Fission Gas Release during Post Irradiation Annealing of BWR Fuels

15 Pages·2008·1.25 MB·English

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 27[11], pp.1002~1016 (November 1990).
Fission Gas Release during Post Irradiation
Annealing of BWR Fuels
Katsumi UNE and Shinji KASHIBE
Nippon Nuclear Fuel Development Co., Ltd.*
Received May 1, 1990
Uranium dioxide fuels irradiated in a commercial BWR during 1 to 4 cycles (burn-up:
6~28GWd/t) have been annealed in an out-of-pile condition and the release rate of 85Kr was
measured continuously. The annealing temperature ranged 1,500~1,800dc. The effects of
temperature and burn-up have been studied on burst release during temperature ramp and on
diffusional release during isothermal annealing.
The burst release evolved on temperature ramp was mainly due to the rapid release of
fission gas from grain boundaries, which had previously accumulated there during base irradia-
tion. With increasing burn-up, the critical temperature for the onset of the burst release
became lower and the fractional burst release increased. The number of fission gas atoms per
unit area of grain boundary, which was calculated from the quantities of the burst released
Kr, was about 2~4 times larger than saturation values estimated from theoretical considera- 85
tions based on the assumption of mechanically equilibrated bubbles for 3 and 4 cycle irradiated
fuels. Effective diffusion coefficient of 85Kr obtained by the isothermal annealing tests appeared
to be independent of burn-up (<30GWd/t).
KEYWORDS: krypton 85, fission products, release, uranium dioxide, BWR type
reactors, nuclear fuels, burnup, diffusion coefficient, swelling, gas bubbles, grain
boundaries, post-irradiation annealing, very high temperature, temperature depend-
ence, microstructures
elucidation of the mechanism of fission gas
I. INTRODUCTION
release and bubble swelling is precise data
Fission gas release and swelling caused on the time dependence of the release rate
by intra- and intergranular bubbles must be and fuel microstructural changes in the course
adequately considered in the thermal and me- of fission gas release. In order to obtain the
chanical design of LWR fuel. Fission gases above data, in-pile measurements of the in-
released in the fuel rod plenum and pellet- ternal pressure in fuel rods have been recently
cladding gap pressurize the cladding, reduce conducted(1)(2), followed by detailed post-irra-
the thermal conductivity of the filling gas, diation examinations (PIE) on retained fission
and thus increase fuel operating temperature. gas within a pellet and on fuel microstructural
The precipitations of fission gas bubbles gen- change. In irradiation experiments, however,
erated in the course of fission gas release not there are often uncontrollable variables such
only swell the fuel pellet and cause a severe as irradiation temperature and other irradia-
pellet-cladding mechanical interaction, but also tion effects which largely influence the data
degrade the thermal conductivity of the pellet, analysis. Moreover, a relatively long duration
which also increases fuel operating tempera- is usually required to complete a series of
ture. Therefore, more precise knowledge of irradiation experiments and subsequent detail-
the above phenomena is required for the fuel ed PIEs. On the other hand, in the post
design, especially at high burn-ups. irradiation annealing experiment using irra -
One of the most useful information for * Oarai -machi, Ibaraki-ken 311-13.
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Vol. 27, No . 11 (Nov. 1990)
1003
diated fuels, fission gas release rate can be
perature and burn-up for UO2 irradiated in a
continuously monitored under controlled and commercial BWR during 1 to 4 cycles (burn-
precise temperature. up: 6~28GWd/t) were systematically examined
Extensive out-of-pile annealing experiments on burst release during temperature ramp and
have been conducted in the past with irradi- on diffusional release during isothermal heat-
ated nuclear fuels. Lawrence(3) and Matzke(4) ing, by means of a post irradiation annealing
have reviewed the data of fission gas diffusion technique. The annealing temperature ranged
in UO2 and (U,Pu)O2 and discussed its me- 1,500~1,800dc. In addition, the microstruc-
chanism. Main factors affecting the diffusion tural changes of the fuels during the annealing
are temperature, fuel density, fuel stoichiom- experiments were examined to elucidate the
etry, fission rate and burn-up. In addition to correlation between fission gas release and gas
these factors, the effect of additives in UO2 bubble formation.
on the diffusion has been reported by Matzke(5)
and Une et al.(6) The range of burn-up in II. EXPERIMENTAL
the above out-of-pile experiments is limited 1. Specimen
to several hundred MWd/t, except for a few The specimens used were taken from UO2
of the more recent studies which used UO2 fuel pellets irradiated in a commercial BWR
irradiated to burn-ups above several GWd/t. during 1 to 4 cycles at maximum linear
Baker & Killeen(7)(8) have measured the rel- heat generation rates between 300 and 370
ease of 85Kr from UO2 irradiated to 11~38 W/cm. The fractional fission gas releases
GWd/t in the AGR and PWR by out-of-pile measured by a pin puncturing test after base
annealing experiments to examine the effect irradiation were 0.2, 0.8, 21.0 and 21.0% for
of temperature, burn-up and annealing atmos- each fuel rod. The burn-ups were 6, 16, 23
phere. They reported that the fractional gas and 28GWd/t for each fuel cycle. Pieces of
release became higher and the effective diffu- a fuel pellet with the rim were mounted in
sion coefficient of 85Kr increased with increas- resin and sliced into slabs about 1 mm thick.
ing burn-up. Small(9) has also heated irradi- Then about 1.5mm diameter specimens were
ated UO2 (burn-up: 18 and 38GWd/t) at various ultrasonically punched from these slabs. The
heating rates to simulate the fission gas release location of the specimens was roughly between
under transient conditions; he measured a the fuel rim and middle. Their weights were
large initial burst release during and shortly 30~40mg.
after the temperature ramp, followed by a 2. Annealing Experiments
relatively slow release during the isothermal Figure 1 gives a schematic diagram of the
period. He interpreted this burst release as annealing equipment and measuring system of
due to the fraction of the gas within the released 85Kr. A Mo capsule (20mm long, 18
grains which diffuses quickly to the grain mm I.D., 1mm thick) containing the specimen
boundaries to form instantaneous and complete was heated by a high frequency induction
interlinkage of the grain boundary bubbles. coil. The capsule was supported by a alumina
Burbach & Zimmermann(10)(11) have reported tube, which was set in a water-cooled quartz
a higher fractional release at higher burn-ups, furnace. Specimen temperature was measured
based on annealing experiments of sealed Mo by a two color radiation pyrometer which had
capsules containing UO2 pellets (burn-up: 4~38 been calibrated in advance by measuring the
GWd/t). In spite of the above elaborate stud- melting points of Ni (1,453dc) and Pt(1,769dc).
ies, the causes of the initial burst release and The accuracy of the pyrometer was within
the higher fractional release of high burn-up 10dc over the above temperature range. +-
fuels, and the burn-up dependence of effective The sweep gas used in the present experi-
diffusion coefficient have not yet been satis- ments was high purity He/2%H2 mixed gas
factorily elucidated. at a flow rate of 60cm3/min. Oxygen poten-
In the present study, the effects of tem- tial of the sweep gas was monitored by both
21
1004 J. Nucl. Sci. Technol.,
Fig. 1 Schematic diagram of furnace chamber and 85Kr measuring system
a dew-point meter and a stabilized zirconia 500cm3, respectively. Therefore, a time lag
oxygen probe heated at 816dc. The ratios and a time constant should exist when detect-
of PH2O/PH2 calculated by the measured dew- ing 85Kr released from the specimen, depending
point of -24dc and oxygen partial pressure on the flow rate of the sweep gas. In order
of 10-20atm at 816dc were both about 3x10-2. to obtain the above data, a standard quantity
Consequently, the oxygen potentials of this of 85Kr was injected instantaneously into the
mixed gas were in the range of -420~-380 sweep gas (flow rate: 60cm3/min) from the gas
kJ/mol at the furnace temperatures of 1,000 inlet of the furnace. The obtained time lag
,800dc. Under these oxygen potentials, ~1 and full width at half maximum (FWHM) of
/M ratios of the specimens during O the an- the 85Kr peak were about 2 and 14min,
nealing would be maintained very close to respectively. After about 50min, b-activity
the stoichiometric composition of 2.000 from of 85Kr dropped to the background level.
thermodynamic considerations(12)(13) of irradi- 3. Dissolution Experiments
ated UO2. In addition to fission gases of Kr After the annealing experiments, the spec-
and Xe, volatile fission products such as Cs imens were dissolved using the equipment
and I released during the annealing were shown in Fig. 2 to measure the residual 85Kr
removed by a trap of silver zeolite and glass in the specimens. The specimen was dissolved
wool. Released 85Kr was passed through an in a solution of 5N-HNO3 at about 80dc for
ionization chamber and its b-activity (energy: 2h. The dissolution atmosphere was high
687keV; half-life: 10.7yr) was continuously purity He flowing at the same rate of 60
measured. Then the released 85Kr was finally cm3/min as used in the annealing experiments.
collected in a charcoal trap cooled by liquid After the volatile fission products in the
nitrogen. Gamma activity (energy: 514keV) of sweep gas were removed by water buffer and
the trapped 85Kr was measured by a Ge de- alkali (NaOH solution) scrubbers, b-activity of
tector after the annealing experiments. Kr was continuously measured by an ioni- 85
The volumes of the furnace and ionization zation chamber, using the same method as
chamber shown in Fig. 1 were about 300 and in the annealing experiments. All of the
22
Vol. 27, No . 11 (Nov. 1990) 1005
Fig. 2 Schematic diagram of dissolution equipment
released 85Kr was collected in a charcoal trar specimen (burn-up: 28GWd/t), which was heat-
and its g-activity was measured by a Ge ed up to 1,800dc at the rate of 100dc/min
detector after the dissolution experiments. and then cooled to room temperature at the
The detection efficiency of the Ge detector same rate, is given in Fig. 3. After a small
for the collected 85Kr in a charcoal trap was burst release around several hundred degrees,
14.6cps/MBq. The correlation between radio- a large burst release appears around 1,500dc.
activity of the collected 85Kr, A(MBq), and In the figure the peak of 85Kr, injected instan-
the corresponding integrated output of the taneously from the gas inlet of the furnace,
ionization chamber, y(mV,s•110-6), was given is also shown. Overlapping the two peaks at
by the equation in the range of 0.3<y<14.0 the point of the abrupt rise, the shape and
FWHM of both peaks appear to be almost
(1)
identical. This indicates that the burst release
The fractional release of 85Kr was evaluated which occurred during the temperature ramp
by the integrated output of the ionization is of a very short duration within 1 or 2 min,
chamber, from which radioactivities of 85Kr and it suggests that the burst release should
released in the annealing and dissolution ex- be mainly due to the rapid release of fission
periments were calculated by Eq. (1). gas retained at the grain boundaries through
4. Microstructure Examination the rapid growth and interlinkage of grain
Before and after the annealing experiments, boundary bubbles and formation of the grain
the microstructure of the specimens was exam- edge tunnel. The fission gas is mostly trapped
ined by an optical microscope and a scanning in the grain boundaries during base irradia-
electron microscope. Measurements of their tion. This assumption is supported by SEM
grain size and porosity were carried out by observations of the specimen fracture surface,
an image analysis of micrographs of the which are described in detail in the next
specimens' polished surfaces. The morphology section. There are may small bubbles on the
of intra- and intergranular bubbles was grain boundaries for as-irradiated 2~4 cycle
observed from the fracture surfaces of the specimens (Photo. 3(b)~(d)) except for the 1
specimens. cycle specimen (Photo. 3(a)), and extensive
interlinkage of bubbles and formation of the
III. RESULTS AND DISCUSSION
grain edge tunnel (Photo. 4(a)) are clearly
1. Burst Release observed for the above specimen of Fig. 3
(1) Effect of Temperature Ramp just after the burst release. Zimmermann(11)
The release curve of 85Kr from a 4 cycle also reported that the high release rate at the
23
1006 J. Nucl. Sci. Technol.,
beginning of the out-of-pile annealing is pre- instantaneous release rate of 133Xe increased
sumably caused by the release of fission gas substantially not only during power rise, but
accumulated at grain boundaries. From the also during power decrease, while that of
in-pile sweeping gas release experiment with 138Xe did not increase . From these results,
a high burn-up fuel rod irradiated in a PWR they suggested that fission gases accumulate
up to the burn-up of 35.5GWd/t, Porrot et during steady-state irradiation on grain bound-
al.(14) measured the release rate of 133Xe (half- aries and then they are released through
life: 5.23d) of long-lived gas and 138Xe (half- the intergranular cracks induced by power
life: 14.17min) of short-lived one during steady- changes.
state irradiation and power transient; the
Fig. 3 85Kr release on temperature ramp of 1,800dc
Figure 4 shows the release curves of 85Kr for the 2 cycle specimen and 1,500dc for the
for the 2 and 4 cycle specimens together with 3 and 4 cycle specimens. In the same figure,
the temperature history, heating to 1,000dc the burst release temperatures of 129I and 137Cs
at the rate of 100dc/min and then raising by as reported by Peehs et al.(15) are also given.
steps of 100dc at the same rate and holding They heated irradiated UO2 in a Knudsen cell
at each temperature. In the case of the 2 in vacuo and analyzed the trapped isotopes
cycle specimen, after a slight release at each radiochemically. The present and their data
temperature rise to 1,500dc, a large burst clearly indicate that the burst release tempera-
release appears from 1,500 to 1,600dc. For ture becomes lower with increasing burn-up.
the 4 cycle specimen, prior to the large burst Same tendency has been reported by Vitanza
releases at the temperatures above 1,500dc a et al.(16), who measured the fuel centerline
small burst release around 200~300dc appears, temperature of instrumented fuel rods in the
which corresponds to the fractional release of Halden reactor. The fractional fission gas
about 1.5%. This low temperature burst release of the rods was above 1%.
release is probably due to the occurrence of This burn-up dependence of burst release
intergranular microcracks induced by thermal temperature is attributable to the increases of
shock. The critical temperature for the onset fission gas inventory in the grain boundaries
of the high temperature burst release is and the density of the grain boundary bubbles
plotted against specimen burn-up in Fig. 5. at higher burn-ups. Namely, higher internal
It is 1,800dc for the 1 cycle specimen, 1,600dc pressures in the bubbles lead to their faster
24
Vol. 27, No. 11 (Nov . 1990)
1007
Fig. 4 85Kr release on stepwise temperature ramp
transient fission gas release and bubble swell-
ing exert a great effect on the thermal and
mechanical behaviors of fuels, but also atten-
dant transient release of volatile fission prod-
ucts of I, Cs and Cd would instantaneously
increase local concentrations of corrosive
agents above the threshold values(17) of PCl/
SCC failure of fuel rods.
(2) Fractional Burst Release
The effects of ramp temperature and spec-
imen burn-up on the burst release of 85Kr
were examined by a series of experimental
Fig. 5 Burn-up dependence of critical tem-
temperature ramps up to 1,600~1,800dc, fol-
perature for onset of burst release
lowed by isothermal annealing at these tem-
growth rates, and the higher densities of the peratures for about 5h. The FWHM of the
bubbles increase the probability of bubble above 85Kr peaks were about 20min and
interlinkage. These factors probably decrease longer than that (about 14min) obtained in the
the burst release temperature. In addition to instantaneous injection test (Fig. 3). This
this, the occurrence of microcracks, based on difference is attributed to a diffusional release
a degradation of mechanical strength of the during the isothermal annealing at the high
temperatures. Accordingly the contribution
grain boundary, would enhance the above
tendency. The rapid burst release is unfavo- of burst release was calculated by subtracting
rable for LWR fuel performance. Not only the contribution of diffusional release from
25
1008 J. Nucl. Sci. Technol.,
the peak. As a first approximation, the con- releases of 85Kr for 1~4 cycle specimens at
tribution of diffusional release was considered the ramp and isothermal annealing tempera-
to correspond to the interpolated curve from tures of 1,800dc. Shortly after the tempera-
the isothermal release curve. The integrated ture ramp, the abrupt rises are seen. The
values of the burst peaks were finally con- cumulative fractional releases up to the heating
verted into radioactivities of the burst released duration of about 60min correspond to the
Kr by using Eq. (1). Fractional burst 85 release burst release. About 70~80% of the total
was defined by the ratio of the integrated cumulative releases after the isothermal an-
burst release to the integrated total amount nealing of about 5h are contributions of the
of 85Kr in the specimens. burst release, except for the value of about
Figure 6 shows the cumulative fractional 25% for the 1 cycle specimen.
Fig. 6 Cumulative fractional release of 85Kr at ramp and
isothermal annealing temperatures of 1,800dc
The burn-up dependence of cumulative ramp power(18). These data can be reasonably
fractional burst release is shown in Fig. 7. interpreted as the burn-up dependence of the
In the figure the data at the ramp tempera- burst release during power transient. Figure 7
tures of 1,600 and 1,700dc are also shown. indicates that the percentage of the quantities
Although there is a relatively large scatter of fission gas trapped in the grain boundaries
of the data, the release clearly increases with becomes larger at higher burn-ups. Unlike
increasing burn-up. The present fractional our proposed mechanism, if the rapid burst
burst release is 4.5% for 1 cycle specimen release is assumed to be mainly due to the
and about 20~30% for 3 and 4 cycle speci- fission gases diffused from the grain interiors,
mens. The latter values are in good agree- as suggested by Small(9), almost the same
ment with previously reported values(15) of fractional burst release should be measured
129I and 137Cs for UO
2 samples with the burn- when fission gas diffusivity is independent of
up of 33GWd/t. Fractional fission gas release fuel burn-up. In fact effective diffusion coeffi-
of segmented fuel rods obtained in power cient of 85Kr obtained in the present experi -
ramp experiments has been reported to in- ments was almost independent of burn-up
crease with increasing fuel burn-up at a given (<30GWd/t), as will be discussed later. The
26
Vol. 27, No. 11 (Nov . 1990) 1009
ORIGEN-2 code, indicating no fission gas
release from these fuel pellets during base
irradiation. In fact the fractional fission gas
release of these fuel rods measured by pin
puncture tests does not exceed 1%. On the
other hand, about 10~20 and 30~50% of the
produced 85Kr are already released from 3
and 4 cycle specimens during their base irra-
diations, respectively. The higher fission gas
releases for 4 cycle specimens are consistent
with the pin puncture fractional release of
21% for the 4 cycle fuel rod. The burst
released 85Kr, which is considered to be
trapped in the grain boundaries during base
irradiation, appears to increase up to the 3rd
cycle irradiation and decrease during 4th cycle
irradiation in the same manner as the total
retained 85Kr.
Fig. 7 Burn-up dependence of fractional
burst release of 85Kr
actual response of Fig. 7 supports our model
for the burst release. For 4 cycle specimens
(burn-up: 28GWd/t) the fractional burst release
seems to be insensitive to ramp temperature,
while it seems to depend on the ramp tem-
perature for 2 cycle specimens (burn-up: 16
GWd/t). This behavior would result from the
probability of the bubble interlinkage and
tunnel formation largely depending on ramp
temperature, especially for the specimens of
lower burn-ups with less fission gas invento-
ries in the grain boundaries. As the specimen
burn-up is increased, the burst release tem-
perature becomes lower as shown in Fig. 5,
and the probability of the bubble interlinkage Fig. 8 85Kr concentrations in UO2
and tunnel formation becomes relatively in- as a function of burn-up
sensitive to ramp temperature.
(3) Grain Boundary Inventory Figure 9 gives the correlation between the
The quantities of total 85Kr retained in burst released 85Kr obtained at the ramp
the specimens after base irradiation and the temperature of 1,800dc and the total retained
burst released 85Kr are plotted against the one. The burst released 85Kr obviously in-
specimen burn-up in Fig. 8. The specific creases with the higher total retained 85Kr.
activities of 85Kr per unit weight of specimen The burst released 85Kr does not seem to be
are corrected to the values at the time of saturated within the upper limit of the pre-
reactor shutdown. The total retained 85Kr sent data. Assuming that the burst released
for 1 and 2 cycle specimens is in good Kr corresponds to the quantities 85of 85Kr
agreement with the values calculated by the accumulated in the grain boundaries during
27
1010 J. Nucl. Sci. Technol.,
Fig. 9 Burst released 85Kr as a function of total retained quantity
base irradiation and that each grain is a as-irradiated 1 cycle specimen having appa-
sphere of 9mm diameter for the present UO2 rently no gas bubbles on the grain boundaries
pellets, the number of fission gas atoms per (Photo. 3(a)). The fact that the estimated
unit area of the grain boundary is also given values of fission gas in the grain boundaries
in the right-hand vertical axis of Fig. 9. In for 4 cycle specimens are relatively close to
the calculation, the isotopic ratio of 85Kr and the theoretically calculated ones, suggests
the Kr/Xe ratio obtained by the ORIGEN-2 that the fission gases trapped in the grain
code were used. The gas trapped in the boundaries mainly exist in the equilibrium
grain boundaries ranges 0.7~1.5x1020 atoms bubbles. In fact, relatively larger Ienticular
/m2 for 3 cycle specimens and 0.7~1.0x1020 bubbles are observed from the fractograph of
atoms/m2 for 4 cycle specimens. These the as-irradiated 4 cycle specimen, as is shown
values are about 2~4 times larger than theo- in Photo. 3(d) of the next section. The pre-
retically calculated saturation values of 0.2~ sent estimation on the fission gas in the grain
0.5x1020 atoms/m2(19)(20), which is based on boundaries indicates that the grain boundary
the assumption of an equilibrium grain face bubbles, precipitated during base irradiation
le:• bubb
at relatively lower temperatures, exist as
non-equilibrium bubbles of p(cid:129)â2g/r and that
(2)
the fission gases in the grain boundaries not
where p is the gas pressure in the bubble, g
only precipitate in the bubbles, but also dis-
the surface energy of UO2 and g the radius
solve in grain boundaries having many defects.
of the bubble. The grain boundary inventory 2. Microstructural Observation
of fission gas atoms of Fig. 9, of course,
(1) Bubble Swelling
include a little contribution of the fission gas Ceramographs of 3 cycle specimens (burn-
atoms diffused from the grain interiors during up: 23GWd/t) before and after the annealing
temperature ramp. This diffusional contribu- experiments at 1,600 and 1,800dc for about
tion may amount to about 20~30% of the 5h are given in Photo. 1. There seems to
burst released fission gases at the most, based be no apparent grain boundary bubbles for
on the fractional burst release (4.5%) for the the base irradiated specimen (Photo . 1(a)),
28
Vol. 27, No. 11 (Nov . 1990) 1011
Photo. 1(a)~(c) Ceramographs of 3 cycle specimens (burn-up: 23GWd/t)
while many fission gas bubbles precipitated average grain size was obtained by an image
along the grain boundaries for the annealed analysis of the ceramographs of etched spec-
specimens (Photo. 1(b) and (c)). Scanning elec- imens. Almost the same grain sizes of about
tron micrographs of the above polished spec- 9mm were measured for both specimens before
imens (Photo. 2) clearly indicate that both and after the above heat treatments, suggest-
inter- and intragranular bubbles grow on ing the suppression of grain growth by the
increasing the annealing temperature. The
precipitation of the bubbles.
Photo. 2(a)~(c) Scanning electron micrographs of polished surface
of 3 cycle specimens (burn-up: 23GWd/t)
Bubble swelling was evaluated by the dif- 1,800dc for 5h, so less dependent on burn-up.
ference in porosity between before and after On the other hand, fractional fission gas
annealing. The burn-up dependence of bubble release has a strong burn-up dependence as
swelling after the annealing at 1,600 and shown in Fig. 6. This remarkable difference
1,800dc for about 5h is given in Fig. 10. between fission gas release and bubble swell-
The bubble swelling is about 7% for the ing indicates that complete grain edge tunnels
specimens annealed at 1,600dc for 5h and formed at the swellings of about 7~10% and
about 9~10% for the specimens annealed at bubble swellings saturated at these levels.
29