Ignition behavior and kinetics of oxidation of the reactor metals, uranium, zirconium, plutonium, and thorium, and binary alloys of each : a status report

208 Pages·1959·20.29 MB·English

by  J G Schnizlein

ANL-5974
I ! Reactors - General
(TID-4500, 14th Ed.)
AEC Research and
Development Report
ARGONNE NATIONAL LABORATORY
P. 0. Box 299
Lemont, Illinois
IGNITION BEHAVIOR AND KINETICS OF OXIDATION OF THE
REACTOR METALS, URANIUM, ZIRCONIUM, PLUTONIUM
AND THORIUM, AND BINARY ALLOYS OF EACH
z
A Status Report
4‘
1
J. G. Schnizlein, P. J. Pizzolato, H. A. Porte, J. D. Bingle,
D. F. Fischer, L. W. Mishler, and R. C. Vogel
Chemical Engine e ring Division
This document is
April, 1959 PUBLICLY RELEASABLE
3.7-
-
v/
Authorizing Official
b -5jl ’&
Date:
Operated by The University of Chicago
under
Contract No. W-31-109-eng-38
DISCLAIMER

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2
ABSTRACT
IGNITION BEHAVIOR AND KINETICS OF OXIDATION OF THE
REACTOR METALS, URANIUM, ZIRCONIUM, PLUTONIUM,
AND THORIUM, AND BINARY ALLOYS OF EACH
A Status Report
The importance of prevention of fires and explosions involving ura-
nium, zirconium, plutonium, and thorium, which are of particular interest
to the nuclear energy program, made imperative the study of their ignition
-
behavior and oxidation kinetics. Methods of measurements of ignition char
acteristics of uranium and zirconium were developed and used to determine
the effects of variables, such as surface preparation, metallurgical history,
specific area (sample size), additives to the metal, and oxygen content and
presence of moisture in the oxidizing gas.
The study of ignition characteristics was supported by study of the
-
effects of similar variables on the kinetics of oxidation of uranium and zir
conium and binary alloys of each. The oxidation of uranium always pro-
ceeded in two linear stages over the temperature range of 125 to 295 C at
pressures of 20, 50, 200, and 800 mm of oxygen. The temperature de-
pendences of both stages indicate activation energy dependent on pres-
;u1
sure. The presence of ten additive elements in uranium metal caused only
very small effects on the oxidation. The oxidation of zirconium was inde-
pendent of pressure and proceeded according to a cubic rate law over the
temperature range from 400 to 900 Cyw ith an activation energy of 42.7 kcal
per mole. For those additives soluble in zirconium at 700 C,the effects on
the initial cubic rate law for oxidation are explained in terms of valency
according to the Wagner-Hauffe Theory. For those additives insoluble in
alpha zirconium, no single theory is felt to be adequate. The breakaway
phenomena observed for many of the twenty alloys is explained in terms of
a 15 per cent deviation of the additive ionic radius from the ionic radius
of zirconium.
Surveys of the literature on the oxidation of plutonium and thorium
are presented in preparation for the study of their behavior.
3
TABLE OF CONTENTS
Page
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I INTRODUCTION 7
. . . . . . . . . . . . .
I1 IGNITION AND ISOTHERMAL OXIDATION OF URANIUM 10
. ..........................
A Some Uranium Ignition Incidents 10
. . . . . . . . . . . . . . . . . .
B Ignition Properties of Uranium and Its Alloys 10
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Apparatus and Materials 11
. ..........................
2 Burning-Curve Experiments 12
. ...........................
a Experimental Procedure 12
.
b Burning Curves and Ignition Temperatures in Oxygen of
............................
Uranium and its Alloys 15
. . . . . . . . . . . . . . . . . . .
3 Shielded-Ignition Experiments in Oxygen 23
. .....................
4 Specific Area Dependence in Oxygen 26
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a Pureuranium 26
....................
(1) Metallurgical History Influence 28
(2) Ignition of Bulk Uranium Metal by Foil Specimen
.........................
("Piggy-Back" Ignition) 29
. . . . .
(3) Extension of Specific Area Dependence to Metal Power 29
h
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b Alloys 30
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Gas Variables 31
.
a Preliminary Ignition Studies in Air and Nitrogen- Oxygen
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mixtures 31
. . . . . . . . . . . . . . .
b Burning Curves in Helium-Oxygen Mixtures 32
. . . . . . . . . . . . . .
c Burning Curves in Nitrogen-Oxygen Mistures 34
. ......................
d Maximum Burning Temperature 34
. . . . . . . .
e Specific Area Dependence and Additive Effects in Air 36
. ....................
f Moisture Content of Oxidizing Gas 37
. ....................
g Burning Curves in Carbon Dioxide 37
. .......................
6 Burning Propagation Rate Studies 38
. . . . . . . . . . . . . . . . .
C Isothermal Oxidation of Uranium and Its Alloys 39
. .................................
1 Literature Survey 39
. .............................
2 Apparatus and Materials 41
. ............................
a Volumetric Apparatus 41
. .................................
b Thermobalance 42
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c UraniumSamples 44
. .................................
3 Oxidation Kinetics 45
. .......................
a Effect of Surface Preparations 46
. . . . . . . . . . . . . . . . . .
(1) Metallographic Type Preparations 46
. . . . . . . . . . . . . . . . .
(2) Cathodic Vacuum Etch Preparation 48
4
TABLE OF CONTENTS @
Page c
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b Base.LineData 50
. . . . . . . . . . . . . . . . . . . . . . . .
(1) Temperature Dependence 51
. . . . . . . . . . . . . . . . . . . . . . . . . .
(a) First-Stage Rates 51
. . . . . . . . . . . . . . . . . . . . . . . . .
(b) Second-Stage Rates 53
.............................
(c) Break Weight 55
.................. ........
(2) Pressure Dependence 56
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
c Metallurgical Variables 58
.
d Comparison of the Data of This Investigation with That of
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D Cubicciotti 59
. . . . . . . . . . . . . . . . . . . . . . .
e Effects of Additives to Uranium 62
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
f Gasvariables 68
. . . . .
(1) Air. 0xygen.Nitrogen. and Oxygen-Helium Mixtures 68
. . . . . . . . . . . . . . . . .
(2) Moisture Content of Oxygen or Air 69
. .......................
4 Mechanism of Uranium Oxidation 71
. ....................
a Identification of Reaction Products 71
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b Marker Experiments 74 .t
\
. . . . . . . . . . . . . .
(1) Metallographically Determined Marker 74
. . . . . . . . . . . . . . . . . . . . . . . . . . . J
(2) Radioactive Marker 78
. . . . . . . . . . . . . . . . . . . . . . . . . .
c Hydridation after Oxidation 79
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d Electron Microscopy 80
. . . . . . . . . . . . . . . . . . . . . . . .
e Hot-Stage Optical Microscopy 85
. . . . . . . . . . .
I11 IGNITION AND ISOTHERMAL OXIDATION OF ZIRCONIUM 87
. . . . . . . . . . . . . . . . . . . . . . . . . .
A Some Zirconium Ignition Incidents 87
. . . . . . . . . . . . . . . . .
B Ignition Properties of Zirconium and Its Alloys 87
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Apparatus and Materials 87
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Ignition in Oxygen and Air 90
. . . . . . . . . . . . . . . . . . . . . . . .
a Effects of Surface Preparation 91
. .................. .....
b Dependence on Gas Flow Rate 92
. .....................
c .Effects of Additives to Zirconium 93
. ....................
d Moisture Content of Oxidizing Gas 93
. . . . . . . . . . . .
3 Ignition Temperature Dependence on Specific Area 93
. . . . . . .
4 Correlation of Ignition Temperatures with Isothermal Data 95
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Electric a1 Heating Ignition 96
. . . . . . . . . . . . . . . . . . . . . . . .
6 Burning Propagation Rate Studies 97
. . . . . . . . . . . . . . . .
C Isothermal Oxidation of Zirconium and Its Alloys 97
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Literature Survey 97
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Apparatus and Materials 100
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Experimental Results 101
. . . . . . . . . . . . . . . . . . . . . . . .
a Effects of Surface Preparation 101
. ..........................
b Temperature Dependence 103
c
.......... ...... ............. ...................... ......
5
TABLE OF CONTENTS
i Page
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
c. Pressure Dependence 106
. . . . . . . . . . . . . . . . . . . . . . . . . . .
d. Sample Shape Influence 108
....................
e. Effects of Additives to Zirconium 108
. . . . . . . . . . . . . . . . . . . . . . . . .
f . Oxidation in Air at 700 C. 123
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Discussion.. 123
. . . . . . . . . . . . . .
a. Literature and Theoretical Considerations 123
. . . . . . . . . . . . . . . . .
b. Correlation of Rate Data with Theory. 125
. . . . . . . . . . . . . . . . . . . . . . . . . . .
c. Breakaway Phenomena 127
. . . . . . . . . . . . . . . . . . . . . . . . . .
IV. THE OXIDATION OF PLUTONIUM 131
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. THE OXIDATION OF THORIUM 134
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. SUMMARY 137
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Uranium Ignition Studies. 137
.....................
B. Uranium Isothermal Oxidation Studies. 138
t C. Zirconium Ignition Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
....................
b D. Zirconium Isothermal Oxidation Studies 141
E. Comparisons of Oxidation and Ignition Behaviors of Uranium
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
and Zirconium 142
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ACKNOWLEDGEMENTS 144
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APPENDICIES 145
I. Fabrication, Analysis, and Metallurgy of Uranium
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
and Its Alloys 145
11. Oxidation, Nitridation, and Ignition Properties of
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
“Fissium” Metal 184
.......................
111. Oxidation of Zircaloy- 3 in Dry Air 189
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES 193
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIST OF FIGURES. 197
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIST OF TABLES 205
7
IGNITION BEHAVIOR AND KINETICS OF OXIDATION OF THE
REACTOR METALS, URANIUM, ZIRCONIUM, PLUTONIUM,
AND THORIUM, AND BINARY ALLOYS OF EACH
A Status Report
J. G. Schnizlein, PoJ . Pizzolato, H. A. Porte, J. D. Bingle,
D. F. Fischer, LoW . Mishler, and R. C. Vogel
I. INTRODUCTION
The over-all safety record of the nuclear energy program has been
better than that of most private industries. Ln 1956 the lost-time injury
frequency was better only for the communications industry of the 42 in-
dustries considered by the National Safety Council. However, 3 of the 8
fatalities experienced that year were associated with metal fires and
explosions, In 1955 nearly fifty per cent and in 1957 seventy per cent of
the property damage losses associated with the nuclear energy program
!!
resulted from spontaneous metal fires. The frequency of these ingitions
c
is great enough to constitute a serious hazard.(l) A study of the circum-
stances surrounding each ignition does not definitely indicate the causes.
The ignitions have occasionally taken place in metal initially at room
temperature and even in apparently bulk metal.
The history of such incidents and the increasing number of nuclear
reactors, with the concomitant processing and handling of metals (often
radioactive) such as uranium, zirconium, thorium, and plutonium, made it
imperative to undertake a basic laboratory investigation. The general ap-
approach was to obtain, first, an understanding of the isothermal oxida-
tion kinetics of the metals. Such an understanding is then most useful in
interpreting the more applied type of experiment which almost invariably
gives data more difficult to interpret. The work reported herein therefore
constitutes an endeavor to understand the factors which influence the pyro-
phoric nature of these metals. The study was initiated on uranium and
zirconium and will be continued on plutonium and thorium.
Two modes of attack have been followed. In a direct approach
several different types of ignition experiments have been developed in
order to study important factors such as metal purity, particular alloying
additives, and specific area, as well as the effects of composition of the
oxidizing gas and of gas impurities. In the second approach, since funda-
mental studies are ultimately more enduring and ignition is basically
oxidation, the fundamental kinetics of isothermal oxidations have been
inve s t igat ed.
8
r
The reasoning behind the study of the kinetics of isothermal oxida-
tion in a study of metal ignition is the following: In a much oversimplified
manner the mechanism of the spontaneous ignition of a metal can be
hypothesized. Consider a bulk piece of metal which has certain surface
irregularities leading to large surface area. The metal irregularities will
eventually suffer accelerating oxidation or ignition if the rate of liberation
of heat due to metal oxidation is greater than the rate of loss of heat. When
this occurs, the bulk metal, in addition to the surface irregularities, may
be completely oxidized.
That such a mechanism is responsible for at least one type of spon-
taneous ignition was rather well demonstrated by an experiment in which a
.*
thermocouple was inserted in a plutonium-uranium alloy sample When
this sample was exposed to laboratory air, the thermocouple indicated that
the sample was very gradually becoming warmer. At a critical point after
42 hours the sample suddenly ignited. In the case of this sample the igni-
tion problem was exaggerated because the material was metallurgically
unsound. However, the slow steady increase of temperature did indicate
that heat was being released by oxidation at a greater rate than heat could
be lost from the sample.
1
Let us consider the general factors influencing the rates of libera- d
tion and loss of heat.
Step Possible Factors Influencing Rate
Metal surface of unit area (1) Metal purity
oxidizing in air. (2) Metallurgical condition
(grain size, strains, etc.)
(3) Temperature
(4) Time
(5) Gas Composition
(6) Type of oxide film formed
(protective or not)
Loss of heat of reaction by (1) Thermql conductivity of metal
conduction to the (2) Thermal conductivity of oxide
.
surroundings coating
(3) Cross-sectional area at right
angles to direction of heat
flow
(4) Temperature gradient
In order to understand the causes of metal ignitions it is necessary to iso-
late the variables and study them separately. If one studies the rate of
*This experiment was performed by L. Kelman of the Metallurgy
.
Division, Ar gonne National Lab0 r at ory
3
oxidation of polished flat specimens under isothermal conditions, it is pos -
sible to study the following variables : metal purity, metallurgical conditions,
temperature, gas composition, and type of oxide film, without having the
difficulty of dealing with a variable surface area. Another approach is to
prepare powders, perhaps by hydriding and dehydriding, and then to meas-
ure the surface area. This is an approach which has been only partially
explored. Disadvantages of the approach are:
(1) the requirement of measurements of surface area,
(2) the difficulty of obtaining particles of uniform size,
(3) the difficulty of obtaining a uniform temperature in a
sample of powder, and
(4) the difficulty of measuring the temperature of a powder.
One phase of the study was, therefore, a consideration of the factors
influencing the rate of oxidation of polished pieces of uranium, zirconium,
thorium, and plutonium. Rather modest differences in rate could make the
difference between the usual slow corrosion and spontaneous ignition (ox-
idation too rapid for heat loss to maintain isothermal conditions); there-
fore, it is appropriate to investigate the causes of these differences.
Conditions which accelerate the oxidation will probably lower the
ignition temperature and increase the possibility of spontaneous ignitions.
For example, it has been shown(2) that a direct relationship exists between
-
oxidation rate and ignition temperature in the magnesium-aluminum sys
tem. The results reported in this document indicate that a similar relation
exists for zirconium and its alloys. Using such a correlation, it should be
possible to predict the likelihood of spontaneous ignition of, at least,
zi r conium.
The discussion in the preceding paragraphs is, by implication at
least, directed mainly at situations in which the metal ignites spontaneously,
without outside stimuli such as heat, radiation, shock waves, etc. It is ex-
pected that conditions which lead to spontaneous ignitions of metals will
also make these systems more susceptible to ignition stimuli of other
types. This hypothesis will need to be investigated thoroughly.