of particles smaller than the
fission fragment range, the radionuclide release can be
considered in a ?monolayer? approximation
. In equations (7) and (11), this where Rf is the FP release rate from fuel pellets to voids
corresponds to the formal transition k1 >> 0 and k2 >> 0 inside the fuel rod; and ? is the escape rate coefficient for
which gives f = g = 1. In this case, the calculated ratios of the given radionuclide from fuel rod into the coolant. The
the normalized iodine release rates should be equal to uni- escape rate coefficient relates the rate of the radionuclide
ty (see the dashed line in Fig. 1). In practice, minor devi- release into the coolant to the total inventory of this
radiations from this value may be observed. This is explained onuclide inside the fuel rod
(Slavyagin et al. 2003a)
by the fact that the value Y depends on the nuclide com- value ? depends on the characteristics of leaking fuel, the
position of the fuel in deposits which is, generally spea- decay constant, and the radionuclide?s chemical
properking, unknown. As a rule, the normalized release rates ties. For surfactant nuclides (iodine, in particular), ? can
are calculated based on radiation measurement results by be much smaller than for the noble gases in the same
leapostulating a certain nuclide composition of deposits. For king fuel rod.
instance, it is accepted in (RD JeO 18.104.22.168.0521-2009 When plugging expression (13) into equations (2), (3),
2016) that only 235U is fissionable in deposits. The diffe- and (1), one can see that a fuel failure may cause the ratio
rence of the assumed from the actual nuclide composition of the normalized 131I and 134I activities to increase
marleads to variation of the ratio between A* for the iodine kedly. This circumstance is used to detect the fuel failure
radionuclides in the limits of up to 30%. in the reactor.
If fuel deposits consist of large-size particles, the ratios According to RD
(RD JeO 22.214.171.124.0521-2009 2016)
of the normalized release rates (12) due to the diffusive the region above the solid curve in Fig. 1 corresponds to
release may deviate markedly from unity. RD
(RD JeO the presence of leaking fuel in the reactor. If the data lie in
presents an upper expert estimate the region between the solid and the dashed curves in Fig.
for the respective ratios of normalized activities (see Fig. 1). 1, the iodine activity can be defined by FP release both
An analog of the dependence from RD
(RD JeO from leaking fuel and from fuel deposits. For this
for large-size particles in the de- on, no firm conclusion can be made as to the presence or
posits can be obtained from equations (1) ? (12). Values of absence of leaking fuel in the core proceeding only from
intrinsic and cumulative yields, as well as decay constants the iodine activities
(RD JeO 126.96.36.199.0521-2009 2016)
from the database in
(Tasaka Kanj et al. 1990)
throughout for the calculations. The highest values for the
ratio of the normalized release rates of 131I to 134I are achie- Noble gas activities as a means for
ved in the event of the diffusive mechanism contributing fuel failure detection
predominantly to the iodine release from fuel deposits.
(RD JeO 188.8.131.52.0521-2009 2016)
obtained if it is assumed in equations (8) and (12) that F = 4?1019
m?3s?1, St/Sg = 100, and St /Sg ? dc << rc. Such parameters can
be achieved if fuel deposits have compacted in the core to
form a structure with a large number of small open pores.
The result of the respective calculation is shown in Fig. 1.
Analysis of iodine normalized release rates and its
constraints. If there is a fuel failure in the reactor, the
source of each radionuclide Rc can be written as
Differences in xenon and iodine behavior. It follows
from equation (13) that the contribution of the leaking
fuel to the cumulative FP source in the coolant is defined
by the ratio between the decay constant ? and the escape
rate coefficient ?. The 133Xe and 131I decay constants are
(Tasaka Kanj et al. 1990)
. If chemical properties of
xenon and iodine were similar, a marked deviation of the
133Xe and 131I activity from the background during fuel
failure would be observed simultaneously. However, due
to iodine adsorption on the cladding inner surface, in a
certain range of the failed fuel characteristics it is possible
that 131I activity is still defined by its release from fuel
deposits but 133Xe activity already exceeds the background
level significantly. The width of this range is the larger
the more intensively iodine is adsorbed inside the leaking
fuel rod (the temperature near the defect is assumed to be
higher than the water saturation temperature; if the defect
is in the fuel?s ?cold? zone and is covered with water,
iodine can dissolve in it and be carried out of the failed fuel
rather intensively). Thanks to such peculiarities, the NG
activities can be a more sensitive failure indicator than the
For efficient detection of fuel failures based on the
ratio of the NG activities, it is desired to use a pair of
gaseous FPs with the following properties.
? The lifetime of the selected FPs should differ
greatly. In this case, much of the short-lived gas will
deRc = R + R = R + Rf ?/(? + ?),
cay having no time to leave the leaking fuel rod, as
the activity of the long-lived gas will grow
markedly after the failure.
? The radiation yields of the selected gases should
depend weakly on the fuel?s nuclide composition. As
the burn-up of fuel deposits is normally unknown,
the use of radionuclides with the release depending
strongly on the fuel?s nuclide composition may lead
to major uncertainties. In particular, this is the reason
for 85mKr and 88Kr being not suitable for analysis.
? The nuclides used should be isotopes of one
chemical element. A part of the gases dissolved in the
coolant changes to a gas phase and is lost in the
sampling process. Since gases have different
solubility, the gas losses during sampling vary for
isotopes of different chemical elements.
The pair of 133Xe and 135Xe satisfies to these conditions
among the NGs whose activities are monitored at NPPs.
Release of xenon radionuclides from fuel deposits.
To describe the activities of 133Xe and 135Xe in the primary
circuit, it is required to take into account the behavior of
the precursor iodines, 133I and 135I respectively. The
balance equation for the xenon radionuclides in the primary
circuit has the following form
dNXe = RXe + RXe ? (?Xe + ?Xe )NXe + (?I + B?I )NI.
The right-hand side of equation (16) may be dealt with
as a family of curves G(?Xe) depending only on the xenon
removal rate. The largest value of the function G, with
the fixed value ?Xe, is reached if the ?diffusion? terms in
the functions f and g are the predominant contributors.
The parameters, with which this condition is fulfilled, are
described above. The solid line in Fig. 2 shows the upper
envelope for the considered family of curves G(?Xe). The
following parameters were varied to plot this with each
value of the xenon removal rate: water flow rate to the
filters, iodine removal efficiency, and nuclide composition
of fuel deposits.
The solid line in Fig. 2 can be looked upon as the upper
limit for the value of the potential ratio of the 133Xe and
135Xe activities if the only FP source in the primary circuit
is fuel deposits.
Determination of the NG removal rate. The absolute
values and the ratios of the gaseous FP activities depend
on the degassing rate.
No measurement of degassing parameters is stipulated
by the actual WWER regulations. The degassing
parameters can be determined by measuring the NG activities
downstream and upstream of the degasser. In this case, the
degassing efficiency a in (15) is calculated from the formula
? = A2Xe/A1Xe,
where A1Xe, A2Xe are the activities of xenon downstream
Designations as in equation (2) are used here; the coef- and upstream of the degasser respectively.
ficient B allows for xenon which enters the core due to The NG activities downstream and upstream of the
decay of precursor iodine in the letdown filters. It is as- degasser are measured, e.g. at the Temelin NPP (twice a
sumed that the ion-exchange filters and the degasser are month).
connected in series in the letdown system, and the coolant A technique to detect fuel failures based on the
flow rate through these is identical. The cleaning rate ?Xe xenon radionuclide activity. Fig. 2 shows the
?threshcan be written in the form old? dependences of the ratios between the 133Xe and
135Xe activities on the degassing rate. The dashed line
?Xe = Q?/M, (15) represents the ?monolayer? approximation, and the solid
line represents fuel deposits comprising large fuel
partiwhere Q is the mass flow rate of water to the ion-ex- cles with a developed surface. If the ratio of the measured
change filters and the degasser; M is the mass of the water xenon activities turns out to be above the solid line, it can
circulating in the primary circuit; and ? is the degassing be stated that there is a fuel failure in the reactor. If the
efficiency assuming the values of zero to unity. The coef- activity measurement results fall within the region below
ficient B relates to the degassing efficiency a as B = 1 ? ?. the solid line, then, proceeding only from the Xe
The behavior of the precursor iodine in (14) is
described by equation (2). If the reactor operates in a steady-state
mode, a steady-state solution can be used for equation (14).
For the xenon radionuclides, as well as for iodines, the
rate of release from fuel deposits can be described using
the functions f and g (see above). It follows from equation
(14) that the ratio between the 133Xe and 135Xe activities
in the reactor, when there is no fuel failure, has the form
yXe-133g (hXe-133, hI-133 ) + ?I-133 + B?I YI-133 f (hI-133 )
?I-133 + ?I
clide activities, no clear answer can be given. In this case,
it is required to analyze other factors: ratios of the
normalized release rates for iodine radionuclides, presence of a
spiking event, and presence of activity escalations during
steady-state reactor operation.
The fuel failure detection procedures can be
formulated as follows.
1. The rate of the xenon removal from the coolant hXe
is determined (see above). If the removal rate has
not been measured, a conservative estimate is used:
hXe = 0.
2. Using the diagram in Fig. 2 (the solid line), the
threshold value is determined for the ratio between
the 133Xe and 135Xe activities corresponding to the
3. The obtained value is compared with the ratio
between the measured 133Xe and 135Xe activities. If the
ratio between the measured 133Xe and 135Xe
activities turns out to be higher than the threshold, this
indicates that there is a fuel failure in the core.
Constraints. The ratio between the 133Xe and 135Xe
activities after the fuel failure depends non-monotonically
on the escape rate coefficient m for the leaking fuel rod.
After the fuel failure, this ratio starts to grow if m
increases. The growth continues for as long as the relatively
longlived 133Xe releases predominantly from the fuel, and the
135Xe activity is still defined by the background. As the
extent of damage to fuel cladding progresses, the time of
the radionuclide ?delay? inside the leaking rod prior to
the release into the coolant starts to decrease. When 135Xe
starts to go out of the leaking fuel in a noticeable quantity,
the ratio between the 133Xe and 135Xe activities starts to
In the event of a severe failure (e.g., when secondary
defects are formed in the failed fuel cladding), the time of
?delay? may become smaller than the lifetime of 135Xe. In
the limiting case of severe cladding degradation, the 133Xe
and 135Xe release from the leaking fuel rod will be such as
if there is no cladding at all and there is a direct contact
of fuel pellets with the coolant. If it is so, there will be no
difference between the FP release from the leaking fuel
with a low power and the FP release from a large mass of
fuel deposits. For high power leaking fuel, when
diffusion is a major contributor to the FP release from fuel
lets, the ratio between the 133Xe and 135Xe activities in the
coolant may also fall within the ?uncertainty zone? shown
in Fig. 2 (between the solid and the dashed lines). Then,
using only the ratio between the 133Xe and 135Xe activities
in some measurements, it is not possible to detect the fuel
failure in the reactor, so other fuel failure symptoms need
to be used.
Table 1 presents average values for the ratios between
the steady-state 133Xe and 135Xe background activities for
several cycles with no fuel failures in different
WWER1000 power units. It can be seen that all of the values are
below the solid line in Fig. 2. This is as it should be when
there is no fuel failure.
Table 2 presents the maximum weekly average ratios
of the 133Xe and 135Xe activities during steady-state reactor
operation in fuel cycles with fuel failures clearly
identified based on other symptoms. It can be seen that all of
the values are above the solid curve. On the whole, the
analysis of the WWER-1000 NPP fuel cycles for the past
10 years shows that, in most cases when fuel failures were
confirmed by the ratio of the normalized release rates foe
131I and 134I, the ratio of the 133Xe and 135Xe activities also
turned out to be higher than the threshold dependence in
One of the examples to illustrate higher sensitivity of
the proposed procedure is cycle N-1. Fig. 3 shows the
ratio between the normalized release rates of 131I and 134I
calculated based on the data on iodine activity in the nominal
mode of the reactor operation. This figure also shows how
the ratio of the 133Xe and 135Xe activities varied during the
cycle. As xenon removal rate from the coolant was not
monitored in this unit, the analysis used the conservative
thershold value for the ratio of the 133Xe and 135Xe
activities, which corresponds to the zero hXe value.
It can be seen from Fig. 3 that the ratio of the
normalized release rates for 131I and 134I did not exceed five.
Accordingly, as shown in RD
(Tasaka Kanj et al. 1990)
no firm conclusion could be made based on the 131I and 134I
activities up to the end of cycle as to the presence of a fuel
failure in the reactor core. However, the ratio of the 133Xe
and 135Xe activities started to grow approximately on the
250th day and was markedly in excess of the threshold
value. This clearly indicates that there was leaking fuel
in the core.
One leaking FA was found after cycle N-1 was over.
Therefore, the analysis of the NG activities made it
possible to detect the fuel failure promptly.
The paper proposes the technique to detect fuel failures
in WWER units based on the activity of radioactive
noble gases in the reactor primary circuit. It has been shown
that the activity of noble gases can be a more reliable
indicator of a fuel failure than the activities of iodine
radionuclides. The proposed fuel failure detection criterion
is based on analysis of the ratio between the 133Xe and
The procedure has a low sensitivity to the primary
coolant sampling technique and to the extent of the water
degassing during sampling, as well as to the actual
nuclide composition of fuel deposits.
Examples of several fuel cycles show that, when there
is no fuel failure in a WWER-1000 NPP, the ratio of the
133Xe and 135Xe activities is below the boundary as it
follows from the obtained criterion. If there is a fuel failure,
the ratio of the 133Xe and 135Xe activities turns out to be
above the criterion boundary in most cases.
It has been also demonstrated that the proposed
criterion is capable to detect fuel failures during steady-state
reactor operation even if the 131I and 134I activities do not
make it possible to make a firm conclusion on the
presence or absence of leaking fuel in the core.
The authors express their gratitude to Oleg
Vladimirovich Khoruzhy for the valuable discussion of the findings.
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