Leachability of Zircon as a Function of Alpha Dose
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LEACHABILITY OF ZIRCON AS A FUNCTION OF ALPHA DOSE RODNEY C. EWING+, RICHARD F. HAAKER+ AND WERNER LUTZE+ +Department of Geology, University of New Mexico, Albuquerque, NM 87131 USA ++Hahn-Meitner Institute, Glienicker Strasse 100, 1000 Berlin 39, Federal Republic of Germany
ABSTRACT The variation
zircons
the leachability of naturally occurring
(ZrSiO4 )
0
an increase of one order
the range of 1016 to 1018 alphas/gm there is
doses in
the weight percent of zircon that is
of magnitude in in
in
a function of total alpha dose has been measured at 87 C. For calculated
as
dissolved and an increase
leach rate of the zircon 2.9 x 10-8 to 2.3 x 10-7 gm/cm2 day.
the
metamict
(dose >
specimens 2
10-6 gm/cm
1019 alphas/gm)
Totally
have leach rates as high as 1.8 x
day.
INTRODUCTION The
effect
radiation damage on the leach rates of proposed high level
of
radioactive
waste
predictions
of
compositionally
leaching
a
factor
behavior
can
that be
must made.
it
report the variations in metamict
zircons
Pb
known
natural
Since
isotopic
zircon
is
a
crystalline
The purpose of this study is
the leach rate of crystalline,
for which age
long term
before
a convenient model for which the effect of radiation
is
on leach rates can easily be measured.
damage
be
simple and relatively conmion phase which occurs in
and metamict forms,
and
is
forms
the alpha-dose
determinations,
or
to
partially metamict and
has been calculated based on U,Th estimated
on
the
basis
of
x-ray
diffraction analysis and density determinations. There has been considerable discussion and controversy concerning radiation effects on the leachability of minerals and borosilicate glass and crystalline ceramic
radioactive waste forms.
altered,
such phases
that
Ringwood8
1,2,3,4,5 Since metamict minerals are often
brittle and glassy, some mineralogists have assumed 6 7 less resistant than their crystalline counterparts. ,
microfractured,
citing
are
the work
9
of Pidgeon and others '
1 0
has
suggested
that
in
phases susceptible to radiation damage, such as zircon, actinides would remain "tightly bound" even in the metamict state. Further work has shown that naturally occurring Pb
and that there is
alpha in
zirconolites only
dose. 1112 Roy and Vance
a crystalline
have behaved
a slight increase
as closed systems for U, Th and in
U leach rates with increased
13 have maintained that the solubility changes
phase due to radiation effects are likely to be quite small.
390 14 In
contrast,
Fleischer
mica,
factor
metamict
of
discordance
that
and
the
is
documented
of
solubility of zircons in a mixture of 6 as a function of degree of metamictization.
increased
sulfuric acid
and
rates
dissolution of altered and
17
hydrofluoric
Increased
ages.
radiometric
are substantiated by the differential
zircon
Also well
Fleischer suggests
and natural glass.
quartz
such as zircon,
14
the
in
dissolution
have described the
phases
in
loss of
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