B. A. R. G.-984

sGOVERNMENT OF INDIA

ATOMIC ENERGY COMMISSION

MBTALLOGRAPHIC EXAMINATION OF IRRADIATEDNUCLEAR FUEL ELEMENTS AT RADIOMETALLUROY

HOT CELL FACILITY

by

D. N. Sah, E. Ramadasan and K. UnnikrishnanRadiomeulliirgyr Division

BHABHA ATOMIC RESEARCH CENTRB

BOMBAY, INDIA1978

B.A. R. C.-984

«> GOVERNMENT OF INEIAT ATOMIC ENERGY COMMISSION

METALLOGRAPHIC EXAMINATION OF IRRAEIATEDNUCLEAR FUEL ELEMENTS AT RAEtOMETALLURGY

HOT CELL FAQ LIT Y

by

D. N. Sah, E. Ramadasan and K. UnnikrishnanRadiometallurgy Division

BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA

1978

INIS Sub.1«ct Cftt«gory i H 6

HOT CELLS

UETALLQGBAPUT

SPENT FUEL-

FUEL CANS

laCBOSTHUCTUHE

ZIHCALOY 2

URSNIUf DIOXIDE

FAILURES

6 M I N SIZE

ETCHING

SAMPLE PREPARATION

CORROSION

AUTOBADIOGRAPHr

HXDRIDATION

PEOTOUICROGRAPHy

METALLOGRAPHIC EXAMINATION OP IRRADIATED NUCLEAR FUEL ELEMENTSAT RADIOKETALLTOGY HOT CELL FACILITY

D.N. Sah, E. Ramadasan and K. UnnikrislinanRadiometal lurgy Div i s ion

Bhabha Atomic Research CentreTrombay, Bombay-400 085.

ABSTRACT

The Hot Cell Facility of Radiometallurgy Division in B.A.R.C.

is equipped for carrying out metallographie examination on irradiated

nuclear fuel elements. Equipment available for metallography and the

procedures followed for carrying out metallographlc examination in thie

facility are described in this report. Some important observations from

recent metallographic investigations on irradiated fuel elements are

also included in this report*

I INTRODUCTION

Metallography plays a very important role in the examination

of irradiated nuclear fuel elements. It provides a large amount of

information regarding the fuel behaviour and fuel element performance

during irradiation. Direct information is obtained by metallography

about structural changes in the fuel and cladding, density variation,

fuel-clad and clad-coolant interactions under the operating conditions

of the nuclear fuel element • Indirect knowledge can also be obtained

by metallography, about the fuel centre temperature, radial temperature

distribution in the fuel pellet cross section, mode of material transport

and distribution of solid and gaseous fission products in the fuel.

Metallography is an essential tool for evaluating the causes of failures

. in irradiated fuel elements*

Irradiated fuel elements are highly radioactive of the order

of 10000 curies of 1 Mev gamma radiation. Samples cut from these

elements for metallography will have activities varying from 10-100

curies* Because of tHis high gamma activity, all metallographic

operations such as cutting, mounting, surface preparation and examination

1 2 J

of the specimens Eire performed, in radiation shielded c e l l s , using

remote control equipmen^m^ster slave manipulators* These c e l l s are

maintained at a lower pressure compered to the adjacent areas, to

avoid any possibi l i ty of release of radioactive particles to other

areas. Al l the equipment inside the c e l l s for various operations have

to be either automatic or properly designed for remote operations.

Al l optical parts in viewing equipment and metallograph are required

to be made of radiation-resistant g l a s s . Further, arrangements for

supply of reagents and other materials to the c e l l s and for handling

and disposal of solid and liquid waste produced during various operations

are also required*

Metallo&raphic investigation i s one of the main examinations

carried out on Irradiated fuel elements at Hadiometallurgy hot c e l l s .

The f a c i l i t i e s available and procedures adopted for carrying out th i s

examination are described in th i s report* Some Important findings

noted during recent examinations are also inoluded.

2 . DESCRIPTION OP KE3AILOGRAPHIC FACILITY AKD BQUIIOTT IAYOUT

The hot c e l l f a c i l i t y of the Hadiometallurgy Division, (HMD)

B J I . R . C . i s equipped for carrying out metallographlc examination of

Irradiated nuclear fue ls and other structural materials* Three c e l l s

are u t i l i s e d for various metallographic operations in th is f a c i l i t y -

the cutting c e l l (ce l l 2 ) , the specimen mounting c e l l (ce l l 3) and

the metallography c e l l (ce l l 6 ) .

The cutting c e l l i s 2.415 x 2.1M In area and capable of

handling 100000 curies of ac t i v i ty . At present th is c e l l accommodates

two diamond cut off wheels for sectioning specimens from irradiated

fuel elements. One of them i s a hl^h speed cut off wheel which can

accept fue l elements of length upto 30 cms and maximum diameter of

25 mm. The other one Is a alow speed cut off wheel designed and fabr i -

cated at RMD for remote operation CPig.i). In th is cut off wheel

J 3 *

arrangements have been made for automatic feeding of the wheel to the

job. This machine ie capable of accepting longer fuel elements with

a diameter upto 17 mm. In addition, a tube cutter ia also available

for sectioning cladding samples from the tubes. Arrangements for

filtration of coolant water, used during cutting and drainage of this

to high level liquid waste tank have been provided. Both transverse

and longitudinal specimens can be cut using these cut off wheels.

Slow speed cut off wheel is used to cut fuel specimens for metallo-

graphy.

The mounting cell has provisions for vacuum impregnation

of fuel pin pieces with araldite as well as araldite mounting of

metallographic specimens under vacuum. This cell has an ultrasonic

cleaner also, for cleaning the mounted specimens. The set up for

vacuum araldite mounting of metallographic specimens is shown in

Pig. 2. Arrangement for araldite impregnation of 10-15 cms long fuel

pin pieces is shown in ¥ig'3* This impregnation of fuel pin pieces

is required to ensure that the cracked fuels do not fall apart during

specimen cutting.

After araldite mounting the specimens are cleaned and

transported to metallography cell through a pneumatic transfer tube

which connects the two cells. The pneumatic transfer device operates

with 100 psi air line and is very quick.

The metallography cell (cell 6) contains facilities for

specimen grinding., specimen polishing, chemical and electrochemical

polishing and etching of specimens and for periscopio examination, micro-

scopic examination, alpha and beta-gamma autoradiography and mierohardness

testing. This cell is 7-211 long, 2.1M wide and is designed to handle

100 curies (1.3 MeV gamma) activity. There is a ©mall steel blister

attached to this cell which accommodates a remotised metallograph.

This blister is made of 23 cms thick steel and has Inside dimensions of

82 cm long x 60 cm wide x 82 cm height. It ia designed to

handle 100 curies (1.3 MeV gamma) activity.

« 4 »

A small lead glass viewing window and a ball-tong assembly have been

provided in the blister to facilitate viewing and operationa respectively.

The blister le connected to the metallography cell by a 15 cms dia port

hole in the cell wall.

Layout of the equipment in the cell is shewn in Fig.4. A

brief description of the equipment available in the metallography cell

is given below.

Grindersi~ 'Buehler' Automat grinders (z Nos.) with simplified specimen

holders have been provided . The original specimen holders supplied

with the equipment have been discarded because of difficulties in remote

operation . Controls for the machine have been fixed in the operating

area. Two specimens can be ground siirailtaneously in this grinder*

Polishersi- 'Syntron' vibratory polishers (2 Nos.) have been placed

inside the cell and their controls are placed in the operating area.

Remotised Metallograph.- A remotised 'Bausch and Iomb metallograph1 is

available which can give magnifications upto 1000 X. The illumination

source is a carbon arc lamp. The metallograph has provision for exami-

nation under bright field and polarised light. Attachments for photography

on 4" x 5" sheet films as well as 35 mm roil film are available. Three

non-browning objectives 5X, 20X, 40X are fitted on the turrst. The stage

can take up specimens mounted in rings of maximum di& of 31 mm* All the

motions in the metallograph have remote controls on the operating face

of the blister.

Microhardness Tester:- A remptised 'Tukon' Microhardneas tester is also

available in this cell. The control unit is in the operating area*

Diamond pyramid indenter is used and the test load can be varied from

25 gms to 50 Kgms. This hardness tester is attached with a microscope

(625X). The magnified image of indentation is transferred to operating

area with the help of a wall periscope and measurements are carried out

with a calibrated micrometer eyepiece.

AutoradiQflraphy set-upi A set up (fig. 5) designed and fabricated at RMD

is available for taking autoradiographs on irradiated fuel specimens.

Exposures of even a few seconds can be given with this equipment. Both

« 5 »

alpha and beta gamma autoradiographe can be taken using this equipment.

The metallography cell also contains an automatic electro-

polishing and etching unit, three ultrasonic cleaners for cleaning of

samples during grinding and polishing steps, a 'Kollmorgen' remote viewing

periscope for macroexamination and photography of specimen surfaces and

a dial gauge stand to estimate the specimen height reduction during

grinding and polishing operations.

All other general cell facilitiee like intercell and external

transfer systems, lighting and viewing, service lines, storage arrangements

etc- are available as described in Ref.1.

3. SEQUENCE OP OPERATORS DT METALLOGRAPHY OF IRRADIATED PUEL

ELEMENTS

The various steps in the preparation of metallographic specimens

are shown in F.^.6. Some importai., ones are briefly described below.

3.1 Speoimen Selection

Selection of locations for metallographie examination, in a

fuel element, is made in the light of observations of preliminary non-

destructive testings. Non destructive testings.like visual examination,

gamma acanr.ing, leak testing and dimensional measurements, reveal

locations of abnormality and failures in the fuel element. Such locationo

may bei regions of excessive dimensional increase, heavy corrosion sites,

position of power peaking, leak locations, and fret marks, ,'hese locations

are selected for metallography to know the cause of their formation and

to assess their effect on fuel element life. In a failed fuel element,

locations of failure as well as nearby locations having no sign of failure,

are selected for examination, so that, reason of failure can be established.

In a fuel element which has not failed, the location of highest power

rating is examined. This location experiences the most severe conditions

in the fuel element during irradiation.

In specimen selection the aim is to examine the minimum number

of specimens that can provide maximum information. Depending upon the

features present at the selected location a transverse or longitudinal

section, which will reveal maximum amount of information, is selected for

t 6 1

examination. Such locations selected In an Irradiated TAPS fuel element

are shown in Fig.7. indicating the basis of their selection. The

locations thus selected are marked on the fuel element with araldlte or

with coloured ink. A cutting plan of the fuel element is made* The

plans also indicate the type of the sample to be cut e.g. transverse or

longitudinal sample.

3-2 Specimen Cutting

The specimens for metallography is cut in two stages* First

a 10-15 cm piece containing the relevant location is cut from the fuel

element* Fig.(8a) shows the cross section before eraldlte Impregnation.

This piece is first Impregnated with araldite using the set up shown In

Fig.3. This step ensures that the cracked UCL fuel pieces are fixed

with araldite. The metallographie specimen is then sectioned from this

pre-impregnated piece of fuel element• The 3ize of the specimens vary

from ^mm in length to 20mm depending upon the fuel burnup and cooling

time. Specimene should not have more than 100 curies(1>3 KeV gamma)

activity to enable their handling in the metallograph blister. Care is

taken for adequate cooling during specimen cutting. For -making the cut

at exact location a telescope ia used to locate the feature whsre

sectioning is to be carried out. Slow speed cut off wheel Is used to

obtain a good plane, cut surface and to avoid spread of contamination in

the cell. When the cutting is to be made at a feature which is so small

that cutting exactly at that location is difficult, then, out is made

slightly away. The specimen is then carefully ground sufficiently to

reveal the feature during the specimen preparation stage.

3.3 Specimen Mounting

Specimen cut from the fuel element is mounted with araldite

in stainless steel ring having 30mm outer dia and 25mm height. The size

of the stainless steel ring has been selected to suit the vibratory

polisher specimen holders and the metallograph specimen holding cup*

Both of these are designed to accept a maximum of 31mm diameter mountings.

Identification numbers are punched on the S.S. ring for identification

purposes.

« 7 «

The Specimens are mounted in S.S. rings using the <?et up

shown in Pig.2. The specimen is placed down (face to be examined) on

the greased surfac« of neoprene sheet- The 8.3. ring is placed

surrounding the specimen. The set up is then assembled with liquid

araldite in the top plastic container. The system is evacuated for

five minutes. The specimen and araldite both get separately evacuated

during this period. Then the steel ball closing the optning in the

araldite container is removed with the help of lever. The liquid

araldite flows down and f i l l s the S.S. ring with ara ldi te . The araldite

penetrates the cracks and voids in the fuel specimen. The system is

then brought to atmospheric pressure* The system is eTaeuated and

brought back to atmospheric pressure three times to ensure complete

penetrations of araldite in the fuel cracks. The specimen i s left for

24 hrs for araldite to cure.

3.4 Specimen Surface ^reparation

3.4.1 Grinding» The mounted specimen ia cleaned with tr i lene to

remove the grease and then cleaned ultrasonically. This specimen i s

then fixed in Buehler Automet grinder. Hough grinding i s carried out

using 240 gr i t self adhesive silicon carbide papers. Water is used as

lubricant during grinding. A weight of 400 gms is placed on the speaimen

during grinding . This rough grinding is continued t i l l the specimen

surface becomes plane and a l l xhe araldi t i from the surface has been

removed. This operation requires about three hours for a specimen taken

from a fuel element exposed to a burnup of about 10,000 MTO/lOT.

The specimen is then removed from the grinding wheel, cleaned

in ultrasonic cleaner and subjeoted to fine grinding using 600 gr i t

silicon carbide paper for on« hour. The specimen ia cleaned and examined

through periscope. This operation removes the deep and wide scratches

produced during coarse grinding. The specimen is then subjected to

polishing.

3.4*2 Polishing 1 Specimen ia fixed in suitable holder and polishing

of the specimen is carried out in Syntron vibratory polishers using

diamond paste or diamond spraying compound. Coarse polishing is carried

i 8 t

out overnight using 4 micron else diamond abrasive. Final polishing la

carried out with one micron diamond abrasive for 46 hours. The specimen

la removed from the holder, cleaned in ultrasonic cleaner and is ready

for examination* Fig* (8b) shows a specimen prepared for metallographic

examination*

Specimens in which only zircaloy cladding is to be examined are

chemically polished after fine grinding using the following solution!

HNO- 45 parts by volume

ILO 45 parts by volume

HP 48?? 10 parts by volume

3.4*3 Etching) Polished specimen is chemically etohed using 90$ by

velum* H^O, and 10$ volume H^SO solution, to reveal uranium dioxide

grain morphology*

Etching of the specimen surface is carried out for 60-90

seconds by immersing the poliehed fuel surface in the above solution*

The specimen ia then thoroughly washed in fresh water and further cleaned

in an ultrasonic cleaner. Some staining has been noted on the specimen

after etching* Etching in this solution is seen to result in stains on

the specimen surface • This is removed by a mild polish on vibratory

polisher with one micron diamond* The specimen is then examined under

miorosoope*

Etching of zircaloy to reveal grain structure is carried out

with a freshly prepared solution of following composition!

HNO, 45 parts by volume

ILO 45 parts by volume

HP (48$) 8 parts by volume

The specimen is swab-etched for 20 seconds with this solution, washed

and dried*

To reveal the hydride distribution in the aircaloy cladding

Bection, the following etchant is utilised:

KLOg 50 parts by volurie

Ethyl Alcohol 25 parts by volume

HNO,, 10 parts by volume

HP 1 part by volume

Swab etching for 45 seconds reveals the hydride distribution clearly*

i 9 i

4 . METALLOGRAPHIO EXAMINATION

General scheme of examination of meta l lographica l ly prepared

•pecimens i s g iven la Fig*9* This f igure a l s o l i s t s down Informations

that are collected during these examinations.

The metallographic examination consists of two stagesiexamination of the as-polished surface, and examination of fuel andolad surfaces after proper etching.

4*1 Examination of Specimen in a Polished ConditionExamination of as polished specimen i« carried out first with

a periscope at a magnification of 10X to reveal the crack pattern on thefuel section end a photograph of this seation is taken. This photographis used to analyse the fuel cracking behaviour and study of dimensionalchanges due to swelling and densification effect. The periscope exami-nation also revealB major defects in the cladding and their location withrespect to fuel cracks. The specimen is then transferred to the remotecontrolled metallograph for mieroexamination at high magnification. Themicroexaminetion consists of the followingt

(i) Thorough examination of the cladding section for externalcorrosion, fuel-cl&cl interaction, deformation, thinning etc*

( i i ) Shape, size and distribution of pores along the fuelpellet radius.

( i i i ) Measurement of oxide layer thickness on inner and outerside of the cladding.

(iv) Measurement of nodule thickness and nodule diameter atnodular corrosion s i tes .

(v) Measurement of fuel-clad gap, crack width etc.(vi) Close examination of cladding and fuel at the failure

location with respect to second phase formation, nature of cracking,fuel cracking, fuel loss etc.

(vii) Hardness measurement on cladding and fuel.

Alpha and fcrfca-gamma autoradiographs are taken on the polishedspecimens. Beta-gamma autoradiographs reveal distribution of fissionproducts OB the fuel cross-section and the distribution of plutonium lmthe fuel cross-section is revealed by alpha autoradiography.

i 10 «

4*2 Mioroexamination of specimen after etching

Fuel miotoatructural changest

The specimen is thoroughly examined at 20QX and 400X to reveal

restructuring in the UC fuel* The epeolmea is scanned along the diameter

on the specimen cross-section. Extent, of restructuring e.g. formation of

structurally different zones are noted. The width of each zone is

measured*

4.2.1 Grain size measurementi The grain size measurement is carried

out on U0_ fuel by linear intercept method. A calibrated filar eyepiece

(12-5X) is used* A minimum of twenty grains are counted as far us

possible * Readings are recorded in three directions and their average

is taken as the grain size of UCL at that location.

Radial distribution of the grain size is also measured to

reveal the radial location where equiaxed grain growth had started*

For this purpose, grain size measurements are made at ten equidistant

locations along the diameter of the fuel pellet (Plg.1O). Such measure-

ments are oarried out on three diameters and average grain size is

calculated at each radial location. The average grain size is plotted

against the pellet radial looations • This plot gives the radial grain

size distribution. Sharp Increase in grain size at any point in the

plot indicates radial locution of the start of grain growth. Equaxed

grain growth In sintered U0? occurs above 1300°C. Thus knowledge of

the radial position of She etart of equiaxed grain growth indicates a

temperature of 1300*0 at that location.

4.2*2 Cladding ezaminationi The sircaloy cladding portion of the

specimen is examined at 4001 under bright field and polarised light*

The grain size measurement is oarried out under polarised light since

it reveals the grain* batter* The cladding spMimen from the weld

region are examined after etching forvasurement of heat affected eone,

weld microstrueture and weld defeets.

The sircaloy oladdlng it suitably etehed to reveal the hydride

distribution. The examination is carried out at 2001 magnification under

bright field. The distribution of glreonium hydride platelets and

» 11 1

massive hydride blisters are examined. The orientation and size of the

hydride platelets are determined. The size of hydride blisters (sunburst

hydriding) and their depth of penetration in the cladding are measured.

locations of massive hydriding and radially oriented platelets are

carefully examined with respect to the fuel cracks end other associated

details to reveal the cause of their formation • The extent of radial

hydriding is determined by estimation of f number, which is the ratio

of the number of radial hydride platelets to the total number of hydride

platelets in a fixed area on the transverse cross-section of the cladding

tube. Hydride platelets oriented between 45~9O° from circumferential

direction are designated as radial hydrides. The f^ number is determined

from photomicrographs. A three inch square area is selected on a photo"

micrograph taken at 200X .for this purpose.

5. SOME IMPORTANT MSTAILOGRAPHIC OBSERVATION IN IRRADIATED

FUEL ELEMENTS

A number of irradiated fuel elements have been examined in

Hadiometallurgy hot cells (Table i). These include experimental fuel

elements of HAPP type, containing natural UO,, and UOp-PuOg fuel pellets

elad in zircaloy-2, and power reactor fuel elements from TAPS, which

oontain enriched tKL pellets in ziroaloy cladding. Metallographio

examinations carried out on specimens from these elements hav« revealed

interesting features* Some of the important observations are presented

in Pigs. 11-18.

Macroscopic features revealed on the transverse and longitudinal

•eotions of an Irradiated zirealoy clad UOg fuel elements are shown In

figures 61a)and (11b). The macrographs show radial and circumferential

cracking of fuel pellet in the transverse section. The longitudinal

eeotion shows cracking of the fuel pellet in transverse direction as well

as axial direction. During irradiation, fuel pellets experience steep

temperature gradient in the radial direction. Thermal stresses produced

by the temperature gradient lead to radial oraoke in the pellet. The

circumferential cracks are formed due to differential contraction of the

central and peripheral regions during reactor shut down. Fuel eraoking

Influences a number of factors e.g. fuel-clad mechanical interaction

during power changes, fission gas release, and heat transfer from fuel*

I 12 >

Fig». 12, 13, 14 ehow three modeu of fuel elements failures

revealed by metallograph J examination. Pig.12 reveals features of a

cladding failure in fuel element IWI-B... The middle photograph shows

longitudinal eeotion at the failure location. The magnified view of

the failure location ie shown on the right hand side. It reveals clad

thinning and subsequent collapse of the clad in the axial gap '"ider the

influence of external coolant pressure. The extent of clad thinning

can be assessed by comparison with the original clad thickness shown on

the left side*

Pig. 13 shows an internal hydriding failure observed in fuel

element WL-P.. The photograph shows the transverse section of tha fuel

element at the failure. A magnified view of the cladding (itched for

hydride is also shown. It shows formation of massive zirconium hydride

whiah led to failure of the zircaloy cladding.

Pig.14 shows longitudinal section at the top end weld location

in a TAPS fuel element. It shows severe cracking of the cladding near

the weld. Bottom photograph shows the same section after hydride etching,

and reveals formation of massive zirconium hydride at the location of

failure*

Pig.15 shows different types of orientations of zirconium

hydride platelets on the transverse section of the irradiated zircaloy

cladding. Three types of orientations e.g. circumferential, radial and

random orientation are shown. The figure on the top right side, shows

localised hydrogen pick up, leading to formation of massive zirconium

hydride blister, with consequent failure of the cladding.

Two micrographs 1A Pig.16, show localised accelerated corrosion

on the outer surface of the ulrealoy oladding, resulting into the formation

of lens shaped zirconium oxide nodules. This was located ia the zircaloy

cladding of TAPS clement. This type of corrosion is termed nodular

corrosion. Nodular corrosion of slxealoy occurs in oxygenated water in

the presence of neutron Irradiation. Since BVRs operate with oxygenated

coolant water, nodular corrosion is common In M R fuel cladding materials.

i 13 i

Pig.17 shows the fuel pellet eroes-seetiob of an Irradiated

TAPS fuel elementt along with a beta-gamma autoradiograph of the same

location. The autbradiograph reveals the distribution of beta-gamna

eotlve fission products in the fuel cross-section. The fuel centre

temperature was about 1600°C and the fuel had experienced a burn up of

12,000 MTO/teU. The fuel had been cooled for more than five rears whenthe autoradiograph was made* The autoradiograph shows, therefore,

137the distribution of fission product Oe only* It shows that the Inner

central regions depleted of cesium. The outer cooler rim of the pellet

shows, concentration of cesium. The volatile nature of cesium (b.p. 690°c)

makes it migrate towards cooler regions In the fuel*

The microstructural changes occurring in uranium dioxide,

during irradiation is shown in Pig.18. The top figure shows a photo-

macrograph of fuel cross-section. Micrographs of fuel periphery mid-*

portion and centre are also shown. It reveals that grain growth has

occurred in the fuel centre. The start of blackish porous region

corresponds to start of appearance of fission gas bubbles at grain

boundaries*

OOHCLUSIOH

Badiometallurgy Hot Cells at B.A.R.O. are fully equipped to

carry out detailed netallographic examination of irradiated fuel elements*

procedures have been standardised for the various steps needed in the

preparation of samples suitable for metallographic observation. Existing

facilities afford estimation of various parameters, like grain alee and

other structural changes in fuel and cladding materials* corrosion aspects

and the various types of hydride formation In aircaloy clad, pellet clad

interaction between fuel and clad which will help In assessing the aotual

behaviour of fuel sleiients during operation*

I U t

The author* wish to acknowledge with thanka the help amoo-operatlon £lr*n *y their colleagues in Hot Cell Group of Radio-'••tallurcr SlTiMlon» Th«y alao rriah to thank Shri J.X. Bahl andShrl K«S. SiTarauafcrithnan, HtaAof Hot Cell Group for the helpfuldiacucaiona and connanta. Thanka are alao due to Shri ?tlt> Roy,Head, Radionetallurjy Diriaion for hie keen Interest in this work.

EEIERENGE

1. Radiometallurgy Hot Cel l s a t BAHC, Trombay, by K.S. Sivaramakrishnane t a l . (BARC/I-425)

TABLE I

CHARACTERISTICS 01? THE IRRADIATED FUEL ELEMENTS EXAMINEDIN RADIOr.'ITCTALUJRGY HOT CELLS

Fuel ElementDesignation Fue l «n -U4 P lace of Burnup

Clacking i r r a d i a t i o n MTO/MTH

PWIr-A1

JWL-B

P»L-C1

H M 1 D2

P.YL-P

KP-OO33

KM-0268

KS-0847

Nat. U02

Nat. U02

Nat. UO2

Nat. UOg

U0.-1.5$Pu0o2 2

Enriched UO-

Enriched UOp

Enriched UO_

Zircaloy-2

Zircalo^-2

Zircaloy-2

Zircaloy-2

Zircaloy-2

Sircaloy-2

Zircaloy-2

Zircaloy-2

P.V1-CIRUS

PffL-OKUS

IWL-OIEUS

PffL-CIRUS

mmmmTAPS

TAPS

TAPS

180

660

545

2176

507

9012

9012

S012

Sound

Failed

Failed

Sound

Failed

Failed

Failed

Sound

Fig. I. Slow speed cul off wheel for specimen cutting from irradiated fuelelements designed and fabricated in Radiometallurgy Division.

HANDLE FOR REMOVINGSTEEL BALL

TO PUMP

CORK

PEBSPEX TUBE

STEEL BALLSTOPPING ARALOITE

FROM FLOWING DAWN

FUEL-SAMPLE

-S.SRING

-NEOPRENE

!PLATE

Fig .2 . Set up for YBCUUTI yiountin.* of netallographic specimen.

tl»H»T

Fig.3. Arranjeuent forla p j ;

nation of 10-15c»slong fuel elenentpiece '»efore cut-ting a netallo-jraphic speoinen.

layout ofaquipnenta Inthe netallo-

cell-

I.DML GAUGE STAND. 7. ACTIVE LWUIO DRAINAGE.I.»UTOR«IO<WWPHtC SET-UP. 8.TRAVS FOR NEEPMBI ABRASIVES,3.AUTOMET POUSHMG MACHINES. ETCHANTS, SOLUTIONS ETC.4.SYNTMN VIBRATORT POLISHERS. S.ETCNIH9 TRAY.9.KAM ORIERS. MITUKON MCROHARDNESS TESTOt.

6.ULTMS0MC CLEANERS. II.REMOTIHD MCTALLOORAPH.

I. SWIVELLING STAOt.

Z. SAMPLE.

S.LOADWi CAM.

4 . L 0 A I M N I M I N a .

9.COM. SPMNO.

•.PARALLEL ARMS.

T.LOCKNW LEVER.

• . F I L M PACKET.

Pig.5. Autoradlojraphj-Bet up.

FIO. 6 STEPS BT MBTAIiLO&KAPHIC EXAMim,TIOH

Specimen Selection and Marking the Location

Cutting a 4~6 inch long piece containingSpeciwen Location/Locations

Araldi te Impregnation of Cracked Fuel

Specimen CuttingI

Specimen Mounting in Steel Hinge with Araldite

Coarse Grinding240 grit SiC Paper

Periscope Examination

Ee^rind ifBpecimen Surfacenot plane

Ultrasonic Cleaning

Fine Grinding ^600 gri t SiC Paper ~\

I EegrindingUltrasonic Cleaning ** necessary

i IPeriscope Sxamination .—1

\Pixin,? in Vibratory Polisher Specimen Holder

Polijhing 4 M diamond (48 hrs.)

Final Polishing 1 yu diamond (48 hrs . ; •*—

Hepolishingif necessary

Prepare* Specimen Ready for as-polished Examination ori [vetallograph

Metallographic Examination

Etching

Metallographic Examination

Storage

SPECIMENSELECTED

LKPOHTAHTFEATURE ATTHE SELECTEDLOCATION

EtSSENCE OFWHITE CIRCUM-FHEt3ET IAIBAUDS

SHOWEDj&xBrou FLUX

BYVISUAi

KCI7-PAHSDHEGIGli, CIOSSTO P A I L S ^LOCATION

tf

FHET 1'AP.KISVIJALSD 2YVIStTAL 3!CAi!.

2SGCIAR7TSID SXAM.

POBPOSE OP TO PIKD THE TO STUDY TIE TO F E D THE COISLEISCETCAUSE OP PCR- FUEL & CUD CAUSS OF '.TITH PAILSD1'ATIOIF OF WHITS BEHAVIOUR EAILUS3 HSGIOKBAUDS kW ITSEFFECTS

DSFTrr OF W3IDP53T, HYB30GEI: BEEAVIOUBPICK-UT

. 7 Se lec t ion of Ueta l losraphic Specimens f roa an I r rad ia tedTAPS Fuel Element.

ZIECALOY-2 MADDINGI

U0 2 FUEL

Fig. S a As cut surface of a specimen taken from irradiated fuel element (befote araklite impre-gnation).

STAINLESS STEEL RING

ARALDITE

ZDtCALOY-2 CLADDING

CHACKED D0 2 FUEL

Fig. 8 b. A metatlographic specimen mounted with aralide in S. S. rings and polished for examination.

F I G . 9 SCKEKE CP LfflTALLOGPAPHIC 3XA.MITATIC1T ON PREPARED SFlEI iSNS AHD ESFORI&TIOKS OBTAEED AT

VARIOUS STAGES OP

1 a.UCRO 3XALHEATI0N

: . A 5 JrGi.-J3r23 ^J,

b . MicHCiU:.:]CIAD

0 . A."?CHAJ}•' of m r

ICG-HAPHT^T i 1 4 T,"7 "A

2 . •3CA

2a

Mi:-A?ic:-i I I :

.MICRO-IS. CL.O

rrcrss CCCTITIOSyaiEATIC!"

2b. ?U3I

1. Fuel CrackPattern

2 . Lfejor clad-ding defects

3« Fuel-loss atdefects

1. Clad thinning 1•clad collapse

2. Details near 2.failure

3. Outer ard 3-inner oxidelayer thick-ness

4• Modularcorrosionfeatures

5• Insipientclad cracks

6. Fuel-cladgap condition

7. Fuel-cladInteraction

8. Fret depth

9. Crud thick-

10. Porosity inthe crud

Fuel oraclediaension3

Dislodgedfuel chips

Radialdistributionof porosity,pore shape,sizes

Central void

Metallic in-clusions

Distri-of

plutonJ.ua

Distributionof fissionproducts

1. Claddingmicrostruc-ture, grainsize

2. Second phasedistribution

3. Hot spotsshowing graingrowth

4. Hature ofclad cracks

5. Disialbutlonof hydrideplateletstheir siaeand orien-tation

6. Weld

condition

7- hydrideblisters,size anddistribution

1. Grain sizedistribution

2. Dimensions ofstructurallydifferentareas in thefuel, viz.,

a) As sinteredgrain region,

b) Equiassdgrain growthregion

c) Coliunmr graingrowth region

d) As caststructure,

e) Central void3- Indication

of melting

4- Porosity dis-tribution indifferentareas

5. Indication offuel centretemperature

*only in the ca3e of Pu bearing fuels.

THE PELLET CROSS SECTION SHOWING DIFFERENT

DIRECTIONS AND THE LOCATIONS ON A DIRECTION

WHERE GRAIN SIZE IS MEASUREO.

DIRECTION-2

d LENGTH MEASURED

n NUMBER OF GRAINS

d.AVERAGE GRAIN -T1

SIZE IN MICRONS = —L

IN

IN

.+

OIRECTION-3

MICRONS

LENGTH d .

"2 "3

THE METHOD Of GRAIN SIZE MEASUREMENT

AT EACH LOCATION.

UNEAR INTERCEPT METHOD OF GRAIN SIZE MEASUREMENT ON

IRRADIATED UO2 FUEL PELLETS-

Fig. 10 Grain Size Keasurement on the Fuel Cross Section

FERENTIAL CRACK

RA3IALCHACK

THANSVERSE CBOSS-SECTION

AXIALCHACK

TEANSVERSECHACK

LONOITUDBM, CROSS-SECTION

Fig. 11. Fuel crack pattern on the irradiated fuel element sections.

INTERFACE

Fig. 12. Features of cladding failure by clad thinning and collapse inelements PWL Bj irradiated in the pressurised water loop of CIRUS.

PLATFLETS n r«c*LO»1-21

CI.AO 'NSIOE

'CRACKED FUEL

Fig. 13. Internal hydriding failure of cladding in PWL P

fhotomacrograph ofTop End W«ld Section

a* polished -nag.I 4*

Failure location after etchingfor hydride at 35X magnification

I'm. 14. H y d r i d e f a i l u r e a t t h e t o p e n d w e l d l o c a t i o n in T A P S e l e m e n t ( K M 0 2 6 8 ) .

Circumferential hydride platelets (200

Uasaire hydridinc leadioj to

ztroaloy clad failure (25 X)

Radial hydride platelets (200 X) Random hydride platelets (200 X)

Fig. 15. Hydriding characteristics of zircaloy-2 cladding in irradiated oxide fuel elements.

Lenticular ErCL Nodule

Fhotooacrograpta

Beta gonnaAuto radiograph(Black indicat?activity)

ZlrcaloyM. Moduiai corrosion on (lie outer surface of zircaloy cladding inTAPS fuel elements

A'. 17. Fission product redistribution in irradiated fuel pelL-i ti

Photomacrographs

Outar dansar r»jlon Blackish porous rationI

Fuel Ptriphery (Btohed)aa »Inter*d nicroatruoture (200 z )

Towards . . . ^porosity inalda jfr'.f.Sthe grains • ' . i[. •

Fuel Centre (Etched)Grain jrowth and f i s s ion gasbubbles at the srain boundaries (200X)

Towards fuel centreAppearance of porositjat (rain boundaries

Start of Porous Hegion (ft.s polished)

F/̂ . IK. Microstruclural changes in UO2 during irradiation. (200 X)