ML20147D930

Cladding Damage Analysis of Core II Burnable Poison Rods

ML20147D930
Person / Time
Site:	Maine Yankee
Issue date:	10/30/1978
From:	ABB COMBUSTION ENGINEERING NUCLEAR FUEL (FORMERLY
To:
Shared Package
ML20147D918	List: ML20147D912 ML20147D930
References
CEN-077(M)-NP, CEN-77(M)-NP, NUDOCS 7812200223
Download: ML20147D930 (31)

Text

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CEN 77 ( M ) NP Revision 1 NP l

CLADDING DAMAGE ANALYSIS of MAINE YANKEE CORE il BURNABLE POISON RODS OCTOBER,1978 4

78122002S1

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COMBUSTION ENGlNEERING INC

k LEGAL NOTICE THIS REPORT WAS PREPARED AS AN ACCOUNT OF WORK SPONSORED BY COM8USTION ENGINEERING, INC. NEITHER COMBUSTION ENGINEERING NOR ANY PERSON ACTING ON ITS BEHALF:

A.

MAKES ANY WARRANTY OR REPRESENTATION, EXPRESS OR lMPLIED INCLUDING THE WARRANTIES OF FITNESS FOR A PARTICULAR l

PURPOSE OR MERCHANTABILITY, WITH RESPECT TO THE ACCURACY, COMPLETENESS, OR USEFULNESS OF THE INFORMATION CONTAINED IN THIS REPORT, OR THAT THE USE OF ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS REPORT MAY NOT INFRINGE PRIVATELY OWNED RIGHTS;OR B. ASSUMES ANY LIABILITIES WITH RESPECT TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE OF, ANY INFORMATION, APPARATUS, METHOD OR PROCESS DISCLOSED IN THIS REPORT.

l s

3 REVISION NOTICE (REVISION 1)

Revised to clearly delineate information held proprietary by Combustion Engineering, Inc.

and to make minor typographical and editorial corrections.

This revision supersedes all previous issues.

l

CLADDING DAMAGE ANALYSIS OF MAINE YANKEE CORE II BURNABLE POIS0N RODS CEN-77 (M)-NP (Revision 1-NP )

By:

N.

Fuhrman D.

B.

Scott Combustion Engineering, Inc.

October 1978

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,l Table of Contents Page Section No.

a No.

1.

INTRODUCTION l

2.

NONDESTRUCTIVE EXAMINATION RESULTS 2

2.1

. Visual Examination 2

2.2 Profilometry 5

2.3-Eddy _. Current Testing 7

3.0 DESTRUCTIVE EXAMINATION RESULTS 8

~

3.1 Visual. Examination of Crack Surface 8

and Cladding Iq, 3.2 Metallography of Longitudinal Cracks 9

3.2.1 Procedure 9

3.2.2 Results 11 3.2.2.1 Metallography of Cladding Crack in JJD-085 11 4

3.2.2.2 Metallography of Cladding Crack in JJ0-042 12 3.2.2.3 Microexamination of Al 0 -B C Pellets 13 2 4 brick from JJD-098 14 Metallography of[0xideFilmThickness]onFractureSurfaces 3.3 16 3.4 4.

CLADDING DAMAGE MECHANISMS 17 4.1 Primary Hydriding 17 4.2 Secondary Hydriding 17 4.2.1 Sources _of Hy,drogen 18 1

4.2.2 Formation of 20

]

4.2.3 Formation of[

]

21 4.3 Clad Cracking 22 4.3.1 Crack Initiation Mechanisms-22 4.3.1.1 PCI Enhancement Factors in Perforated Rods 22 4.3.1.2 Effect of Thermal Cycling 24 4.3.2 Crack Formation 25

)) Crack of JJD-098 Longitudinal Cracks 25 4.3.2.1 25 4.3.2.2 l

5.

SUMMARY

.AND CONCLUSIONS 26 5.1 Cause of Clad Cracking 26 5.2 Overall Conclusions 28 6.

REFERENCES 30 q

J

ABSTRACT

.p hot cell examination of representative perforated !;urnable poison rods from Maine Yankee Batch E was conducted to determine the cause of cladding cracks observed during inspection at the spent fuel pool.

The underlying mechanism' established for the cracking phenomenon was the secondary hydriding of the cladding following water ingress through.a perforated primary nydride sunburst.

The major source of hydrogen for the secondary hydriding was the chemical reaction between the B C particles in the burnable poison pellets and the incoming water.

4 Summarized herein is the observed pattern of secondary hydriding and the clad cracking mechanism believed to be operating.

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,o CLADDING DAMAGE ANALYSIS OF MAINE Y \\NKEE CORE II

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BURNABLE POIS0N RODS 1.

ETRODUCTION Early in the Maine Yankee refueling outage of April-May 1977, the planned' routine visual examination of several assemblies revealed that some of the burnable poison rods in Core II had indications of hydride blisters.

These observations were expected as a result of previous experience at both St. Lucie-I and Calvert Cliffs-I with poison rods fabricated in the same manner.

In one assembly, however, a longitudinal claddingcrack,[

]wasobscrvedonapoisonrod.

This finding prompted an expanded visual examina* ion program for assemblies containing poison rods to evaluate the frequency of such cracks.

These included [ 3 assemblies from Batch E and[

assemilies from Batch F.

The nominal poison pellet B C contents for the two b, tches were 3.2 w/o and '

4 1.7 w/o, respectively.

As a result of the additional inspection at the spent fuel pool, perforationswereobservedon[

< poison rods in the Batch E assemblies.

Longitudinal cladding cracks were-observed on(

of the perforated rods.

Two of the cracks were approximately[

[long; the remainder did not exceed Subsequent to the assembly inspection,[ ] individual Batch E poison rods were visually examined after being removed from the assemblies, and in addition, most of these were[

The results confirmed the perforation statistics established earlier and the presence of hydride blister-type cladding damage.

The short

[longitudinalcladding-cracks were found to be associated with blisters or bulges, either propagatir;g away from or passing through them.

A.

. crack appeared to be free _ of such indications except for a bulge at ene end.

The,

, data 4

indicated the presence of

.] indications

.]along some of the perforated rods.

Other perforated rods exhibited

, indications similar to intact rods.

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y 9

In ' view of the results from this poolside inspection, a hot E

cell examination program was undertaken to characte-ize the poison rod cladding damage further and to detennine the cause of the longitudinal cladding. cracks.

A representative group of Batch E poison rods was shipped to the Battelle-Columbus hot cell facility to accomplish these objectives.

The rods examined are described in Table 1.

The results of the hot cell program and the conclusions stemming from the overall study of poison rod cladding damage are presented in this reoort.

2.

NONDESTRUCTIVE EXAMINATION RESULTS 2.1 Visual Examination All' 5 perforated rods listed in Table 1 tere visually examined in the hot cell at low magnification,and salient ftatures were photographed.

All of the observable fracture surfaces on the 3 rods with longitudinal cladding cracks exhibited brittle fracture characteristics.

Other signifi-cant observations for each rod are presented below.

JJD-042.

A prominent crater-type cladding perforation, indicative of a mature hydride blister, was found at(

above the bottom of the rod and is shown in Fig. 1.

longitudinal cladding cracks,

)wereobservedbetween above rod bottom.

Fig. 2 illustrates the crack configuration and shows an obvious cladding bulge intersected by the cracks.

No other significant cladding anomalies were observed.

JJD-085.

A crater-type cladding perforation was also found on

~

thisrodat[

]abcve rod bottom. [

]itssizewascomparable to that of the similar perforation in JJD-042,

[asshowninFig.3.[

[lonoitudinal cladding cracks were observed on the rod.

' as shown in Figs. 4 and 5, respectively.

The

- h-

] passed through a'large ciadding bulge,[

] terminated at a small blister.

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t Table 1

.[

iMaine Yankee Core II, Batch E Burnable Poison Rods' Examined at the Hot Cell to Characterize l

~C1' adding Damaae l

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Rod'N6.

'Assembiv

'Descriotion*

=

JJD-042 6D JJD-085 71 JJF-112 80 JJF-195 80 r

JJD-098 6J

[

Based on observations and results from examination at th*e Maine Yankee spent fuel " pool.

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CRATER - TYPE PERFORATION ON JJD-04(

FIGURE 1

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1 The only other cladding anomalies present consisted of several small discolored cladding bulges, some of which were detected later by profilometry.

1 JJF-112.

The only cladding perforations on this rod consisted of a cratered blister at[

]above rod bottom and a probable broken blister at[

]aboverodbottomindicatedbyseepageof a solution from the rod interior, as shown in Fig. 6.

No other significant cladding anomalies were observed.

JJF-195.

The only cladding anomaly observed on this rod was an innature cracked blister, shown in Fig. 7, at [

laboverod bottom.[

]

JJD-098.

[Themajorcladdingdamagefeaturewasalongitudinalcladding crack which ran from

] above rod bottom as shown m

in Fig. 8.

Fracture surface detail illustrated inFig.8exhibitedthetypical(.

3 characteristics of brittle behavior.

Only at' the top of the crack was there an indication of intersectionwitha'localcladdingbulge.[

[wasobservedonone, of the fracture surfaces

)

as shown in Fig. 9.

More importantly, a crackedblistershowninFig.10wasfoundat[

]aboverodbottom.

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FIGURE 7 IMMATURE CRACKED BLISTER ON JJF-195

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u en FRACTURE SURFACE DETAll AT(

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2.2 Profilometry Continuous profilometry measurements were made on 4 rods (i.e.,

all but JJD-098) to characterize diametral variations and to pinpoint the location of anomalous cladding features.

The Battelle profilometer and calibration standard employea were the same as used in the Maine Yankee Core I fuel examination and have been described previously Briefly, the axial location of any measurement is known to within +1/16 in., and the accuracy and precision of the measured diameter is 0.0002 in, (95% confidence).

Each rod was profiled with a rotational pitch of 8 revolutions / inch which produced an envelope of maximum-minimum diameters on a strip chart recorder trace.

The rods selected for profilometry represented the 2 perforated rod populations exhibiting

as noted in Table 1.

JJD-042 and JJFell2 were from the population with indications while JJD-085 and JJF-195 were from the population with

., indications, similar to an intact rod.

The diameter profile traces of the former confirmed the presence of cladding bumps or rod ridges (typically 2 mils larger than the adjacent diameters)[

) This is illustrated in Fig. 11 which shows the differences between the profiles of the two ' types of perforated rods at the same elevation.

The profiles not only [

'The significance of this result is that The key profilometry results are summari::ed in Table 2, which L

also includes pertinent rod growth data from the poolside inspection.

The significant conclusions from these data are as follows:

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AXIAL LOCATION,IN. ABOVE ROD BOTTOM 0.442

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COMPARISON OF PROFli.OMETRY TRACES OF PERFORATED RODS AT LOCATION OF MAXIMUM UNIFORM CLADDING CREEPDOWN OBSERVED IN INTACT RODS

'JJD 042 EXHIBITS LOCAL CLADDING DEFORMATION (

JJD 085 EXHIBITS NONE j

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Table 2' Results of' Hot' Cell Profilometry of Perforated Maine Yankee Core II Rods Lonaitudinal Local'C1 adding Max. Uniform C1 adding Rod Growth

• Rod'No.

Clad Cracks Deformation Creepdown, mils-

% al/L

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JJD-042-Yes JJF-il2

.No

- JJ0-085-Yes JJF-195 No

=

From poolside' length measurement results.

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The' maximum uniform cladding creepdown range from,

,[ indicates that the occurrence of initial perforations was[,

'] (The. range of creepdown found among intact rods at the same exposurewas(

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The likelihood that many of the fiaine YankeeCore11 poison ~rodswere(

)

helps explain the absence of the St. Lucie-I type reactivity and power anomalies during the Maine Yankee Core'II operation.

2.3 Eddy Current Testing The two rods with[

[longitudinalcladdingcracks,JJ0-042 and JJD-085, were eddy current tested at the hot cell with an encircling coil to precisely locate the end of the cracks preparatory to sectioning for metallography.

In all cases, the eddy current signal locations agreed with the visually determined ends of the cracks indicating that there were no transitions to partial cladding penetrations of any significant depth or length.

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3.

DESTRUCTIVE EXAMINATION RESULTS-3.1 Visual Examination of[

Crack Surface and Cladding ID The[

. crack observed in JJD-098 was further visually examined to characterize the fracture surface and cladding features which could be related to the mode of cracking. This involved cutting out the sectionfrom[

]aboverodbottomandexaminingthe

> crack under, higher magnification than previously available. The only evidence of a possible hydride blister intersected by the crack was found[

3as shown in Fig. 12.

Tne previously observed brittle [

] characteristics of the fracture surface-were confirmed.

In addition, a pattern wasevidentasshowninFig.13,(

(2) l

., section from J above rod bottom was A,

cut from the original larger section and slit longitudinally 180 away from the ' crack to expose the cladding ID for examination.

The two clamshells were then twisted apart breaking the web of cladding material at the bottom of the crack holding the two pieces together.

Thefresh[

]

surface produced adjacent to the original brittle crack surface as shown in Fig.14 helps explain why the original crack prcpagation was arrested.

The cladding ID examination supported the previous observation that the crack did not intersect a hydride blister or bulge excep't at the top.(

)

A fresh crack, probably taused by hot cell manipulator handling of the clamshells could be seen passing.through one of these regions as shown in Fig. 15,

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hRACK IN JJD-098[

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- FIGURE 13 VISUAL EARANCE OF CRACK FRACTURE SURFACE OF JJD-098

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3.2 Metall'ographyof(

Lonoitudinal Cracks At the Maine Yankee spent fuel pool and at the hot cell, visual examination of the[

]longitudinalcracks[

[ indicated that'they always~ intersected cladding blisters and bulges presumably of localized hydriding origin. The strong inference s;emming from these observations was that the brittle fracture characteristics exhibited by the cracks were the result of. hydrogen embrittlement of the cladding following an initial primary hydride perforation allowing water ingress into the_ rod.

To characterize the cladding hydrogen composition and distribution in such cases, 4 metallographic specimens from 2 typical

[

]cracksweretaken,i.e.,in2adjacentsectionsineachofProds as described in Table 3.

Note that one of the selected cracks was from Well below a cratered blister believed to be the initial perforation while the other was from well above such a perforation.

l The metallographic procedure employed, anj the results are l

described in the following sections:

3.2.1 Procedure The specimens were 1-in, long transverse sections cut with a diamond wheel.- They were raounted at room temperature inside backup stainless steel cylinders in Bakelite mounts with epoxy resin.

The azimuthal orientation I

was maintained by use of a hole drilled in the mount.

The specimens were ground and polished incrementally to permit examination of many surfaces with progress followed by measuring the mount length with a micrometer when a new surface of interest was reached.

Coarse grinding was performed with a resin-bonded'60 pm diamond disc.

The fine grinding was carried out on silicon carbide papers, ranging from 120 to 600 grit, with final polishing using a slurry of 1 um y-alumina in 25 chromic acid ~ solution. Water was used as-the medium in all grinding'and polishing operations.

The cladding i

etchant to reveal the hydride phase distribution consisted of a freshly mixed solution of 48'v/o H 022(30%),48v/oHNO3 (M) and 4 v/o HF (48%).

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'Y 3.2.2 R_esul ts Table 3 sumarizes the key observations from the microexamination.

In the following sections, the results from the two pairs of specimens

'from JJD-085 and JJD-042, respectively,.are discussed in detail.

3.2.2.1 Metallography of Cladding Crack in JJD-085 It was imediately evident that additional cladding damage, including opening of longitudinal cracks, was introduced when the two specimens were sectioned from the. lower crack in JJD-035 for mounting.

This can be seen in Fig.16 which shows macrophotographs of each of the( ) cross sections examined during the incremental grinding and polishing through the specimens.

The significant features observed during the cladding examination are as. follows:

Described herein are the detailed metallographic results for the specimens from the cladding crack in JJD-085, indicating the hydroce distribution and crack pattern.

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. Considerably less additional-cladding damage was incurred i

during 'the sectioning nf the crack region in JJD-042.

This is evident in Fig. 20 which shows macrophotographs of each of the[ ] cross sections examined as well as the. visually discernible external appearance of the pair of cracks.

The significant features observed during the cladding examination' are as follows:

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Described herein are.the detailed metallographic results for.the specimen from the cladding crack in JJD-042, indicating the hydrogen distribution and crack pattern.

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SECTION OF SPECIMEN 645 OF JJD-085 (

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FIGURE 19 TYPICAL HYDROGEr$ DISTRIBUTION OF SPECIMEN 644 OF JJD-085

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,s 3.2.2.3 tiicroexamination ofl Al 0 -8 C Pellets 23 4 Evidence of Al 0 -B C pellet erosion was observed.in the cross 23 4 sections of specimens taken from both JJD-085 und JJD-042 as shown in Figs.16-and 20, respectively.

In,both cases, the erosion had occurred' mo'stly near the: crack opening; however, more erosion was prevalent in the cross sections from JJD-042..Most importantly, the cross sections of Figs'16and20also.show[

](ThespecimensfromJJD-085,.whichweresectioned from well below the cratered blister on the rod.[

)Ontheotherhand,the specimens from JJD-042, which were sectioned from above the cratered blister.

'] Figs. 23 and 24, respectively, present higher magnification views of the pellet structures which illustrate the(

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3.2.2.3 flicroexamination of Al 0 -8 C Pellets 23 4 Evidence of Al 0 -8 C pellet erosion was observed in the cross 23 4 sections of specimens taken from both JJD-085 and JJD-042 as shown in Figs. 16 and 20, respectively.

In both cases, the. erosion had occurred

,mostly near the crack opening; however, more erosion was prevalent in the cross' sections from JJD-042. Most importantly, the cross sections of Figs 16and20also-show(-

3 The specimens from JJD-085, which were sectioned from well below the cratered blister on the rod,[

On the other hand, the specimens;from JJD-042, which were sectioned from above the cratered

blister,

) Figs. 23 and.24, respectivelyipresent higher magnification views of the pellet structures which illustrate the 4

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C PELLET STRUCTURE OF SPECIMtN FIGURE 23 PHOTOMICROGR APH,S OF Al2 03*B4 644 FROM JJD-035.

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2 647 FROM JJD--042.,,

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4 4

13.3; Metallography'of

[Crackfrom'JJD-098-Based on the visual' examination results, it was suspected that-the[

( cladding crack.in JJD-098 represented a form of hydrogen embrittlement different from that of the(

]longitudinalcracks.

To characterize-the cladding hydrogen distribution and the crack morphology in.this. case, 5 transverse. sections were' cut from the crack region:asLlisted in Table 4; i.e,', 2 full rod cross sections.and 3 sections

'from the clamshells resulting from the slit cladding examination. The metallographic procedures were similar to those employed for the[

]

cracks, except that no Al 0 -8 C pellets were present for evaluation and 23 4 only one cladding surface was examined per specimen.

The significant observations, summarized.in Taole 4, are as follows:

Described herein are the detailed metallographic results for the cladding specimens from JJD-098 indicating the hydrogen distribution and crack pattern.

4

{.

• ] hydride layers have been produced ex-reactor by exposing Zircaloy.to high hydrogen pressures at elevated temperature (3l, -

~

h Table 4 Metallographic Examination Results -[

Crack From-JJD-098 -

.. Met.

"Axfal Location

• Spec. Specimen

'(in. above rod flo.

Type bottom)

Basis for Selection

Key Observations

~

648-Full

.m

. Transverse

Cross Sectio 1

649 Full

--Transverse CrossSection I

.650

- Clamshell

~

,g-

.Section

~

t-651 Clamshell

~

i1

~

.:- Section i

652 Clamshell

-Section Transverse sections incrementally ground from' first indicatdd

. axial location.

_r z

,, _. _.. ~. ~,, _ _ -

.m

a.x,.

l l

1 i

sun FIGURE 25 PHOTOMICROGRAPHS OF TRANSVERSE SECTIONS F JJD-098{

M

u a

.w r

..n>,

,e r

.c.-s.

m o ae.p

,4..

s

..n

,p.a4...

.a ya,,ya.

a-as+es_m ewr m

w w

4-

..Y

.9

' 4-E eq 1

i d'

s i

i 7

- FIGURE 7G PHOTOMICROGRAPHS OF TRANSVERSE SECTIONS [

j

]OF JJD-098[

J

' l 4

..~..-. -

E t_..

A e

'4 N

g i

~

+

+

5 l's t

1' A

i PHOTOMICROGRAPH MONTAGE OF SPECIMEN 649 FROM[

)

FIGURE 27 JJD-098[

3 k

o h

n p

s.

4--e.

+

a

a

-3.4 0xide Film Thickness on Fracture Surfaces The oxide film thicknesses observed on the fracture surfaces of the cracks provided a means of determining the time of occurrence, specifically, the length of time the surfaces were exposed to the hot primary. coolant.

For example, the crack visible on the clad ID of the clamshell, shown in Fig.15, was suspected of being introduced by. handling-in the hot cell.

Metallography of a transverse section through the region clearly showed the absence of an oxide film, thus confirming its occurrence af ter removal from the reactor.

On the other hand, oxide layers on 'the fracturesurfaceswereobservedonthesectionsthroughthe(

l' longitudinal cracks of JJD-085 and JJD-042 and the

] crack of JJD-098.

In principle, the observed oxide film thickness can be converted to a time of operation at the coolant temperature assuming published Zircaloy oxidation rate data are applicable.

Because the oxide film on the hydride phase tends to be thicker

~

than on the metal phase due to enhanced oxidation kinetics of the hydride, only film thicknesses away from the hydride phase were relevant.

Suitable regions for thickness measurement could be found only on the cladding cracks in JJD-042 and JJD-098..The thicknesses were converted to equivalent weight gain.

Using the pre-and post-transition isothernal Zircaloy I4 )

oxidation rate data of Hillner

, the respective exposure times were computed.

The results were as follows:

LMetallography Fracture Surface Oxide Estimated Crack Rod No.

Specimen tio.

Film Thickness, u Exposure Time, days

~

JJD-042 646 JJD-098 648 A comparison of the estimated exposure times with the Maine Yankee Core II operating history indicates that the occurrence of the crack of JJD-098 could have The'crackexamined'inJJD-042apparentlyoccurred[

'].

I I

l

!=

.w

4. '

CLADDING DAMAGE MECHANISMS l

4.1

' Primary Hydriding In addition to the above results from Maine Yankee Core II

burnable poison rod examination the results of C-E's hot cell examination of St. Lucie-I poison rods. (conducted in September 1976) are pertinent to establishing the' cladding damage mechanisms operating in the rods.

The St. Lucie-I program proved that, in poison rods fabricated in the same manner as the Maine Yankee Core II rods, the initial perforation was caused by primary hydriding due to high moisture content.in the hot pressed Al 0 -B C 23 4 pellets. The program involved the destructive examination of representative rods after a brief core exposure of approximately 800 Mwd /MTU and operation at power for about 8 weeks.

Intactrods(i.e.,rodscontainingthehelium prepressure introduced in fabrication) were found with many primary' hydride sunbursts

inonecaseat as many as 13 such sites. A small fraction of the rods exhibited evidence of only a single primary hydride sunburst, More importantly, no hydrogen or water was found in the gases collected from intact' poison rods with primary hydride damaae, so that the observed distribution of hydride sunbursts can be considered a starting condition for the secondary hydriding attack following water ingress through an initial perforation.

Because of the relative low. cladding ID-temperature and small radial thermal gradient across the cladding, primary hydride sunbursts in an intact poison rod would not be expected to disperse and homogenize into the cladding as rapidly as in a fuel rod.

Thus, primary hydride cladding damage would be'likely to persist relatively late-in-life in the form of hydride phase sunbursts, as well as non-perforating cladding cracks.

l l

4.2 Secondary Hydridina The level of hydrided cladding damage observed.among Maine Yankee Core II burnable poison rods destructively examined at the h0t cell, as described in Section 3, clearly represented a progression beyond the St. Lucie-I primary hydride damage.

Neitherthe[

}

[

nor:the(

.]wereinevidenceamongtheSt.Lucie-Irods, 17 j

g so that it is reasonable to conclude that they represented secondary.

hydriding phenomena resulting from coolant water ingress through a cladding perforation. This section discusses the possible sources of hydrogen for secondary hydriding and presents likely hydriding mechanisms which account for the observed hydrogen distributions in the cladding, j

4.2.1 Sources of Hydrogen i

In the context of secondary hydriding in Zircaloy-clad fuel, Markowitz( } concluded.that the source of hydrogen is the radiolytic 1,

i decomposition of water induced by fission fragment recoil to produce l

2 H 0+H2+H0*

molecular hydrogen and hydrogen peroxide, i.e.,

2 22

'In addition, the H 022 is removed by reaction with U02 to produce more molecular hydrogen so that the net effect is to provide a sufficiently high ' molecular hydrogen flux under irradiation to support a buildup of a localized hydride phase on the cladding ID.

Although fission fragments are not present in burnable poison rods for similar H2 generation, the nuclear reaction 810 (n, a) Li7 also and H 0 However, the promotes the radiolysis of water to produce H2 22 H0 is not as readily removed as in the fuel' rod case so that' this 22 source of hydrogen may not provide a sufficient hydrogen flux to support localized hydriding. Moreover, there is no other significant radiation-induced mechanism possible in poison rods to produce molecular hydrogen; 10 therefore, as the B is depleted by the nuclear reaction, the radiolytically derived hydrogen is correspondingly reduced.

Thus, if this were th.e only i

source of hydrogen in a poison rod,-a late-in-life perforation would not result 'in secondary hydriding damage. But, the hot cell examination uncovered at least one rod perforated relatively late-in-life with such damage, namely, JJD-085.

J ;

-l L.

')

s In' searching for.other sources of hydrogen, a chemical reaction unique to breached burnable poison rods was identified as the most potent source available, i.e., the oxidation of t:

B C particles in the porous 4

Al 02 3 pellet matrix by the incoming water. The reaction of H O with 2

8)

B C has.been previously studied in one case, the hydrogen 4

produced has been monitored to follow the oxidation of B C particles in 4

U) a porous graphite matrix In an excess of water, the following reaction occurs:

B C + 12 H O + 6 H2 + 4 H 803+C 4

2 3

The 6 mols of molecular hydrogen produced for every mol of 8 C present 4

translates into a potential generation of about

.,in a Maine Yankee Batch E poison rod, which contains[

as fabricated.

Based on the St. Lucie-I observations, it is estimated that

] The corresponding rate of H production is[-

= per hour.

This is equivalent to 2

. per hour assuming no loss f rom the rod.

(The potential steady state hydrogen charging rate unifornly into all the' cladding of a poisonrod,then,is[-

per hour; so -that a high average hydrogen level consis ent with brittle behavior could easily be reached in a few days withot onsuming much of the generated hydrogen.)

The signifisance of this chemical source of hydrogen is that radiolytically produced hydrogen is not required for the observed secondary hydriding pattern.

Thus, the potential for such cladding damage is 10 still present in a late-in-life rod perforation after the B is depleted.

o

~

4 2.2 Formation of Based on the hot cell examination results, the predominant form.

of secondary hydriding apparently produces

.]onthecladdingID. These[

]are believed to develop from preexisting primary hydride sunbursts by' the following mechanism:

a.

In poison rods with a relative large initial breach of the cladding, at a primary hydride blister, liquid water quickly fills the rod to the perforation level with a steam-helium mixture present above the perforation.

b.

A high hydrogen activity in both the liquid and steam phases is then produced by the oxidation of the B C particles.

4 c.

The presence of water and steam generally protects the Zircaloy cladding surfaces from new nucleation of a localized hydride phase, by maintenance of an oxide film impermeable to hydrogen (9 ; but any preexisting primary _ hydride sunburst sites are not similarly protected.

d.

These sites pick up' additional hydrogen, and the hydride phase tends to grow as long as the. hydrogen flux tc the site is greater than the diffusion of hydrogen in the cladding away from the site.-

e.

Because of the relatively small radial thennal gradient across the cladding and;the

.] Based on theory and observation (10)

~

f.

Some hydrogen also diffuses toward the cladding OD. f 1 '

t

~l J

?.

'l l

g. (-

i.

~

. 4. 2. 3.

Fonnation of a

, pm,

I y

e 1

i This section presents the secondary hydriding mechanism believed to have operated in JJil-098.

r I

l l

l 1

m i -

k

..)

p; 7

R

(

I r

i i

t 4.3 C' lad Cracking 4.3.1 Crack Initiation' Mechanisms 4.3.1.1 PCI Enhancement Factors in Perforated Rods The evidence from the nondestructive examination of perforated rods described in Section 2, particularly, the finding of C3 I-It appears that the conditions developed in I

perforated rods introduced (

]which intensified the cladding tensile hoop stresses in hydrided regions to provide a clad cracking driving force, s

E

1 e

22-'

p u

y.,

,,.c-

,.,,~,,,

- =.. - ~. -

[

In the liquid water-filled portion of-the perforated rod the oxidation of the B C 4

particles.in the presence of excess wa produces-H 80 0F HB0 which 3 3 2

.are soluble at the operating temperature, b

In the steam-filled portion of the rod (i.e., above the initial perforation) B C particles are also oxidized but more slowly than in 'the 4

liquid water-filled section.

Initially, the pore structure remains open to permit transport of the oxidation products out of the pellet to the pellet-clad gap.

J In addition to the metallographic results, supporting evidence for the

.(,was obtained from a preliminary examination-of pellets removed from a perforated burnable poison rod i

representing 100", B depletion. The results confirmed the presence of l

10

'[

[Tniswasinsharp contrast to the I

~ found on pellets from a 10 St. Lucie-I perforated rod which had experienced little B depletion.

I L

j l

A relatively minor contributor to PCI enhancement in perforated rods is. the gap closure associated with the formation of the hydride phase f

on the cladding ID.

Not.only does the cladding eventually bulge outward as the hydride phase thickens at the expense of the metal; but tnere is also an observable expansion into the pellet-clad gap. as welI as a thicker than nonnal ID oxide layer which contributes to gap closure,

.23-

.c t

v

~

1 p- - -

All of. the above mechanisms for PCI enhancement are consistent with the evidence from the hot cell examination and clearly indicate that-

'I thecladdingtensilehoop'stressesarisingfromthe(

,could increase to levels sufficient to initiate longitudinal. clad cracking.

4.3.1.2 Effect of Thermal Cycling Another possible cause of crack initiation is a hoop stress intensification accompanyir.g startup from a cold shutdown. A cladding hoop stress would be expected to develop in the region of radially. oriented hydride platelets.on.startup~ as a consequence of the greater thennal expansion of the adjacent ID hydride phase relative to that of the metaP.

An addittonal possible contributton to the hoop stress involves the

,into the pellet-clad gap of perforated rods as a result of the cold shutdown, Coolant water is also drawn into the rod'on shutdown; so that on return to power, the water volume expansion might be constrained by the reduced communication causedbythe(

}Suchawaterloggingphenomenoncouldthen add to a hoop stress intensification on startup.

As noted earlier, dating calculations based on fracture surface oxide film thickness measurements show that the occurrence of the[

T

• Note that the thermal expansion of zirconium hydride is 50% to 100% greater than that of Zircaloy-4 depending upon the 1

specific bulk hydride phase present on the 10.

I o

I f' p]

_p.

'4.3.?

Crack' Formation-4.3.2'.1- [

. Longitudinal crac'ks In the case of.the

] cracking probably(

]The crack breaks through l

1

-..g 4.3.2.2

]CrackofJJD-098 It is possible that the

,in'JJ0-098,exhibitingthe(.

[ cracked during hot'operationbyamechanism[

] Another possibility,['

t w4

.f 1

J l!.

!i l

I -

k 1

ll li -.

i

{.

25-L

p

. 5.

SUMMARY

'AND CONCLUSIONS 5,1 Cause of Clad Cracking.

Based on the results from the hot. cell examination and'the analysis'of possible cladding damage mechanisms, the following sequence of events is believed-to have occurred in the Maine Yankee. Batch E burnable. poison rods-to cause the observed cladding cracks:

a.-

Just as in the St. Lucie-I case,.the internal surface of the Zircaloy cladding was subjected to a localized primary hi riding attack due d

to the presence of excessive moisture in the burnable poison gn11ets. This produced a variety of primary hydride sunburst distributions among the poison rods..

b.

The cladding was initially breached by breakthrough of a brittle primary hydride sunburst. (..

8' k

4 c.

Coolant water entered the rod through the cladding breach leading to {

secondary hydriding. (

,}themajorsourceof hydrogen for the attack was the oxidation of the 8 C. particles by water 4

penetrating the porous A102 3 pellet matrix which produced molecular-hydrogen directly.

~

l'

+

Sumarized herein are the secondary hydriding and clad cracking m,echanisms detailed in previous sections.

4 M

D.

5.2 Overall Conclusions At the outset of the program, the poolside observation of burnable. poison rod' cladding cracks [

' ]Twocausalfactorshave now been 4

i

[

t l

~.

i

. While there is no evidence (other than the poolside observation) of a cladding damage threshold, the following conclusions concerning the evolution of cladding damage in burnable poison rods subject to ^ primary hydride perforations are indicated by the analyses of the post-irradiation

. examination results:

Because the major sources of hydrogen for secondary hydriding, a.

once water enters the rod, are the oxidation of B C by H O and the radiolytic decompositi'on' of. H 0 induced by. the B10(n,a)Lii 2

2 reaction; M

b, [Y

}

~

i e

{ -

.t1 N

.- 1,l.

-(

t k

y.,

-y28--

-j

.y

$[,

s

,1

...4

..,_m

1 f

c..

In any-given. fuel batch containing burnable poison rods

. subject to' prinary hydride perforations,:three major classes of rods.

corresponding to'different stages of cladding damage are possible:

(1) Intact'rodsDwith primaty hydride damage which may yet be breached 4

to. allow water ingress; (2) perforated rods with secondary hydride damage,consistingof(:

t

,]which are susceptible to clad cracking; and (3) perforated rods with at'least 'some of the secondary hydride damaced areas already cracked. '

i d.,

Chelspectrum of claddi,ng damage could include longitudinal cracks [

[' reflecti.ng the secondary hydriding

.[

[reachedbytheobserved[

J V

E I.

i l

,1 s !

L a...

1.

~

>r.

' 6, REFERENCES l.

' N Fuhrmen,'et al.. " Evaluation of Fuel Performance in Maine Yankee Core I", Final Report, EPRI hP-218, Projet.t 586-1, November 1976.

l 2.

"Fractographic Features Revealed by Light Microscopy", Metals _

Handbook, Vol. 9 Fractograohy and Atlas of Fractoaraphs, pp. 27-48, ASM (1974).

l 3.

A. Sawatsky, "The Diffusion and Solubility of Hydrogen in the Alpha-Phase of Zircaloy-P.", J. Nuc_ lear Materials, Vol. 2, No.1, pp. 62-8 (1960).

l 4.

E. Ilillner, " Corrosion of Zirconium-Base Alloys --An Overview",

l L

presented'at che ASTM Symposiur on Zirconium in the Nuclear Industry, l

August 10-12, 1976, Queb'ee City, Canada (Published by ASTM as STP633, A. L. Lowe, Jr. and C, W. Parry, Editors).

5.

J. M. Markowitz, " Internal Zirconium Hydride Formation in Zircaloy Fuel Elenent Cladding Under Irradiation", WAPD-TM-351, May 1963.

l 6-E. J. Hart, W. R. McDonnell and S. Gordon, "The Deccmposition of Light and Heavy Water Boric Acid Solution by Nuclear Reactor Radiations",

Proc. 1st Int. Conf. Peaceful Uses of Atomic Energy, Vol. 7, p. 593, Geneva (1955).

7.

Public Service Company of Colorado 330-fMe) High Tertperature Gas-Cooled Reactor Research and Dcvelopment Program, Quarterly Report l

for;the Period Ending March 31,1971, p. 52, GA-10560 (April 30 1971).

.i i

\\

r

-30..

.).

.p.

.. ~

i 1

8.

L, M.' Litz and R. A. Mercuri

" Oxidation of Boron Carbide by

- Air, Water and Air-Water Mixtures at Elevated Temperatures", J.

Electrochemical Society, Vol. 110, No. 8, pp. 921-5 (1963).

9.-

D. W. Shannva, "Effect of Oxidation. Rate on Hydriding of

~ Zirconium Alloys in Gas Mixtures'Containing Hydrogen", Corrosion,

.Vol'.19, pp. 414t-420t (1963). '

10.

C. E. Ells and C. J. Simpson, " Stress Induced Movement of Hydrogen in Zirconium Alloys'.', in Hydrogen in Metals, edited by I. M. Bernstein and A. -W. Thompson. Proceeding of Seven Springs

)

Conference, September-.23-27,1973, American Society for Metals (1974).

11-R.: A. Proebstle,-et al., "The tiechanism 'of Defection of

-l Zircaloy Clad Fuel Rods by Internal Hydriding" Proceedings of Joint Topical Meeting on Commercial Nuclear Fuel Today, 75 CNA/ANS-100, April 28-30,1975, Toronto, Canada.

l 12.

P. Cohen, Water Coolant Technology of Power Reactors, pp. 220-4, Gordon and Breach, Science Publishers, Inc., New York, New York (1969),

t i

i

)

[

. J-a i.

-m

., - ~ _ - - -

.... ~ -,.

-