Concentration profiles of fission products in the coating layers of irradiated fuel particles

Journal of Nuclear Materials 66 (1977) 55-64 0 North-Holland Publishing Company

CONCENT~TION PROFILES OF FISSION PRODUCTS IN THE COATING LAYERS OF IRRADIATED FUEL PARTICLES

K. FUKUDA and K. IWAMOTO Divisionof Nuclear Fuel Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaragi-ken,Japan Received 9 August 1976

The distributions of 13’Cs, 90Sr and 144Ce in the coating layers of several kinds of TRISO coated fuel particles for HTGR were determined by method of removing the coating layers stepwise after irradiation. The distributions in Sic coating layer are influenced by its density and irradiation temperature. High dense SE coating layer showed good retention of the fission products at such high irradiation temperature as 153O”C, while the retention of low dense one would change at 13OO’C; above the temperature, it would be lost. There are several patterns of the distribution in outer PyC coating layer such as high accumulation of the fission products in a certain position of the layer, outerward rises in the concentrations and high concentrations in the layer than those in some positions of SIC coating layer. The post irradiation annealing was also carried out for one kind of the particle. La distribution de 137Cs, *Sr and f44Ce dans les couches de rev&ements de plusieurs types de particules de combustible recouvertes d’un d&pi% pour le reacteur HTGR a 6th dt%erminie par la mkthode d’abrasions successives des couches de d8pcit apres irradiation. Les distributions dans les couches de rev&tement en Sic sont influencees par leur densitd et la tempirature d’irradiation. Une couche de revdtement de SiC de haute densite pr6sentait une bonne retention des produits de fission B des tempkratures d’irradiation aussi ilevees que 153O”C, tandis que la ritention d’un d&pat moins dense disparaissait au-dessus de 1300°C. 11y a plusieurs types de distribution dans un revbtement externe de PyC telles que l’accumulation des produits de fission s’&ve i des concentrations plus &levies dans une certaine region du dCpbt que ceIle observ&es dans quelques rkgions d’un dip% de SE. Une sorte de particules a Bt&soumise i un recuit apris irradiation. Die Verteilung von ’ 37Cs, 9oSr und 144Ce in den Schichten verschiedener TRISO-beschi~hteter Brennstoffte~chen fiir Hochtemperaturreaktoren wurde nach der Bestrahlung stufenweise durch Schichtabtrag untersucht. Die Verteilung in der SiC-Schicht wird durch deren Dichte und durch die Bestrahlungstemperatur beeinflusst. Hoch dichte Sic-Schichten haben ein gutes Riickhaltevermijgen fir die Spaltprodukte bei derartig hohen Bestrahlungstemperaturen wie 153O”C, wIhrend das Riickhaltevermiigen der niedrig dichten Schichten sich bei 1300°C iindern diirfte; oberhalb dieser Temperatur diirfte es verschwinden. Es gibt mehrere Formen der Verteilung in der lusseren PyC-Schicht, wie eine starke Ansammlung der Spaltprodukte an einer bestimmten Stelle der Schicht, ein Konzentrationsanstieg an der Aussenseite und hahere Konzentrationen in der Schicht als an einigen Stellen der Sic-Schicht. Eine W~rmebehandlu~ nach Bestrahlung wurde an einer Partikelart ebenfalls durchgef~hrt.

1. Introduction

sible to restrain the release of fission products from the UOz kernel by adding ceramic inclusions to it such as A1203 and SiOz, which form stable oxides with fission products such as Sr, Ba and Cs within the kernel [S-7]. Results of this attempt showed good retention of these fission products as in the case of SiC coating layer [S--7]. The coated fuel particles with Sic coating layer, i.e. the TRISO type, however, are most widely used as the driver duel of HTGR at present. In irradiation test of

PyC coating layer in the HTGR coated fuel partides is effective for retention of gaseous fission products [1,2], but only more or less time delaying for the solid fission products as Sr, Ba and Cs. Retention of the solid fission products can be improved particularly in two ways: first is due to addition of Sic coating layer sandwiched between PyC coating layers, which retains especially Ba and Sr [3,4]. In the other way, it is pos55

K. Fukuda, K. Iwamoto / Concentration

56

the coated fuel particles, attention is paid to the release of fission products as important as breakage of the coating layers during irradiation. Also migration behavior of the fission products requires analysis of their distributions in the coating layers. Various methods of removing the layers are applied to obtain the distributions; ion sputtering [&lo], chemical etching [ll-151,fusedalkali orsalt [16-18],and direct measurement by electron probe microanalysis [19,20]. Among the marked characters of fission products in the distribution are that Cs displays a maximum concentration at the interface between Sic and inner PyC coating layer [B], constant concentration in inner PyC coating layer [ 191, the profile depending on the fuel burn-up [ 161, and the accumulation in defects of Sic coating layer [lo]. Maximum concentration of Sr in Sic coating layer [20] and maximum concentration of

Table 1 Characterization

profiles of fission products

Ru at the interface between Sic and inner PyC coating layer [8,20] are also noticeable. On the whole, higher concentration of some fission products toward the particle surface in outer PyC coating layer are seen in several studies [8,9,14,16]. Sr behavior is, however, hardly reported, although Sr is important as ingestion hazard. It is due to a complicated experimental technique to determine the distribution of 90Sr because it is only P-emitting. In the present study, the distributions of 137Cs, 90Sr and 144Ce in the TRISO coated fuel particles irradiated under different conditions are measured in order to see the migration behaviors, with high concentration particularly in outer PyC and Sic coating layer. Furthermore, one of the particles is annealed in connection with change of the distribution and the release of the fission products.

and irradiation condition of coated fuel particles

____Specimen 1

Specimen 2-Aa)

Specimen 2-Ba)

Specimen 2-Ca)

Specimen 3

UC2 20

UO2 20

UC2

UC2 20

uo2

20

Buffer PyC: Density (g/cm3) Thickness (p)

1.1 35

1.21 43

1.21 43

1.21 43

1.18 39

Inner PyC:

Density (g/cm3) Thickness (p)

1.7-1.8 34

1.76 34

1.76 34

1.76 34

1.80 26

Sic:

Density (g/cm? Thickness (M)~)

3.13 29.2

3.19 28.7

3.19 31.0

3.19 29.6

3.15 33.7

Outer PyC:

Density (g/cm3) Thickness (/L)~)

1.7-1.8 43.6

1.76 43.5

1.76 39.2

1.76 44.5

1.9 48.2

1560-1495

1460-1200

12.1

12.1

Coated fuel particles

Kernel:

Material 235U enrichment (%)

8

Irradiation condition

Temperature (“C)

max. 1300

Time (days)

21.6

Fuel burn up (% FIMA)

1.84

mean 1140 (1170-1100) 30.3 2.02

Fast neutron dose (n/cm?, E > 1 .O MeV)

4 x 10’9

5 x 10’9

Reactor

JMTR

JMTR

a) Taken from same batch of coating operation. b) Measured for individual particle by X-ray microradiography.

0.7

___~

0.13

6 x lo’*

4x

1019

JRR-2 __._~

JRR-2

K. Fukuda, K. lwamoto j Concentration profiles of fission products

2. Experimental Three kinds of the TRISO coated fuel particles to totally five specimens were used in the experiment, as characterized in table 1 where specimen 2-A, 2-B and 2-C were taken from the same batch of coating operation. The buffer PyC in all specimens was deposited by pyrolysis of Ca Hz under a carrier gas of Ar; both the inner and outer high dense PyC’s, deposited by pyrolysis of CsHa at a temperature between 1400’ and 1700°C under the carrier gas; and the Sic, deposited by pyrolysis of CH$iCls at a temperature between

Fig. 1. Typical cross section of coated fuel particle irradiated at 1140°C up to 2.02% FIMA.

B. KOH-Na2C~ treatment

Dissolution

time (min.)

Fig. 2. Variation of coating layer thickness in dissolution process.

51

1500’ and 1600°C under the carrier gas of Ar or He. Thicknesses of the Sic and the outer PyC layers in each specimen were measured individually by X-ray microradiography . Irradiation conditions of the specimens are summarized also in the table. Specimen 2-A and 2-B were irradiated at a relatively low temperature (114O’C) up to 2.02% FIMA in fuel burn-up. But specimen 2-C was irradiated at the highest temperature (1560-1495”(Z), although the fuel burn-up was low (0.7% FIMA). Typical cross section of the coated fuel particle irradiated at the same capsule as specimen 2-A and 2-B is shown in fig. 1. For determination of the fission-product distribution, the outer and inner PyC’s of a single particle were removed stepwise by dissolving in KaCrsO,-HNOs solution at 14O”C,and the Sic by fused KOH-NazCOs mixture at 800°C. The latter method was reported in detail previously f 171. The removal temperature of 800°C hardly affected the distribution in the SiC [17] and the inner PyC!.By a simple estimation, Sr movement distance in the inner PyC, for instance, during the Sic removal would be only 1 /J or less for 40 min if the diffusion coefficient in the PyC at 800°C is taken to be 7 X 10-‘2 cm’/sec [21]. Typical removal process with a function of the dissolution time is shown in fig.2, where it took from 5 to 30 mm for the removal in each step. Most particles were broken in the way of dissolving the inner PyC. The solutions obtained from the removal process (in case of the SIC removal, mixed alkali was dissolved in 6M-HNOs) were radiochemically analyzed, and 13’Cs, 90Sr and 14”Ce in the removed layer were measured for their radioactivities (7 activity for 13’Cs and 144Ce, and /3activity for 90Sr). Decrease of the layer was measured by X-ray microradiography after each removal step. Fig. 3 shows the typical X-ray microradiographs taken in several steps, where numbers in photos correspond to those in fig. 2. In fig. 3, it is seen that the layer removal proceeds uniformly around the surface in any step, while breakage of the inner PyC is seen in photo (5). In case of specimen 2-B, a particle contained in a long graphite crucible was first annealed such that the temperature was raised stepwise from 1200’ to 18OO’C with 100°C interval, except only 13OO”C,kept for 2 h in each stage. 13’Cs, 90Sr and ‘44Ce released and absorbed in the graphite crucible in each annealing stage were collected by burning off the crucibles in oxygen

K. Fukuda. K. Iwamoro / Concenrrarion profiles of fission products

58

Fig. 3. Typical X-ray microradiographs in dissolution process; numbers in photos are identical to those shown in fig. 2.

atmosphere at 1000°C and radiochemically analyzed, followed by measurement of the activities: Then the particle was analyzed for its fission product distribution

3. Results 3.1. Fission product distribution after irradiation Fission product distributions in the coating layers of specimen 1 are shown in fig. 4 where the ratio of fission product concentration in the coating layer to that in the kernel is plotted against radius of the particle. In the figure, it is seen that very high concentrations of all the fission products are in the middle of the outer PyC. ’ 37Cs and 144Ce are very low in concentrations less than detectable close to the surface of the particle, while 90Sr shows relatively equal concentration throughout the layer. In the Sic, its ability as barrier for the fission-product migration would be retained during irradiation in spite of its low density (3.13 g/cm3), since all the concentrations reduce rapidly toward the outerside. Among the fission products,

90Sr show the highest concentration in the Sic. Fig. 5 shows the distributions in specimen 2-A. In the outer PyC, 137Cs displays several distinctions which are a outward rise and the highest concentration close to the surface. But, its concentration is less than detectable at the interface between the outer PyC and the Sic. Although rather constant concentration is seen in both 90Sr and ‘44Ce, the concentrations are highest close to the surface as well. In the Sic, on the other hand, outstanding decrease in the concentration toward the outer surface is observed for all the fission products, among which the ’ 37Cs gradient is most steep and the ‘44Ce gradient is rather gentle. At the interface between the Sic and the inner PyC, the 144Ce concentration level, however, is much less than those of the others. In the inner PyC, all the concentration gradients may be more gentle than those in the Sic. The distributions in specimen 2-C taken from the same batch of the deposition with specimen 2-A but irradiated at the highest temperature (mean 1530°C) are shown in fig. 6. In comparison with fig. 5, the difference of the distributions in fig. 6 are especially noticed in the outer PyC. In the figure, the concentrations

K. Fukuda, K. Iwumoto / Concentmfion profires of fission products , OO1c;Oulqr-PY:j

i

I

$l”_ % 0 =

vlc34--1

_

lb 378

of three fission products in the outer F’yCare about two orders of magnitude greater than those in specimen 2-A. Even though the outward rise in the outer PyC is also seen slightly in this case, the steep rise close to the surface (as seen in fig. 5) would not appear but the extremely low concentration of 137Csis still distinct close to the interface between the outer E’yCand the Sic. In the inner portion of the Sic, the inward steep rises of all the fission products are seen again as well as in specimen 2-A. This indicates that the ability of the Sic as the barrier for the migration is still effective in such high temperature irradiation. Difference in 13’Cs and “Sr distribution is not apparent in the SIC. The ‘44Ce concentration at the interface between the inner PyC and the Sic is notably high in comparison with that in specimen 2-A. In the outer portion of the Sic, that tendency is vice versa; the rise faces toward the outer side as if the fission products migrate from the outer PyC. Specimen 3, having low dense SIC (3.15 g/cm3) indicates high concentration and relatively constant distribution throughout the outer PyC and the SIC as shown in fig. 7. In the outer PyC, the rise of 137Csin concentration toward the surface is found and this

,~“,~, i

-4

.________/

1

ld

~.L___.j

,.

59

p-1.

3%

Xii

Radius of particle fpt) Fig. 4. Fission product distribution

in coating layers of specimen 1 irradiated at below 1300°C up to 1.84% FIMA.

Radius

of particle

($11

Fig. 5. Fission product distribution in coating layers of specimen 2-A irradiated at 1140°C up to 2.02% FIMA; shaded region is initial 235U contamination level.

Radius Of particle

(p)

Fig. 6. Fission product distribution in coating layers of specimen 2C irradiated at 1560°C up to 0.7% FIMA.

K. Fukuda, K. Iwamoto / Concentration profiles of fission products &

Outer-PyC ------do

I

,

,

,:I

SIC -4

%

I

:

9*sr

i144ce /137@

I

t

I 820

Radius of particle ( J.I)

Fig. ‘7. Fission product distribution in coating layers of specimen 3 irradiated at 146O’C up to 0.13% FIMA. Anneahnd temperature (“C)

tendency is also visible slightly in two other fission products. The gradient of the concentration in the Sic is the greatest in 13’Cs, followed by ‘44Ce and then 90Sr although these are all gentle. As for the concentration, 9o Sr is the greatest among the fission products. These indicate the poor ability as the barrier in this irradiation. 3.2. Annealing effect The accumulated fractional releases in the isochronal annealing of specimen 2-B from 1200” to 1800°C are shown in fig. 8 as a function of the annealing temperature. All the fission products are rapidly released at 12OO”C, the initial annealing temperature, followed by the moderate at higher temperatures. 90Sr in the figure, however, shows a steep rise in the release at 18OO”C, suggesting onset of diffusion release from the kernel. The levels of the release are ranging from order of 10e4 in 90Sr to lo-’ in ‘44Ce and then IOW6 in 13’cs. The dist~bution of specimen 2-B after annealing is’ shown in fig. 9. The r3’Cs distribution in the outer portion of the outer PyC is nearly the same as that of specimen 2-A (fig; 5); high concentration close to the surface is still kept in spite of the annealing. This corresponds with the low release of r3’Cs. On the contrary,

Fig. 8. Fractional release of fission products from specimen 2-B in post irradiation annealing.

the concentration in the inner portion of the layer becomes quite high. The “Sr concentration in the outer portion is fairly low in compar~on with that of specimen 2-A, although that in the inner portion is nearly the same as that in specimen 2-A. The ‘44Ce distribution in the outer PyC hardly changes by the annealing. Gradients of the concentrations of all the fission products in the Sic are slightly steeper by annealing; the concentrations close to the interface between the outer PyC and the Sic reduce, and those close to the interface between the Sic and the inner PyC increase, where the ‘44Ce is particularly noticeable.

4. Discussion Among the distributions obtained in the present study, there were some strange behavior of the fission products in the coating layers. They were as follows; (1) accumulation of all the fission products at the central portion in the outer bC (specimen l), (2) outward rises in the concentration in the outer PyC (specimen 2-A, 2-B and 3), particularly high increase of 13’Cs close to the surface (specimen 2-A and 2-B), and higher con-

K. Fukuda, K. Iwamdto / Concentration profiles of fission products

llf ?-

Sic---+

I

1

35v

400 Radius

of particle

(JJ)

Fig. 9. Fission product distribution in coating layers of men 2-B annealed after irradiation.

speci-

centration of all the fission products in the outer PyC than in the some portion of the Sic (specimen 2-C and 3). With regard to (l), such accumulation in the PyC has not been reported anywhere, but according to Coen et al. [l 11, Cs tends to be accumulated in the defects (striated structure) of the Sic coating layer. Although visible defects in the outer PyC of specimen 1 can be hardly noticed in the ceramographs of the post-irradiation examination, similar effects to those in the Sic might occur in the present case. The outward rises in the concentrations (2) were mostly common in the distribution in the outer PyC coating layer [8,9,14,16] which had been obtained in different ways. In the study [8], this trend in the 13’Cs and l”Ce was still kept after the annealing at 18OO’C as well as the present case (fig. 9). The steep rise close to the surface of the particle as seen in the case of 13’Cs (figs. 5 and 9) is displayed only in 95Zr in the literature [ 141. There would be no common feature of the fission product behaviors among the various distributions reported. Although such outward rises would be partially derived from the “‘U contamination on the surface as explained in [8], there seems to

61

be another factors causing this phenomena. According to the contamination measurement [22], the surface contamination ranging from 1 X lo-’ gU/g particle to’ less than 0.3 X lo-’ gU/g particle is generally several times larger than that within the outer PyC [22]. The surface contamination of specimen 2-A, 2-B and 2-C is 5 X 10e6 gU/g particle, which leads to the fission product concentration, Cl,,, & kernel25 2 X lo:‘, in average within the fission range from the surface, if the range is taken to be 14 p in high dense PyC [23]. Furthermore, taking the above value (5 X 10b6 gU/g particle) as the contamination within the outer PyC eventhough it may be maximum estimation, the average concentration generated from the contamination in the outer PyC would be - 1 X lo-‘, which is illustrated as the shaded region including the concentration by the surface contamination in fig. 5. In comparison of the fission product concentrations with that of the contamination in the figure, it is found that the 144Ce concentration is nearly the same as the level of the comtamination through the outer PyC, while the 90Sr one is about one order of magnitude higher than that. The profile of 13’Cs is, however, quite different from that. Considering the diffusion coefficients, where Sr is 3 X lOma cm2/sec [24], Ce 3.8 X lo-l4 cm’/sec [ll] and Cs 4.4 X lo-l2 cm’/sec [25] at 12OO”C, 90Sr is most mobile and l”Ce is most immobile in the PyC, and here it would be found that 144Ce is mostly attributed to the 235U contamination but 90Sr and 13’Cs are apparently different from that. Thus, it appears that the outward rises (and also the concentrations throughout the outer PyC) are not only due to the initial 235U contamination. This is also suggested by keeping the outward rises even after the annealing which would cause the distribution change so as to conform to the diffusion law. According to the sutdy made by one of the authors [26], surface accumulation of fission product on graphite is possible by forming certain surface compounds. However, the compounds are assumed to accumulate only on the surface, and so it would be sure in the present case that this outward rise close to the surface is not relevant to the surface compounds. Since the migration of 13’Cs in dense PyC takes place mainly via gas phase in spite of the formation of CsClo [l 11,location of defect such as micro-pores would influence strongly the distribution. On the other hand, although Sr migrates rapidly than Cs in PyC, it is less volatile and absorbed firmly in the pores [27].

K. Fukuda, K. Iwamoto / Concentmtion profiles of fission products

62

Table 2 Comparison of released amount of fission product in annealing and total amount of it in coating layers Fission product

Accumulated fractional release in annealing (-)

Total amount of fission product in outer PyC

Total amount of fission product in Sic

Specimen 2-A

Specimen 2-B

Specimen 2-C

Specimen 2-A

Specimen 2-B

Specimen 2C

1.4 x 10-S

3.1 x 10-S

4.9 x 1o-5

1.3 x 1o-3

2.8 x 1O-2

3.9 x 10-2

1.4 x 10-2

%r

3.4 x 10-4

1.9 x lo+

8.3 x 1O-5

9.4 x 10-2

1.4 x 10-Z

4.0 x 10-2

2.8 x 1O-2

‘%e

3.1 x 10-S

2.9 x 10-S

1.4 x 10-S

6.9 x’ 1O-3

7.6 x 10-4

1.1 x 10-Z

3.4 x 10-S

‘37cs

With regard to the fission product release behavior, all the releases tend to level off with increasing temperature except the 90Sr at 1800°C (fig. 8). Although such tendency is alleged to be caused by the 235U contamination in the coating layer [20], the initial contamination level in the outer coating layer would not be necessarily same as those of the fission products as shown in the fig. 5. Furthermore, it seems that since the gradients in the concentrations in the Sic of specimen 2-B (fig. 9) become steeper than that of specimen 2-A (fig. 4), the fusion products are by far more mobile during irradiation than the. post-irradiation annealing if the diffusion theory is considered. In table 2 the comparison between the total amount of released fission products in the annealing and those. remaining in the outer PyC and the Sic of specimen 2-A, 2-B and 2-C is useful to see the cause of the release, where the latter were obtained from figs. 5,6 and 9, and converted to the same unit as the release. The amounts in the outer PyC of specimen 2-A minus that of specimen 2-B is nearly the same as the total amount released although the ’ 37Cs is reverse. The balance should, of course, take account of the amounts mi-

Table 3 Fraction of breakage of coated fuel particles by irradiation Fraction of breakage * Specimen 1

<1x104

Specimen 2-A

3.75 x 10-3

Specimen 2-C

3.18 X 1O-3

Specimen 3

<1x

l

Obtained by HN03 leaching.

10-4

grated into the outer PyC from the BC during the annealing. However, above fact well indicates that all the releases depend heavily upon the amount of the fission products having resided in the outer PyC after irradiation. Thus, it appears that the release in the post-irradiation annealing is caused not only by the initial tission product attributed from the contamination but also greatly by the fission product having migrated Into the outer PyC during irradiation. In addition it is distinctive in table 2 that the amount of 137Cs and 90Sr in the Sic of specimen 2-A is fairly the same as those of specimen 2-C although the former was irradiated at by far lower temperature than that of the latter (table 1). As for 144Ce, the low concentration in the SIC of specimen 2-A and 2-C is certainly affected by formation of stable Ce20s in the U02 kernel [ 111. Conspicuously high concentrations in the outer PyC of specimen 2-C and 3 (figs. 6 and 7) are also one of the features in the present study, although these in specimen 3 might be attributed to degradation of the Sic by irradiation. It might come up for discussion that these behaviors are caused by the transport of the fission products released from the broken particles into the intact outer PyC via the particle surface or gas phase in the capsule during irradiation. The fraction of the breakage in the irradiated coated fuel particle obtained by HN03 leaching is listed in table 3. As seen in the table, the fractions are 3 X 10e3 or less which would be too low to make such high concentrations in the outer PyC. It would be, however, possible if specimen 2-C particle happened to be in contact with a broken particle in the capsule, although this possibility would be very low. It is, therefore, not determinable at present whether the high concentrations are caused by the neighboring broken particle in the

X. Fukuda, K. Iwamoto / Concentration profiles

of fission products

63

Table 4 Partition coefficient at interface between outer PyC and SC coating layer ---Specimen 1

Specimen 2-A

Specimen Z-B

(49.3)


0.90

%r

0.06

0.26

2.65

*+Ze

0.02

0.09

0.48

l3’CS

capsule or the other effect, for instance, anomalous behavior of the fission products during irradiation. Partition coefficient defined as the concentration discontinuity at the interface of different layers [29, factor to estimate 301, QI= Cz(alfC 1(a > is an ~portant the fission product release from the coated fuel particles, where Q,is the coefficient, C, the concentration at the interface in the outer layer, Cr that in the inner layer and a the radius of the interface. The coefficients of each fission product at the interface between the outer PyC and the Sic obtained from the present distributions are summarized in table 4. In the case of t3’Cs, the coefficients are less than 1 if that in specimen 1 is disregarded, while those of “Sr and 144Ce widely range up to 153. Comparing the coefficients of each fission product in specimen 2-A with those in specimen Z-B, it is found that the annealing makes them higher. In specimen 3 which has low dense SIC layer, all are nearly 1.

5. Conclusion

The present study regarding to the dist~bution of the fission products in the coated fuel particles leads to the following conclusions: (1) The method adopted in the present study for removal of PyC with KsCr203-HNOs and Sic with fused KOH-NaaC03 is useful for determination of the distribution in TRISO coated fuel particles. (2) The distributions in the Sic coating layer are influenced by its density and irradiation temperature. The high dense Sic coating layer prevents migration of the fission products even at such high irradiation temperatures as 1530°C. This ability as the barrier in the fow dense Sic coating layer tends to be mostly influenced by the irradiation temperature. At the low temperature less than 1300°C, the layer exhibits rela-

Specimen 2-C 0.24 153 4.87

Specimen 3 0.64 0.70 1.15

tively good retention while above the temperature, it would lose its ability. In the high dense Sic coating layer, there is no difference between 13’Cs and 90Sr distribution, but in the low dense Sic, 90Sr shows the higher concentration. (3) There are several patterns of the distributions in the outer PyC; high accumulations of all the fission products in a central portion of the layer, outward rises in the concentrations and higher concentrations in the layer than those in some position of the Sic coating layer. These phenomena in the outer PyC seem to be attributed not only to the initial fission products derived from the contamination but also to the fission products having migrated during irradiation. (4) The partition coefficients at the interface between the outer PyC and the Sic obtained range widely from less than 0.001 up to 153. The annealing, however, tends to make them higher. (5) In post irradiation annealing, “Sr release is the greatest among the three kinds of the fission products. The “?Ze concentration close to the inner surface of the Sic increases with increasing temperature both in irradiation and annealing. In addition to the concentration of 235U contamination, the fission products having migrated into the coating layers during irradiation would play an important role for the release in the high temperature post irradiation annealing.

Acknowledgement The authors wish to express their thanks to Dr. S. Nomura, Head of the Division of Nuclear Fuel Research of Japan Atomic Energy Research Institute for his interest and encouragement. Thanks are also due to Mr., H. Ito, Chief of Hot Laboratory of the institue for his aid in the present study.

64

K. Fukuda, K. lwamoto / Concentration profiles of fission products

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