Fission gas release from UO2-dispersed graphite during irradiation

Journalof NuclearEnergy,Vol. 26,pp. 333to 348. PergamonPress1972.Printed in Northern Ireland

FISSION GAS RELEASE FROM UO,-DISPERSED GRAPHITE DURING IRRADIATION K. SHIBA, M. HANDA, S. YAMAGISHI,T. FUKUDA,Y. TAKAHASHI, T. TANIFUJIand S. OMORI Division of Nuclear Fuel Research, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken, Japan (Received 26 October 1971 and in revisedform 14 February 1972) Abstract-In-pile release of B5mKr, s7Kr, saKr, IrsXe, 13&Xeand 13*Xe from natural graphite was measured over a temperature range of 100 to 950°C and that of Ias and IS61was estimated. The effects of fission rate and fission density on the fission gas release were examined. The B-decay process of iodine within and outside the graphite was observed to control the gross release of xenon. Possible mechanisms of an anomalous sink of asKr in the plot of log R/B versus log A were discussed. A new model for knock-out release was proposed on the basis of its temperature dependence found. These results were compared with those of post-irradiation experiments and explained by a mechanism whereby fission gas is trapped in defects created in graphite by fission and B-decay energy, and released through annealing of the defects. 1. INTRODUCTION

The fission gas release from natural graphite which fission products have recoiled into has been well characterized in post-irradiation experiments (YAJIMAet al., 1961, 1962,1963,1966a). A series of experiments have been carried out in order to investigate the in-pile behavior of fission gas release from the graphite with this background of research. We have reported some preliminary results of an in-pile release of fission gas from UO,-dispersed natural graphite (YAJIMAet al., 1967), where a lack of the data based on the absolute release rate prevented a quantitative discussion. In addition, a new phenomenon has been found on the same kind of graphite that xenon is released from the graphite in the event of p-decays of the precursor iodine (KAMEMOTO et al., 1970). In the present paper, we will evaluate the in-pile fission gas release from the natural graphite, compare this data with the post-irradiation data and try to elucidate the mechanism of fission gas release from natural graphite. Data of in-pile release of fission gas from pyrocarbon have been published by ZUMWALTet al. (1962), REAGANet al. (1965 and 1967) and BAUMANNet al. (1969). For those who want to obtain the knowledge of the in-pile release of fission gas from other ceramic fuels, a review article by CARROLL(1965) would be helpful. Studies on fission gas release from graphite using post-irradiation techniques have recently been reviewed by YAJIMA(1971). 2. EXPERIMENTAL 2.1 Apparatus Fission Gas Release Loop in the Japan Research Reactor No. 3 was used throughout the experiments. Complete details of the apparatus have been described earlier (YAJIMA et al., 1966b). In brief, it consisted of a specimen chamber capable of being heated to lOOO”C,in which specimen was exposed to a thermal neutron flux of 6 x lOI* n/cm*/sec. A slow stream (60 ml/min at 500 mmHg) of purified helium passing over the specimen transported fission gas released from it through internal traps (a-alumina and silver mesh) for trapping fission halogen to a sampling device. The helium gas was periodically sampled there into glass samplers (20 ml) for y-assaying. It took 30 min for the fission gas released from the specimen to reach the sampler. A water loop for cooling the specimen chamber was utilized for continuous flux monitoring (KAMEMOTO et al., 1967b). The y-rays of lBN in water were measured outside the pipe of the loop by a ratemeter with an NaI crystal. 1

333

334

K. SHIBAet al.

2.2 Specimen

The specimen was composed of 4 pellets of UO,-dispersed graphite: each pellet contained O-25g of UOBpowder (natural U) and 7.06 g of natural graphite powder, with a size of 22 mm in O.D. and 2.7 mm in I.D. by 10 mm long. Both materials used were of particle size of about 1 pm. The surface area of the pellets was measured by BET method to be 5.6 m*/g. Under these conditions, about 90% of the fission products were experimentally found to be captured in the graphite by fission recoil. In addition, since a fractional release of fission gas from the graphite is much larger than from the UOa, the release from the mixture is referred to as being governed by the release from the graphite. The specimen was externally heated by a platinum resistance furnace. The temperature of the specimen was measured with a Pt Pt-Rh thermocouple and controlled by a PID controller and saturable reactor. 2.3 Determination of absolute release rates offission gas A sampled gas was measured 12 times for about 60 hr by a 400 channel y-ray spectrometer with a 3 in. x 3 in. NaI crystal. The y-ray spectrum of fission gas showed intense photopeaks at O-08,0.15, O-19,0*25and 0.40 MeV, which were respectively assigned to IssXe, ls*Xe and ssmKr, %r, l*aXe and rssXe, and *‘Kr by decomposing the decay curves (KAMEMOM) et al., 1967a). These photopeaks were used for determiningtherelease rates of fission gas. All the counts were corrected for the travel time (30 rain) from specimen to sampler. The normalized counts (~-ray&c/ml) were multiplied by conversion factors involving the flow rate of helium, half-life, y-ray abundance of a nuclide and counting efficiency, of which the last was determined by using the nuclides as standard; “OTrn (@084), rssSm (O-103),‘We (O-145).rlamIn (0*192),Wr (0.320), loaAu (0.412) and ls7Cs (W662). The pertinent data are summarized in Table 1. A y-ray abundance means unconverted photons for each 100 disintegration of a nuclide. 3. RESULTS

3.1 Release at constantJux

and temperature

The specimen temperature has been raised to a given temperature and kept constant before irradiation. The sweep gas was sampled every 5 or 8 hr during irradiation. Simultaneously with reactor shutdown, the heating was stopped and sampling was carried out every 2 hr. The fission gas in the samplers was quantitatively determined in the manner described in Section 2.3. A typical plot of the gross release of fission gas is shown in Fig. 1. The gross releases of s5mKr, *‘Kr, geKr and lssXe increased sharply in the initial stage of irradiation, then reached an equilibrium value as irradiation was continued, and stopped with shutdown of the reactor and the furnace. On the other hand, the gross releases of xenon nuclides except lasXe increased gradually, saturated at an equilibrium value much later than 4 nuclides above-mentioned, and, immediately after the reactor and the furnace had been shutdown, decreased rapidly with half-lives of 21 and 6.7 hr, respectively. These figures correspond to the half-lives of precursors 1331and 1351. The phenomena should be attributed to the iodine having deposited on the internal traps and decaying to xenon. In addition, apparent half-lives obtained from the release of xenon (see Table 2), which are tentatively referred to as a saturation curve [l - exp (-0.693 t/Tl,&] in production of a radioactive isotope with a half-life Tl,2 under neutron irradiation, are in good agreement with the half-lives of the precursors rather than those of xenon. This also implies the importance of the release behavior of iodine in the gross release of xenon. Assuming that all xenon, which is a decay product of the iodine deposited on the internal traps, was instantaneously released, the release of iodine was estimated by extrapolating the residual release rate back to the shutdown time. The gross xenon release minus the iodine release gives a net xenon release under irradiation. Although the experimental limitations made it difficult to apply the above analysis to the release of krypton and l=Xe and to estimate apparent half-lives accurately, they

Kr Br Kr Br Kr Br Xe I

Xe I Xe I

85

135

6.7 hr 15min ssec

9.13 hr

4.4 hr 3 .O min 78 min 55 set 2.77 hr 16-O set 5.27 day 20-B hr

Half-life TX/, 4.38 3.85 1.48 1.26 6.95 4.33 l-52 9.27 (3.9 2.11 2.87 7.70 1.39

x x x x x x x x x x x x x

10-S 10-s 1O-4 lo-$ 1O-6 10-a 1O-B 10-B lo-s*) 10-S 1O-5 10-a 10-r

Decay constant 1 (set-I)

* Under irradiation (neutron flux: 6 x 10la n/cm2/sec). t Theoretical values (WAHL, 1962).

138

133

88

87

Element

Mass no.

0.081

0.191

0.403

0.150

y-Energy (MW

36.4

34.6

(Z)87

77.9

y-Abundance (%)

TABLE l.-NUCLEAR DATA OF FISSION GAS

6.67

3.56

2.50

1.30

0 0.10 0.14 1-l 0.6 2.0 0.03 o-2

Fission yield (%) Chain Independent?

3.61 3.60 3.32 1.74

7.31 7.31 1.41 1.33 2.01 1.67 3.74 3.73

x x x x

x x x x x x x x

10B lOa lo0 100

108 108 100 100 100 loo 100 100

Production rate B (atoms/xc)

336

K. SHIBAet al.

v 40

reactor

shut down

0

IRRADIATION

Fm. 1.-Fission TABLE 2.--APPARENT

TIME

1hr 1

gas release from UO,-graphite during irradiation at 300°C. HALF-LIFE

DETERMINED

FROM

GROSS

RELEASE

CURVE

OF XENON

Nuclide lssXe (hr) la6Xe (hr)

10

11

20 6.3

18 7.0

Run no. 12 25 6.7

14

15

27 8.0

24 7.5

Average 23 7.1

were found to be very short, i.e. less than 1 hr even for g6mKr. This suggests an important part played in the gross release of fission krypton by its precursor bromine, which has a short half-life.

3.2 Efict

of neutronj?ux (j&ion rate)

The reactor power was increased step by step from 1 to 10 MW at a temperature of 200°C and kept constant at each stage for 12 hr. The range of the power corresponded to a neutron flux range from 6 x loll to 6 x 1012n/cm2/sec. The release rates of krypton and 138Xewere plotted vs. neutron flux in Fig. 2. Results on lUXe and 13sXe were discarded because equilibrium could not be attained for the nuclides in 12 hr. All the lines in Fig. 2 have the same slope of 45 degrees : the release rate of fission gas at 200°C is directly proportional to neutron flux. A similar dependence for identical specimens at 1000°C was already reported in a previous paper (YAJIMA et al., 1967). A reasonable conclusion is that the linear relation between the fission gas release and the fission rate holds for the natural graphite specimen over the entire range of temperature studied.

Fission gas release from UO,-dispersed graphite during irradiation

NEUTRON FIG. 2.-Relation

FLUX 1 n/cmVsec

337

1

between neutron flux and release rate of krypton from UO,-graphite irradiated at 200°C.

3.3 E#ect of temperature

The specimen temperature was changed from 100 to 950°C at a flux of 6 x 10’” n/cm2/sec. A temperature 85°C was a steady temperature resulting from only self-heat generation of the specimen under irradiation, such as fission heat, decay heat, etc. An irradiation time for a run ranged from 48 to 130 hr. The release rates of krypton, xenon and iodine at reactor shutdown were determined according to the analysis described in Section 2.3 and shown in Table 3 together with the irradiation conditions. Irradiation times were long enough for the release of krypton and l*Xe to attain an equilibrium value but not necessarily for the releases of iodine and xenon except 13*Xe. A release rate R of fission gas divided by the production rate B gives a fractional release of fission gas, which is presented in terms of percent in Table 3. The overall errors in determining absolute release rates were estimated to be &5% for 135Xe, A7 ‘A for sSmKr and f8 % for others. Figures 3 and 4 show the plots of the release rates of fission gas vs. temperature. The releases increased significantly with increasing temperature and did not reveal a temperature-independent release which had usually been observed on most ceramic fuels (Cmo~~etal., 1965; JACKSON~~~~.,1964; MELEHANet al., 1963,1964; STUBBS et al., 1962; SOULHIER,1966). Other important features of the figures were that the temperature dependence for the gross krypton release was larger than that for the gross xenon release but that there seemed to be no difference in temperature dependence among the nuclides of an element. The fractional releases of iodine and xenon, which are a combination of knock-out and diffusion release, are plotted in Fig. 4. The iodine release increased exponentially with temperature. On the contrary, the xenon release was a complex function of

100

98

200

200

200

300

300

450

450

650

640

700

950

10

11

8

9

16

6-l

12

7-l

14

6-2

15

7-2

13

5.21 l-39 5.24 1.40 7.15 1.91 7.78 2.08 7.55 2.02 8.01 2.14 11.3 3.03 15.0 4.01 17.5 4.68 24.7 6.61 21.7 5.80 28.7 7.68 35.6 9.5

ISaXe 4.48 1.20 4.54 1.21 5.34 1.43 5.74 1.53 5.51 1.47 5.48 1.64 6.75 1.80 8.02 2.14 8.62 2.31 17.0 4.55 13.0 3.48 18.4 4.92 19.8 5.3

133I 0.73 0.20 0.70 0.19 1.80 0.48 2.04 o-55 2.04 0.55 2.54 0.68 4.61 1.23 7.01 1.87 8.88 2.37 7.68 2.05 8.68 2.32 10.4 2.78 15.8 4.2

rSsXe,t 3.29 0.91 3.57 0.99 4.62 1.28 4.74 1.31 4.71 1.31 7.78 2.15 6.24 1.73 11.0 3.05 9.81 2.72 14.5 4.02 12.7 3.52 16.7 4.63 21.3 59

136Xe 2.77 0.77 3.06 0.85 3.69 1.02 4.05 1.12 3.53 0.98 4.98 1.38 4.43 1.23 7.24 2.01 6.19 1.72 12.4 3.43 9.05 2.51 11.9 3.30 13.3 3.7

MI

::z 1.31 7.9 2.2

0.53 0.14 0.51 0.14 0.92 0.26 0.68 0.19 1.18 0.33 2.81 0.78 1.81 0.50 3.76 1.04 3.62 l*OO 2.08 0.58 3.62

la5Xe, 0.63 0.86 0.68 0.93 1.01 1.38 1.04 1.42 0.93 1.28 l-79 2.45 1.36 1.86 2.93 4.02 2.27 3.12 4.61 6.31 3.69 5.06 6.82 9.33 10.6 14.5 0.90 0.64 0.88 0.63 1.26 0.90 1.39 0.99 1.32 0.94 2.60 1.86 1.76 1.26 3.72 2.66 3.08 2.20 6.33 4.53 4.56 3.26 8.78 6.30 13.0 9.2

*Kr 0.94 0.47 1.02 0.51 1.60 0.80 1.43 0.72 1.37 0.69 2.93 1.47 1.95 0.98 459 2.30 334 1.77 7.49 3.75 5.47 2.74 10.6 5.31 15.9 7.9

8%.

DURMG IRRADIATION

5.58 1.68

4.76 1.43

3.02 0.91

2.41 0.73 2.56 0.77

1.69 0.51 1.87 0.56

la*Xe

88.5

time (hr)

110

49

82.5

48

130

49

108

47.5

108.5

97

50

104.5

brad.

* Run no. means the order of experiments and the present experiments were started with run no. 5 (irradiation time of run No. 5: 98 hr). t Xe, is a net release of xenon. z Upper values are release rates in terms of 10’ atoms/set and lower values fractional release R/B in terms of percent.

Temp. (“C)

Run no. *

TABLE ~.-EQ~J~L~EI~IUMRELEASERATE OF FISSION GAS PROM UOI-~~~~

% Q z

$

P

F

Fission gas release from UO&ispeised graphite during irradiation

TEMPERATURE

Fro. 3.-Grass

release rate of krypton from UOp-graphite irradiated at various temperatures. (Dotted lines indicate data from runs 6-S).

TE’MPERATURE

FIG. I.-Release

1% 1

t°C 1

of xenon and iodine from UOI-graphite irradiated at various temperatures. (Xe and Xe, mean gross and net xenon release respectively and the rates of the isobar 135 are reduced by a factor of 10)

339

340

K. SHIBA et al.

temperature. The release pattern had something like a peak at temperatures around 500°C. Apparently any exact coincidence in temperature dependence was not found among gross xenon, gross krypton, net xenon and iodine release. 3.4 Eflect offission density Figure 3 shows that data of krypton obtained from runs 9 to 16 are on solid lines while data from runs 6 to 8 are on dotted lines. The latter always lies over the former. This indicates an effect of irradiation on the release. An appropriate couple of runs permits us to calculate the resultant decrease in release, which is given in Table 4. TABLE 4.--EFPECT OF FISSION DENSITYON THE PISSIONKRYPTON RELEASE FROMUO+ZRAPHITE

Run no. 6-1: 6-2: 7-1: 8:

Difference in irrad. time (hr)

Temperature (“Cl

Ratio *

546 887 755 847

300 650 450 200

0.70 0.72 0.77 0.96

12 15

14 16

* Ratio means a release at the latter run divided by a release at the former and is an average of the ratios for ssWr, *Kr and Wr.

Clearly irradiation brings about retardation of the fission gas release. However, a longer irradiation does not always give a higher decreasing ratio (compare 650°C data with 300°C ones). This situation is remarkable for data obtained at 200°C. A reasonable solution to the problem is to assume that the effect of fission density is readily saturated in the beginning of irradiation and that further irradiation makes little difference in release. The assumption is the same as one of the conclusions drawn from the post-irradiation experiment (YAJIMA et al., 1962) and implies a promising applicability of the release model to the analysis of the in-pile fission gas release. 3.5 E@xt of decay constant An equilibrium fractional release R/B changes with decay constant il, as is shown in Fig. 5. This indication is that the release is a process depending on time. Slopes of log R/B vs. log 2 for iodine, gross xenon and krypton are almost the same in all the experiments. Since the release rate of 13sXe is not completely saturated under the present, experimental conditions, however, the true equilibrium value of lsaXe would be higher than indicated in the figure. The points of krypton are too scattered to be defined as a straight line seemingly because of an anomalous sink of 8sKr release which will be discussed later. In addition, we can see a tendency that the krypton release surpasses the xenon release with increasing temperature. 3.6 Separation of knock-out and dl@ision release of xenon The xenon release Xe, release

Xe, .

diffusion

An

release.

attempt

is a combination was made

The specimen

of knock-out

to determine

continued

by switching

off the furnace.

and diffusion

release free from

to be heated at the same temperature

during irradiation for 10 hr after the reactor was shut down, temperature

release Xek,

a knock-out

and then cooled

Results are shown in Fig. 6.

a as

to room

Fission gas release from UO,-dispersed graphite during irradiation

rcf3

‘33xe _’

16”

ICP DECAY CONSTANT

FIG.

5.-Plot

tO-3

10A (sac+ 1

of log (fractional release) vs. log (decay constant) for UO&raphite.

TIME FIG.

341

FROM START OF IRRADIATION

(hr 1

6.--Separation of knock-out and diffusion release of xenon.

As soon as the reactor had been shut down, the release of 133Xedropped abruptly, while the release of 135Xedid not drop to an accurately measurable extent. Since the difference between before and after the shutdown is only the presence of neutron irradiation (in this case the temperature distribution across the specimen can be neglected), the sharp drop should result from the knock-out process. Data obtained at 200 and 640°C are given in Table 5 (for the case of 640°C there was an interruption of heating for a period of 8 hr after the reactor shutdown and the knock-out release

,342

K.

SHIFIA et al.

TABLE~-KNOCK-OUTRELEASE

(“Cl

Releaserate XekO (atomslsec)

200 (Run No. 16) 640 (Run No. 15)

0.83 x 10’ 2.67 x 10’

Temperature

OF lssXe FROM UO+ZRAPHITE

Fractional release (%)

Saturation factor s

0.22 0.72

0.42 0.39

(xeko/B) x 100

Fractional knock-out rate Observed Calculated XedlBS

(set-I)

(EC-‘)

0.083 x lo-? 0.14 x lo-’ 0.28 x lo-’ 1.2 x lo-’

was obtained by the subsequent heating). The fractional knock-out rate is a knock-out release divided by the xenon atoms present in the specimen. Evidently the knock-out release does depend on temperature. This observation is in glaring contrast to the assumption that a knock-out release is independent of temperature, which seems to have been accepted as being general (CARROLL, 1965; CARROLLet al., 1965 ; JACKSONet al., 1964; MELEHANet al., 1963,1964; SOULHIER, 1966). During the heating period after the shutdown, the xenon release Xe, decreased with the decay rate of precursor iodine as the release ai room temperature was the case (Section 3.1), and no krypton could be detected in the sweep gas. As discussed elsewhere in detail (KAMEMOTO et al., 1970), these observations indicate that the xenon release originates in the iodine trapped by imperfections of the specimen, that is, a part of xenon is released in a short time after the iodine has decayed to xenon in the graphite. The concord between the release and the decay curve leads to the conclusion that a probability of xenon being released by the p-decay of iodine is constant at a temperature. Since the knock-out release of 135Xeis negligibly small, the gross release observed is governed only by the instantaneous release accompanying B-decay of the precursor iodine trapped in the graphite as well as the iodine that had been released and trapped in the internal traps. The importance of the iodine behavior during irradiation should be borne in mind in considering the xenon release. 4. DISCUSSION 4.1 The release mechanism of iodine-xenon chain Before discussing the in-pile release of fission gas, it would be pertinent to recall Yajima and coworkers’ results obtained in post-irradiation experiments from the same graphite as in the present work. Figure 7 shows typical heating curves of fission xenon and iodine release from the natural graphite irradiated at 40°C (YAJIMAet al., 1962, 1963). These curves are considered to present distributions of xenon and iodine atoms trapped in lattice defects, which are produced in graphite by nuclear process such as fission, p-decay, etc. and have a specific activation energy for annealing. In other words, the curves are referred to as indicating a probability for fission xenon and iodine to be trapped in defects. Heating the graphite with such distribution at a temperature Ti causes the defects corresponding to the shaded area in Fig. 7 to be annealed, releasing the fission gas trapped in them. The distribution is a function of fission density; the higher the fission density, the richer the high temperature fraction. Table 6 gives fractional releases of laaXe, integration of a distribution curve at a fission density of 1.9 x 1017f/cm”, around which the latter part (from Run 9 on) of the present experiment was carried out.

Fission gas release from UO&ispersed graphite during irradiation

343

A : Xe 0.7 x lO’3 f/cm3 B : Xe 1.9 x i0” f/em3 C : I l~3xld4f/cm3

0

500

Ti

TEMPERATURE FIG. ‘I.-Heating

1500

1000 PC)

curves of fission xenon and iodine release from natural graphite in post-irradiation annealing (5 deg/min).

TABLE6.--FRACTIONAL RELEASE OF '*'xe FROM NATURAL GRAPHITE IN POST-IRRADIATION EXPERIMENTS

Temp. (“C)

200

300

400

500

600

640

700

800

900

Fractional release (%)

0.8

l-4

2.3

3.8

6.0

6.9

8.5

12

17

The rate-determining process for fission gas release during irradiation also is expected to be trapping of fission gas by defects and release from them. In discussing the gross xenon release, the distribution of iodine in defects will become important because most of xenon atoms are not produced primarily by fission but by decay of the precursor iodine atoms which have long lives. Our model of the iodine-xenon release for the in-pile experiment is schematically shown in Fig. 8. Let us assume an isobar of iodine-xenon chain in which only iodine is regarded as a primary fission product. Fission fragments plunged into graphite by fission recoil energy lose the energy along their paths and at the ends stop with dissipation of the residual energy as heat, which brings about local melting (or sublimation) followed by resolidification within lo-l1 sec. This process creates many lattice defects in the graphite. Some of the fragments (iodine atoms) are captured by defects not annealed at irradiation temperature, while the others, which should have been caught by defects preservable at lower temperatures, are released as iodine from the graphite. The iodine is trapped in internal traps and decays to xenon, which gives Xe, presented by equation (1) Xe, =

P(T) dT,

where T,; irradiation temperature P(T);

distribution of iodine and

-P(T)dT=

s0

1.

344

K. SHIBAet al.

r-l I

released trapped 1 internc

and in

( B-

(knockSout

1

I h#

xeko

I % FIG. S.-Xenon

:e

P

release mechanism.

On the other hand, the iodine trapped in the defects also decays to xenon and a local region around them is subjected to a rapid heating by /?-recoil energy followed by a rapid cooling as in the case of the fission recoil. At the first stage, a fraction of the xenon resulting from the B-decay is instantaneously set free due to the degradation of the defects and the chemical effect of the change of iodine to xenon. At the second stage, some of the free xenon is retrapped by new defects, while the others are released from the graphite. Consequently, the fractional release of xenon (Xe& by this process can be expressed by the following equation: Xe, = K(T,)

f

mW’M%, dT

Ti

where I$“‘)~$; probability for xenon to be set free instantaneously from a defect in the #I-decay process, probability for the free xenon to be released in the redistribution W’& process of b-decay. Needless to say, K(T,) is related to the second stage and the integration term of equation (2) to the first stage. The distribution curves of iodine P(T) may be obtained by differentiating the release curves of iodine in Fig. 4 and are shown in Fig. 9. These curves have a peak at 750°C which is considered to correspond to the peak of iodine observed in the post-irradiation experiments. Then, the gross xenon release (Xe) is

Fission gas releasefrom UO,-dispersedgraphite during irradiation

r&-----J 0 500 TEMPERATURE

345

IO00 (“cl

FIG. %-Differential curves of iodine release from UO,-graphite. (‘Ibe curve of la51is reduced by a factor of 10. The plots at 0-100°Care obtained by dividingthe fractional release at 100°Cby 100deg.

given by the following equation: Xe = Xe, + Xe, + (Xe,,).

(3)

The knock-out release (Xe,,), which is related to the fate of the xenon still trapped in defects, will be discussed in the following Section. In the above discussion is not taken into consideration a time of fission gas being released after its production, although L dependence of R/B evidently shows the presence of such a process. For quantitative discussion, a term of the time should be added to the equations. Comparing the data in Table 3 and 6, we see that the fractional gross release of fission gas during irradiation is, at almost the same fission density, in agreement with the fractional release of ls3Xe in the post-irradiation experiment. The data of the latter reflect the overall effects which l=Xe receives during the course from production as iodine through /?-decay to release, while the gross xenon release totalizes fission gas releases at each stage during irradiation. The agreement between the post-irradiation and the in-pile data is interesting from the practical and the theoretical point of view. Figure 9 shows that iodine is released even at lOO”C, amounting to l-2 % for laaI and 041 ‘A for 1351in terms of R/B. The 1 dependence of R/B for 100°C is the same as for the higher temperatures (see Fig. 5). This seems to indicate that the similar mechanism operates for all the temperatures, that is, a contribution of recoil process to the release of iodine at 100°C is relatively small. 4.2 A proposed model for the knock-out release A knock-out release, or a recoil-activated release, has been described as follows. When a fission fragment passes across the surface of a fuel by fission recoil, a volume of the fuel near the surface is evaporated and all fission products there are released

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K. SHIBAet al.

(MELEHAN,1963). If the knock-out release were as above, the present data would lead to the improbable conclusion that the volume evaporated at 640°C amounted to a volume about 3.4 times larger than at 200°C. Another model should be proposed. We regard the knock-out release as being an escaping and redistribution process of fission gas trapped in the defects not only on the surface but also inside of fuel in agitated region by fission fragments by analogy with the b-decay process already discussed. The main features of the difference between the knock-out and the b-decay release are the absence of the chemical effect as well as the substantially larger volume of the agitated region in the former. Then, the following calculation was made. When the region subjected to irradiation of a fission fragment is taken to be a fission spike in graphite, 13 ,um (WALTON, 1962) and of 25 A radius, its volume is O-5 x 10-16 cm3/fission. The total volume irradiated is calculated to be 0.28 x 10M4 cm3/sec by multiplying by the fission rate 5.62 x lOlo f/set. On the other hand, the volume of the specimen is 15 cm 3. Consequently the fractional volume irradiated is 1.8 x 10-s/sec. This corresponds to a fractional knock-out release rate where all xenon present within the spikes is released. Assuming that a fractional release of xenon from the spike region is equal to the fractional release of lssXe given in Table 6, we can calculate a fractional knock-out release rate to be 1.2 x lo-‘/set for 640°C and O-14 x IO-‘/see for 200°C. These values agree with ‘those observed in the order of magnitude (Table S), although the agreement might be fortuitous due to choice of spike parameters. This supports the present model for the knock-out release revealing the temperature dependence. 4.3 Release behavior of bromine-krypton chain On the basis of the foregoing discussion, we will try to elucidate the release behavior of krypton, especially the release sink of ssKr in Fig. 5. It has been observed that the release rate of krypton reaches an equilibrium value soon after the irradiation starts and drops as soon as the reactor is shutdown. In common sense, there is a distinct similarity in nature between bromine and iodine as well as between krypton and xenon. Therefore, it is reasonable to assume that the same mechanism operates for the release of krypton as for that of xenon which is schematically shown in Fig. 8. Let us adopt the same notation of suffixes for krypton as for xenon. Because all the nuclides of krypton in concern have shorter half-lives than 135Xe, we can neglect the Krko process. Independent yields of krypton are so small that the recoil process is reserved for later discussion. Then, all we have to do is to consider the release processes of Kr, and Kr,. If the Kra is the only process of release for krypton, that is to say, no bromine is released with krypton in the event of p-decay of bromine in the specimen, the fractional release of krypton will be observed to decrease in the order of 85mKr, **Kr, and s7Kr. On the other hand, if only bromine is released (Kr,) with no release of krypton from the specimen the fractional release of krypton will decrease in the order of 85mKr, *‘Kr and ssKr. In this case, it is possible to observe a release sink of krypton when the fractional releases of krypton are plotted vs. the decay constants of krypton. As a matter of fact, both processes should be responsible for the krypton release as we have seen that such is the case for the xenon release. Assuming that the Kr, is more predominant than the Kr,, such a release sink will be observed for krypton. This is equivalent to the assumption that an average time for bromine to be released after the

Fission gas release from UOp-dispersed graphite during irradiation

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production is shorter than the half-lives of bromine or comparable to those. Although we conveniently plotted the gross releases of fission gas vs. the decay constants of krypton and xenon, there is no support to warrant such plots. Another factor is considered in relation to fission yields. The independent yield of ssKr comes to 0.6 per cent and composes 17 per cent of the cumulative yield, the largest among the krypton nuclides (6 % for 87Kr and 0 % for 85mKr). Since a /?-decay process generally gives an additional release, the release of **Kr owing to p-decay of bromine is decreased by an amount corresponding to 17 per cent. This effect may be minor but should be added to the explanation of the anomaly. So far we have discussed the behavior of krypton on the basis that the used figures of fission yields, decay schemes, etc. are right, However, some uncertainty in them might exist and so might explain the anomaly of release. Our preliminary experiments show that products of fission yield and y-abundance of *‘Kr and **Kr seem to be smaller than the figures given in Table 1 when they are normalized to the data of 85mKr. However, more precise, quantitative experiments to determine fission yields, decay schemes etc. of krypton will be needed. A careful examination of our data indicates that the gross release of 85mKrtends to surpass that of 135Xeat high temperature in spite of the shorter half-life of the former. Such was also observed on pyrocarbon under irradiation (REAGAN et al., 1965). These results correspond to the facts observed in post-irradiation experiments by WALKER et al. (1966) that the fractional releases of argon, krypton and xenon decrease with increasing the atomic number (size). The effect of atomic size may result from the difference in probabilities of the fission gases to be trapped by a defect. authors would like to thank Prof. S. Yajima (Tohoku Univ.) for allowing us to read his review before publication. They are also indebted to Mr. N. Yamabayashi (JAERI) for the contribution in determination of absolute release rates.

Acknowledgements-The

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