<s>
A	O
radioisotope	B-Application
thermoelectric	I-Application
generator	I-Application
(	O
RTG	O
,	O
RITEG	B-Application
)	O
,	O
sometimes	O
referred	O
to	O
as	O
a	O
radioisotope	O
power	O
system	O
(	O
RPS	O
)	O
,	O
is	O
a	O
type	O
of	O
nuclear	B-Application
battery	I-Application
that	O
uses	O
an	O
array	O
of	O
thermocouples	O
to	O
convert	O
the	O
heat	O
released	O
by	O
the	O
decay	O
of	O
a	O
suitable	O
radioactive	O
material	O
into	O
electricity	O
by	O
the	O
Seebeck	O
effect	O
.	O
</s>
<s>
RTGs	O
have	O
been	O
used	O
as	O
power	O
sources	O
in	O
satellites	B-Application
,	O
space	O
probes	O
,	O
and	O
uncrewed	O
remote	O
facilities	O
such	O
as	O
a	O
series	O
of	O
lighthouses	B-Application
built	O
by	O
the	O
Soviet	O
Union	O
inside	O
the	O
Arctic	O
Circle	O
.	O
</s>
<s>
The	O
first	O
RTG	O
launched	O
into	O
space	O
by	O
the	O
United	O
States	O
was	O
SNAP	B-Application
3B	I-Application
in	O
1961	O
powered	O
by	O
96	O
grams	O
of	O
plutonium-238	O
metal	O
,	O
aboard	O
the	O
Navy	O
Transit	O
4A	O
spacecraft	O
.	O
</s>
<s>
Systems	B-Application
for	I-Application
Nuclear	I-Application
Auxiliary	I-Application
Power	I-Application
(	O
SNAP	O
)	O
units	O
were	O
used	O
for	O
probes	O
that	O
traveled	O
far	O
from	O
the	O
Sun	O
rendering	O
solar	O
panels	O
impractical	O
.	O
</s>
<s>
RTGs	O
were	O
also	O
used	O
for	O
the	O
Nimbus	O
,	O
Transit	O
and	O
LES	O
satellites	B-Application
.	O
</s>
<s>
In	O
addition	O
to	O
spacecraft	O
,	O
the	O
Soviet	O
Union	O
built	O
1007	O
RTGs	O
to	O
power	O
uncrewed	O
lighthouses	B-Application
and	O
navigation	O
beacons	B-Protocol
on	O
the	O
Soviet	O
arctic	O
coast	O
by	O
the	O
late	O
1980s	O
.	O
</s>
<s>
The	O
lighthouses	B-Application
were	O
not	O
maintained	O
for	O
many	O
years	O
after	O
the	O
dissolution	O
of	O
the	O
Soviet	O
Union	O
in	O
1991	O
.	O
</s>
<s>
In	O
1996	O
,	O
a	O
project	O
was	O
begun	O
by	O
Russian	O
and	O
international	O
supporters	O
to	O
decommission	O
the	O
RTGs	O
in	O
the	O
lighthouses	B-Application
,	O
and	O
by	O
2021	O
,	O
all	O
RTGs	O
are	O
now	O
removed	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
small	O
"	O
plutonium	O
cells	O
"	O
(	O
very	O
small	O
238Pu-powered	O
RTGs	O
)	O
were	O
used	O
in	O
implanted	O
heart	B-Device
pacemakers	I-Device
to	O
ensure	O
a	O
very	O
long	O
"	O
battery	O
life	O
"	O
.	O
</s>
<s>
The	O
Mound	O
Laboratory	O
Cardiac	O
Pacemaker	B-Device
program	O
began	O
on	O
1	O
June	O
1966	O
,	O
in	O
conjunction	O
with	O
NUMEC	O
.	O
</s>
<s>
Its	O
half-life	B-Application
must	O
be	O
long	O
enough	O
so	O
that	O
it	O
will	O
release	O
energy	O
at	O
a	O
relatively	O
constant	O
rate	O
for	O
a	O
reasonable	O
amount	O
of	O
time	O
.	O
</s>
<s>
The	O
amount	O
of	O
energy	O
released	O
per	O
time	O
(	O
power	O
)	O
of	O
a	O
given	O
quantity	O
is	O
inversely	O
proportional	O
to	O
half-life	B-Application
.	O
</s>
<s>
An	O
isotope	O
with	O
twice	O
the	O
half-life	B-Application
and	O
the	O
same	O
energy	O
per	O
decay	O
will	O
release	O
power	O
at	O
half	O
the	O
rate	O
per	O
mole	O
.	O
</s>
<s>
Typical	O
half-lives	B-Application
for	O
radioisotopes	O
used	O
in	O
RTGs	O
are	O
therefore	O
several	O
decades	O
,	O
although	O
isotopes	O
with	O
shorter	O
half-lives	B-Application
could	O
be	O
used	O
for	O
specialized	O
applications	O
.	O
</s>
<s>
As	O
Sr-90	O
,	O
Cs-137	O
and	O
other	O
lighter	O
radionuclides	O
cannot	O
maintain	O
a	O
nuclear	O
chain	B-Application
reaction	I-Application
under	O
any	O
circumstances	O
,	O
RTGs	O
of	O
arbitrary	O
size	O
and	O
power	O
could	O
be	O
assembled	O
from	O
them	O
if	O
enough	O
material	O
can	O
be	O
produced	O
.	O
</s>
<s>
Plutonium-238	O
has	O
a	O
half-life	B-Application
of	O
87.7	O
years	O
,	O
reasonable	O
power	O
density	O
of	O
0.57	O
watts	O
per	O
gram	O
,	O
</s>
<s>
This	O
lowering	O
of	O
the	O
neutron	O
emission	O
rate	O
of	O
PuO2	O
containing	O
normal	O
oxygen	O
by	O
a	O
factor	O
of	O
five	O
was	O
discovered	O
during	O
the	O
cardiac	O
pacemaker	B-Device
research	O
at	O
Mound	O
Laboratory	O
in	O
1966	O
,	O
due	O
in	O
part	O
to	O
the	O
Mound	O
Laboratory	O
's	O
experience	O
with	O
production	O
of	O
stable	O
isotopes	O
beginning	O
in	O
1960	O
.	O
</s>
<s>
While	O
its	O
half	B-Application
life	I-Application
of	O
28.8	O
years	O
is	O
much	O
shorter	O
than	O
that	O
of	O
238Pu	O
,	O
it	O
also	O
has	O
a	O
lower	O
decay	O
energy	O
with	O
a	O
power	O
density	O
of	O
0.46	O
watts	O
per	O
gram	O
.	O
</s>
<s>
This	O
isotope	O
provides	O
phenomenal	O
power	O
density	O
(	O
pure	O
210Po	O
emits	O
140	O
W/g	O
)	O
because	O
of	O
its	O
high	O
decay	O
rate	O
,	O
but	O
has	O
limited	O
use	O
because	O
of	O
its	O
very	O
short	O
half-life	B-Application
of	O
138	O
days	O
.	O
</s>
<s>
As	O
Po-210	O
is	O
a	O
pure	O
alpha-emitter	O
and	O
does	O
not	O
emit	O
significant	O
gamma	O
or	O
X-ray	B-Library
radiation	I-Library
,	O
the	O
shielding	O
requirements	O
are	O
also	O
low	O
as	O
for	O
Pu-238	O
.	O
</s>
<s>
While	O
the	O
short	O
half-life	B-Application
also	O
reduces	O
the	O
time	O
during	O
which	O
accidental	O
release	O
to	O
the	O
environment	O
is	O
a	O
concern	O
,	O
Polonium-210	O
is	O
extremely	O
radiotoxic	O
if	O
ingested	O
and	O
can	O
cause	O
significant	O
harm	O
even	O
in	O
chemically	O
inert	O
forms	O
,	O
which	O
pass	O
through	O
the	O
digestive	O
tract	O
as	O
a	O
"	O
foreign	O
object	O
"	O
.	O
</s>
<s>
A	O
common	O
route	O
of	O
production	O
(	O
whether	O
accidental	O
or	O
deliberate	O
)	O
is	O
neutron	O
irradiation	O
of	O
,	O
the	O
only	O
naturally	O
occurring	O
isotope	O
of	O
Bismuth	B-Algorithm
.	O
</s>
<s>
It	O
is	O
this	O
accidental	O
production	O
that	O
is	O
cited	O
as	O
an	O
argument	O
against	O
the	O
use	O
of	O
lead-bismuth	O
eutectic	O
as	O
a	O
coolant	O
in	O
liquid	O
metal	O
reactors	O
.	O
</s>
<s>
However	O
,	O
if	O
a	O
sufficient	O
demand	O
for	O
Polonium-210	O
exists	O
,	O
its	O
extraction	B-Application
could	O
be	O
worthwhile	O
similar	O
to	O
how	O
tritium	B-Language
is	O
economically	O
recovered	O
from	O
the	O
heavy	O
water	O
moderator	O
in	O
CANDUs	O
.	O
</s>
<s>
Although	O
241Am	O
has	O
a	O
half-life	B-Application
of	O
432	O
years	O
which	O
is	O
more	O
than	O
238Pu	O
and	O
could	O
hypothetically	O
power	O
a	O
device	O
for	O
centuries	O
,	O
missions	O
with	O
more	O
than	O
10	O
years	O
are	O
not	O
subject	O
of	O
the	O
research	O
until	O
2019	O
.	O
</s>
<s>
Compared	O
to	O
Plutonium-238	O
,	O
Curium-250	O
provides	O
about	O
a	O
quarter	O
of	O
the	O
power	O
density	O
,	O
but	O
100	O
times	O
the	O
half-life	B-Application
(	O
~	O
87	O
vs	O
~	O
9000	O
)	O
.	O
</s>
<s>
Most	O
RTGs	O
use	O
238Pu	O
,	O
which	O
decays	O
with	O
a	O
half-life	B-Application
of	O
87.7	O
years	O
.	O
</s>
<s>
NASA	O
has	O
developed	O
a	O
multi-mission	O
radioisotope	B-Application
thermoelectric	I-Application
generator	I-Application
(	O
MMRTG	O
)	O
in	O
which	O
the	O
thermocouples	O
would	O
be	O
made	O
of	O
skutterudite	O
,	O
a	O
cobalt	B-Algorithm
arsenide	O
(	O
CoAs3	O
)	O
,	O
which	O
can	O
function	O
with	O
a	O
smaller	O
temperature	O
difference	O
than	O
the	O
current	O
tellurium-based	O
designs	O
.	O
</s>
<s>
They	O
are	O
barely	O
useful	O
for	O
a	O
genuine	O
nuclear	O
weapon	O
,	O
but	O
still	O
can	O
serve	O
in	O
a	O
"	O
dirty	B-Application
bomb	I-Application
"	O
.	O
</s>
<s>
The	O
Soviet	O
Union	O
constructed	O
many	O
uncrewed	O
lighthouses	B-Application
and	O
navigation	O
beacons	B-Protocol
powered	O
by	O
RTGs	O
using	O
strontium-90	O
(	O
90Sr	O
)	O
.	O
</s>
<s>
The	O
launch	O
was	O
successful	O
and	O
Cassini	O
–	O
Huygens	O
reached	O
Saturn	B-Device
.	O
</s>
<s>
They	O
are	O
surrounded	O
by	O
a	O
layer	O
of	O
iridium	O
metal	O
and	O
encased	O
in	O
high-strength	O
graphite	B-Application
blocks	O
.	O
</s>
<s>
Surrounding	O
the	O
graphite	B-Application
blocks	O
is	O
an	O
aeroshell	O
,	O
designed	O
to	O
protect	O
the	O
entire	O
assembly	O
against	O
the	O
heat	O
of	O
reentering	O
the	O
Earth	O
's	O
atmosphere	O
.	O
</s>
<s>
The	O
plutonium-238	O
used	O
in	O
these	O
RTGs	O
has	O
a	O
half-life	B-Application
of	O
87.74	O
years	O
,	O
in	O
contrast	O
to	O
the	O
24,110	O
year	O
half-life	B-Application
of	O
plutonium-239	O
used	O
in	O
nuclear	O
weapons	O
and	O
reactors	O
.	O
</s>
<s>
A	O
consequence	O
of	O
the	O
shorter	O
half-life	B-Application
is	O
that	O
plutonium-238	O
is	O
about	O
275	O
times	O
more	O
radioactive	O
than	O
plutonium-239	O
(	O
i.e.	O
</s>
<s>
Particularly	O
at	O
risk	O
is	O
the	O
skeleton	B-Application
,	O
the	O
surface	O
of	O
which	O
is	O
likely	O
to	O
absorb	O
the	O
isotope	O
,	O
and	O
the	O
liver	O
,	O
where	O
the	O
isotope	O
will	O
collect	O
and	O
become	O
concentrated	O
.	O
</s>
<s>
A	O
launch	O
failure	O
on	O
21	O
April	O
1964	O
in	O
which	O
the	O
U.S.	O
Transit-5BN-3	O
navigation	O
satellite	B-Application
failed	O
to	O
achieve	O
orbit	O
and	O
burned	O
up	O
on	O
re-entry	O
north	O
of	O
Madagascar	B-Application
.	O
</s>
<s>
The	O
Nimbus	O
B-1	O
weather	O
satellite	B-Application
,	O
whose	O
launch	O
vehicle	O
was	O
deliberately	O
destroyed	O
shortly	O
after	O
launch	O
on	O
21	O
May	O
1968	O
because	O
of	O
erratic	O
trajectory	O
.	O
</s>
<s>
The	O
failure	O
of	O
the	O
Apollo	O
13	O
mission	O
in	O
April	O
1970	O
meant	O
that	O
the	O
Lunar	O
Module	O
reentered	O
the	O
atmosphere	O
carrying	O
an	O
RTG	O
and	O
burned	O
up	O
over	O
Fiji	B-Language
.	O
</s>
<s>
It	O
carried	O
a	O
SNAP-27	O
RTG	O
containing	O
of	O
plutonium	O
dioxide	O
in	O
a	O
graphite	B-Application
cask	O
on	O
the	O
lander	O
leg	O
which	O
survived	O
reentry	O
into	O
the	O
Earth	O
's	O
atmosphere	O
intact	O
,	O
as	O
it	O
was	O
designed	O
to	O
do	O
,	O
the	O
trajectory	O
being	O
arranged	O
so	O
that	O
it	O
would	O
plunge	O
into	O
6	O
–	O
9	O
kilometers	O
of	O
water	O
in	O
the	O
Tonga	O
trench	O
in	O
the	O
Pacific	O
Ocean	O
.	O
</s>
<s>
The	O
cask	O
is	O
expected	O
to	O
contain	O
the	O
fuel	O
for	O
at	O
least	O
10	O
half-lives	B-Application
(	O
i.e.	O
</s>
<s>
The	O
US	O
Department	O
of	O
Energy	O
has	O
conducted	O
seawater	O
tests	O
and	O
determined	O
that	O
the	O
graphite	B-Application
casing	O
,	O
which	O
was	O
designed	O
to	O
withstand	O
reentry	O
,	O
is	O
stable	O
and	O
no	O
release	O
of	O
plutonium	O
should	O
occur	O
.	O
</s>
<s>
Many	O
Beta-M	O
RTGs	O
produced	O
by	O
the	O
Soviet	O
Union	O
to	O
power	O
lighthouses	B-Application
and	O
beacons	B-Protocol
have	O
become	O
orphaned	O
sources	O
of	O
radiation	O
.	O
</s>
<s>
The	O
US	O
Department	O
of	O
Defense	O
cooperative	O
threat	O
reduction	O
program	O
has	O
expressed	O
concern	O
that	O
material	O
from	O
the	O
Beta-M	O
RTGs	O
can	O
be	O
used	O
by	O
terrorists	B-Device
to	O
construct	O
a	O
dirty	B-Application
bomb	I-Application
.	O
</s>
<s>
Nuclear	O
power	O
reactors	O
(	O
including	O
the	O
miniaturized	O
ones	O
used	O
in	O
space	O
)	O
perform	O
controlled	O
nuclear	O
fission	O
in	O
a	O
chain	B-Application
reaction	I-Application
.	O
</s>
<s>
Chain	B-Application
reactions	I-Application
do	O
not	O
occur	O
in	O
RTGs	O
.	O
</s>
<s>
Heat	O
is	O
produced	O
through	O
spontaneous	O
radioactive	O
decay	O
at	O
a	O
non-adjustable	O
and	O
steadily	O
decreasing	O
rate	O
that	O
depends	O
only	O
on	O
the	O
amount	O
of	O
fuel	O
isotope	O
and	O
its	O
half-life	B-Application
.	O
</s>
<s>
Other	O
isotopes	O
for	O
RTG	O
were	O
also	O
examined	O
in	O
the	O
study	O
,	O
looking	O
at	O
traits	O
such	O
as	O
watt/gram	O
,	O
half-life	B-Application
,	O
and	O
decay	O
products	O
.	O
</s>
<s>
One	O
mission	O
proposed	O
using	O
the	O
electricity	O
to	O
power	O
ion	O
engines	O
,	O
calling	O
this	O
method	O
radioisotope	B-Application
electric	I-Application
propulsion	I-Application
(	O
REP	O
)	O
.	O
</s>
<s>
Name	O
and	O
modelUse	O
Maximum	O
outputRadioisotopeMax	O
fuel	O
used	O
(	O
kg	O
)	O
Mass	O
(	O
kg	O
)	O
Electrical	O
(	O
W	O
)	O
Heat	O
(	O
W	O
)	O
Beta-M	O
Obsolete	O
Soviet	O
uncrewed	O
lighthouses	B-Application
and	O
beacons	B-Protocol
10	O
230	O
90SrTiO3	O
0.26	O
560Efir-MA	O
30	O
720	O
?	O
</s>
