Evaluation Of Reactor Coolant System Specific Activity To
Determine Fuel Integrity At NPP Krško
Martin Chambers
Nuklearna Elektrarna Krško
Vrbina 12
8270 Krško, Slovenia
[email protected]
Dejvi Kadivnik, Bojan Kurinčič
Nuklearna Elektrarna Krško
Vrbina 12
8270 Krško, Slovenia
[email protected], [email protected]
ABSTRACT
In recent fuel cycles NPP Krško experienced some leaking fuel rods that degraded during the
operational cycle. The leaking fuel rods were tight in nature thus overall Fuel Reliability
Index was not significantly impacted. In all cycles, leaking fuel was initially determined by
evaluation of the Reactor Coolant System (RCS) isotopic specific activity. Typically, the first
indication of leaking fuel is observed in RCS specific activity of noble gas isotopes,
particularly the Xe-133 isotope that will increase by more than an order of magnitude when a
fuel rod is leaking. Using the RCS isotopic activity evaluation during the cycle, it is usually
possible to determine other properties of the leaking fuel rod(s) such as the fuel rod defect
size, failure mechanism, relative power, core position and burnup. Here, we present the RCS
chemistry evaluation and explanation of how the attributes of leaking fuel were determined
for NEK operational cycle 25. The RCS chemical evaluation results are compared to number
of leaking fuel assemblies determined by fuel examinations (telescopic sipping, visual fuel
assembly and ultrasonic fuel rod inspections) during and after core offload.
1
INTRODUCTION
Nuclear power plant Krško (NEK) is required to monitor fuel integrity during operation
according to NEK technical specifications [1] and procedures [2]. The (WANO performance
indicator) Fuel Reliability Indicator is used for comparison between plants in similar reactor
types (PWR) with the aim of operation without fuel defects. In addition, leaking fuel requires
significant plant resources to determine and correct. Outage doses may be increased affecting
outage maintenance or modification activities. A significant amount of resources are utilised
to determine the failed fuel with Telescopic sipping, ultrasonic fuel rod inspections, visual
inspections and repair the fuel assembly (replacing the leaking fuel rods with a stainless steel
filler rods) as well as corrective actions and analyses to determine the cause of leaking fuel
rods and prevent or reduce the probability of reoccurrence. If the Fuel Assembly (FA) is to be
reloaded in the next cycle, then the new core may have to be redesigned during outage.
Radiochemical determination of leaking fuel and its properties can aid in reduced impact of
leaking fuel.
1018.1
1018.2
Monitoring of the RCS radiochemistry is usually performed daily, where specifically the
activities of xenon, krypton, iodine and caesium isotopes are of interest in the fuel
performance monitoring program. Analysis to determine whether leaking fuel is present or
not in the reactor is usually relatively straightforward, when elevated isotopic activity values
are observed. However, quantification or estimation of the number of defective fuel rods can
be very subjective and difficult to estimate, especially so when the leaking rods typically have
small tight defects or a mixture of tight and open defects. A significant amount of experience
from past behaviour (previously prevalent mechanisms) and analysis may also be required.
Radiochemical analysis is a sensitive and relatively developed technique, such that
determination of a minor fuel cladding defect is possible. There are detailed references [3]
dealing with fuel failure properties determination and the use of radiochemical analysis is an
established technique in the industry. However, variation of the many fuel properties of
design (rod diameter, rod power, absorbers (IFBA, gadolinium), axial blanket, …) and
operation (rod power, enrichment, burnup, peaking factors, GH system usage, …) may
require in-depth analysis on a plant level basis to determine leaking fuel defect properties.
The first indication of leaking fuel in the cycle is usually observed in the rapid increase of the
Xe-133 isotope specific activity. The Xe-133 isotope has a relatively longer half-life (≈ 5.25
days) and higher yields compared to the other noble gases (krypton and xenon) that are
monitored.
Depending on the type of fuel rod cladding defect, the leaking fuel rods are generally
classified as tight or open (to water penetration). Where only gaseous isotopes are present, the
leaking fuel rods are typically classed as tight (i.e. xenon and krypton isotopes). Open defects
are typically determined on the ability of water to penetrate the fuel rod and release water
soluble fission products (i.e. iodine isotopes). Previous leaking fuel rod examinations of GridTo-Rod-Fretting (GTRF) tight leakers have shown cladding perforations of roughly 0.1-1
mm2 that may be determined in radiochemical analysis.
The size of the fuel cladding defect can be determined using i) relationship between different
isotopes of xenon or iodine with different half-life times (the yields are similar), ii) Increase in
caesium isotopes above background, iii) Solid particles such as Np-239 that indicate water
penetration into the fuel rod with changes in the Mo-99 activity.
The defect mechanism may be determined by the typical characteristic behaviour that has
been previously observed, such information is available from fuel vendors. Typically for
NEK, prevalent mechanisms of debris fretting and GTRF have been observed. Debris fretting
usually occurs early in the operating cycle, showing a large simultaneous increase in Xenon
and iodine isotopic activities indicating an open defect. GTRF usually occurs late in operating
cycle and an increase in xenon isotopic activity is observed with a possible later change in
iodine isotopic activity.
The relative power of the leaking fuel rods can be inferred by many methods, i) the steady
state isotopic activity levels of iodine and xenon is related to the rod power, ii) release to birth
plots of iodine and xenon may be useful for open defects.
The tramp value is typically calculated using the Iodine-134 specific activity. Correction of
other isotopes to remove tramp values may also be performed using Iodine-134.
The burnup of the leaking fuel rods may be determined, where possible, by use of the Cs-137
to Cs-134 ratio. With tight leaking fuel defects, the caesium activities may not be sufficiently
high to yield reasonable accuracy. When there are multiple leaking fuel rods, the determined
burnup value may not be accurate.
1018.3
Computation programs are also available and used in NEK, however many of these programs
are based on open leaker models. The effectiveness for the case of tight leakers that are
prevalent at NEK has not been very good.
2
RESULTS
The RCS measured specific isotopic activities of NEK operational cycle 25 are shown in this
section with a brief explanation of the usefulness.
Iodine and caesium spiking behaviour during significant power and/or pressurisation changes
can sometimes be observed due to changes in the fuel rod internal pressure compared to the
RCS pressure. Depending on the defect size, different behaviour can be observed. The
shutdown measurements for cycle 25 (period between final decrease of reactor power to RCS
system depressurisation) are omitted here, but are also of interest due to associated iodine and
caesium spiking that occurs.
Xenon Isotopes (Figure 1): The Xe-133 isotope shows a step level increase at 335 days, 415
days, 452 days and 498 days from the beginning of the cycle. After 508 days, the Waste Gas
Processing System (GH) is turned on and the effective gaseous clean up rate increases,
lowering the steady state Xe-133 level. Compared to iodine and other isotopes, the Xe-133
abrupt specific activity change for around an order in magnitude at 335 days was the first
indication of a leaking fuel rod. The four Xe-133 specific activity changes would indicate four
leaking fuel rods. As the increase in Xe-133 is more than the shorter half life Xe-135 and Xe138 isotopes, the defect size is relatively small (tight). For all events, the specific activity
level is relatively smaller than would be typically observed for relative core power leaking
fuel rods and would indicate a lower than core average power fuel rod.
Figure 1: NEK operational cycle 25 daily measured specific activity values for xenon isotopes.
Iodine isotopes (Figure 2): The first I-131 event at 343 days is relatively long in duration and
not highly pronounced. After the event, the I-131 activity level returns to roughly to the
background value. The second event at 454 days is shorter in duration and higher in
magnitude. The I-131 activity value then stabilises at a higher value than previously observed.
The other iodine isotopes activity changes are less pronounced or clear than that of I-131. As
the increase in I-131 is more than the shorter half life iodine isotopes, the defect size is
relatively small (tight). The activity level is relatively smaller than would be expected for
relative core power leaking fuel rods and would indicate lower than core average power fuel
1018.4
rods. The I-134 specific activity, that is commonly used to represent the tramp component,
hardly changes over the operational cycle.
Figure 2: NEK operational cycle 25 daily measured specific activity values for iodine isotopes.
Isotopic Ratios (Figure 3): Various ratios can be used to increase the visibility of leaking fuel
rod events so that a determination of leaking fuel may be performed more easily. The first
event around 335 days (in Xe-133) is clearly visible, however further small changes are not as
easily observed and the specific isotopic activity values are more easily used for
determination of subsequent events.
Figure 3: NEK operational cycle 25 daily measured specific activity values for ratios of iodine, xenon
and krypton isotopes.
Main isotopes (Figure 4): The changes in I-131, I-133 and Xe-133 show that the Xe-133
increase usually occurs before the I-131 change for the mechanism shown here (primarily
GTRF). The I-133 activity change is less sensitive than that of the I-131 due to the half-life
difference and tight defect size. The annotations show the steady state level changes in the
isotopes. From the level changes it is estimated that there are 4 leaking fuel rods with tight
defects. One leaking fuel rod is more open than the other three, observed by the iodine
increase.
1018.5
Figure 4: NEK operational cycle 25 daily measured specific activity values for I-131/I-133 ratio, I-131, I-133
and Xe-133 isotopes.
The institute of Nuclear Power Operators (INPO) and World Association of Nuclear
Operators (WANO) have a defined a reportable indicator called the Fuel reliability indicator
(FRI) [3-4]. The FRI is defined in equation 1 as the steady-state primary coolant Iodine-131
activity (Bq/g or µCi/g), corrected for the tramp uranium contribution and power level, and
normalized to a common purification rate and Linear Heat Generator Rate. The FRI provides
a general measure of the extent to which the reactor coolant activity is increased as a result of
fuel defects. Based on information obtained from fuel vendors, a reactor core containing one
or more defects is likely to produce indicator values (under steady state conditions) greater
than 19 Bq/g (5 x 10-4 µCi/g) for pressurised water reactors (PWR). A core that has a FRI
value equal to or less than this value has a high probability of containing no steady-state fuel
defects.
3

 LN 100  2

FRI   A131  kA131
(1)
LHGR
P
0




A131 is the average steady state activity of Iodine-131 normalized to a purification constant of
2 x 10-5 s–1. The tramp correction coefficient, k is used to correct for the tramp contribution.
The linear heat generation rate is normalised by the normalisation value LN (5.5kW/ft) and the
actual average linear heat generation rate (LHGR) at 100% power (kW/ft). The P0 is average
reactor power in percent at the times the activities were measured, P0 scales the LHGR.
The reported FRI values are the average of all recorded values above 85% rated thermal
power for 3 days preceding the day of measurement. In Figure 5, the individual daily
measurement values greater than 1E-6 µCi/g are shown (values below 1E-6 µCi/g are
artificially corrected to 1E-6 µCi/g). The FRI limit value of 5E-4 µCi/g was determined by
INPO to represent a high probability of leaking fuel in PWR plants. For NEK in the
operational cycle 25, the calculated values for the iodine spike were above the limit, otherwise
all values were below the FRI limit, despite having an end of cycle prediction of 4 leaking
fuel rods.
1018.6
Figure 5: NEK operational cycle 25 Fuel Reliability Indicator calculated based on INPO/WANO reporting
requirements. The Fuel Reliability Indicator is calculated based on the I-131 activity value, I-134 activity value,
reactor power and reactor coolant system letdown purification rate [3].
To determine the number and type of defects (Figure 6), sometimes the I-131 magnitude
plotted against ratio of I-131/I-133 values is used. This assumes an I-131 magnitude per fuel
rod and that the ratio is responsive to the defect size due to the half-life difference. In the
operational cycle shown here, as the defects are tight in nature, the I-131 magnitude per fuel
rod and the iodine ratio is very low, yielding that this method is not effective.
Figure 6: NEK operational cycle 25 I-131 vs I-131/I-133 ratio for quantification of the number and type of fuel
defects.
Caesium Isotopes (Figure 7): The caesium isotopic activity remains relatively uneventful. The
specific activity values remain close to background apart from the spike that occurs at the
same time as the iodine spike (see Figure 7 at 460 days). Two further spikes are observed at
EOC during shutdown (not shown) relating to the reactor power decrease and RCS
depressurisation. The EOC spikes are significantly higher in magnitude around 1E-3 µCi/g.
When a sufficiently high caesium isotopic activity is measurable in the RCS (related to
measurement uncertainty), the accuracy of the caesium ratio burnup determination by
comparison of measured RCS caesium radioisotopes to actual leaking FA burnup is typically
within a ±20% uncertainty range. The Cs-134/Cs-137 ratio was determined from the 460 days
spike as well as the EOC power reduction and pressure reduction spiking as 26,150
MWD/MTU, 24,700 MWD/MTU and 34,400 MWD/MTU respectively. The relative change
1018.7
in burnup between the two dates is low compared to the core average burnup, this also
suggests low power fuel rods.
Figure 7: NEK operational cycle 25 caesium isotopic activity.
The FA cycle burnup, total burnup as well as relative core power are shown in Figure 8, here
the leaking FA locations that were estimated from radiochemistry are shown. As the core is
designed on an quarter core rotation, the FA properties can be represented on the quarter core
diagram. There is no difference between the rotational locations in the rest of the core and
those shown in the quarter core representation. The specific locations were chosen based on i)
low relative core power locations, ii) burnup values that are within the burnup range
determined from the isotopic caesium ratio and iii) previous GTRF leaking fuel cycles
preference of baffle facing FA locations. In Figure 8, it can be noted that most core baffle
facing locations are also low relative core power locations (average value less than 1).
Figure 8: NEK operational cycle 25 quarter core fuel assembly values of fuel assembly burnup (left image) and
relative core power (right image). The predicted leaking FA from radiochemistry are highlighted grey and a red
dot ( ) is used in the figure to denote the symmetrical location of the telescopic sipping determined leaking FA.
Telescopic Sipping: As the FA is moved from the core to the spent FA storage area, the water
that is surrounding the FA is constantly sampled. The sampled water is degassed and the gas
is examined continuously for presence of Xe-133. As the FA is moved from the core, the
water depth is reduced (i.e. water pressure is reduced), in leaking fuel, the pressure change
causes a small amount of the gaseous contents of the rod to be expelled into the water
surrounding the FA, which is detected via the telescopic sipping system. During offload of the
operation cycle presented here, 3 leaking FA were identified via telescopic sipping. The
1018.8
leaking FA locations were correctly estimated as being located in the region 26A positions
shown Figure 8.
3
CONCLUSION
The first indication of leaking fuel was observed in a Xe-133 increase.
The increase in I-131 occurs at a later time than that of the Xe-133 increase, indicating that
the defects are tight in nature.
The Xe-133 and I-131 activity level changes supported the determination of 4 leaking fuel
rods.
The low Xe-133 and I-131 activity per fuel rod, indicates low powered core regions (i.e.
baffle facing FA locations).
The isotopic behaviour is typical for Grid-To-Rod-Fretting observed in previous cycles in
Krško where a Xe-133 increase is followed by a later I-131 increase at a cycle duration
greater than 300 days.
The Fuel Reliability Indicator threshold value of 5E-4 µCi/g appears to be too high to identify
leaking fuel which has small tight defects for the case of NEK.
Plots of I-131 to the I-131 / I-133 ratio that is based on open leakers does not provide useful
insight for these low power tight leakers experienced at NEK.
As the defects are tight in nature, little caesium activity is present during the cycle apart from
one peak late in operational cycle. During the EOC shutdown (power and pressure
reductions), the measured activity values during iodine and caesium spiking were relatively
larger in magnitude. The change of caesium calculated burnup supports low relative core
power fuel determination and provides an expected burnup range of around 26-40
GWD/MTU.
The characteristics determined from radiochemical behaviour of i) low relative core power
locations, ii) burnup values that are within the burnup range determined from the isotopic
caesium ratio and iii) previous GTRF leaking fuel cycles preference of baffle facing FA were
used to identify core locations. For the operational cycle presented, the radiochemical
prediction was confirmed during the leaking FA inspections found during offload using
telescopic sipping.
REFERENCES
[1]
NEK Technical Specifications, revision 154
[2]
NEK procedure REP-5.125 “Evaluation of Daily Radiochemical Analysis”
[3]
Inter Atomic Energy Association Nuclear Energy Series, Review of Fuel Failures in
Water Cooled Reactors (NT-T-2.1), Vienna, 2010.
[4]
Institute of Nuclear Power Operations (INPO), Detailed Description of Overall
Performance Indicators, INPO (1993) 96–003