Fission-fusion correlation by fission reactor irradiation with improved control

327

Journal of Nuclear Materials 174 (1990) 327-351 North-Holland

Fission-fusion with improved

correlation control *

by fission reactor

irradiation

M. Kiritani ‘, T. Yoshiie 2, S. Kojima ‘, Y. Satoh’ and K. Hamada 2 ’ Department of Nuclear Engineering, School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan 2 Department of Precision Engineering, Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan

The necessity for the elimination of exposure to neutrons at lower temperatures during start-up and shut-down of the reactor is confirmed by experiments which compare the result of irradiation of metals with conventional and improved temperature control in JMTR. Only several percent of exposure to neutrons at lower temperatures is found to result in a one hundred percent difference of radiation induced microstructures in some cases. All differences can be understood from the microstructural development mechanisms, i.e. from the temperature dependence of the stability of point defect clusters and from the relationship of the transient temperature to the temperature for nucleation and growth. Fission neutron irradiation data with improved control are compared with fusion neutron irradiation data from RTNS-II. The differences of vacancy and interstitial clusters formed directly from cascades, observed in samples irradiated as thin foils, are understood when the difference in the primary recoil energy spectrum and the thermal stability of the clusters are taken into consideration. The defect structures which are developed and/or modified by the reactions of freely migrating point defects, such as vacancy clusters interstitial type dislocation structures and voids, observed in samples irradiated as bulk, are remarkably different in the two cases. Factors to be applied to the fission neutron irradiation dose to introduce microstructures equivalent to those by fusion neutrons are found to range widely, depending on the kind of microstructure, materials and irradiation temperature, from a value less than one up to 30 in the scaling of damage energy per atom.

1. Introduction

At the workshop on “Radiation Damage Correlation for Fusion Conditions” at Silkeborg in October 1989, the present authors discussed a variety of topics relative to microstructural evolution during neutron irradiation. Progress of experiments and analysis of fusion neutron irradiation were evaluated, the central theme belong the recoil energy spectrum analysis of the formation of cascades and subcascades [1,2]. Topics emphasized as specific to cascade damage were the role of impact effect to cascade produced point defects from other cascades [2,3] and the bias effect which automatically comes from the difference in the extent of localization between vacancies and interstitials in the nascent collision cascade [3,4]. In the latter half of the presentation, the author emphasized the need for controlled fission reactor irradiation [S], by showing preliminary data from a fission reactor irradiations. The analysis of fu* This work was supported Education,

by the Grant-in-Aid Science and Culture, Japan

0022-3115/90/$03.50

of Ministry

0 1990 - Elsevier Science Publishers

of

sion neutron irradiation has already been published in several papers indicated as references, and the issue of obtaining an irradiation correlation from improved fission reactor irradiation will be dealt with in this paper. In order to anticipate the change of materials in the fusion environment from knowledge obtained from fission neutron irradiation testing, the so called fissionfusion correlation which tells us how to convert fission data to that of the fusion environment should be known. It is rather illogical to aim at obtaining the correlation without having a high flux fusion neutron irradiation facility, in which the fusion irradiation data can be produced to be compared with that of fission. However, an effort should be made to use our presently available low fluence data. An analysis of the present situation with each irradiation data set is a good starting point. At present. the direct data from fusion neutron irradiation come solely from RTNS-II, the rotating tritium target deuterium ion neutron source at LLNL, which was available up to 1987 [6]. The irradiation dose achieved in this 14 MeV neutron source was a little less than 1 X 1O23 n/m2, and the irradiation temperatures

B.V. (North-Holland)

328

M. Kiritani et al. / Fission-fusion correlation byfismon reactor irrudiation

ranged from 15 to 823 K; the materials tested included almost every conceivable possibility. Post irradiation examination of the ~crost~ctur~ development is now reaching the stage at which all characteristic radiation effect due to fusion neutrons can be analyzed by decomposing them into the recoil energy spectrum. Let us keep in mind that the irradiation conditions were well controlled and monitored, including the neutron flux and the temperature of samples. Samples were never exposed to neutrons at temperatures different from those desired. On the other hand, when one examines the data from fission neutron irradiation tests, he will be surprised and deeply disappointed, not only because of the inadequate data acquisition but also by the deficiencies in the control of irradiation conditions, especially those in the correlation history of the neutron flux and temperature. As already pointed out by one of the present authors [5], irradiation tests in fission reactors almost always contain some exposure of samples to neutrons at temperatures different from that desired, typically during the start-up of a reactor where the heating by y-rays is insufficient to raise the temperature up to that selected. Only when these deficiencies are eliminated, can the fission data be brought to the common reference point to be compared with fusion data. In this paper, the experimental details of improved control of irradiation tests in the JMTR, Japan Material Testing Reactor, will be described. Comparison of defect microstructures in various metals will be made between tests with conventional and improved control, and interpretations based on defect structure development mechanisms will be given to explain the differences found between the two modes of control. Finally, the data from the irradiation tests with improved control will be compared with those of fusion neutron irradiation tests in RTNS-II, to obtain a broader understanding of the differences and similarities between fission and fusion irradiation environments.

2. Improvement of temperature control in JMTR irradiation tests 2.1. Uppralion mode of the JMTR perature control

and conventional tem-

The Japan Material Testing Reactor (JMTR) at Japan Atomic Energy Research Institute at Oarai light water reactor, which was designed especially the irradiation testing of materials. The reactor with power of 50 Mw normally operates with a 12-14

the is a for full day

cycle. About ten hours are required for its start-up, with stepwise increase in power, and several hours are required for shut-down to zero power. The most common technique to control the sample temperature during reactor in-core irradiation used for years is by the balance between the heating by y-rays and the cooling by the coolant flowing through the irradiation rig. The control was done by changing the heat conduction through the heat-gap between the samples and the coolant. The heat conduction was altered by changing the helium gas pressure in the heat gap. With this conventional technique, although the temperature of samples at full reactor power can be maintained and controlled at a desired temperature by a proper design of the irradiation rig, it is impossible to avoid the exposure of samples to neutrons at temperatures lower than that selected, especially during the start-up and shut-down of reactor. None of reports made on irradiation of materials in the JMTR has ever mentioned such an actual temperature history. A similar situation is found in many other reactors [5]. 2.2. Design and construction of in-core irradiation rig for improved temperature control The simplest method to avoid the exposure to neutrons at temperatures different from that desired is to maintain the sample temperature with a strong heater which can heat up the sample to the designed temperature without any help of y-heating. Fig. 1 is the vertical cross-section of the in-core irradiation rig, designed and constructed to obtain improved temperature control. The inside of the rig is divided into four sections, each of which can be controlled independently. Each of four sections is made of a solid aluminum rod thermal medum, at the center of which are situated the samples to be irradiated. Surrounding the aluminum rod along a helical groove, an electric heater wire is embedded. The heater capacity is greater than the heat generated in the samples and thermal medium by y-rays at full power, for example in the present experiment at 673 K the power needed was 3 KW (equivalent to y-ray heating of 3 W/g). Two thermocouples were installed at the center of the sample assembly. The aluminum rods are placed in a tube of 304 stainless steel with a gas gap designed for each desired temperature. Two tubes are used for higher temperatures (> 573 K) and single tube is required for lower temperatures. The thickness of the gas gap varies for different desired temperatures, narrower for lower temperatures (normally the gas gap and the rig internals are filled with helium gas of one atmospheric pressure. Sometimes is pumped to lower

329

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

Insulator

Heater

Thermocouple Sample

Fig. 2. Cross-section of an aluminum block for the mock-up test of y-ray heating. These are four thermocouples to measure the radial distribution of the temperature.

Holder

Aluminum

2 for

Rod

are examples of heating to the same temperature with the outer and inner heater, respectively. Fig. 3(c) shows the capability of maintaining the temperature during alternate on-and-off cycles of the two heaters, which correspond to the most severe change of reactqr power anticipated. The temperature inside the aluminum rod, except at a position near its surface, could be maintained within the allowed deviation.

Heater Inner

Tube

Outer

Tube

2.3. Simultaneous

irradiation

with different

modes of con-

trol

IO cm

With the present improved irradiation rig, the correlation history between temperature-neutron flux with conventional control can be reproduced easily. In order to examine the difference between conventional and improved temperature control, two blocks in an irradiation rig at positions with the same neutron flux, e.g. symmetric positions with reference to the center of the reactor, block No. II and IV in fig. 1, were given the conventional and improved control, respectively. Fig. 4 shows the record of one cycle of irradiation, in which the temperature of improved control position was maintained at the desired value before start-up of the reactor and was kept at the temperature until the reactor power was reduced to zero, whereas the temperature during conventional control followed almost directly the reactor power. The control of temperature in the conventional control position was started after the reactor reached full power. Two cycles of irradiation were given, i.e. a total irradiation of 12 X 2 days; the same

1

Fig. 1. Vertical cross-section of the in-core irradiation rig of JMTR for improved temperature control. Each of the four sections can be controlled independently.

pressure to help the heating by the heater, particularly at the period of low reactor power. During the design of the present irradiation rig, one of the serious problems to be overcome was the elimination of the over- and under-shooting of the sample temperature with a sudden change of reactor power. One of the mock-up tests is quoted here. Exactly the same geometry and dimension of one block of the irradiation rig was provided with additional heaters (inner heaters) embedded in the aluminum rod as in fig. 2 to simulate the heating by y-rays in the reactor. Four thermocouples were aligned along the radial direction from the center as shown in the figure. Fig. 3(a) and (b) 1.283

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Fig. 3. Temperature distribution in an aluminum rod during the mock-up test. (a) Heating with heater 1 (corresponds to low reactor power), and (b) heating with heater 2 (corresponds to high reactor power). (c) Alternate on-and-off of heater 1 and heater 2 (corresponds to a sudden change in reactor power). Note that the temperature is constant at positions 1, 2 and 3.

M. Kiritani et al. / Fission-furion correlation by f&on

330

start-up

Reactor 600-

Improved

Control I

,,??+____________I

15

,-’ IO #

0

2

4

Control

30 ,__I’ ’

8

6

-

Conventional

?q.*-’

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I2

Time

14

16

I8

20

22

24

Reactor

1

Shut-down

_ ___

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------,

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Control

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Control

Control

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2

3

4

1

5 Time

2.4. Packaging

of samples

6

7

8

9

In order irradiate large numbers of small samples, they were loaded in elongated cube shaped vacuum-tight copper boxes [7]. In each box the samples were tightly packed in holes in a copper block for good thermal contact. The temperature of each sample does not exceed the temperature of the aluminum block when thermal contact is maintained even if the inside of the box is kept in vacuum to protect pre-thinned samples, but it is better if the box is filled with pure helium. In practice, four copper sample boxes (5 X 10 X 25 mm3) were placed in one block of the aluminum thermal media. One box can hold 600 three mm diameter disks or 150 miniaturized tensile test samples. 2.5. Materials

(82M-32Ul

-,ruyuV

G ;

(hour)

of Cycle

600-

trol. namely at 473 and 623 K. The accumulated neutron dose was measured with the dosimetry foils loaded at several places in the aluminum blocks, and also sometimes with the irradiated samples whenever appropriate and necessary. The neutron irradiation dose at each temperature is listed in table 1. The dose customarily reported from JMTR is for energies greater than 1 MeV and it can be converted to any other notation using the neutron energy spectrum given later in fig. 26.

(hour) r

600

reactor irradrurron

I

IO

II

1

I2

I3

I4

(day)

Fig. 4. Recorded temperature for the irradiation to compare conventional and improved control. The variation of the temperature in conventional control is observed to reflect the variation of the reactor power.

temperature control was applied during the two cycles. The irradiations to compare the conventional and improved control so far were at 573 and 673 K. The neutron dose at temperatures lower than desired in conventional control was estimated to be about 3% of the total dose. Other blocks in fig. 1 were utilized for irradiation at different temperatures with improved con-

irradiated

Almost the same materials as those irradiated with 14 MeV fusion neutrons during 1982-1987 in the rotating target neutron source RTNS-II [6] were tested, because one of the major purposes of the present series of irradiation experiments in a fission reactor is to obtain a so-called fission-fusion correlation. They included pure metals (Al, Ag, Au, Cu, Ni, Fe, V, MO), dilute alloys (Cu-0.05-2 at% Si, Ge, Ni, Sn; Ni-0.05-2 at% Si, Ge, Cu, Sn), model alloys (base alloys of 316 SS, FeeCr with varied composition) and some candidate alloys for fusion application (modified 316 SS, and several kinds of ferritic steels). Shape, size and treatment of samples are categorized into three kinds. Two are disks of 3 mm in diameter and

Table 1 Neutron dose by JMTR irradiation for 24 days

( X1024/m2

(E >l.O

MeV))

by the

Temperature

Conventional control Improved control

473 K

573 K

623 K

673 K

0.25

0.59 0.37

1.10

0.92 0.96

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

331

standardized miniature tensile test samples. Samples which had been already prepared for electron microscopy observation were also irradiated. The usefulness of the irradiation of these thin foil samples for the understanding of microstructure development mechanisms has already been recognized during the course of extensive study of fusion neutron irradiations [8,9].

3. Comparison of defect structures in irradiations with conventional and improved temperature control In this section, a comparison of microstructures developed by conventional and improved control at 573 and 673 K will be made. The microstructures at two other temperatures, 473 and 623 K, with improved control are included for the purpose of understanding the variation with the irradiation temperature. 3.1. Pure nickel Thin foil irradiation of pure nickel (Johnson Matthey 99.998% pure, annealed for 1 h at 1200 K) is one of the best materials, to demonstrate the pronounced difference introduced by the differences in temperature control. Fig. 5 compares the dislocation and void structure at 673 K. Defect microstructures introduced by conventional control show a complex variation as the foil thickness increases. With bright field observation no observable defect are visible in the thinnest region (top part of fig. 5, thickness of < 100 nm). Large dislocation loops elongated along the foil plane, the majority of which are unfaulted, are observed at a little thicker region (loo-260 nm). Farther out one sees no observable defects (260-340 nm). At a region of greater thickness (340-400 nm), one sees a high density of voids without dislocation structure. In the thickest part in the figure (bottom part, > 400 nm), a high density of voids coexists with deformed loops and tangled dislocations. In improved control, the defect microstructures show rather simple variation with foil thickness. Defects are not observed up to fairly thick regions (< 350 nm). Voids of less number density than in the conventional control are formed in the thicker region (> 350 nm), with a low number of scattered dislocations. At a lower temperature of 573 K, the difference between the two modes of control is not so drastic as at 673 K, both specimens showing the formation of interstitial loops. However, a quantitative difference is noticed as in fig. 6. The size distribution of loops in the improved control is rather narrow, whereas that in the conventional control is broad including larger ones.

Fig. 5. Comparison of defect microstructures in nickel between (a) conventional and (b) improved temperature control. Irradiated as thin foils at 673 K. Note the complex variation in conventional control with the sample thickness (thickness increases from top to bottom of figures).

Vacancy type small clusters were revealed by dark field electron microscopy at the very thin region wheras no defect structure was observed with bright field electron microscopy. Noticeably, many more clusters are seen in the conventional control than in the improved one. The number density in the improved control will be compared later with those in the RTNS-II irradiation in section 5.2.

ht. Ktn‘tani et al. / Fissron-fusion correlation by fission reactor irradation

332

Ni,

Thin foil,

(a) Conventional

(b)

Improved 9x10*’

Size

of Loops

573

K

Conrfol

Control loops/m”

_

inm)

Fig. 6. Comparison of the size distribution of interstitial type dislocation loops in nickel irradiated at 573 K between (a) conventional and (b) improved temperature control. Data from RTNS-II fusion neutron irradiation (c) are accompanied.

Fig. 8. hi, irradiated as bulk at 573 K. Inserted dark field micrographs of higher magnification show the void formation all over the matrix. (a) Convential control: (b) improved con

trol.

Figs. 7 and 8 show defect microstructures developed in bulk samples (0.1 mm thickness) at 673 and 573 K, respectively. At 673 K, the number of voids in conventional control is much greater than that in the improved control. At 573 K, a remarkable microstructure is observed consisting of curved dislocations accompanied with small interstitial loops, and voids are formed all over the matrix as observed in dark field micrographs. The measured number density and size of voids at the two temperatures are shown in figs. 9 and 10. Figs. 11(a) and (b) show dislocation loops and voids at 473 and 623 K, respectively, both irradiated in bulk with improved control. These data are useful when placed in series with the data already shown.

Fig. 7. Comparison of defect microstructures in nickel between (a) conventional and (b) improved temperature control. Irradiated as bulk at 673 K. Note the large difference in void formation.

Figs. 12 shows comparison between the conventional and improved control irradiations of a dilute nickel alloy with 2 at% Si at 673 K, irradiated as thin foils. The difference between the two modes of control is not so drastic as in pure nickel (fig. 5), and interstitial loops

-

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation NI, JMTR,

0

(b) Improved

L

L20

Bulk,

573

333

K

Control

l.7x102’voids/m3

l-l

i

IO -

OO

4

2

Size

Fig. 9. Comparison

6 of

6 Voids

IO

12

(nm)

of the number and size of voids in bulk nickel at 573 K. Fig. 11. Ni, irradiated with improved

are formed in both. The number of interstitial loops in the alloy with Si is found to be much greater (5 X 10”/m3) in the conventional control than that in the improved control (2 X 101’/m3). The size of the loops has an inverse relation to the number density. The complex variation of the defect microstructure along the direction of increase of foil thickness, which was explained in pure nickel, is also noticed in the sample irradiated with conventional control.

as bulk control.

at two different temperatures (a) 473 K; (b) 623 K.

proved control. The number and size of voids at 573 K are not different at first glance between the two modes of control. However, with a closer observation as in the insert micrograph in fig. 14(a), the tiny voids are observed in conventional control. Fig. 15 compares the number density and size distribution of these voids. Figs. 16(a) and (b) shows the microstructures developed in bulk copper at 473 and 623 K, both irradiated

3.3. Pure copper Figs. 13 and 14 compare the void formation in copper (Johnson Matthey 99.999%, rolled to 0.1 mm thick and annealed at 1250 K for 20 min), between conventional and improved temperature control, irradiated in bulk at 673 and 573 K, respectively. Large voids were formed at 673 K with conventional control, whereas no voids was found when irradiated with im-

Ni,

JMTR,

Bulk,

(0) Conventional 2.7x

“0

IO

20 Size

Fig. 10. Comparison

of

30 Voids

673K Control

-

102’ vo,ds/m3

40

(nm)

of the number and size of voids in bulk nickel at 673 K.

Fig. 12. Ni-2 at% Si, irradiated as thin foils at 673 K. To be compared with fig. 5. (a) Conventional control; (b) improved control.

M. Kiritani et al. / Fission-fusion correlation

334

Fig. 13. Difference of void formation in copper irradiated bulk at 673 K. No voids at all in improved control. Conventional control; (b) improved control.

as (a)

byfission reactor rrradiutm

Fig. 14. Cu, irradiated as bulk at 573 K. Inserted enlarged micrograph show the coexistence of tiny voids in conventional control. (a) Conventional control; (b) improved control.

improved control, showing no voids at either temperature. Fig. 17 is shown to compare the interstitial cluster formation near dislocations in bulk copper irradiated at 573 K. Localized formation of dislocation loops near pre-existing dislocations is observed in the conventional controf, whereas they are absent in the improved control. This localized formation of dislocation loops is observed at 473 K with improved control as shown in fig. 16(a), but not observed at 673 K as shown in fig. 13(b). Vacancy clusters in the shape of stacking fault tetrahedron observed by dark field electron microscopy are not significantly different between the two modes of control at 573 K. However, in their size distributions a greater population of smaller size is observed within the conventional control as shown in fig. 18.

with

/

r

I

r--

ib) 573K,

I

Improved 9x10”

Control Voids /m3

I-

/ (cl

I

673K,

Conventional 2 Y IO”

Control

Voids/m3

3.4. Copper aifsVs 0

19 compares the influence of pre-existing dislocations on the formation of interstitial clusters in Cu-2 at.% Ni alloy, irradiated in bulk at 573 K. With conventional control, dislocation loops are observed throughout the dislocation free matrix with a higher density near Fig.

0

20

40 SIX

of

Voids

60 (nm)

Fig. 15. Comparison of the number and size of voids in bulk copper. Bimodal size distribution is observed in conventional control at 573 K. Voids were not formed at 673 K when irradiated with improved control.

335

hf. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation Cu. r

1

(

Y

I

(b)

c_

L

JMTR. 11

Thmfoll.

Improved

573K 101

Control

IO -

c-c., 6

0. 0

2

4 Size

of

SFT

I 8

I

(run)

Fig. 18. Comparison of the size distribution of vacancy clusters formed in copper thin foils at 573 K between two modes of temperature control. Note the more population at smaller size in conventional control. Fig. 16. Defect microstructures introduced in bulk Cu at two different temperatures, both irradiated with improved control. (a) 473 K; (b) 623 K.

preexisting dislocations. In improved control, the loops are formed only near pre-existing dislocations. The upward shift of the temperature limit for the occurrence of these phenomena with the addition of solutes will be discussed later. A noticeable difference in this alloy from pure copper at this temperature is the absence of voids. In addition, no voids were observed in this alloy at any temperatures examined.

Fig. 17. Comparison of interstitial cluster formation formed around pre-existing dislocations between conventional (a) and improved control. (b). Cu, irradiated as bulk at 573 K. All

Fig. 19. Cu-2 at% Ni, irradiated as bulk at 573 K. Voids are not observed in both. (a) Conventional control; (b) improved

defects scattered in the dislocation free matrix are voids.

control.

336

M. Kiritani et al. / Fission-fusion

1

;

Cu-Zat.%Ge,

I

1

* JMTR. , Thin 8 foil,( 573KI (0) Conventlonol Control

I

I

D

,

(b)

LL

I

Improved

(

correlation by fissron reactor irradiatron Au,

(b)

I

JMTR,

573

Thin

foil

K

Control

7 IO -

I 8

0 0

2

4 Size

of

6 SFT

,

(nm)

Fig. 20. Comparison of the size distribution of vacancy clusters formed in Cu-2 at% Ge thin foils at 573 K between two modes of temperature control. The difference is similar to that in pure copper.

The size distribution of SFT in thin foils exhibited the same tendency as in pure copper as shown in fig. 20. 3.5. Gold Vacancy type stacking fault tetrahedra are the sole defects observed in thin foils of pure gold (Johnson Matthey 99.999%, rolled down to 0.1 mm thickness, annealed at 1000 K for 1 h in air) irradiated at 573 K. No noticeable difference is found between the two modes of temperature control as shown in fig. 21. Figs. 22 compares the data of stacking fault tetrahedra for-

0

2

4

6

Size

of

IO

0

SFT

(nm)

Fig. 22. Comparison of the number density and size distribution of SFT among three temperatures in thin foil gold, all irradiated with improved temperature control.

mation in thin foil gold at 473, 573 and 673 K, all with improved control. Fewer larger SFT are formed at higher temperature. Fig. 23 compares the size distribution of SFT in bulk gold at 573 K. Again no noticeable difference is found. 3.6. Silver The same description as in the case of gold can be for silver (Koch Chemical 99.99%, rolled down to

given

Au, 301

I

30

1

IO 0 of

SFT

(nm)

Fig. 21. Comparison of SFT size distribution in thin foil gold at 573 K between conventional and improved temperature control. Data from RTNS-II fusion neutron irradiation (1.4~ 10” n/m*) are accompanied.

’

’

’

2

573

K

( Conventional ’ ’ ’ Control’ ’ (0)

’

1bl

4

Size

Bulk,

’

Improved

t-.--d5

0 Stze

1

6

of

SFT

8

’

Control

-

’

IO

(nm)

Fig. 23. Comparison of SFT size distribution in bulk gold at 573 K between conventional and improved temperature control. Data from RTNS-II fusion neutron irradiation (1.4X lo’* n/m*) are accompanied.

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation Ag, JMTR, Thinfoil,

573K

I::Imyj

n

I

r

0

2

4

6

8

IO

12

14

Size of SFT (nm)

Fig. 24. Comparison of SFT size distribution in silver at 573 K between conventional and unproved temperature control.’

0.1 mm thickness and annealed at 900 K for 1 h). One example of the SFT size distribution is given in fig. 24. The silver samples become strongly radio-active by fission reactor irradiation, and were dropped from systematic investigations.

4. Point defect reactions caused by the difference between conventional and improved control irradiation The first aim of this section is to understand the point defect reactions through which the difference of defect microstructures in conventional control from those in improved control were introduced by the transient low temperature exposure to neutrons. An understanding of the results with conventional control during irradiation as a series of different temperatures gives a better understanding of defect structure development mechanism during fission neutron irradiation, and consequently it will provide confidence in the result from irradiation with improved control. 4.1. Vacancy clusters formed

directly from cascades

When the number of vacancy clusters formed from cascades increases proportionally with irradiation dose, they are supposed not to be affected by freely migrating point defects [9]. Although the variation with irradiation dose was not examined in the present fission neutron irradiation experiment, the experience on 14 MeV neutron irradiation tells us that stacking fault tetrahedra formed at very thin part of thin foil of copper and nickel belong in this category (see figs. 30 and 31). On the other hand, in the thicker part of thin foil samples

331

and of course in bulk samples, the vacancy clusters formed from cascades are strongly modified by freely migrating point defects coming from other cascades. Generally their increase slows down with dose, typically with the square root of the irradiation dose [9] (see figs. 30, 31 and 32). This latter case will be dealt with in the next sub-section. Subcascades in those materials in which subcascades are widely separated as in Cu and Ni cannot cooperate to form larger vacancy clusters at elevated temperatures; they simply become unable to form smaller size clusters [9]. This aspect is the origin of the observed difference in the comparison of the size distribution of SFT at 573 K between the irradiation with the conventional and improved control in figs. 18 and 20 for Cu and Cu-2 at% Ge. In both figures, the size distribution of larger size SFT is not different between the two modes of control, but there is a greater population of smaller ones with conventional control than with improved control. These smaller vacancy clusters are believed to be formed at lower temperatures during the temperature transient. The fraction of smaller clusters in conventional control in excess of those in improved control is more than ten percent of the total, and is much more than one would expect from exposure to neutrons at lower temperatures of only several percent. However, results are reasonable when we recall the formation of a much greater number at low temperatures. An example of a great number at a lower temperature will be shown later in fig. 30. 4.2. Vacancy clusters formed from cascades and modified by freely migrating point defects As irradiation proceeds, vacancy clusters formed directly from cascades are modified by the reaction of freely migrating point defects sent from other cascades. Typical is the irradiation of bulk samples in which the vacancy clusters are continuously annihilated by freely migrating interstitials, and consequently only a very small fraction of originally formed clusters survive. This occurs even in a thin foil sample for electron microscopy starting at thicker zones. Actually the increase in the number of vacancy clusters with irradiation dose decelerates starting from the central part of the specimen foil [9]. A comparison of defect structures belonging to this category between the conventional and the improved control is made using the formation of SFT in silver and gold at high temperatures. Before comparing conventional and the improved control, one must consider the variation of vacancy cluster formation with irradiation temperature within

338

M. Kiritani et al. / Fission-fusion

the irradiation with improved control in fig. 22. The understanding of the formation of SFT from closely spaced subcascades, obtained from fusion neutrons, is found to be also valid in fission neutron irradiations. The distribution at 473 K has its large peak at small size, and the smallest size at 573 K is larger than the peak at 473 K. In addition, a greater population of larger SFT is seen in 573 K irradiation than at 473 K. These tendencies are more pronounced at 673 K. At a higher temperature at which a small vacancy cluster formed from a subcascade is not thermally stable, vacancies in closely spaced subcascades such as in Ag and Au can cooperate to form a larger cluster. The size distribution of vacancy clusters formed in thin foils of gold and silver at 573 K was compared in figs. 21 and 24 (RTNS-II data are accompanied in fig. 21 for later discussion) between conventional and the improved control. The higher population of smaller size clusters observed for copper at the same temperature is not found in either metal. The reason why we do not find small SFT formed during the transient lower temperature irradiation is clear when we observe the variation of these SFT with irradiation dose, which will be shown later in fig. 31, even though they are obtained by fusion neutron irradiation. A very rapid slow down of the increase of the observed number density is telling us the continuous an~lation of SFT by freely migrating interstitials, and the smaller SFT formed at the beginning of irradiation through temperature transient could not survive through a prolonged irradiation at a high temperature. This situation is the same in bulk irradiation as shown in fig. 23. 4.3. Point defect clusters formed by freey migrating point

defects

correlation by fission reactor irrudiatron

expected for the present case of a low point defect production rate. At least, the observed strong temperature dependence of the dislocation loop formation indicates that the formation process includes the thermally activated reaction of interstitials. A typical temperature dependence of nucleation is known to be inversely proportional to the square root of the jump frequency of interstitial atoms. On the other hand, the growth of interstitial loops is carried out by freely boating interstitials simply because their size can easily become larger than that expected from a single cascade. The growth speed during the competing process between interstitials and vacancies is known to be faster at higher temperatures again from the electron irradiation study [13]. With this knowfedge of the temperature dependent nature of dislocation loop formation, a comparison between two modes of control is made below. The most pronounced difference was found in pure nickel irradiated at 673 K (fig. 5). in which an appreciable number of interstitial loops was formed in samples irradiated with conventional control. whereas absolutely no interstitial clusters were seen in samples irradiated with improved control. The understanding of this difference is straightforward when one considers the general nature of the nucleation and growth of point defect clusters. Nucleation is generally enhanced at medium temperatures, whereas the growth is faster at higher temperatures. This general situation is schematically illustrated in fig. 25. Of course growth is inhibited at much higher temperatures because of the faster thermal shrinkage. The temperature 673 K in nickel is in the high temperature region, C, in the figure, and no interstitial clusters are formed because of no nucleation, though the growth speed is faster at that temperature. With the conventional temperature control, the sample

4.3.1. Interstitial type dislocation structures At low temperatures (< 20 K), interstitial clusters in the form of dislocation loops are formed by a high concentration of interstitials in cascades [lO,ll], and their number density reaches as high as that of vacancy clusters made directly from cascades. At elevated temperatures as in the case of the present experiment, the number density of dislocation loops is much smaller than that at low temperatures but still they are not expected to nucleate from the homogeneous reaction of interstitials which have been released from cascades. Their nucleation is still believed to be caused by the highly localized interstitials at cascades. We have good knowledge of the formation of interstitial clusters from the homogeneous generation of interstitials during electron irradiation [IZ], from which the nucleation is not

Temperature

Fig. 25. Schematic illustration of the temperature dependence of nucleation rate and growth speed of point defect clusters.

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

was exposed to neutrons at temperatures through A and B during which the nucleation of interstitial loops took place and they grew large during a prolonged irradiation at the higher temperature C. By the addition of solutes in nickel, for example 2 at% Si as in fig. 12, the same temperature of 673 K shifts to the position B from C in fig. 25, at which the nucleation of interstitial loops is suppressed but not completely inhibited. An appreciable number of interstitial loops appears even in the improved control, and they grow large by the supply of freely migrating interstitials. In the case of conventional control, a large number of interstitial loops nucleated at temperatures through A, but their size stayed smaller than those in the case of improved control because the same amount of freely migrating interstitials must be shared by a greater number of loops. The temperature of 573 K in pure nickel is situated at B in fig. 25 as 673 K in the nickel alloy above mentioned, and no notable difference is observed because both the nucleation and the growth have appreciable speed, though a minor difference in the size distribution was observed in samples irradiated as thin foils as in fig. 6. Dislocation structures, with high density (> 10i3/m2) and of irregular curved shape, are introduced in bulk nickel at 573 K as in fig. 8, both with conventional and improved control. These high density of dislocations cannot be expected to have developed from pre-existed low density (< 10r”/m2) dislocations in the annealed samples, and their development is understood to have started from the nucleation and growth of interstitial loops. 4.3.2. Interstitial

clusters in the vicinity of edge disloca-

tions

An additional remark on interstitial loops can be made on those formed in the vicinity of edge dislocations in the bulk irradiation. They are known to be formed on the dilatation side of edge dislocations, and they are most remarkably observed at the irradiation temperature which is slightly too high for the formation of interstitial loops in the defect-free matrix [14,15]. The dislocation structures and associated loops are similar in nickel at 573 K between conventional control and improved control as in fig. 8, but the loops associated with dislocations are observed only for conventional control in copper at the same temperature as in fig. 17. The addition of solutes in copper shifted the temperature upward. At 573 K, formation of dislocation loops becomes possible in the dislocation-free matrix in the conventional control, and their formation in the vicinity

339

of dislocations becomes possible even in the improved control as in fig. 19. Dislocation loops formed in the vicinity of dislocations in case of conventional control at the temperature at which such dislocation loops are absent for improved control are simply understood to have nucleated during the temperature transient. Such a decoration of dislocations with dislocation loops may change the sink efficiency of dislocations and might produce a large effect on further irradiation. 4.3.3. Formation of voiak Among the temperatures examined in the present experiment, 473, 573,623 and 673 K, all with improved temperature control, the temperature 573 K is the unique temperature at which the formation of voids was observed in pure copper. From these data, the voids observed after irradiation at 673 K with conventional control are easily attributed to voids nucleated during the transient through 573 K at the start-up of the reactor. A closer inspection of fig. 14(a) with 673 K irradiation with conventional control informed us of the existence of tiny voids ( - 8 nm in diameter) in addition to those of larger size (15-35 nm in diameter), and their size distributions were shown in fig. 15(a). These tiny voids observed in conventional control are believed to be formed at the end of irradiation during the stepwise shutdown of the reactor as in fig. 4(b). The number of vacancies in a tiny void is about 1% of those in a large void, and their formation during step-cooling is expected. The formation of voids in bulk nickel was compared in figs. 9 and 10 at two temperatures with both conventional and improved control. At first glance, there is an . oppostte dtrection of difference between the two temperatures; the void size is larger with conventional control at 573 K, whereas it is larger with improved control at 673 K. However, this can be reasonably understood when their number densities are considered from the standpoint of the stages of their nucleation processes. Voids of an almost equivalent number allowed to nucleated at 573 K are thought to have already nucleated during the transient temperature rise in conventional control and they grew without further nucleation. Consequently the population of smaller voids is missing with conventional control, compared with those with improved control. In the 673 K irradiation, the number of voids which nucleate during reactor start-up is one order more than those to be nucleated at 673 K, and they cannot grow to the size in the case of improved control simply because of the higher number

340

M. Kiritani et al. / Fission-fusion correlation @ /ision reactor irrudration

density. The difference in the width of the size distribution at 673 K is also understood from the difference in the stages of void nucleation. The voids in the case of improved control are thought to have nucleated progressively during irradiation. In the discussion above on void formation, the difference in the bias effect of dislocations should have been incorporated. Greater bias is expected in the case of conventional control which resulted from the growth of the interstitial loops nucleated during the temperature transient. On the other hand, void formation proceeded in thin foil irradiation without any influence of dislocations, and another source of bias which is the localization of point defect production by cascades will be discussed in the next section. 4.4. Consideration induced bias

thicker

region (260-340 nm), vacancies formed in become effective to eliminate all interstitial loops by the cascade localization induced bias effect, but not effective enough to form their own clusters as voids. (4) At an even thicker region (340-400 nm), the vacancy predominance is so powerful enough to form clusters as voids after eliminating interstitial loops. (5) At the very thick region of the foil (> 400 nm), the formation of a greater number of voids resulted from the coexistence of the dislocation structure. Although the initial aim of the comparison of conventional and the improved control was to obtain confidence in the need for improved control, the analyses of the microstructure development through the temperature transient in the conventional control has given us useful information about CLIB effects in a typical example. cascade

of the effect of cascade localization 5. Comparison with fusion neutron irradiation

The more localized formation of vacancies compared with interstitials by cascade collision has been considered to automatically give rise to the predominance of vacancy reaction with existing defect structures, and it has been named as the effect of cascade localization induced bias (CLIB) [3,4,16]. The origin of the CLIB effect is different from that of “production bias” proposed by C.H. Woo and B.N. Singh 1171 in which the formation of interstitial clusters have been postulated to give rise to the predominance of freely migrating vacancies. The CLIB effect is expected regardless of point defect cluster formation. One typical example showing the existence of the effect of this cascade localization induced bias is found in the irradiation of pure nickel thin foil with conventional control. The formation of voids in a nickel thin foil at 673 K with improved control (fig. S(b)) should be understood to occur without any help from dislocations, and the vacancy predominant atmosphere is attributed to CLIB. The complex variation of defect structures developed by irradiation with conventional control at 673 K {fig. 5(a)) should be understood from the fairly homogeneous high density of dislocation loops nucleated during the start-up of the reactor as the initial condition. (1) At a very thin part of the sample (< 100 nm thick), the number of vacancy clusters made from cascades are greater with conventional control than in the case of improved control, as explained already. (2) At a little thicker region (100-260 nm), the foil surface is still the predominant sink and absorbs vacancies and interstitials equally. The dislocation loops nucleated through the temperature transient simply grow with their stronger attraction to interstitial% (3) At a little more

Irradiation with fission neutrons in the JMTR discussed in this paper supplied us with confidence to compare the results of the irradiation with improved control with fusion neutron irradiation. In 14 MeV neutron irradiation in RTNS-II, the neutron dose was varied about three orders of magnitude within the same irradiation run by placing a number of samples at various distances from the closest position to the neutron source. The longest irradiation (about 6 months) was performed at four temperatures, 363, 473, 563 and 723 K reaching 6 x lo’* n/m* at the position closest to the source. Two of these temperatures are the same or very cIose to two temperatures in the present fission neutron irradiation, 473 and 573 K. The irradiation at other temperatures, at about 50 degrees intervals between room temperature and 823 K, were performed up to 1 x lo** n/m*. To be compared with these 14 MeV neutron irradiations, the neutron doses given in the present fission neutron irradiation with JMTR are higher as listed in table 1. However, these differences are not large when we scale with damage energy as will be seen. Some of the fusion data here has been published and some others were from two doctorate theses [18,19]. The rest is newly obtained from the re-examination of stored samples of RTNS-II irradiation. 5.1. Recoil energy spectrum and damage energy Before making a comparison of experimentally observed defect structures produced by fission and fusion neutrons, theoretically estimated collision parameters will be compared. The first is the recoil energy spec-

h4. Kiritani et al. / Fission-fusion

trum, which should serve as the basis for all the analyses. The second is the damage energy, which should be used as the conversion parameter for the irradiation dose between irradiations with a fission reactor (JMTR) and with a fusion neutron source (RTNS-II). The neutron energy spectrum at the m-core position in the JMTR is shown in fig. 26 [20]. From this neutron energy spectrum, the recoil energy spectra were calculated for various elements by Shimomura et al. by using SPECTER Code [21]. Recoil energy spectra due to 14 MeV neutrons were quoted also from Shimomura et al. which are a revised version of those by Logan and Russell [22]. These two collision cross-section spectra against PKA energy are shown in fig. 27(a) adopting the case of copper as an example. One of the practical and proper ways to compare the effect of the differences in recoil energy spectra is to compare them at the same total damage energy. In order to do this, several steps of conversion are necessary. First, the PKA energy spectrum is obtained as the dotted lines in fig. 27(b) by applying the PKA energy to the cross-section spectrum in fig. 27(a). Secondly, the damage energy spectrum is obtained as the solid lines in fig. 27(b) using considering the energy loss to the electron system by the Lindhard theory [23]; the fraction of the loss is generally larger for larger energy. Finally, in order to compare the two spectra at the same total damage energy, the scalings for JMTR and RTNS-II are proportionally shifted in such a way that the total integrated area between the two is equal with each other as in fig. 27(c). Here the multiplier p to be applied to the nominal neutron dose of JMTR in order to have the same total damage energy as by RTNS-II is defined as the multiplication factor /3. Dosimetry data from JMTR are generally supplied on the basis of neutrons whose energy is higher than 1.0 MeV, and the multiplication

JMTR,

In-Core

1

6-

correlation by fission reactor irradiation

200

0

:

IO2

IO’

Energy

IO6

IOB

(eV)

Fig. 26. Neutron energy spectrum at in-core irradiation posi-

tion in

JMTR

[20].

1000

I200

1400

1.0

(c)

200

400 f=KA

600

800

Energy

, , 1000 Ep

1200

1400

(KeV)

Fig. 27. (a) Collision cross-section spectra along PKA energy in a fission reactor (JMTR) and in a fusion neutron source (RTNS-II), (b) PKA energy spectra (Er.du/dEEp dotted lines) and damage energy spectra ( Er,da/d E,, solid lines), and (c) the comparison of the damage energy spectrum at the same total damage energy. The factor is the factor applied to JMTR values in order to have the same total damage energy as in RTNS-II.

factor for four materials determined by the procedure above mentioned is listed in table 2. In this paper, as the scaling factor for neutron irradiation, the damage energy per atom (DEPA) is adopted. The reason why we cannot adopt the neutron dose itself is obvious from the large difference in energy between the two to be compared. The major reason why we do not adopt the conventional DPA (displacement per atom) scaling is the strong process sensitivity of the

dose /3 to give the same

damage

energy,

/ ‘14 MeV ).

a)

Neutron

800

poJp_&

( OJMTR

IO0

600

(b)

Table 2 Ratio of neutron

lo-2

400

(E>l.OMeV) (E > 0.1 MeV)

Ag

Au

cu

Ni

1.6 3.6

2.3 5.0

2.0 4.3

1.7 3.7

a) When the nominal dose is expressed as the dose of neutrons whose energy is larger than the value indicated.

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

342

efficiency of damage energy to displaced atoms especially when the process includes cascade collision as in the present case. Also by adopting DEPA unit, which is the parameter one step earlier than DPA, we can avoid the inclusion of ambiguity such as the threshold energy for displacement and the validity of the Kin&in-Peace approximation. Those who still prefer DPA scaling may suppose the present DEPA scaling as a proportional shift of DPA. The comparisons of damage energy spectra for four kinds of materials are shown in fig. 28. By the procedure mentioned above, they are all plotted so as to have the same integrated area. Primary recoil with a lower energy than the threshold for displacement of atoms do not contribute to damage and should be excluded in these discussions, but their inclusion changes by only several tenths of percent the total energy and should not be considered seriously. The most remarkable difference between 14 MeV neutrons and neutrons in the fission reactor is the difference of the primary recoil energy from which the major part of the damage energy is

Oh

II

1

lz

(b) RTNS-II IO-

/ 3x

1023SFT/rn3

-

I

0 0

2

4 Size

of

6 SFT

8

(nm)

Fig. 29. Comparison of the size distribution of vacancy clusters produced in thin foils of copper at the same temperature of 473 K, between fission (JMTR) and fusion neutron (RTNS-II, 1.1 X lo** n/m*) irradiation.

deposited. With 14 MeV neutrons, the major part comes from the peak at high primary recoil energy, whereas that with fission neutrons comes from the smaller energy recoil, which monotonically increases towards smaller energy. 5.2.

Vacancy

clusters formed

from

cascades

Although point defect clusters directly formed from cascade damage are disclosed better at lower temperatures than at the elevated temperatures of the present fission neutron irradiation, some direct comparisons with the case of 14 MeV neutrons can be made.

200

400

600

800

200

400

600

1300 1000 1200 1400

PKA

Energy

1000 1200 1400

Ep

( KeVl

Fig. 28. Comparison of damage energy spectrum plotted against primary recoil energy between JMTR (fission neutrons in a reactor) and RTbJS-II (14 MeV neutrons). A proper factor p for each material is multiplied so as to have the same integrated area (integrated total damage energy) between the two.

5.2.1. Copper as a high temperature case In fig. 29, the size distribution of vacancy clusters in copper irradiated at 473 K as thin foils is compared between JMTR and RTNS-II. The distribution in RTNS-II is noticed to extend to a larger size than that in JMTR. A large difference, as great as 5 times, is found when the number density is compared at the equivalent damage energy by extrapolating the RTNS-II data toward higher dose as in fig. 30. If the same manner of increase of SFT with irradiation dose as that for RTNS-II (a little slower increase than proportional to the dose) is assumed for JMTR irradiation, an irradiation of 8 times more in DEPA scaling is required for JMTR irradiation to produce SFT equivalent to RTNSII irradiation. In materials such as copper and nickel in which subcascades made from a large energy recoil are widely separated from each other, each subcascade is known to

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

clusters while some subcascades produced from a small primary recoil below this energy may form vacancy clusters, it can be said that a large fraction of damage energy of about 80% does not contribute to the formation of these vacancy clusters in JMTR irradiation. Implications to the formation of freely migrating defects from these smaller primary recoil will be important.

300K

.

c

102’

I

c

I

lci2

I

Id DEPA

343

I

100

I

IO'

(eV/atom)

Fig. 30. Comparison of the number density of vacancy clusters produced in copper by fission and fusion neutrons.

behave independently even at high temperatures. Smaller subcascades gradually lose their ability to form vacancy clusters at higher temperatures [l]. Keeping this nature of vacancy clustered defects in copper in mind, their formation in JMTR and RTNS-II is compared. When the damage energy is integrated above a certain lower limiting value of PKA, in such a way that the integrated energy can account for the observed number of vacancy clusters when it is divided by this lower limiting value, the energy is obtained to be 180 keV and 150 keV for JMTR and RTNS-II, respectively. These two values are regarded to be same when the errors in experimental data acquisition and analysis are taken into account. However, this lower limiting energy does not mean at all the energy to form a vacancy cluster. It does mean merely that one vacancy cluster is produced on average per this energy. From our past work [2,15,24], a large primary recoil energy in copper is known to be divided into subcascade energy of 20 keV on average. These subcascades should be understood to have their size distribution such that the largest one tenth of the total can form stable vacancy clusters at a temperature of 473 K. An important consequence of the analysis above is that the formation of vacancy clusters directly from cascades is given a consistent interpretation between fission and fusion when the difference in the recoil energy spectrum is taken into account. Another importance is in the primary recoil below this energy. The fraction of energy smaller than the above unit energy is about 80% and 12% for fission and fusion, respectively. Although many subcascades produced from a large primary recoil above this energy may not form vacancy

5.2.2.

Nickel

as a low temperature

case

The same scheme of comparison is made in fig. 31 for vacancy clusters formed in nickel thin foil at 473 K. A 13 times smaller number density is observed for JMTR in comparison with the value extrapolated from the lower dose in RTNS-II. In the case of nickel, the irradiation dose in JMTR required to produce defect clusters equivalent to RTNS-II is also 13 times smaller, because the functional dependence of the irradiation dose is exactly proportional. The temperature 473 K in nickel is not high enough to lose the ability to form vacancy clusters from subcascades, and this continues up to 573 K as understood from an additional plot of the number density in fig. 31. At a one hundred degrees higher temperature of 673 K, the number density of clusters decreases appreciably as seen in the same figure. A numerical analysis with the same scheme as in the case of copper in the preceding section was made, but it was not successful. As stated in the last part of the discussion on copper, subcascades produced from a

Ni,

473

K

loz5%

I

I

I

10-l

I00 DEPA

1

IO'

I

(eV/a?om)

Fig. 31. Comparison of the number density of vacancy clusters produced in nickel by fission and fusion neutrons.

344

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation

I

1

’

Au, 563-573

K

TtyAo------“E 210

22

0

-

5.3.2.

/

I:

JMTR,

/

6 h .z :

573K RTNS-II,

-

563K

/’ I

I

0.1

0.5 DEPA

1.0

5.0

(eV/alom)

Fig. 32 Comparison of the number density of vacancy clusters produced in gold by fission and fusion neutrons.

large primary recoil are not necessarily large and vice versa. A quantitative analysis which introduces the size distribution of subcascades is left to a future exercise. 5.3.

Vacancy clusters formed

by freely

smaller difference of the JMTR value from the extrapolation of RTNS-II has also the same tendency as in the bulk case. The implications of this will be discussed in the next section too.

migrating

from cascades

and modified

point defects

5.3.1. Irradiation as thin foil At temperatures lower than those in the present experiment, closely spaced subcascades are known from 14 MeV neutron irradiation experiments to produce separate small vacancy clusters, and they form larger clusters at higher temperatures as in the present experiment. In fig. 21, the size distribution of SFT formed in gold by 14 MeV neutrons at 563 K were compared with those due to fission neutrons at 573 K. The occurrence of larger SFT in RTNS-II suggests their formation from larger subcascade groups produced by large PKA. In fig. 32, the number of large SFT in gold thin foil at 573 K in JMTR irradiation is compared with the variation of the same type of defects at 563 K in RTNS-II irradiation with neutron dose. The same discussion cannot be made for gold as for copper and nickel in section 5.2, because the increase of the number density with neutron dose is much slower than a proportional relationship. indicating that the SFT produced by large cascades are not preserved but are continuously annihilated. The temperature 573 K is below the annealing temperature of the present size of SFT in gold, and the annihilation is attributed to the reaction of freely migrating interstitial atoms. This resembles the case of bulk irradiation explained in the next section, and the

Irradiation

of bulk material

The discussion above has been made on the basis of the observation of defects produced in thin foil samples assuming that the defect clusters formed directly from cascades are not modified much by freely migrating defects sent from other cascades. This expectation was fulfilled for copper and nickel at 473 K in both of which the number density of SFT increased proportionally with the neutron dose. However, in gold at 573 K the situation was found to be similar to the bulk irradiation. When irradiated as bulk material, vacancy clusters produced from a large cascade are expected to grow by absorbing vacancies sent from other cascades and are expected to shrink and disappear by absorbing freely migrating interstitials. When the same type of defects in the bulk samples are compared with those in thin foils and also between fission and fusion, information on the difference of the reaction of freely migrating defects will be obtained. The method and usefulness of this type of analysis has been proposed by one of the present authors during the analysis of a 14 MeV neutron irradiation study [9]. The size distribution of small vacancy clusters introduced by the irradiation of pure copper as bulk is compared between JMTR and RTNS-II in figs. 33 and 34 at 473 and 573 K, respectively. The number density of vacancy clusters observed in JMTR bulk irradiation of copper at these two temperatures was compared with

8 20F

T

I

I

’

It

I

’

1

‘-

lb1 RTNS-II 6x102’SFT/m3 IO -

Size

of

SFT

lnmi

Fig. 33 Comparison of the size distribution of vacancy clustered defects in copper between fission and fusion (1.1 X 1O22 n/m’), irradiated as bulk at 473 K.

M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation Cu, I

1

I

8

1

573K, 1

Bulk

I

I_

(a)JMTR 3.5~10" SFT/m3

(b) RTNS -11 (563K)

0

2

4 6 Size of SFT (nml

8

Fig. 34. Comparison of the size distribution of vacancy clustered defects in copper between fission and fusion (6.5 x lo** n/m*), irradiated as bulk at 573 K.

RTNS-II irradiation in fig. 29. In a strong contrast to the much less number of survived defects in bulk compared with those in thin foil in RTNS-II irradiation, almost the same number was found to stay in JMTR irradiation as those in thin foils. The same series of data have been obtained for nickel for comparison of fission with fusion. The number density of vacancy clusters in bulk irradiation at 473 K is superposed in fig. 31. The value is on the extrapolation to those after fusion neutrons, but this is rather fortuitous. The importance is in the fact that the decrease of vacancy clusters in bulk from the level in thin foil is much less in JMTR than that in RTNS-II. 5.3.3. Production

and destination of freely migrating

inter-

stitials

Considering the difference between JMTR and RTNS-II mentioned above, a much smaller number of vacancy clusters in fission neutron irradiation originally formed from cascades (thin foil irradiation) when compared at the same damage energy but with less annihilation (bulk irradiation), gives us important information on the difference between fission and fusion. The first half of this information is naturally accepted by the difference in primary recoil energy between the two irradiations, and the second half is giving us information concerning the role of freely migrating point defects. In the case of 14 MeV neutrons, at the temperature at which appreciable formation of vacancy clusters from cascades takes place, the freely migrating interstitial atoms in excess of vacancies resulted from this vacancy

345

cluster formation may play a major role, whereas, in the case of fission neutron irradiation, a greater number of both interstitials and vacancies are thought to be released as freely migrating defects and their competing processes to modify the existing structure may become important. More efficient production of point defects by smaller energy recoils has been derived by Heinisch from his computer simulation work [25]. The efficient production of point defects in Heinisch’s calculation exists up to the primary recoil of 50 KeV in the case of copper, and the population of primary recoil in this range in JMTR is notably higher than in that RTNS-II (see fig. 28). However, one encounters a difficulty to attribute the difference between fission and fusion to this efficient production of point defects from smaller primary recoil in fission, because the extent of the annihilation of vacancy clusters by freely migrating defects is greater in RTNS-II but not in JMTR. Another source of difference should be sought in the difference in point defect processes. When a vacancy cluster is formed in a cascade, exactly the same number of interstitials as the number of vacancies in the cluster is thought to be sent out as freely migrating interstitials in excess of freely migrating vacancies. If all of these point defects migrated and reached vacancy clusters, all the vacancy clusters would be annihilated and no microstructure would be developed. However, as clearly analyzed before by the present authors [9], the annihilation of freely migrating point defects is shared between point defect clusters and permanent sinks. The fraction of freely migrating defects which succeed to reach vacancy clusters is expected to be greater in fusion neutron irradiation because more vacancy clusters are formed from cascades. On the other hand, in fission neutron irradiation in which less vacancy clusters are formed from cascades, a greater fraction of freely migrating point defects escapes to permanent sinks without reaching vacancy clusters, consequently resulting in less difference between thin foil and bulk irradiation. Here, the importance is in the complex combination of the production and reaction of point defects that determines the final microstructures. 5.4. Evolution of interstitial type dislocation structures The extent of the role of freely migrating interstitials has been detected by the change of vacancy clusters produced from cascades in the preceding section. Other reactions of interstitials are in the evolution of various dislocation structures, by the nucleation and growth of interstitial type dislocation loo@ or by the climb of pre-existing dislocations. Experimental data and their

346

M. Kiritani et al. / Fission--fusion

Fig. 35. Interstitial loop formation with 14 MeV neutrons (a-c) and with fission neutrons (d) in thin foil nicket at 473 K. (a) 0.9 x 10” n/m*; (b) 1.6 X 1O22 n/m*, (c) 4.3 X 10” n/m*; (d) 2.5 X 1O23n/m2.

correlation by fission reactor rrradiation

increased with irradiation dose in RTNS-II, but they do not seem to have grown appreciably. Defect microstructures introduced by JMTR irradiation look alike with those by RTNS-II. Although the existence of the interstitial loop free thin region (the top part of fig. 35 with almost no interstitial clusters is about 70 nm in thickness, gradually becoming thicker at the bottom) indicates the existence of interstitial loop free layers along specimen surfaces even at the thicker region, one can obtain the bulk number density of these defects, not strongly affected by the surfaces, by measuring the increase of area number density with the increase of specimen thickness. The number density data thus obtained are plotted in fig. 36 with the damage energy per atom as the variable. The variation of the size of interstitial loops is also shown in the figure. When the observed proportional increase of the number of interstitial clusters with irradiation dose is combined with the observed no appreciable increase of their size with the dose, they are understood to be formed by localized interstitials produced in cascades. A similar comparison of RTNS-II and JMTR as in the case of vacancy clusters can be made here for these interstitial loops. A factor of 6 times more irradiation dose, in the damage energy scaling, is required for JMTR to introduce an equivalent number of interstitial loops as in RTNS-II. Interstitial cluster nucleation is said to be more efficient in fusion neutron irradiation

Ni,473

analyses will be presented for nickel at two temperatures, 473 and 573 K. The lower temperature case represents the nucleation of interstitial clusters at cascades, and the higher temperature case represents the role of freely migrating interstitials. It should be noticed in advance that each of these two temperatures is situated below and above the temperature range for an efficient thermally activated reaction of vacancies in this material.

5.4.1. Nucleation of interstitial clusters at cascades As explained already, a very thin region of foil samples is suitable for the detection of vacancy clusters formed directly from cascade damage. When one goes to a thicker region, interstitial clusters in the form of dislocation loops are observed. Fig. 35 compares interstitial cluster formation in nickel at 473 K in three different neutron doses with RTNS-II and one with JMTR. The number of interstitial clusters seems to have

K, Thin

foil

-

$ E

I

:

0’ DEPA

0

IO’

\:“/atoin,

Fig. 36. Comparison of the number and size of interstitial loops in nickel at 473 K between fission and fusion neutron irradiation.

347

M. Kiritani et al. / Fission-furion correlation by fission reactor irradiation

than in fission irradiation, and the similar line of discussion can be made as in the case of vacancy cluster formation in section 6.2 in terms of the recoil energy spectrum difference between the two. 5.4.2. Growth of dislocation Ioops at high temperatures At higher temperatures, 573 K and above, in nickel, much less numbers of interstitial clusters are nucleated at cascades and the role of freely migrating interstitials becomes important in microstructural evolution. The large size of interstitial loops in nickel at 573 K in fig. 6 and those in a nickel alloy at 673 K (fig. 12) are not expected to be formed from a single cascade, but the major contribution to their growth is attributed to the freely migrating interstitials released from cascades. One should recall the analogy to the dislocation loop growth under electron irradiation, in which the loops grow large steadily at vacancy mobile high temperatures whereas they stay small at lower temperatures [12,13]. The amount of interstitial atoms forming dislocation loops in nickel irradiated as thin foils at 573 K (fission) and 563 K (fusion) is compared between JMTR and RTNSII. The number density of loops by fission neutron irradiation to a dose of 3.7 X 1O23 n/m2 (> 1 MeV) (6.8 eV/atom DEPA) is 9 x 1021 loops/m3 and that by fusion neutron irradiation to a dose of 6.2 x 1O22 n/m2 (2.0 eV/atom DEPA) is 4 X 102’ loops/m3 (fig. 6). Assuming the same manner of increase of the number of loops with irradiation dose for both irradiations as that for 473 K irradiation with RTNS-II (fig. 36), the irradiation of 1.5 times more in DEPA scaling is required for JMTR to produce. the same number of loops as RTNS-II. The average loop size in the pair of irradiations above is 12.5 nm in JMTR and 5.3 nm in RTNS-II, respectively. From the number and size of dislocation loops, the amount of interstitial atoms which formed these dislocation loops can be estimated. If an additional assumption is made that the amount of interstitials increases with irradiation dose with the same manner as assumed above, the irradiation dose with JMTR to produce equivalent loops is obtained to be 0.28 times of RTNS-II. Here, it is concluded that the reaction of freely migrating interstititals at 573 K in nickel is more dominant in fission neutron irradiation than that in fusion neutron irradiation. Concerning the curved dislocations in bulk nickel produced by JMTR irradiation at 573 K (fig. 8), it was confirmed that the compression side is inside the curved dislocation. If we regarded these dislocations to have developed by interstitital atoms, a strong reaction of freely migrating interstitials in JMTR irradiation is concluded again.

of voids

5.5. Formation

Formation of voids has been clearly observed in bulk samples of copper and nickel both in fusion and fission irradiation, even at the low doses of the present experiment. The most remarkable results to the authors were the formation of voids in thin foil samples of nickel at 673 K, which was never observed by RTNS-II irradiation, though the irradiation dose was smaller. The present authors propose the existence of a driving force for void formation under irradiation with cascade damage, resulting from the difference in the localization of the nascent distribution of vacancies and interstitials (named as the cascade localization induced bias (CLIB)). A prolonged discussion is required to understand the origin of the differences and similarities of void formation between fission and fusion including this CLIB effect, and therefore, only the phenomenological comparison of the observed results of fission reactor irradiation with the former RTNS-II results will be given below. Number density of voids and amount of swelling in copper at 573 K in JMTR irradiation are compared in fig. 37 with the extrapolation of those in RTNS-II irradiation. RTNS-II data show a continuous nucleation of voids, but the size distribution in fig. 38 [19] is telling us that these voids do not grow continuously but stop their growth at a size characteristic of the temperature. The size distribution in JMTR irradiation already shown in fig. 15(b) is the same as those in RTNS-II irradiation at the same temperature. The number density of voids

Cu, I

I6

I

573K

1

100 DEPA

1

OCJE

I

I

10-l

Bulk I

I -/ I

563--573K,

,

J

IO'

(eV/otom)

Fig. 37. Comparison of void formation in copper between fission (573 K) and fusion (563 K) neutron irradiation.

M. Kiritani et al. / Fission-fusion correlation by @ion

348

Cu, RTNS-II,

563K,

Bulk

I

t 1

I

1

(a) 6x10~‘“/m~ adoi voids/m3

--I

P

N~,Bulk, 563~593K

/

I

IO -

reactor irradiation

16’ -

l-

-

?

-0

/

g

JMTR 573 ii

/O

P

p-’ 8

’ /

RTNS-II 563 K

- id26

l

-

/y-----

(Cl

6x10’*“/m2

6 x IO”

voids/m3-

1

iO-3

I

I

0

10

20 Size

40 of

I 50

Voids (nmi

l

.’

i E

--r

JMTR 573K

a. .r

I -Id’

/

t 0

; rz 8

o-1

I

100

I

IO’

OEPA ieV/otom)

Fig. 39. Comparison of void formation in nicket at 563 and 573 K between fission and fusion.

Fig. 38. Size distribution of voids in copper at three different irradiation dose of 14 MeV neutrons at 573 K.

and the amount of swelling in JMTR irradiation are smaller than the extrapolation of RUNS-II irradiation, but the difference is not so remarkable in this metal. Void formation in nickel as found in fig. 39 is seen to be considerably different from that in copper. Void nucleation continuously slows down in RTNS-II irradiation, but the swelling is almost proportional to the irradiation dose. This indicates that the voids in nickel are growing continuously, not ceasing their growth as in copper. The number of voids in JMTR is much less than the extrapolation of RTNS-II irradiation.

5.6. Consideration

of damage rate

The damage rate in the present JMTR irradiation should be incorporated into the above discussion in comparison with the damage rate in RTNS-II. By JMTR, 5 X lo*’ n/m2 (g > MeV) was reached in 24 days, which corresponds to 4 eV/atom in damage energy with the damage rate of 2 X 10e6 eV/atom s (by 2.4 X 10” n/m* s (E > MeV)). In RTNS-II at the closest position to the neutron source, 1 X 1O22 n/m2 (14 MeV) was reached by 30 days which corresponds to 0.16 eV/atom and 6 x lo-* eV/atom s (by 4 x 10” n/m2 s (14 MeV)). The damage rate in JMTR is thus 33 times

larger than that in RTNS-II. A greater difference in damage rate occurs for samples in RTNS-II irradiation which were placed at a distance from the neutron source. To what extent are the discussions in this paper on the comparison of JMTR and RTNS-II irradiation altered by this difference in the damage rate? Defect clusters formed directly from cascades in 6.2. are not expected to depend on the damage rate, at least in the case of very thin foils in which they are not modified strongly by the freely migrating defects sent from other cascades. When the reaction of freely migrating point defects comes into the microstructure evolution, generally a radiation rate dependence is expected. When a succeeding cascade is generated in the volume of influence of the preceding cascade within its relaxation time by diffusion, the overlap of freely migrating point defects occurs. One of the present authors made a general formulation for the criterion of the occurrence of damage rate dependence [26], the relation among dose rate, jump frequency of point defects and diffusion distance for their annihilation. In the present regime of irradiation temperature, the radiation rate dependence is not expected in copper, gold and silver in all of which even vacancies move fast enough not to overlap with those produced by other cascades, except in the case of nickel in which the slower motion of vacancies plays the role in microstructure evolution.

M. Kiriiani et al. / Fission-fusion

In an extreme case of a high damage rate, one enters the mutual annihilation dominant regime between vacancies and interstitials. In this extreme case the reaction of point defects with existing defects becomes proportional to the square root of the damage rate as is well known from the high damage rate study of electron irradiation [13]. When the irradiation rate dependence of the reaction speed is proportional to damage rate to a power smaller than unity, the integrated amount of reaction up to the same total dose with a larger damage rate is suppressed in comparison with that of a smaller damage rate (261. Although the damage rate, even with JTMR in the present case, is not expected to reach to this extreme, the interstitial loop growth which is the result of the difference of the arrival of freely migrating interstitials and vacancies should be suppressed by the larger damage rate when compared at the same total damage. This consideration of the damage rate dependence tells us that the interstitial loop growth and the climb of dislocations observed in JMTR fission neutron irradiation may become greater when the irradiation is performed at a smaller damage rate as in the case of RTNS-II fusion neutron irradiation. The greater fraction of vacancy clusters (in bulk) survived from those directly formed from cascades (in thin foil) in JMTR irradiation compared with that in RTNS-II irradiation might partly come from the higher damage rate, because a greater fraction of point defects disappear by the mutual annihilation when the damage rate (the production rate of freely migrating defects) is higher. Here, only several important considerations on the effect of damage rate were mentioned briefly, and a general description can be made only after the irradiation experiments of varied flux and fluence as emphasized in the following conclusion.

An assumption was made in obtaining the factors in table 3. Microstructures in fission neutron irradiation were assumed to develop with the same functional dependence on irradiation dose as in the fusion neutron irradiation, because data on the dose dependence in fission neutron irradiation are not available at present. An important remark which should accompany with table 3 is that each of the factors in the table do not necessarily reflect the difference in the strength of fission and fusion neutrons. Some reflect directly the reaction of defects introduced by cascades, but many others are the result of the variation of other defect processes. Those which are directly related to cascades are (a), (f) and (h). Those which started from cascades but modified strongly by freely migrating defects are (b), (c), (g), (i), (1) and (m). Finally, those which went through complex reactions of cascades and freely migrating defects are (d), (e), (j) and (k).

Table 3 Nominal factors of fission-fusion correlation for a variety of microstructure developments. The factor indicates how many times more DEPA by fission neutrons is required to give the microstructure equivalent to that by fusion neutron. Material

Microstructure

Temperature

Factor

(K)

cu

(a) (b) (c) (d) (e)

5.7. Nominal factors of fission-fusion correlation Ni

The present authors do not retain any hope of obtaining a single conversion factor between irradiations with fission and fusion neutrons. Some defect reaction processes are faster with fusion neutron irradiation in comparison with fission neutron irradiation, with a wide variation of the degree of difference, but some others are faster with fission. In order to demonstrate that the fission-fusion correlation factor is strongly structure and material sensitive, nominal factors to obtain fission neutron irradiation dose to give the microstructures equivalent to those by fusion neutrons, calibrated in the scaling of damage energy per atom (DEPA), are listed in table 3.

349

correlation by fission reactor irradiation

Au

Number of SFT in thin foil Number of SFT in bulk Number of SFT in bulk Number of voids in bulk Swelling of bulk

Number of SFT in thin foil (g) Number of SFT in bulk (h) Number of interstitial loops in thin foil (i) Amount of interstitials in loops in thin foil (i) Number of voids in bulk (k) Swelling of bulk (1) Number of SFT in thin foil (m) Number of SFT in bulk

473

8

413

3.8

573

12

573 513

2.1 1.8

473

9.5

413

1.5

413

6

(f)

> 573

0.28

573 573

21 13

513

17

513

6.5

350

M. Kiritani et al. / Fission-fusion correlation ty fission reactor irradiatron

6. Summary and concluding remarks

6.3. Proposals for the further

6. I. Con@olled reactor irradiation

(1) The extent of the influence of transient low temperature exposure was clarified at least for low neutron dose at around 1O24n/m*. The same scheme of irradiation as the present one is strongly desired for higher dose irradiation, especially finding the effects of irradiation of many reactor cydes. Discussion and negotiation to utilize JOY0 (a fast breeder reactor in PNC (Power Reactor and Nuclear Fuel Development Corporation)) is in progress. (2) In order to obtain a confidence in the differences and similarities between fission and fusion neutron irradiations, experimental information over a wide range of irradiation doses and damage rates is required. For this purpose, a multi-section and multi-division irradiation rig is now under construction for JMTR irradiation, in which each part of the sample assembly can be removed from the reactor at any desired time during reactor operation.

(1) The necessity of the elimination of the exposure to neutrons at lower temperatures during start-up and shut-down of a reactor was confirmed by the comparison of ~crost~ct~es introduced by irradiations with conventional and improved temperature control. (2) Only several percent of exposure to neutrons at lower temperature was found to result in one hundred percent difference in the final defect structures in some cases, while it causes only a slight difference in others. Generally speaking, the differences between conventional and improved control are greater at higher irradiation temperatures. (3) All the large and small differences between two modes of control were understood from the microstructure development mechanisms, i.e., from the position of the transient temperature relative to the temperature for nucleation and growth of each type of point defect cluster. (4) Experimental data on microstructure evolution in Ag, Au, Cu and Ni at a series of temperatures of 473, 573, 623 and 673 K have been accumulated at the fission neutron irradiation dose of about 5 X 10z3 n/m2 (E>l MeV).

6.2. Fission-fusion

correlation

(1) Fission neutron irradiation with improved controi in JMTR allowed a comparison with fusion neutron irradiation in RTNS-II to be made with confidence excluding experimental ambiguities. (2) Point defect cluster formation directly from large cascades, both vacancy and interstitial type, is greatly different between the two, but the difference is reasonably accepted when the difference in the primary recoil energy spectrum was taken into ~nsideration. (3) The extent of the reaction of freely migrating interstitials at high temperatures is greater in fission neutron irradiation as compared to fusion neutron irradiation when normalized on the basis of damage energy. (4) Fission neutron doses to produce the equivalent radiation-induced microstructures as by fusion neutrons were obtained. The factor, in the scaling of damage energy per atom, ranged widely from the value less than unity up to 30 depending on the kind of microstructure, the kind of material, the kind of microstructure and on the irradiation temperature.

progress

of research

Acknowledgements

This work was performed as a research program at the Oarai Branch for JMTR Utilization, Tohoku University. The authors are grateful to the members of Material Irradiation Division of JMTR, Oarai Research Establishment of Japan Atomic Energy Research Institute; without their cooperation this work could not be performed. The part on fusion neutron irradiation was performed under the Japan-USA Fusion Cooperation Program, Collaboration on RTNS-II Utilization, sponsored by the Ministry of ~ucation, Science and Culture, Japan.

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M. Kiritani et al. / Fission-fusion correlation by fission reactor irradiation [8] M. Kiritani, T. Yoshiie and S. Kojima, J. Nucl. Mater. 141-143 (1986) 625. [9] M. Kiritani, Mater. Sci. Forum 15-18 (1987) 1023. [lo] Y. Shimomura, H. Fukushima, M.W. Guinan and M. Kiritani, J. Nucl. Mater. 141-143 (1986) 816. (111 R. Rauch, J. Peisl, A. Schmalzbauer and G. Wallner, J. Nucl. Mater. 168 (1989) 101. [12] M. Kiritani, Proc. Int. Conf. Fundamental Aspects of Radiation Damage in Metals, Gatlinburg, 1975, USERDA CONF-751006, p. 695. [13] M. Kiritani and H. Takata, J. Nucl. Mater. 69 & 70 (1978) 177. (141 S. Kojima, T. Yoshiie and M. Kiritani, J. Nucl. Mater. 155-157 (1988) 1249. [15] Y. Satoh, I. Ishida, T. Yoshiie and M. Kiritani, J. Nucl. Mater. 155-157 (1988) 443. [16] T. Yoshiie, Y. Satoh, H. Taoka, S. Kojima and M. Kiritani, J. Nucl. Mater. 155-157 (1988) 1098.

351

[17] C.H. Woo and B.N. Singh, Phys. Status Solidi B159 (1990) 609. [18] S. Kojima, Doctorate Thesis, Hokkaido University, 1988. (191 Y. Satoh, Doctorate Thesis, Hokkaido University, 1989. [20] JMTR Irradiation Handbook (Department of JMTR Project, Oarai Research Establishment, JAERI, 1987). [21] Y. Shimomura, R. Nishiguchi, P.A. Hahn, M.W. Guinan and M. Kiritani, unpublished. [22] C.M. Logan and E.W. Russell, University of California, Report UCRL-52903 (1976). (231 J. Lindhard, V. Nielsen, M. Scharff and P.V. Thomsen, Mat. Phys. Medd. 33 (1963) 1. [24] Y. Satoh, S. Kojima, T. Yoshiie and M. Kiritani, J. Nucl. Mater. (1990) in press. [25] H.L. Heinisch, Radiat. Eff. and Defects in Solids 113 (1990) 53. [26] M. Kiritani, J. Nucl. Mater. 169 (1989) 89.