NUCLEAR ENGINEERING AND DESIGN 33 (1975) 219-229. © NORTH-HOLLAND PUBLISHING COMPANY

Z I R C A L O Y C L A D D I N G TUBES: M A N U F A C T U R I N G T E C H N I Q U E S AND

A C H I E V A B L E Q U A L I T Y - A TUBE M A N U F A C T U R E R ' S VIEW

Roar ROSE, Knut LUNDE and Steinzr ,~ AS Institutt for A tomenergi, K/eller, Norway

Received 23 March 1975

The behaviour of the Zircaloy fuel cladding in water-cooled power reactors depends on a variety of properties deter- mined during the cladding tube fabrication pro~ss. This does not necessarily imply that variations in the fabrication process will determine the fuel pin integrity, although the considerable number of tube specifieatton changes intro- duced over the last few years may well be interpreted in that way.

1. Introduction

Because of the possible economic consequences of fuel defects, great care has been taken to avoid them. All steps in fuel pin and fuel assembly design and pro- duction are under cons t~ t critical analysis, and strict quality assurance systems govern all production steps. Since a fuel failure necessarily means that it is the cladding which has failed, it is easy to understpnd - at least from a psychological point of view - that this has led to a tightening of the cladding tube specifications. In most cases, however, the cladding defect is the last step in a series of events which was initiated, not because of poor or wrong cladding properties, but because of inadequate fuel design or fabrication prac- tice, or even plant,operation or characteristics. We are referring to such potential defect sources as: interna-' tional corrosion or hydriding; mechanical interaction between fuel and cladding; cladding collapse after fuel densification; stress corrosion; fretting; strain fatigue; brittle end cap welds; crud deposits; and external corrosion.

It is doubtful whether changes in the cladding tube specifications will lead to a reduced number of defects of the types mentioned. This is not saying that cladding tube fabrication defects will not result in fuel failures. Naturally, specitica/ioa changes aimed at eliminating defects in cladding t~bes are well justified.

The tightening of cladding tube specifications is in

conflict with the price reduction that has taken pl over recent years. The immediate result of the pric reduction - wi~ch the tubing manufacturers have to accept as a result of mill overcapacity - has bee that tubing supply contracts have been closed at z In the long run such contracts wal undoubtedly b to a reduction in tub:rag quality.

The aim of this presentation is to clarify the e: to which cladding tube properties or quality can changed by altering fabrication variables, and at , cost. An attempt will also be made to evaluate w there is an interrelationship between changes in t fabrication procedures and fuel pin integrity.

The cladding tube properties determined by t fabrication processes are shown in table 1. The also gives typical values as specified today f~3r P~ BWR fuel cladding. ~ e properties in question a surface finish and roughness; dimensional toleral grain size; hydride orientation; corrosion rate; aJ tensi!e, creep and burst test properties.

2. Manuf~tur'mg process

The starting material in the cladding tube reduc plant is 'tube hollow~', which in most cases are ered with the outside surface pround ~nd the iv honed. The fabrication process comprizez three four tube reductions in pilger marlines cr Rus'

220 ?. Rose et aL, Ztrcaloy cladding rabes

Table 1. Typical Zircaloy cladding tube specification data.

Properties specified PWR BWR

Surface: Surface treatment

Surface roughness (Ra) max.

Dimeli~:ion : Inner diameter (ram) Outer diameter (ram) Wall thickness (ram) C~ali*.y, Ld. (mm) Ecceameity Straightness ma~.

Structure: Grain size, ASTM no. Hyd~de orientation, F n number Texture

ground/sand-blasted

0.7-1.0

± 0.04 ± 0 . 0 4 - 0 . 0 5 NS ± 0.05 rain. ± 0.025 NS ( 0 . 1 0 - 0 . 1 6 ) NS 0.25/300-0.6/600

ground/sand-blasted pickled/sand-blasted pickled/pickled 1.2

± 0.04-0.05 ± 0.05-0,06 NS ± 0.04-0.08 min. ± 0.03-0.05 NS (0.10 -0.16) NS 0.4/500-0.6/600

6(fully annealed)- 7 7-13 < 0.3-0.4 0.1 < F n < 0.45, < 0.3

NS yes NS

19-22

ET 22-38 20-32 10-18 NS

ColTosion : Corrosion weight gain (mg/dm 2) max. 22

Mechanical: Tensile test RT UTS tkp/mrnZ) rrdn. 49 -60 YP (kp/mm 2) rain. 35-46 Elongation (%) min. 12-18 Uniform (%) min. NS

Burst test Burst pressure (%) min. yes NS NS T~tal elongation (%) ram. 12 NS NS Uniform elongation (%) :ain. NS 2.5

Creep elongation (%) max. NS 1

Defect: Cracks~ flaws, inclusions

RT ET 46-53 23-29 NS 29-44, 5 12-22.5 NS 15-30 20-30 NS

NS NS

yes NS NS 16-24 NS NS

NS NS NS

NS NS NS

5% min. wall 5-10% wall

N S = not ~ecified.

HPTRs with intermediate dressing, cleaning and vacuum annealing. After the fma! reduction ~ e tubes are cleaned, inside pickled, vacuum annealed to meet specified mech- anical properties, outside pickled or ground and inside ~ t blasted before the final testing apd control. A typical fabrication flow-sheet is shown in tables 2 and 3. The critical steps ~- the process are tube reduction, inside pickling, vacu.m annealing and straightening.

The important parameters in the reduction process are tool design and the Q t~cter. The d~sign of the rolls/dies and mandrels influe,~ces surface properties avd dimensional tolerances~ anti is by most tube sup-

pliers considered proprietary. The Q factor, i.e. the ratio between the wall and the outer diameter reduc- tion in the last reduction determines the 'texture' of the product. The most commonly used Q factors are in the range 1.5 fgiving a texture angle of around 60 °) to 3.5 (giving a texture angle of around 20-30*). The texture resulting from Q factors of 1.5, 2.5 and 3.5 are shown in fig. 1.

The critical step in the pickling process is the trans- fer from the pickling bath to the rinsing bath. If the transfer time, i.e. the time the pickled surface is ex- posed to alr, exceeds some seconds, the formation of

R. Rose et aL, Zircaloy cladding tubes 221

Table 2. Flowsheet of fabrication procedure.

Tube hollow

Cold reduction in tube reducer, 65-70°//0

Intermediate annealing, 700°C

Surface inspection and conditioning

Final reduction in HPTR

Inside pickling

/

I Annealing to specified mechanical properties ]

I

Roll straightening

l Outside sv (face treatment belt polishing or [ pickli ag .j

, - - - - - - - - - - - - - - - ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' . . . . " ' ' ' ' ' ' ' ' ' l I

l Inside grit-blasting ' ,

~resting and control ' ~

1 ] c.,.is ]

'~'

zirconium fluorides and a drastic reduction of corro- sion properties may well result. In modern plants this diffic~ty has been overcome.

The critical parameter in the ?.rnealing process is betzer than 10 -4 mm the vacuum, which should be '

Hg. If the vacuum is poorer than this, absorption of oxygen and nitrogen result, alteNng the mechanical pro~rties and irapoved ening the corrosion resistance.

Table 3. Flowsh~t of fabrication procedure after final annealing.

Amount of tubes for I lot

Annealing

Straightening t

Outside grinding

Degreasing rinsing drying

Specially marked test tubes

l ,= ]

Inside sand-blasting ! Inside cleaning j !

r Visual control ] i l i !

I - - Inner diameter measurement

Control of outer diameter and wall thickness Ultrasonic control

,,~ 6- . . . . .

Dimension m¢~,~uremefit

Testing for chemical mechanical corro~.on structure surface

properties

Release/reject

Cleaning Visual inspection

Packing

The important variables in the straightening process are deflection and compression. If these exceed certain values, local cold working, shift in the hydride orien- tation and reduced burst-testing elongation may result.

If ali processes are well ix, hand it should be possible for any tube manufacturer to achieve an average tube yield of around 95%. If the tools are properly designed, tubes should rarely be rejected because of ultrasonic indications or surface ir:egularities. Dimensional devia- tions leading to rejection seem to be difficult to avoid,

222 R. Rose et al., Zircaloy cladding tubes

as seen from table 4, where the test results from large and medium series of tubing supplied by Raufoss in 1974 are listed.

There are several reasons for difficulties with the dimensions. One is eccentricity in the tube hollow. In such cases the mandrel will tend to move towards the centre of the rolling force field whereby ~ e already er.isting dimensional deviation is m a i n t a i n e d . . ~ t h e r ~ason is of a more purely practical nature. If tubes are

lying on top of each other on the annealing trays and not pe~ectty augned, there ,,vill be poir, twi3e contact and Ic ~',d deformation will result. If the rolling process is .~ ~pped while a tube is in the machine, occasionally we see ,~hanges in the diameter at the position of the roils large enough so that the tube will have to be scrapped.

Q-FACTOR , ,

Q= t.5

O= 2.5

O =3.5

TYPICAL POLE FIGURE

!

F~ I. Q factor: 0 = reduction of wall _ ~w.t./w.t. reduction of o.d. Ao.d./o.d."

Table 4. Causes of rejection of cladding tubes.

Order n o .

Causes of rejectio~ (%)

Outside Ultra- Dimension Other kreguiarities sonic

Tube yield (%)

1 1 2 4 2 2 0 0 4 1 3 0 0 4 1

9! 95 95

It must be emphasized that the yields listed L-t the. table have been achieved without redressing, reworking or repair of any tubes. A prerequisite for yields on such a basis is stringent in-production control, ~n important part of any quality assurance system.

3. Surface f'mish and roughness

The cladding tube is usually picked on the outside, while grit-blasting in combination with pickling before the final heat treatment seems to be a preferred inside treatment. In some cases belt polishing is specified as the last treatment of the other surface. The surface treatment processes in question give roughness numbers in the range 0.3-0.5 Ra, which is well below the upper roughness limits that according to table 1 are usually specified in the range 0.7-1.2 Ra.

Inside grit-blasting, which is a process in addition to rite pickling that in most cases has been carried out, may increase the tubing costs by as much as 5%. Of the two outside treatment methods, belt polishing is the least e~pensive one and may reduce the tubing costs by as much as 5% compared to pickling.

Most specifications are today specifying grit-blasting as the only acceptable inside surface treatment process after heat treatment to meet specified mechanical pro- perties. This is obviously an attempt to eliminate fluo- ride contamination, wtdch above a certain level and in combination with moisture from the fuel may lead to sunburst formation.

Many have speculated on possible methods for im- proving the resistance of the bore surface against local hydriding. As the danger of the attack emerges from its local character, maximum resistance is expected with a uniform cladding surface with minimum accelerating local contaminants. Pickman et al. [ 1 ] stat~ that fuel pins in the SGE~qR ~ t h an autoclaved internal surface were inferior to those with a sand- blasted surface, presumably because a defect in the autoclaved oxide was selectively attacked. Out-of.pile experiments by Lunde [2] indicate that a sand-blasted surface is more resistant.to sunburst defects than a pickled one and that fluoride contamination is especia~lly dangerous. Expe.,~nents with moisture and fluoride- doped fuel rods in HBWR [3] with pickled and sand- blasted surface gave no clear evidence either for or against this. It is therefore believed to be of secondaq,

• R. Rose et al., Zircai~y cladding tubes 223

importance whether the cladding inside is pickled or sand-blasted. We have concluded, as have others, that sunburst failures can only be eliminated by carefully controlling the moisture contents of the pins.

With regard to the twc: rrethods applied for outside surface treatment, both zeem to be equally good from the viewpoint of in-reactor behaviour. We can conclude therefore that the least expensive of the available methods should be applied for the final cladding tube surface treatment.

We think it is relevant to question if any surface treatment of the finished tubing is at all necessary, except for a careful outside and inside cleaning of the tube to get rid of grease, stains and dust before the final heat treatment of the tube.

4. Dimensional tolerances

4.1. Eccentricity

The equipment used for the reduction of cladding tubes from tube hollows to final size, i.e. the pilger machine and the HFFR, is not capable of improving the centricity of the starting material. If, therefore, the eccentricity of the hollow is, say 0.10, the finished tube will also have roughly the same eccentricity.

Most Zircaloy cladding tube specifications do not contain explicit ecventricity requirements. By com- bining i.d., o.d. and wall thickness tolerances it ,:s found, however, that eccentricities in the range 0.10- 0.16 are tolerated. Fig. 2 shows the relationship be- tween total and uniform elongation and eccentricity in a closed end burst test. It can be seen that elongation

32 . ~ 28.

o.~ 2I?

._ :o 12 8

uJe --i " o,o;

..Eccentricity distributiP~n

~ T E : ~ . , UE

094 0.06 0~08 0.10 e. Wrmx-Wmir,

"Wplax÷Wmin

Fig. 2. Total (TE) and uniform (tiE) dosed end burst-testing elongation at room temperature versus eccentricity and eccentricity distribution diagram.

is drastically reduced as eccentricity increases. It has been found that tubes with localized wall thickness variations corresponding ~o eccentricities belmv 0.03 are capable of taking a total circumferential strain of 1.5% without necking after irradiation. With wall thickness variations corresponding to eccentricities of 0.06 and 0.18, the maximum circumferential strains were reduced to 1.25 and 0 . 5 ~ . This demonstrates that although eccentricities may be acceptable accor- ding to specified dimensional tolerances, its effect on elongation is not. It can also be seen that burst-test elongation data hardly have any relevance if the eccen- tricity is not given at the same thue. This picture alsG gives the eccentricity dist:ibution in a large number of tubes. This curve is most likely also representative for the eccentricity distribution in the tube hollows that have been used as starting material. The distribution curve indicates that a change of the maximum telerable eccentrici:y from today's 0.1 to a value of 0.06 would lead to about 5% increase in the tube cost, or a change to a maximum eccentricity of 0.04 will increase it about 10%, disregarding improved eccentricity of the tube hollows. The costs of improving t[.~e eccentricity of the hollows are not known.

4.2. Ovality

Most specifications accept ovalities in the range 0.08- 0.10. As can be seen from fig. 3, tubes usually fulfil this requirement. It can be questioned, however, if the specified ovalities do suffice regarding in-pile behaviour.

Mogard et al. [4} for example have indicated that this probably may not be the case, since an ovality of 0.10 seemed to be the main reason for a fuel fadure in an over-power test. The consequences for the cladding tube cost of establishing lower limits than those used today can be seen from fig. 3. A lowering of the maxi-

X ~

~70 z'6C

m 4C

o 20

<i

b

L o.62 0.0~. 0.o6 o08 &ID {IDmax- IDmin) "--~

Fig. 3. Ovality distribution diagram (tubing B).

224 R. Rose et al,, Zircaloy cladding tubes

mum ovality limit to 0.06 will only slightly influence cost wh~rea~ lowering to 0.04 would lead to a 15-20% cost increase.

4.3. Straightness

Specffication~ usually accept cladding tubes with an overall straightness better than 5 mm over the full tube length. An additional requirement is often that the straightness ~hall be better than, say 0.5/500, over a short tube length. To meet these requirements all tubes are proce.,sed through amechanical precision roll straightener where they are exposed to the combined action of deflection and compression. It is known that this process may lead to local formation of radial hydrides in the tubing surface, and that this probably happens wi.th tubes that deviate the most from the straightness requirements, so that a considerable degree of mechanical deformation has to be applied before the tubes have become sufficiently straight. Toinvesti- gate the possible effects of this mechanica! deforination a series of exp~.iments were carried out at Kjeller with soft annealed Zircaloy cladding tubes straightened by applying different degrees of deflection and coml:~ression. After straightening, the tubes were hydrided and burst- tested at room temperature. The results are shown in table 5, g hich indicates that the burst-testing total elongation can be reduced drastically as a result of the straightening process. Metallographic examination has revealed tttat the tubes with reduced burst-test elonga- tion in some cases exhibited a considerable fraction of radial hydrides.

Table 5. Effect of conventional roll straightening procedures on burst- test elongation of hydrided Zr-2 tubes.

Adjustment of roll Straightness H Total straightener (ppm) burst

elongation Deflection Compression (%) (ram) (mm)

8 0 0.9/1000 8 g2 '100 27.3 8 0.2 0.411000 200 ( ~

x2 o 0.5110o0 2oo 3o.7 0.2 0.2/1000 200

12 0.2 300 16 0 0.3/1000 200 24.3

not straightened 200 35.7

Table 6. Burst-test elongation of hydrided Zr-2 tubes after rede!~gned straightening tools.

Adjustment of roll Straightness Aw.t. Total straightener (mm) bu~rst

elc,ngation Deflection Compre~on (~:) (mm) (ram)

16 0.2 0.3/1000 0.02 30.2 16 0 0.311000 0.05 24.3 16 0 0.311000 0.09 19.6 16 0.2 0.3/1000 0.03 27.6 16 0.4 0.311000 0.04 24.2 16 0.8 0.3/1000 0.03 26.9

To eliminate the reduced burst-test elongation, and hence the possible in-reactor consequences, modifica- tions were introduced to the roll straightening machine. The modification comprised a redesign of the rolls so that the compression pressure was more evenly applied. The burst-testing results after hydriding of tubes straight ened in the modified machine are shown in table 6. The table indicates that straightening conditions that earlier would reduce the ourst-test elongation properties dras- tically do not now seem to have any influence at all when the modified rolls are being used. It is believed that the variations that are seen in total eloagation in table 6 rather are a result of tubing eccentricity, in accordance Nith fig. 3.

5. Tensile and burst-testing properties

As we can see from table I, there is a remarkably large difference in the mechanical properties specified for PWR and BWR cladding tubes and for various types of BWR cladding tubes. This difference c~m hardly find its justification in a similar difference in the fuel operating or load characteristics. We feel, therefore, that the difference in specified mechanical properties is more a question of different design philosophies or even a matter of taste.

The tensile and burst testing properti,es of a cladding tube are to a large extent determined by the tempera- ture the tube is annealed at after the fntal reduction. The specified mechanical properties are, therefore, in most cases met by proper annealing. There are also other factors that influence the mechanical properties such as texture, ingot properties, oxygen contents,

R. Rose et al., Zircaloy cladding ntbes 225

'T "/[~

e D

60

8~. so f 4O

30-

20

10

0

100

~ ^ 9o

Ultimate ten- 70 sile strength .

60 Yietd strength o

~EE 50 ~ Total o~ v

elongation ~- 40

~ Uniform elongation

I . I I i , I ~'~ s see s2s sso sTs see

Annea!Ing temperature. 'C , .

Fig. 4. Room temperature tensile testing data ver~as annealing temperature.

final cross ~ction reduction and dimensional tolera~.~es. Under certain circumstances these may have to be adjusted to meet the specified properties after anneab ing at a selected temperature. Tensile and closed en4 burst-testing properties as a function of annealing tem- perature are shown in figs 4 and 5, respectively. The two figures present most of the test data that Raufoss has compiled over the last three years, from the testing of PWR, BWR and ,';GHWR cladding tubes. Most of the tubes m question were manufactured with a final reduc- tion of around 70% and with a Q fa,~tor in the range 2.0-2.5. With regard to chemistry and gas contents, all tubes have been well within specified limits. The data exhibit a considerable scatter. We can see, for instance, that the c~ifference between the high and low yield strength after the same heat treatment is some 10-12 kp/mm 2. For the total tensile, elongation the diffe fence is in the range 8"15% and for the burst-test elongation even more pronounced. It is our impression that one important reason for the scatter is a difference in ingot properties ~Lhat we, and others, know very little about, it is, for example, impossible to explain on the basis of very accurate chemical analyses why the metal in ingots with a marginally higher content of recycled

7- ~ 6urst strength

_ ~ Elongalion

t0 elongation

0 .A~LJ i l i i .l _ "i5 500 525 550 57¢., 600

Annealing temperature 'C

Fig. S. R o o m tempera tu re c losed end burs t - tes t ing data versus annealing temperature.

scrap is ha, der and stronger than the metal in the other ingots.

The finer detai!s of the strccture of the Zircaloys are poorly understood, that is, which intermetallic phases are present and when and how do they form or disappear. It must also be remembered that the proper- ties in question are realized by annealing in a tempera- ture range where properties change rapidly with tem- perature arid. time. Unavoidably this gives a larger statistical spread in properties than in e temperature range where the curve~ are flat.

In the narrow range c~f crystallographic textures Raufoss has been working, i.e. corresponding to Q factors from 1.5 to 3.5, texture has hardly any influ- ence on tensile and burst-testing properties. As can be seen from figs 6 and 7 a small difference may be observed in the cold-worked material, but this difference seems to disappear after annealing at temperatures above, say 520°C. Irradiated material tubi~g with a texture corresponding to a Q factor between 2.0 and 2.5 seems to mahatain its ductility better than other textures.

The considerable scatter in the burst-testing elongation

2 2 6 R. Rose et a~, Z~,caloy cladding tubes

g0

8C- (RT}

- - T E S T E D AT RT ¢~ ~YS ---TESTED AT 343 E RT} - O = 1,4 o~ . z 7 0 _ x Q=2,1

o O=3,4

$ =;6o -

8 ~ - - - ~ _ UT S (RTI

t i t %

sc ',JUTS ~

Lo.,(3~3) t ~ " g - - - - -~ - YS

40 (hT)

~'o 6 L T E ~\ R:P. " "" "- (343)

3C ) . X

~ / ~' --~-__~._ urs

)~J \x TE ~'t

(343) " , , ~l"Ji~ TC "--~ . . . . ~. YS

" {3/.3)

10-

Anneahr~g temp .°C

Fig. 6, Tensile properties versus annealing temperature for tubing with different textures.

data is, in accordance with previous observations, partly caused by variations in the tubing eccentricity. Our experience has made tL~ conclude that the scatter of tensile and burst-tes~ properties shown in figs 4 and 5 is a reality both cladding tube manufacturers and fuei designers have to accept and live with, until we have accumulated more basic knowledge and gained a

better understandi~g of the Zircaloys. This means that if you want to avoid a cost penalty, specified values

50

l,C

20

• TESTED AT RT - - - T E S T E D AT 343

• C} = 1,4 x I) = 2.1 o 0=3 .4

/ / CE

/ !13~.31

//o b x / /

/ /

// / / / 4 " " / ¢, /

~/ / , ~CE / ~ / o " ~(RT) / -

: f o !(, : /

L

s(b Annealing tgmp.. °C ----, ,-

Fig. 7. Closed end bu.rst-testing properties versus annealing temperature for tubing with different textu~e~

for tensile and bmst-test properties should not be appreciably highe~ than the lower values in the scatter band. Sensible values for hard, semi-hard and soft tubing are given in table 7.

If you increase the minimum yield streng~ of a semi-hard BWR robing from 45 to 50 kp/mm 2, while w~intaining the other tensile properties, this would mean an increase in the tubing cost of about 20%.

For a more precise impression of the spread in test values, table 8 :;bows a statistical analysis of a very large number of test results. As an example, if the specification L¢. a min~num yield strength of 14.8 kp/mm 2 at 300OC, then the tube manufacturer must centre his processes around 17 kp/mm 2 at the 99%

I~. Rose et al., Zircaloy cladding tubes 227

confidence level, with all the consequences that this implies.

It is tempting, since the yield point is mentioned, to consider the significance of specifying a yield point. Cladding with a high yield point will be less strained, theoretically, than soft cladding, because it will deform the U02 more. Experimental results (e.g. table 9 from a Halden test) shows that there is only a marginal dif- ference between a hard and a soft cladding in this re- spect. Our conclusion is tha', there will always be points

along the rod with sufficiently high stresses to plastically deform the cladding. Realizing this, should not much more emphasis ~e laid on ductility than on strength?

Table 7. Cladding tube mechanical properties - upper limits.

Tensile Closed end burst

UTS YS TE LIE BS TE UE (kg/mm 2) (kg/mm 2) (%) (%) (kg/mm2) (%) (%)

PWR (hard) 65

BWR (1/2hard) 60

BWR (soft) 52

48 14 5 75 9 3.5

45 16 7 65 16 4

37 25 10 63 24 8

6. Creep properties

Creep properties are only specified for PWR tubing. We tend to hold the opinion Lhat this is perhaps the only traditional mechanical V:operty which it is neces- sary to specit~. But even here there are pitfalls.

Figure 8 shows the'creepproperties of cladding tubes after 10 days at 400°C as a function of annealing tem- perature. The results are based on creep testing of BWR and PWR tubes, applying hoop stresses of 15, 13.6 mid 10 kp/mm 2 , respectively. The two upper,

Table 9. Deformation at the first cycle, for IFA 215 [5].

Pin 6 Pin 7

Cladding properties: Heat treatment 450°C 575°C Yield point at RT 64 kp/mm 2 44 kp/mm 2 UTS at RT 75 kp/mm 2 55.7 kp/mm 2 Total elongation 13.5% 20.5% Uniform elongation 3.7% 7.8%

1 power ramp: Max. heat load Hongation at max. heat load

P~rmanent deformation

480 W 480 W 0.82 mm 0.80 mm = 0.164% = 0.16% 0.1 mm 0.2 mm = 0.02% = 0.04%

Table 8. Statistical analysis of Zirca!oy-2 quality.

Test Mean value Stealdard Minimum value with deviatioa 99% confidence level

Tensile test at R T UTS (kp/mm 2) 54.6 2.2 49.0 YS (kp/mm 2) 41.3 2.4 35.0 Total elongation (%) 31.2 2.1 25.8

Tensile test at 300 ~ C UTS (kp/mm 2) 27.4 1.0 24.9 YS (kp/mm 2) 17.0 0.85 14.8 Total elongation (%) 48.1 1.9 43.3

Closed end burst test at R T Burst streng~! (kp/mm 2) 66.6 1.2 63.6 Total elongation (%) 38.5 6.7 21.3 Uniform elongation (%) 15.2 2.6 8.6

Closed end burst test at 300°C Burst strength (kp/mm 2) 33.7 0.5 32. 3 Total elongation (%~ 65.8 8.4 44.0 Uniform elongatiov. (%) 15.8 2.5 9.4

228 R. Rose et al., Zircaloy cladding tubes

3.0 m

2 0 BWR6"H =15kplmm O PWFI~x -15kp/mm' ]

2.5 a, PWRC'H = 13 kp/mm 2 I \ x BWR6"" :i0 kp/mm2 " / / ~ ' ~ .... Frenkei et ol. i / ~ "%.

<[~oo ~°zz 2.01.5 G' H -13.6 kp/mm z. tffi251/~ ~ l l ~ ,

.-J IlJ w m 1.0

O.E %

~. . I f , I i I RT'" 450 500 550 600 650

ANNEALINO TEMPERATURE "C

Fig. 8. Effect of annealing temperature on circumferential creep elongation. Unirradiated Zr-2 and 4 tubing tested at 400"C for 240 hr.

1~: that the ann~ qing temperature should be around 570 rather than 500°C.

7. Grain size

As shown in table 1, cladding tube specifications accept i'rain sizes in the range 6-11. With reduction and an- nealing practices applied today, grain sizes in ~te range 11-12 areusuaily 0b-tained. A smaller grai~l SLze would necessitate modified fabrication processes. The rele- vance, if any, of explicitly specifying the grain size is not easy to see. A fine-grained material seems to maintain its ductility better after-irradiation, a coarse- grained material has better creep proper~Lies than a fine-grained one. If, therefore, creep is rL concern, one should not necessarily strive towards s matt i ~ with as coarse gr',dns as possible. It is not known to what extent the improved properties are an effect of grain size as such. One conclusion is, therefore, that as long as the tubing material has the specified mechanical properties, the grain size as such should be of little concer, l.

orawn curves are the results of a standard creep accep- tance test carried out at Kjeiler with BWR and PWR tubing, applying a stress of 15 kp/mm 2. The lower, drawn curve i ~, also based on tests at KjeUer, applying a hoop stress of 10 kp/mm 2. The dotted curve is taken from Frenkel and Weisz [6] and shows the creep data after I 0 days of applying a hoop stress of 13.6 kp/mm 2 . Whereas the 10 kp/mm 2 does not reveal variations in creep properties with annealing temperature, the results; with a hoop stress of 13.6 kp/mm 2 indicate improved creep properties when the annealin$ temperature is increased. The improved creep properties after anneal- mg in the high temperature range seem to be maintai~.t.'d after irradiation. The creep curves based on the resultz wi~h a hoop stress of 15 kp/mm 2, which is commonl) - sp,~cified for PWR tubes, has a pronounced minimu m so:newhere in the range 500-520°C and a pronounced ~ x i m u m somewhere around 580-600°C. The curve~; sh,Jw clearly that to meet the creep properties specified for PWR tubing, the annealing temperature has to be seiectcd in the range 490-520~C. However, since the heop stress in the PWR cladding is closer to 10 kp/n~a 2 because of internal pressuriz~:tion, a correct test shor~ d slecify this latter stress level. The consequence wov/id

8. Defects

The maximum size of cracks or similar flaws permitted in the cladding wall as delivered from the mill is 5-10% of the wall thickness. As we have seen, very few tubes, if any, are rejdcted because of such imperfections. We can also add that smaller flaws are scarcely even detect- able in ultrasonic testing. This is probably one impor- tant reason why very few fuel failures, if any at all, have been reported to originate from flaws introduced during tube manufacture.

Pickman et al. [I ] have reported two fuel failures in the SGHWR related to microsize holes penetrating the cladding wall. It is believed that these failures originated from inclusions or some other abnormali- ties in the materials. Defects of this nature are rare indeed and hardly justify improved and more expen- sive detection methods.

9. Concluding remarks

In this paper we have attempted to convey two main viewpoints. One is that an overwhelming majority of

R. Rose et aL, Zircaloy cladding tubes 229

the number of fi~el defects are a result of deficiencies in the fuel design, of inadequate fabrication methods and of improper plant operating conditions. They have not been caused by cladding tubes of poor quality. In spite of this, we have seen an almost unending series of changes in cladding specifications. It is in the nat.are of such a situati,Jn that these changes mean tightened requirements, making it more difficult to meet the specifications, requiring an increased number of quali'ty control action's. The end result can only be increased cost and not necessarily better in-reactor fuel perfor~ manee. This brings us to the other point we want tc. make, namely that we are not convinced that the specifications always specify the right type of proper- ties. We do not profess to have all the answers to this latter problem, but we think we can assist in imp,cov- ing the understanding of fuel behaviour, performance and life-limiting phenomena. We hope this paper will

promote a fruitful discussion with fuel designers and fuel users.

References

[1] D.O. Pickman, D.H. Willey and V.W. Eldred, SGHWR fuel element performance, BNES Conference ~n Water Reactors, London, Oct. (1973) 51.

[2] L. Lunde, i.~¢alized or uniform hydr~ding of Zircaloy - some oi'se~ations on the effect of surface conditions, J. Nuc~L Mate~. 44 (1972) 241-245.

[3] HPR-145. OECD Halden Reactor Project, Quarterly Progress Report, HPR-145, Oct.-Dee. (1971).

[4] H. Mogard, S. Aas and S. Jungkxans, 4th Geneva Conference, 1971, Paper 314.

[5 ] OECD Hal ~en Reactor Project, Quarterly Progress Report, Jan.-Apr. ~(1974).

[6] J.M. Frenkel and M. Weizz, The influence of various metallurgical parameterb and neutron flux on creep of Zixealoy 4 cladding tubes, BNES Conference on Water Reactors, London, Oct. (1973) 90.