Table Of ContentVOIDS FORMED BY
IRRADIATION OF
REACTOR MATERIALS
PROCEEDINGS of the
British Nuclear Energy Society
European Conference held at Reading University
on 24 and 25 March 1971.
Edited by Mr. S. F Pugh,
Mr. M.H.Loretto, Dr. D.I.R. Norris.
© The British Nuclear Energy Society 1969
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CONTENTS
PREFACE
The Phenomenon of Void Formation
The Mechanism of Void Formation by Irradiation
Report of Visit
SESSION ON VOIDS PRODUCED BY FAST NEUTRON IRRADIATION
Paper No, Author(s) Title
1 K.Q. Bagley, Fast Neutron Induced Voidage in
J.I* Bramman, Non-Fissile Metals and Alloys
C. Cawthorne
2 J.I. Bramman, Density Changes in Cladding
C, Cawthorne, Materials Irradiated in DFR
E.J. Fulton,
W.D.J* Sinclair
3 C. Cawthorne Electron Microscope Observations
E.J. Fulton of Voids in Cladding Materials
J.I. Bramman Irradiated in DFR
G.A.B. Linekar
R.M. Sharpe
4 P.J. Barton Effect of Stress During Irradia
P.H. Nutter tion on Void Formation in Nickel
5 J.D. Elen Some Electron Microscope
Observations on Voids in Vanadium,
Niobium and Molybdenum
Miles V. Levy Voids Formed by Irradiation of
J. Mathie Reactor Materials
MM. A. Risbet
R. Levy,J.P. Poirier
N.H. Packan The Effect of Neutron Dose on Void
Formation in High Purity Aluminium
SESSION ON THEORY OF VOID FORMATION
8 R. Builough The Theory of Void Growth in
R.C. Perrin Irradiated Materials
K. Malen, The Void Lattice in Molybdenum
R. Builough
10 A.J.E. Foreman The Temperature Range and Dose
Dependence for Void Growth in
Irradiated Materials
11 J.R. Willis The Interaction Between a Void
R. Builough and a Dislocation Loop
12 S.B. Fisher A Theoretical Model for Void
K.R. Williams Formation in the High Voltage
Microscope
(i)
CONTENTS CONTINUED
Page Number
SESSION ON VOIDS PRODUCED BY QUENCHING AND OTHER METHODS
Paper No. Author(s) Title
13 R.E. Smallman The Formation and Behaviour of
K.H. Westmacott Quenched-in Voids in Close
Packed Metals 159-176
14 H.L. Fraser Voids in the Intermetallic
M.H. Loretto Compound NiAl 177-182
H.E. Smallman
R.J. Wasilewski
15 E. Ruedl The Formation of Voids by
Quenching or Deformation of
Metals 183-190
SESSION ON THE PRODUCTION OF VOIDS BY ION BOMBARDMENT
16 R.S. Nelson Void Formation During Ion
D.J. Mazey Irradiation of Metals at
J. A. Hudson Elevated Temperatures 191-203
17 T.M. Williams The Temperature Dependence of
Void Swelling in Type 316
Austenitic Stainless Steel
Irradiated with 20 MeV C** Ions 205-212
18 J.A. Hudson Void Formation in Nickel,
D.J. Mazey 316 Steel and Nimonic PE16
++
R.S. Nelson During 20 MeV C Irradiation 215-221
19 G.P. Walters The Temperature Dependence of
Void Swelling in 18/8/Ti Steel 223-230
20 G.P. Walters The Effect of Cold Work on the
Swelling of Type 316 Steel
Irradiated with 20 MeV Carbon
Ions at 525°C 231-237
21 S.A. Manthorpe The Production of Voids in
S.N. Buckley Cobalt during Ion Bombardment
and Electron Irradiation 239-246
SESSION ON THE PRODUCTION OF VOIDS IN THE HVEM
22 D.I.R. Norris The Simulation of Fast Reactor
Irradiation Effects in the High
Voltage Electron Microscope 247-268
23 M.J. Makin The Production of Voids in
Copper by Electron Irradiation in
a 1 MV Electron Microscope 269-274
24 K. Urban Growth of Interstitial and Vacancy
Agglomerates in Nickel During
Electron Irradiation 275-282
25 J.T. Buswell The Growth of Voids in Stainless
Steel 283-291
(ii)
CONTENTS CONTINUED
Page Number
SESSION ON THE PRODUCTION OF VOIDS IN THE HVEM (CONTINUED)
Paper No. Author(s) Title
26 J.E. Harbottle The Influence of HVEM
Irradiation on Neutron
Induced Voids in Nickel 293-300
SESSION ON THE ENGINEERING IMPLICATIONS OF VOID FORMATION
27 J.F.W, Bishop The Significance of Fast
Neutron Induced Voids in the
Design and Performance of
Fast Reactors 301-318
WRITTEN CONTRIBUTIONS TO THE EVENING DISCUSSION ON
NUCLEATION OF VOIDS
28 D, Kaletta The Atomic Interaction
Potential for Nickel 319-322
29 B.L. Eyre A Comparison of the Damage
J.H. Evans Structure in Fast Reactor
and Accelerator Irradiated
Molybdenum 323-331
30 J.E. Harbottle The Role of Gas Atoms in
Nucleating Voids 333
31 H• Migge Knock-on of Gas Atoms Into
Materials During Irradiation
in a Gas Atmosphere 335-336
32 U» Bergenlid Some Observations of Thermal
Release of Helium From
Irradiated Nickel 337-338
33 I.R. Birss A New Source of Helium in
W.E. Ellis Cladding Materials 339-345
SUMMARIES OF DISCUSSIONS 347-358
COMMITTEES 359
LIST OF DELEGATES 360-365
(iii)
VOIDS FORMED BY IRRADIATION OF REACTOR MATERIALS
PREFACE
The British Nuclear Energy Society organized a two day European Conference
on the subject of Voids formed by Irradiation of Reactor Materials, which
took place at the University of Reading on March 24th and 25th 1971* 126
delegates attended the conference including about 40 from outside the U.K.
Papers presented at the conference are reproduced in this volume, together
with summaries of the discussions that took place. A list of delegates is
appended. For readers who are not familiar with the subject an introduction
has been provided which summarizes the phenomenon and mechanism of void
formation* The conference was arranged in sessions each devoted to a
specific aspect of void formation..
The main technological interest in void formation arises from its incidence
in structural materials exposed in the cores of fast reactors and other high
flux reactors, where bombardment by fast neutrons gives rise to the forma
tion of helium by transmutation and displaced atoms* The first session is
therefore devoted to presentation of the data on production of voids by fast
neutron bombardment* Much of the present experimental work on void formation
is however undertaken using charged particle irradiation, and to relate the
findings of this work to the effects produced by fast neutron irradiation
it is necessary to draw considerably on the quantitative theories of void
formation. The session devoted to theory is therefore second, followed by
the sessions on void formation by other techniques such as quenching and
oxidation, ion bombardment and electron irradiation*
Each session starts with a review paper establishing the present state of
knowledge in that particular area, followed by a series of short original
contributions in the same area. The discussion of each session is presented
immediately after that session in the proceedings* Since all the electron
irradiation experiments have been performed using high voltage electron
microscopes, which have the added advantage of allowing observation of the
specimen during irradiation, these experiments have been allotted a separate
session*
(iv)
THE PHENOMENON OF VOID FORMATION
The phenomenon of void formation in cladding and other structural materials
was discovered at a late stage in reactor technology. The first prediction
that such a phenomenon might occur was made in 1959 in a paper by Foreman
9
Greenwood and Rimmer of A.E.R.E. Harwell, but it was not until 1967 that
Cawthorne and Fulton, of the Dounreay Experimental Research Establishment,
published the first paper in the literature reporting the observation of
voids in stainless steels irradiated in the Dounreay Fast Reactor. The
phenomenon was discovered at this late stage because very high fast neutron
doses are required to form an observable quantity of voids in those
structural materials normally exposed in the cores of nuclear reactors.
In some pure metals voids appear at total fast neutron doses one or two
orders of magnitude lower. The high fast neutron dose is in itself not a
sufficient condition for void formation; only in a narrow temperature range
of about 200 to 300°C in width are voids formed. It is interesting to note
that in this same temperature range other interesting phenomena caused by
radiation induced diffusion also occur. These phenomena are collectively
caused by the production of vacancies by irradiation followed by thermal
migration. The temperature range for these phenomena is restricted
therefore to the one in which vacancies are mobile but where the concentra
tion of vacancies formed thermally is very low and is significantly less
than the concentration of vacancies built up by irradiation. Other radia
tion induced diffusion phenomena include structural changes in alloys such
as order/disorder or precipitation reactions and radiation induced creep.
The temperature range is different for different materials and for annealed
stainless steels in a fast reactor the peak void formation rate occurs at
about 525°C. Volume increases of up to 15% can occur in some materials at
high doses so there is clearly a need to chose reactor structure materials
that are resistant to void formation, and to have a detailed knowledge of
the incidence of voids in reactor materials for design purposes.
THE MECHANISM OF VOID FORMATION BY IRRADIATION
The main factors in void formation are the production of displaced atoms by
radiation and the subsequent migration of the vacant sites and interstitial
atoms to sinks. The random migration of the point defects would cause an
equal number of interstitials and vacancies to be annihilated at any
particular sink. Calculations have shown however that the elastic inter
action between interstitials and dislocations causes an attraction which
(v)
imposes a slight drift on the movement of interstitials to dislocations.
Any other structural feature which has a long range stress field is likely
to impose a preferential drift on interstitials. The corresponding inter
action with a vacancy is considerably less and hence there is a small excess
in the numbers of interstitials reaching dislocations compared with the
number of vacancies. Experiment confirms that during void formation there
is a corresponding formation of interstitial loops which grow by capturing
an excess of interstitials. Thus an excess of vacancies is left and
providing these are mobile they can cause the growth of voids.
Vacancies do not normally form three dimensional clusters but rather form
flat mono-atomic layers which collapse into rings of edge dislocation
together with a stacking fault. Collapse of the cluster removes most of the
volume increase except for the very small effect of the dislocation loop.
Calculations indicate that the three dimensional arrangement of vacancies in
clusters too small to be visible in the electron microscope also would not
be stable and would collapse into a dislocation loop. It is therefore
supposed that something is required to stabilize the void nucleus* For
example, a few inert gas atoms would quite readily prevent the collapse of
a three dimensional arrangement of vacancies and would provide a surface in
the material having effectively the same free energy as that of a free
surface. It would therefore be energetically favourable for other vacancies
to condense on such a complex nucleus and continue the growth in the three
dimensional geometry of typical voids.
Since the vacancies formed by irradiation are less mobile than the
interstitials, the temperature variation of void formation is largely
dominated by vacancy formation rate and vacancy mobility. Void formation
decreases at temperatures above the peak because the concentration of
vacancies arising from thermal activation becomes comparable with that caused
by irradiation and eventually exceeds it; also voids can lose vacancies by
thermal evaporation. The void formation rate falls off at temperatures
below the peak because the decrease in vacancy mobility results in a higher
instantaneous vacancy concentration in the lattice and hence a higher rate
of annihilation of the fast moving interstitials by vacancy-interstitial
recombination. If there were no preferential sinks such as dislocations
present in the structure to impose a drift on the interstitials, then no
void formation would be expected since the vacancy supersaturation in the
(vi)
lattice would not arise. If there is an extremely high dislocation density
present then the dislocations compete with voids as sinks for vacancies and
swelling is greatly reduced. There is therefore an optimum intermediate
dislocation density when the rate of void formation is a maximum. Any sinks
introduced into the structure which show no preference for a particular type
of defect will cause an increase in vacancy interstitial annihilation and
hence a decrease in the rate of void formation.
VISIT TO A.E.R.E. HARWELL
About 40 delegates visited Harwell for the morning of Friday the 26th March
and of these five delegates chose to spend the morning discussing the theory
of void formation. Some of this time was spent discussing void nucleation
and strongly reached the conclusion that the collision cascades were not
an important feature in void nucleation. The remainder, in four parties,
saw in turn five separate topics. Dr. M.J. Makin showed the 1MV A.E.I,
electron microscope in operation, and also a display of the results of
studies of void formation using the high voltage microscope. B.L. Evre
and his Group discussed their findings on void formation in the refractory
metals, particularly in molybdenum, where the temperature range for void
formation is much higher than that in steels. Dr. R.S. Nelson and his
Group showed the heavy ion accelerator coupled to an electron microscope
for studying radiation damage in situ and equipment for studying radiation
creep using ion bombardment. Dr. D.R. Harries and his Group showed results
obtained using reactor irradiation for void production. Finally,
Mr. JmH. Worth and his team showed the target equipment and discussed
techniques for irradiating specimens in the Variable Energy Cyclotron.
(vii)
