k-60884
ACCELERATOR-DRIVEN
ASSEMBLY
FOR
PLUTONIUM
TRANSFORMATION
(ADAPT)
Gregory
J.
Van Tuyle, Michael Todosow, James Powell, and Donald Schweitzer
Brookhaven National Laboratory
Department of Advanced Technology
Upton,
New
York
11973
(5
16) 282-7960
Abstract
A
particle accelerator-driven spallation
target
and corresponding blanket region are
proposed
for
the ultimate disposition of weapons-grade plutonium being retired fiom
excess
nuclear
weapons
in
the
US.
and
Rwsia.
The highly
fissile
plutonium is
contained
within
.25
to
.5
em
diameter
silicon-
carbide coated graphite beads, which are cooled by helium,
within
the slightly
subcritical
bW
region.
Major
a&mtaps
include
very
high
0116p8ss
bumup
(over
WA),
a
high
integrity
waste
form
(the
coated
beads),
and operation
in
a
subcritical mode, thereby minimizing the
vutnerability
to
the
positive reactivity feedbacks often associated
with
plutonium hel.
INTRODUCTIO
N
Of the technology options currently under discussion for possible disposition
of
the
highly
fissile
plutonium being retired fiom nuclear weapons, the large particle accelerator
offers
some
potentid
advantages
in
comparison to better-known technologies such
as
use
as
&el
in
nuclear
reactors
or
vitrification
and
burial.
For
example, one
can
hope
to
hion
all
of
the plutonium
safely
in
one
or
two
accefeaatodriven devices
without
the
creation
of
additional waste actinides, and to leave
the
fission
products
in
a
form
suitable for safe disposal
within
a few years. Further,
owing
to
the
large
amount
of fission
emqy
deased
by
the plutonium, the dewice
can
be
a
net
power generator, i.e., the power
generation
can
greatly exceed
that
required
to
run the accelerator.
A
suitable high-power particle accelerator (a
Linac)
has
been designed for
the
Meraior
Production
of
Tritium
(APT),
and
can
be
assumed
applicable (with only minor
modifications)
to
the
plutonium disposition mission (Todosow,
et
al.,
1993).
Sily, work done on the
APT
target
technologies
has
been significant, and provides a basis for initial
target
work for the plutonium
mission (Van
'Fa@e$g+
al.,
1993).
BNL
is
proposing to use the
APT
Spallation
Induced
Lithium
Conversion
(S&C-<-=Zarget
neutron source design,
with
a plutonium-graphite &el
form
in
the
subcritical blanket r&on based on several
years
of
work on the Particle Bed Reactor
(PBR).
The
components
of
the technology proposed by BNL are largely
in
hand,
although
the
proposed
integration
of
the technologies
is
new and unique. We propose to name the
target
technology
"ADAPT",
for Accelerator-Driven Assembly
for
Plutonium Transformation.
THE
ADAPT
TARGET
TECHNOLOGY
The
ADAPT
target
technology
is
based on a neutron spallation target (the source), which converts
each high-energy proton
(-1-3
GeV)
into approximately
20
(lower energy) neutrons, and
a
blanket
containing plutonium
in
a subcritical configuration. The reference source
is
based on the
APT
SILC
Target
source
region,
and
is
composed
of
an
array of aluminum pressure tubes containing aluminum-
clad lead
pins
(1-
diameter)
cooled
by
heavy
water.
Alternative source concepts are also possible,
e.g., tungsten cooled by helium gas, which would allow much higher temperatures
than
those
allowable
in
the
SILC
source. The challenge for
this
alternative source
is
to overcome
the
large
neutron-capture
cross
section of tungsten by using a high-leakage geometry and a hard neutronic
spectrum.
The
ADAPT
blanket region
is
based on
a
fuel
form
that
offers
two
major desirable attributes
in
the
proposed applications:
it
can
be
run
to very
high
bumups (approaching
lW!),
and
is
in
a
form
where
direct disposal
(after
cooldown) is a genuine option.
In
the
original
application in the
PBR,
small
fuel particles
(0.5-mm
diameter) produced
high
power densities and operated
at
very
high
temperatures (Ludewig,
et
al.,
1994).
In
the proposed plutonium disposition application,
the
operating
emironmeat
is
far
less
challenging,
and
larger
&el
beads
(0.5-
diameter) are proposed.
The beads
are
contained between
two
porous
"fits"
within
&el
elements,
with
the
helium
coolant
passing
downward
through
the outer
annular
region in the clement,
radially
inward through
the
fiits
and
beds
of
fuel
beads,
and then upward through the
coolant
duct
in
the
center
of the
fuel
dement.
The subcritical blanket is composed
of
about
50
such
&el
elements, and contains about
300
kg
of
plutonium.
It
is
expected
that
in a single
fbll
power
(3000
to
3600
MWt)
Unit
about
25
kg
of
plutonium would be fissioned each week
With the low inventory and
high
burnup, the
first
simcant
challenge
is
to
develop
a
practical
reloading
scheme.
Here,
theuse
offuesbeads
is
amajor
advantage
in
that
one
could reload
or
shuffle
the beads weekly, possibly
during
periods
when the accelerator would
be
down for
maintenance.
However, the working goal
is to
extend the period between reload/shufEhg
to
besween
3
d
6
weeks, ifpossiile. Large
reactivity
swings
due
to
Pu
burmap are
expected,
so
a scheme for
wing
hel
shuag, variable plutonium
loading,
burnable
poisons,
control rods, dor variable
accelerator
current
to compensate must be worked out.
It
is
important
to
realize, however,
that
this
will
be
an
optimization effort, because the options
are
clearly available
-
one simply
has
to
find
the
most
practical means
of
compensating for the reactivity swings.
SAFETYCO
NSIDERATIONS
Another
keysq
_<_
-_
is
safety
and the proposed machine would have some attractive
safety
characteristics, &i&Eng subcritical operation (compensates for some dif€idt plutonium reactivity
.
characteristics~&&@nall inventories
of
plutonium and fission products. However, the blanket is
essentially
a
hi@Ggw&density
subcritical
reactor, and in
many
respects the
safety
requirements
will
be those needed for large nuclear reactors.
In
the
case
of
emergency coolant
systems
and
containment requirements, the similarities
will
be
strong.
In
the reactivity
control
areas,
however,
there
will
be major differences.
In
particular, by remaining subcritical, one
gains
the ability
to
shut
down the machine quickly by tripping the proton beam. The
key
question
will
be how far one
must
stay subcritical, which
will
be driven by the maximum credible reactivity- insertion accident. While
it
is
easy
to
develop
this
philosophy,
it
is
fhr
more difEcult
to
assess various potential accidents
at
this
time,
so
the
maximum
acceptableIC,may remainuncertainuntil a
conceptual
design
can
be
evaluated
in
more detail.
This
has
the unfortunate impact of leaving the optimal accelerator current and
energy
DISCLAIMER
Portions of this document may be illegible
in
electronic image products. Images are
produced from the best available original
document.
undetermined,
because the proton beam energy and the &essentially determine the blanket power
level, and the power level determines the plutonium fission rate.
WASTE
STREAM
Given the "waste" disposal
mission,
an
essential question
is
what is the anticipated waste
stream
from the
ADAPT
target. It is currently believed the waste form
will
be
nearly ideal for long term
storage
in
repositories
(Lotts,
et
al.,
1992),
but
a
more
careful
evaluation
is
necessary.
Because
ofthe
low plutonium
loading
in
the
porous
graphite
matrix,
it
is expected
that
fission
gasses
accumulated
through
ahnost
complete bumup
of
the
Pu
inventory
could
be
accommodated
within
the
silicon-carbine
and
perhaps
pyrolytic carbon
coated
beads.
While the anticipated
gas
pressures would
be within the capability
of
the
coatings
if
unirmht
ed,
it remains to
be
demonstrated
that
fidly
irradiated coatings could accommodate the
gas
pressure
at
lW!
burnup.
If
this
can
be
demonstrated, the next step
will
be to
show
the
irradiated
beads
@om
as
well
in
leach
testing
as
prior
testing
would
suggest.
Once
the
high fission product retention
has
been
confhnexi,
many
spent
beads would be imbedded
in
tar
and
placed
in
a
storage canister for the ultimate
esnpkement
in
a
waste repository.
TEIE
ERBIUM
OUES
TION
The
primary rationale for
operating
the
ADAPT
Target
in
subcritical
mode,
as
an
accelerator
target
rather
than
as
a reactor,
is
concern
over positive reactivity
feedbacks
in
highly
fissile plutonium,
The
tendency
of
plutonium
to
fission more effectively
in
a
harder neutronic
spectrum
raises
stabii
and
control
issues,
and
introduces the possibfity
of
power
excursions.
Subcritical
operation
provides
some
extra
margins
and
options for deaiiq with
this
potential threat, but
at
the
cost
of
considerable
additional complexity.
Erbii
has
been
proposedud
as
a
burnable
poison
in
some
reactor designs,
and
has
a
potentiauy
usefuj
tendency
to
capnue
neutrons
in
the
"resonance" energy range.
This
introduces
the
possii
of
using
Erbium
to
counter
the
plutonium preference
for
fhter
neutrons.
That
is,
by
using
enough
erbium
and
an
appropriate
neutronic
spectrum,
a
designteammaybe
able
to
obtain
negative reactivity
feedbacks.
If
this
could be achieved throughout the bumup cycle,
as
the plutonium
and
erbium
inventories change, the advantages of subcritical operation would be greatly
diminished,
if
not
analyses
required to determine the viability
of
the erbium option are very
extensive.
At
the very least, we
expect
to be able to reduce the reactivity feedback issue
using
erbium, which would allow operation closer
to
criticality.
"E
END-OF-LIFE
ISSUE
The
other
driver for subcritical operation
is
the nddesire
to
fission essentially
all
of
the weapons
plutonium. Obviously, at some point the remaining
mass
of
plutonium would be less
than
that
required
for
criticality,
and
one
could
then use an accelerator to push the subcritical pile to complete
burnup.
This
has
been
proposed by
GA
for the
HTGR
plutonium disposition option (Baxter,
A.
M.,
et
al.,
1993).
Alternate
options
would include leaving the balance
of
plutonium at moderately
high
burnup levels,
or using a
&.of
uranium isotopes include U-233 and/or U-235 to supplement the remaining
weapons plutonium.
SUMMARY
BNL is currently evaluating
an
accelerator driven
subcritical
target,
designated ABC/ADAPT,
as
an
uttimate
disposition technology for
weapons
plutonium. The
target
is based on a
fiels
technology
that
was
developed for the particle
bed
reactor, and
that
&el provides excellent characteristics
regarding
high
burnup and
a
high
integrity waste
form.
This
work was performed
under
the
auspices
of
the
U.S. Department
of
Energy.
References
Todosow,
M.
,
et
al.,
(1993) "Nuclear Charactenstics
of
an
Accelerator-Driven Target-Blanket
System", Trans
ANS,
69,424, November 1993.
Van
Tuyle,
G.
J.,
et
d.,
(1993) "Topical
Report
on
a
Preconceptual
Design
for
the
Spallation-Induced
Lithium Conversion (SILC) Target for
the
Accelerator
Production
of
Tritium",
Brookhaven
National
Laboratory
Report,
BNL-52401, September 30,1993.
Ludewig,
H.,
et
at.,
(1994) "Design
of
Particle Bed
Reactors
for
the
Space
Nuclear
Thermal
Propulsion Program", Brookhaven National Laboratory
Report,
BNL-60306,
May
1994.
Lotts,
A
L.,
et
al.,
(1992)
"Options
for Treating High-Tempefature
Gas-Cooled
Reactor
Fuel for
Repository
Disposal",
Oak
Ridge National
Laboratory
Report,
ORNVIU-12027, UC-522,
UC-
810, UC-811,
February
1992.
Baxter,
A
M.,
&*-(1993) "Cornbig
an
Accelerator
and
a
Gas
Turbine
Modular
Helium
Reactor for
N$ar-X&al
Destruction of Weapons Grade Plutonium", Presented
at
the
ADTT
Conference,
Las
Vegas,
NV,
July 1994.
DISCLAIMER
This report was prepared
as
an account of work sponsored by an agency of the United States
Government. Neither the United States Government nor any agency thereof,
nor
any
of
their
employees, makes any warranty, express
or
implied,
or
assumes any legal liability
or
responsi-
bility for the accuracy, completeness, or usefulness of any information, apparatus, product,
or
process disclosed,
or
represents that
its
use would not infringe privately owned rights. Refer-
ence herein to any specific commercial product, process,
or
service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or
imply
its
endorsement, recom-
mendation,
or
favoring by the United States Government
or
any agency thereof. The views
and opinions of authors expressed herein
do
not necessarily
state
or
reflect those of the
United States Government
or
any
agency thereof.