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Journal of Nuclear Science and Technology

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Reduction in Degree of Absorber-CladdingMechanical Interaction by Shroud Tube in ControlRods for the Fast Reactor

Takako DONOMAE , Kozo KATSUYAMA , Yoshiaki TACHI , Koji MAEDA ,Masaya YAMAMOTO & Tomonori SOGA

To cite this article: Takako DONOMAE , Kozo KATSUYAMA , Yoshiaki TACHI , Koji MAEDA ,Masaya YAMAMOTO & Tomonori SOGA (2011) Reduction in Degree of Absorber-CladdingMechanical Interaction by Shroud Tube in Control Rods for the Fast Reactor, Journal of NuclearScience and Technology, 48:4, 580-584

To link to this article: http://dx.doi.org/10.1080/18811248.2011.9711736

Published online: 19 Apr 2012.

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Reduction in Degree of Absorber-Cladding Mechanical Interaction

by Shroud Tube in Control Rods for the Fast Reactor

Takako DONOMAE�, Kozo KATSUYAMA, Yoshiaki TACHI, Koji MAEDA,Masaya YAMAMOTO and Tomonori SOGA

Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Ibaraki 311-1393, Japan

(Received August 2, 2010 and accepted in revised form October 22, 2010)

Research and development of a long-life control rod for fast reactors is being conducted at Joyo. One ofthe challenges in developing a long-life control rod is the restraint of absorber-cladding mechanicalinteraction (ACMI). First, a helium-bonding rod was selected as a control rod for the experimental fastreactor Joyo, which is the first liquid metal fast reactor in Japan. Its lifetime was limited by ACMI, whichis induced by the swelling and relocation of B4C pellets. To restrain ACMI, a shroud tube was insertedinto the gap between the B4C pellets and the cladding tube. However, once B4C pellets cracked and brokeinto small fragments, relocation occurred. After this, the narrow gap closed immediately as the degree ofB4C pellet swelling increased. To solve this problem, the gap was widened during design, and sodium wasselected as the bonding material instead of helium to restrain the increase in pellet temperature. Irradiationtesting of the modified sodium-bonding control rod confirmed that ACMI would be restrained by theshroud tube regardless of the occurrence of B4C pellet relocation. As a result of these improvements, theestimated lifetime of the control rod at Joyo was doubled. In this paper, the results of postirradiationexamination are reported.

KEYWORDS: sodium bond, control rod, Joyo, B4C, irradiation, shroud tube, burnup, ACMI,crack, cladding tube

I. Introduction

A challenge in the development of a long-life control rodfor fast reactors is the restraint of absorber-cladding mechan-ical interaction (ACMI). In Japan, research and developmentof a long-life control rod has been carried out through irra-diation testing at the experimental fast reactor Joyo.1) Heli-um was selected as a bonding material inside absorber pinsin the control rods at the early stage of development. Thehelium-bonding control rod was, however, revealed to havea limited lifetime up to the burnup of boron carbide (B4C)pellets of about 50� 1026 cap/m3 due to cladding deforma-tion by ACMI,1,2) although the theoretical lifetime of B4Cpellets is approximately 260� 1026 cap/m3 of burnup fromthe nuclear point of view.

ACMI is induced by a combination of the swelling andrelocation of B4C pellets and their fragments. During irra-diation, cracking occurred in the B4C pellets and then frac-tured into small pieces. The fragments relocated to a gapregion between the B4C pellet and the cladding tube, wherethey caused cracks in the cladding tube by the mechanicalinteraction, as shown in Fig. 1(a). In order to restrain relo-

cation for the reduction in the degree of ACMI, a shroudtube was inserted into the gap. A wider gap was employed toenhance the shroud tube effect on the restraint of ACMI. Inaddition, sodium was selected as the bonding material in-stead of helium for the reduction in pellet temperature,which would be increased by the wider gap. The shroudtube is expected to hold the B4C pellet fragments, as sche-matically shown in Fig. 1(b). This role has already beenpartially observed in previous irradiation tests carried outin several overseas countries.2)

The first irradiation test of sodium-bonding control rods inJapan has been conducted at the experimental reactor Joyo.Seven absorber pins in the control rod were successfullyirradiated. All the irradiated pins were withdrawn from theJoyo core for postirradiation examination. Thus far, thebehavior of sodium-bonding control rods was investigatedby X-ray computed tomography (X-ray CT) as a nondes-tructive examination.1) The appearance of cracks in B4Cpellets was observed by the X-ray CT test.

In this study, the detailed effects of a shroud tube onACMI were investigated by a series of destructive examina-tions. Seven absorber pins in irradiated sodium-bondingcontrol rods were examined by metrological inspection. Adetailed observation was performed on several samples hav-ing different irradiation conditions using metallography.

�Atomic Energy Society of Japan

�Corresponding author, E-mail: donomae.takako@jaea.go.jp

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 48, No. 4, p. 580–584 (2011)

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II. Experimental

The configurations of the absorber pins in the control rodand B4C pellets in the absorber pin are illustrated in Fig. 2.The B4C pellets were fabricated by a hot-pressing methodusing isotopic B4C with 90% 10B enrichment. The density ofB4C pellets was 90%TD. The specifications of B4C contentare shown in Table 1.

Specimens for postirradiation examination were takenfrom control rods irradiated in the Joyo MK-III core. Themaximum burnup and temperature of the B4C pellets were100� 1026 cap/m3 and approximately 800�C, respectively.Burnup was calculated using the HESTIA code.3) The max-imum temperature at the pellet center was calculated by theuniversal computer code HEATING-5 (Heat Engineeringand Transfer in Nine-Geometries, Vol. 5).4,5)

The absorber pins were removed from the irradiatedcontrol rods after withdrawal from the Joyo core. First, thediameter profiles of cladding tubes were measured by a laser.

Shroud tubes and B4C pellets were contained inside thecladding tubes during measurement. The shroud tubes withB4C pellets were then pulled out from the cladding tubes.The diameter of the shroud tubes was measured using thesame technique used for the cladding tubes. The shroudtubes with B4C pellets were cut to a length of approximately250mm using a diamond blade cutter in order to obtain aprofile of diameter change. Small fragments of pellets andshroud and cladding tubes were taken for metallographicexamination. Burnups were approximately 20, 40, 60, 80,and 100� 1026 cap/m3. The specimens were polished andchemically etched for metallography.

III. Results and Discussion

1. Measurement of Cladding Tube DiameterThe diameters of all cladding tubes were measured. No

significant cracks on the outer surface of the cladding wereobserved on visual inspection. Figure 3 shows a typicalexample of an axial profile of cladding diameter. The profilewas nearly flat and there was no significant deformationaround the B4C pellet region.

2. Measurement of Shroud Tube DiameterFigure 4 shows the results of shroud tube diameter meas-

urements.6–8) As shown in the figure, burnup increases to-ward the pin bottom. For irradiation above an approximateburnup of 50� 1026 cap/m3, cracks appeared on the surfacesof the shroud tubes. The crack length was measured to beapproximately 50mm. The diameter increased significantlytoward the pin bottom at approximately 60� 1026 cap/m3 ata distance from the pin bottom (D.F.P.B.) of approximately

Cladding tube

He

B4 C pellet

(a)

Shroud tube

Na

(b)

Low burnup High burnup

Fig. 1 Mechanism of ACMI from low burnup to high burnup: (a) helium- and (b) sodium-bonding rods

Control rod

Absorber pin

Protecting tube

Handling head

Dash ram

Shroud tubeB4C pellet

Cladding tube

Vent hole Vent tube

Sodium

Fig. 2 Illustration of control rod and absorber pin

Table 1 Specifications for boron and carbon in the B4C pellets

Specification B4C specimen

10B concentration (wt%) 90:0� 1:0 90.1

Total B (wt%) 77:0� 1:0 76.8

Total B+Total C (wt%) =99:0 99.2

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200mm. In comparison with the inner diameter of the clad-ding tube, the maximum shroud tube diameter was larger ata burnup of approximately 75� 1026 cap/m3 as shown inFig. 4, when the wall thickness of the cladding tube wasassumed to be the same during irradiation. This suggestedthat the gap between the shroud and cladding tubes wasclosed and such tubes would contact each other during irra-diation. Then, it should be noted that shroud tube diameterwas measured after withdrawing the shroud tube from thecladding tube. It is assumed that by withdrawing the shroudtube, the stress would be free and the shroud tube diameterwas observed to be larger than the cladding tube innerdiameter. However, its contact did not affect the claddingtube around the cracking region of the shroud tube.

3. Metallographic ExaminationFigure 5 shows the appearance of the (a) cladding tube,

(b) shroud tube, and (c) B4C fragments at a burnup of50� 1026 cap/m3. There is a Cr coating layer that restrainsthe corrosion of stainless steels under the B4C-sodium-stain-

less-steel system. After the irradiation, Fig. 5(a) reveals nocracks on the wall of the cladding tube; however, there wereshort cracks that were limited to the inner layer. Figure 5(b)shows short cracks in the layer of the inner and outer sur-faces, and these cracks were also limited to these surfaces.The B4C fragments shown in Fig. 5(c) had a maximum sizeof approximately 8mm.

Figure 6 also shows the appearance of the (a) claddingtube, (b) shroud tube, and (c) B4C pellet fragments at aburnup of 100� 1026 cap/m3. In this case, the cladding tubewas not breached, and some short cracks, which were limitedto the layer, were seen. On the other hand, the shroud tubewas breached by large cracks. The shroud tube wall ap-peared to have been squeezed, suggesting the ductibility ofthe shroud tube. The apparent ductibility suggests that, in theevent that B4C pellet cracks occur, the shroud tube shouldrestrain the relocation of B4C pellets by holding the B4Cpellet fragments in their positions. During irradiation, thedegree of swelling of B4C pellets was increased, and theshroud tube was squeezed by B4C pellets; thus, shroud tube

18.618.7

18.818.9

19.019.119.2

19.319.4

19.519.6

0

Axial Position From Pin Bottom(mm)

Dia

met

er (

mm

)Weld zone Weld zone

B4C pellets area Absorber pin

140012001000800600400200

Fig. 3 Measurement of cladding tube diameter with illustration of B4C absorber pin position

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

100

Distance from pin bottom (mm)

Dia

met

er (

mm

)

Outer diameter of shroud tube

Inner diameter of cladding tube

Outer diameter of cladding tube

80 30

Burn up ( × 1026cap/m3)

40506070

500400300200

Fig. 4 Measurement of shroud tube and cladding tube outer diameters

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diameter would be larger mechanically. The B4C pellet frag-ments remained in their positions. The maximum size of theB4C fragments was approximately 5mm.

By comparing Figs. 5 and 6, the fragment size in Fig. 5 isobserved to be larger than that in Fig. 6. This difference isdue to the irradiation conditions of average burnup and theaverage temperature of B4C pellets. The burnup for thespecimen shown in Fig. 6 is larger than that shown inFig. 5. The maximum temperature of B4C pellets is limitedto below 800�C by a design criterion. The temperature of thespecimen shown in Fig. 6 is lower than that shown in Fig. 5.Helium is generated by the (n,�) reaction of 10B and isretained in the B4C pellets, which leads to the accumulationof helium with increasing burnup. This helium accumulationcauses microcracks in the pellets. These microcracks couldplay a role in preparing helium release paths to the outside ofthe pellets.

Figure 7 shows a comparison of the appearance of B4Cpellets obtained under different irradiation conditions. Thepellets were irradiated at different temperatures to a similarburnup. The temperature in Fig. 7(a) is 200�C lower thanthat in Fig. 7(b).

It is well known that an increase in temperature causes theoccurrence of microcracks in B4C pellets.1) From Figs. 5–7,it is suggested that microcracks in B4C pellets are generatedas burnup and temperature increase. These results suggestthat pellet cracking could make helium release easier. Whenthe burnup condition is the same, it is established that B4Cpellet fragments become larger under a lower irradiationtemperature. These results correspond to the previously re-ported data for helium retention as a function of burnup, asshown in Fig. 8.9) It is seen that the features of heliumretention depend on the temperature; i.e., the degree ofhelium retention decreases with increasing temperature.

Helium retention is, however, almost saturated above100� 1026 cap/m3.

The lifetime of helium-bonding control rods was limitedto a burnup of 43� 1026 cap/m3.1,2) The lifetime of so-dium-bonding control rods was greatly extended to approx-imately 100� 1026 cap/m3 by the following reasons, whichwere elucidated from the present results. It is suggestedthat pellet fragments could be held well by the shroud tubefor sodium-bonding control rods owing to a larger frag-ment size than that in the case of helium-bonding controlrods.

IV. Conclusions

According to the results of these improvements, it ispossible to double the lifetime of control rods in Joyo.Above the irradiation of 100� 1026 cap/m3, the lifetime ofcontrol rods would be extended by shroud tube improve-ment.

(b) Shroud tube (etched)

(a) Cladding tube (etched)

50 µ m

Inner Cr layer

(c) B4C fragments50 µ m

InnerCr layer

OuterCr layer

B4C pellet

Fig. 5 Optical micrographs ((a) and (b)) and appearance (c) ofirradiated specimens (burnup: 50� 1026 cap/m3)

(a) Cladding tube (etched)

50 µ m

Inner Cr layer

(c)B4C fragments

(b) Shroud tube (etched)

50 µ m

InnerCr layer

OuterCr layer

Fig. 6 Optical micrographs ((a) and (b)) and appearance (c) ofirradiated specimens (burnup: 100� 1026 cap/m3)

(a) (b)

Fig. 7 Appearance of materials after irradiation (burnup:50� 1026 cap/m3) of about (a) 700�C (pellet center temperature)and (b) 1,000�C (pellet center temperature)

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Acknowledgements

The authors express their deep appreciation to Dr. Osaka,Dr. Ishii, and Mr. Arii for valuable discussion on the presentpaper. Grateful acknowledgements are due to Mr. T. Inoue,Mr. H. Fukasaku, and Mr. S. Misawa for their assistance inconducting the study. Also grateful acknowledgement isgiven to Mr. Nagamine, General Manager of the Fuels Mon-itoring Section, and Dr. Akasaka, Deputy General Managerof Fuels Monitoring Section, and Mr. Asaga, Director ofFuels and Materials Department, for their help in giving usthe opportunity to write this paper.

References

1) T. Maruyama, ‘‘Development of control rods for fast breederreactor,’’ Taikabutsu, 60[10], 514–526 (2008), [in Japanese].

2) T. Soga, M. Yamamoto, T. Sekine, T. Aoyama, ‘‘The develop-ment of a long life control rod in the fast reactor (1) Develop-ment of the control rod for fast reactor and its irradiation test inJoyo,’’ Proc. Spring Mtg. of Atomic Energy Society of Japan,Osaka, Japan, Mar. 26–28, 2008, 311 (2008), [in Japanese].

3) T. Aoyama, T. Sekine, S. Maeda, A. Yoshida, Y. Maeda, S.Suzuki, T. Takeda, ‘‘Core performance tests for the JOYOMK-III upgrade,’’ Nucl. Eng. Des., 237, 353–368 (2007).

4) M. Soroi, R. Kitamura, T. Mitsugi, T. Sekine, T. Aoyama,

‘‘Development of irradiation technology for MK-III,’’ JNCTech. Rev. Extra Ed., 21, 111–120 (2003), [in Japanease].

5) T. Maruyama, S. Onose, T. Kaito, H. Horiuchi, ‘‘Effect of fastneutron irradiation on the properties of boron carbide pellet,’’J. Nucl. Sci. Technol., 34[10], 1006–1014 (1997).

6) K. Katsuyama, M. Yamamoto, K. Abe, K. Maeda, T.Nagamine, Y. Nakamura, ‘‘The development of a long lifecontrol rod in the fast reactor (2) Non-destructive PIE resultsof sodium bonded type control rod irradiated in Joyo,’’ Proc.Spring Mtg. of Atomic Energy Society of Japan, Osaka, Japan,Mar. 26–28, 2008, 312 (2008), [in Japanese].

7) K. Abe, K. Katsuyama, T. Soga, K. Maeda, T. Nagamine, Y.Nakamura, ‘‘The development of a long life control rod in thefast reactor (3) Visual observation of shroud tube of sodiumbonded type control rod irradiated in Joyo,’’ Proc. Fall Mtg. ofAtomic Energy Society of Japan, Kohchi, Japan, Sep. 4–6,2008, 804 (2008), [in Japanese].

8) T. Donomae, K. Katsuyama, Y. Tachi, K. Maeda, T. Inoue, H.Fukasaku, ‘‘The development of a long life control rod in thefast reactor (5) Dimensional measurement results of shroudtube of sodium bonded type control rod irradiated in Joyo,’’Proc. Spring Mtg. of Atomic Energy Society of Japan, Tokyo,Japan, Mar. 23–25, 2009, 73 (2009), [in Japanese].

9) T. Dohnomae, K. Katsuyama, Y. Tachi, K. Maeda, M.Yamamoto, T. Soga, ‘‘Development of long-lived control rodsfor the fast reactor,’’ Proc. FR09, Kyoto, Japan, Dec. 7–11,2009 (to be published).

Hel

ium

Ret

enti

on (

m3–S

TP

/ m3

– B

4C

)

Burnup (1026cap / m3)

IrradiationTemp. (°C)

0 50 100 150 2000

100

200

300

Full Retention< 600

600– 800800–1000

1000–1200

500

400

Fig. 8 Burnup dependence of helium retention of irradiated specimens. These specimens are separated by irradiationtemperature.2)

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