- AECL--10687 UA9300967

AECL-10687

ATOMIC ENERGY •/. V 7 ENERGIE ATOMIQUE OF LIMITED /^% DU CANADA LIMITEE

AN OVERVIEW OF THE EXAMINATION OF FUEL AS FOLLOW-UP TO THE 1988 NOVEMBER OVERPOWER TRANSIENT IN PICKERING NGS-A UNIT 1

UN APERQU DE L'EXAMEN DU COMBUSTIBLE COMME EXAMEN COMPLEMENTAIRE DU TRANSITOIRE DE SURPUISSANCE DE NOVEMBRE 1988 DANS LA TRANCHE (UNITE) 1 DE LA CENTRALE ELECTRONUCLEAIRE DE PICKERING A

M.R. FLOYD, R.J. CHENIER, D.A. LEACH and R.R. ELDER

Paper presented at the Third International Conference on CANDU Fuel, Pembroke, , Canada 1992 October 4-8

Chalk River Laboratories Laboratoires de Chalk River

Chalk River, Ontario KOJ 1J0

October 1992 octobre AECL Research

AN OVERVIEW OF THE EXAMINATION OF FUEL AS FOLLOW-UP TO THE 1988 NOVEMBER OVERPOWER TRANSIENT IN PICKERING NGS-A UNIT 1

by

MR. Floyd,* RJ. Chenier, D.A. Leach and R.R. Elder •Ontario Hydro staff, attached to

Paper presented at the Third International Conference on CANDU Fuel Pembroke, Ontario, Canada 1992 October 4-8

Fuel Materials Branch Chalk River Laboratories Chalk River, Ontario KOJ 1J0 1992 October

AECL-10687 EACL Recherche

UN APERÇU DE L'EXAMEN DU COMBUSTIBLE COMME EXAMEN COMPLÉMENTAIRE DU TRANSITOIRE DE SURPUISSANCE DE NOVEMBRE 1988 DANS LA TRANCHE (UNITÉ) 1 DE LA CENTRALE ÉLECTRONUCLÉAIRE DE PICKERING A

par

M.R. Floyd, R.J. Chenier, D.A. Leach et R.R. Elder

RÉSUMÉ

Un nombre important d'éléments de combustible qui ont soutenu des augmenta- tions rapides de puissance au cours d'un transitoire de quarante minutes dans la Tranche (Unité) 1 de la Centrale électronucléaire A de Pickering, ont présenté des ruptures de FCC (SCC)*. Ces dernières se sont toujours trouvées aux points de contact entre les pastilles au bout des éléments qui ont atteint la plus grande augmentation relative de puissance. En outre, on a observé qu'il y a une préférence pour la matière (métal) alpha recristallisée (zone de transition influencée par la chaleur). Le seuil de défaut a été influencé par la géométrie des pastilles.

La durée du transitoire a été trop courte pour influencer la déformation résiduelle du milieu de la gaine ou la croissance des grains. Les stries circonférentielles aux points de contact entre les pastilles ont été généralement agrandies et les inventaires existants de gaz de fission aux joints de grains ont été libérés dans le volume libre.

Combustible et Métaux Laboratoires de Chalk River Chalk River (Ontario) KOJ 1J0 1992 octobre

* Fissuration par corrosion sous contrainte (Stress-Corrosion Cracking)

AECL-10687 AECL Research

AN OVERVIEW OF THE EXAMINATION OF FUEL AS FOLLOW-UP TO THE 1988 NOVEMBER OVERPOWER TRANSIENT IN PICKERING NGS-A UNIT 1

by

M.R. Floyd,* RJ. Chenier, D.A. Leach and R.R. Elder

ABSTRACT

A significant number of fuel elements that sustained power ramps during a forty-minute transient in Pickering NGS-A Unit 1 exhibited SCC* failures. These were always located at pellet-interface locations at the end of the element that achieved the largest relative increase in power. In addition, a preference for recrystallized-alpha (heat-affected-transition-zone) material was observed. The defect threshold was influenced by pellet geometry. The duration of the transient was too short to influence residual midpellet sheath strain or grain growth. Circumferential ridges at pellet-interface locations were generally enlarged and existing grain-boundary fission-gas inventories were vented to the free volume.

Fuel Materials Branch Chalk River Laboratories Chalk River, Ontario KOJ 1 JO 1992 October

1 S.tress-£orrosion Cracking.

AECL-10687 TABLE OF CONTENTS

1. INTRODUCTION 1 2. THE EFFECT OF FABRICATION PARAMETERS ON PERFORMANCE 1 3. PRIMARY DEFECT CAUSE 1

4. SHEATH DIMENSIONAL CHANGES 2 5. FISSION-GAS RELEASE 2 6. GRAIN GROWTH 3

7. CONCLUSIONS 3 ACKNOWLEDGEMENTS 3 REFERENCES 3 TABLE 4 FIGURES 5 -1-

1. INTRODUCTION

In 1988 November, a reactor trip occurred in Pickering NGS-A Unit 1. During the trip recovery, all adjuster rods were withdrawn from the core and reactor power was raised to 87% of full power for 40 minutes. This was outside the range of normal operation; reactor power is usually limited to 65% in this situation. As a result of the transient, approximately 200 fuel bundles in 40 centre-core channels sustained large power ramps. Following the transient, coolant radioiodine levels indicated the presence of many defective fuel elements. Subsequently, fuel was discharged from the 40 centre-core channels that sustained large power ramps. One-hundred and forty-five fuel bundles from 33 of these channels were visually inspected in the Pickering NGS-A Irradiated Fuel Bay. Approximately 290 outer-element failures were observed in 36 fuel bundles. Some bundles experienced failures in all 16 outer elements. The extent of failures was higher than predicted by the existing power-ramp defect correlations.

In 1989, fuel elements from eight fuel bundles that experienced power ramps during the transient were examined in the Hot Cell Facilities at Chalk River Laboratories.!1] Failures were observed in outer elements that sustained peak linear powers in excess of 50 kW/m, having SCC as their primary cause. Higher-than-expected values of strain and fission-gas release (FGR) were observed in intact elements. Anomalous grain-growth behaviour was also observed. The defect threshold has been correlated with power history, resulting in a new failure-probability formalism. The results show that power-ramp failures would not have occurred had the reactor been operated within its normal range during the trip recovery. In 1990, an additional 29 fuel elements were examined at Chalk River, to assist in further understanding the fuel behaviour exhibited during the 1988 November transient. Both ramped and unramped elements were examined, in order to determine the effect of the 40-minute transient on fuel behaviour. This paper reviews the examination that was conducted on fuel examined in 1989 and 1990, and discusses the impact of the transient on performance parameters such as sheath strain, grain growth and FGR. 2. THE EFFECT OF FABRICATION PARAMETERS ON PERFORMANCE The majority of bundles that exhibited SCC were power-ramped at outer-element bumups of 120 to 215 MWh/kgU.Ul All of the bundles in-core that were ramped in this range of bumup had virtually identical fabrication parameters. Two bundles, H29243C and H31138C, exhibited single outer-element failures at bumups of 77 and 88 MWh/kgU, respectively. Both achieved peak outer element linear powers of 60 kW/m. In contrast, bundles H59398W, H76155W and H78879W did not exhibit failures, having similar power histories (Table 1). These bundles had a different pellet geometry ("B", Table 1) from that of bundles H29243C and H3.1138C ("A").** Pellet geometry "B" incorporated a smaller length-to-diameter ratio (1/d) and larger land width than pellet geometry "A" (Figure 1). Similar trends in fuel performance have been exhibited elsewhere in fuel elements with different pellet geometries,!2.3] and may be important to the optimization of existing fabrication parameters.

3. PRIMARY DEFECT CAUSE The primary defect cause was SCC. The defects were characterized as follows:!!]

The UO2 pellet densities for all five bundles were the same, but slightly lower than those ramped at bumups >100 MWh/kgU. -2-

1. SCC was limited to the end of the element adjacent to an adjuster rod withdrawn during the overpower transient. This end of the element experienced the largest increase in power during the transient. 2. SCC was limited to pellet interfaces where circumferential ridging resulted in peak strains. 3. SCC was limited to recrystallized-alpha material located in the transition zone between the braze-heat-affected and as-received sheath structures. A similar structure preference has been observed in SCC-related endcap failures.!3-4! 4. SHEATH DIMENSIONAL CHANGES Residual midpellet diametral sheath strains showed no dependence on the peak power achieved during the forty-minute ramp (Figure 2), nor was there any relation between residual (plastic) midpellet strain and the onset of defects. Apparently, the ramp duration was too short to permit conversion of elastic strain to plastic strain, and hence the residual strains observed were better related to operation before and after the transient. This is substantiated by the fact that unramped and ramped elements exhibited similar residual tensile sheath strains (up to 1%). We can conclude that residual midpellet tensile sheath strains*** are occurring in 28-element fuel irradiated in Pickering NGS within the normal operating envelope. This was not the case in the past with fuel that had lower uranium masses,!5! and was not predicted by the fuel-modelling code ELESIM (ModlO).

Total residual strains of up to 2% were observed at the pellet interfaces. Circumferential ridging at the pellet interfaces was influenced by the peak power achieved during the transient (Figure 3). In addition, unramped elements generally exhibited smaller ridges than those ramped. Figure 3 also shows that elements from bundles exhibiting failures tended to exhibit the largest ridges. Hence, unlike residual midpellet strain, pellet-interface ridging exhibited a dependence on the transient and the onset of defects. The correlation between ridges and defects is consistent with the exclusive occurrence of SCC at pellet interfaces (Section 2).

5. FISSION-GAS RELEASE From Figure 4, the following FGR observations can be made: 1. For a given peak power, unramped outer elements exhibited less FGR than those that were ramped (0.1 to 1.0% versus 1 to 5%, respectively). This suggests that the forty- minute transient had a significant role in venting existing grain-boundary fission-gas inventories. 2. For a given peak power, ramped intermediate elements showed FGR that was an order of magnitude less than for ramped outer elements (from different bundles). This is apparently due to the fact that the intermediate elements operated at lower powers prior to the transient. This resulted in smaller fission-gas inventories at the grain boundaries available for release upon ramping.

3. There is no simple correlation between FGR and peak power. FGR is dependent on fuel temperature, which varies with fuel power, however, the duration of the transient was apparently too short to allow a significant amount of enhanced fission-gas diffusion to the grain boundaries.

Relative to the as-fabricated diameter. -3-

From the above observations, it is concluded that the role of the transient with respect to FGR was to vent existing grain-boundary inventories to the free volume of the elements. The magnitude of grain-boundary fission-gas inventories was determined by the pre-transient power history.

6. GRAIN GROWTH

Grain growth did not exhibit any dependence on the peak power achieved during the ramp, being better correlated with the pre/post-transient steady-state power history. Apparently, the transient duration/fuel temperature was insufficient to induce grain growth (consistent with FGR observations, Section 5). 7. CONCLUSIONS 1. The observed primary defects were caused by SCC at the end of the elements, which sustained the largest relative increase in power.

2. These defects were always at pellet-interface locations and showed a preference for recrystallized-alpha material. 3. The defect threshold appears to have been influenced by pellet geometry; those elements incorporating pellets with smaller 1/d and larger land widths exhibited higher defect thresholds. This may be useful in optimizing existing fabrication parameters. 4. The duration of the transient was too short to influence residual midpellet strain or grain growth. Pellet interface ridges were generally enlarged as a result of the power ramp, and existing fission-gas grain-boundary inventories (determined by the pre-transient power history) were vented to the free volume. ACKNOWLEDGEMENTS A portion of this work was funded by the CANDU Owners Group (Working Party 9). REFERENCES 1. M.R. Floyd, R.L. da Silva, M.J.F. Notley, RJ. Chenier and R.R. Elder, "Characterization of Fuel Failures Resulting from the 1988 November Overpower Transient in Pickering NGS-A Unit 1", proceedings from the Eleventh Annual CNS Conference, Toronto, Canada, 1990 June 3-6. 2. TJ. Carter, "Experimental Investigation of Various Pellet Geometries to Reduce Strains in Zirconium Alloy Cladding", Nucl. Technol., 45, 166 (1979). 3. M.R. Floyd, et al., "Behaviour of Brace NGS-A Fuel Irradiated to Bumups of -500 MWh/kgU", paper presented at this conference. 4. R. Sejnoha, et al., "Performance of End Cap Welds in 37-Element CANDU Fuel", proceedings from the Second International Conference on CANDU Fuel, 1989 October 1-5, Pembroke, Canada, pp. 148-157. 5. M.R. Floyd, "The Effect of Increasing Uranium Mass on Sheath Strain in Bruce and Pickering Fuel", paper presented at this conference. -4-

Table 1: Power History Data for Bundles Having Different Pellet Geometries

Outer Element Burnup @ Ramp Peak Power Pellet Bundle (MWh/kgU) (kW/m) SCC Failures Geometry H29243C 77 60 1 element "A" H31138C 87 60 1 element "A"

H59398W 67 62 none "B" H76155W 95 61 none "B" H78879W 99 56 none "B" -5-

"LAND" WIDTH

DIAMETER (d)

NOT TO SCALE

Figure 1: Schematic CANDU Fuel-pellet geometry showing parameters that influence performance. -6-

AVERAGE MIDPELLET STRAIN (%) 1

0.8 H ALL INTACT ELEMENTS B

• 0.6

•

0 X 0.4 L a \ m e m m a C m

•

-0.2 i i i ! 1 35 40 45 50 55 60 65 70 PEAK LINEAR POWER (kW/m)

• OUTER ELEMENTS • INTERMEDIATE ELS. * x OUTER ELEMENTS * 0 OUTER ELS.-UNRAMPED

• FROM DEFECTIVE BUNDLES

Figure 2: Midpellet sheath strain versus peak power. -7-

AVERAGE RIDGE HEIGHT (urn) 50

ALL INTACT ELEMENTS

20

10

0 35 40 45 50 55 60 65 70 PEAK LINEAR POWER (kW/m)

• OUTER ELEMENTS D INTERMEDIATE ELS. • x OUTER ELEMENTS * 0 OUTER ELS.-UNRAMPED

* FROM DEFECTIVE BUNDLES

Figure 3: Pellet-interface ridge height versus peak power. -8-

FISSION-GAS RELEASE (%) 10 y h •

— u - 1 • B t m

x; •

X. 1 z 0 X

0 0

- 0 B 0 -

• • 0.1 0 a

r\ 4 i 1 1 1 35 40 45 50 55 60 65 70 PEAK LINEAR POWER (kW/m)

OUTER ELEMENTS a INTERMEDIATE ELS.* OUTER ELS.* 0 OUTER ELS.-UNRAMPED

* FROM DEFECTIVE BUNDLES

Figure 4: Fission-gas release versus peak power. Cat. No. CC2-10687E No. aucat. CC2-10687E ISBN 0-660-14769-6 ISBN 0-660-14769-6 ISSN 0067-0367 ISSN 0067-0367

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