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74 Bulletin of the Atomic Scientists January/February 2003
tions, Mods 0 through 5. Three of
these versions, Mods 1, 3, and 4,
were upgraded with improved characteristics
and safety features. Mods
0, 2, and 5 have been retired and dismantled.
Programs planned for three
other upgrades (Mods 6, 8, and 9)
were canceled. The B61-10 is a converted
Pershing II missile warhead.
For more than 30 years, the B61
bomb has been the bread and butter
of the Los Alamos National Laboratory
in New Mexico. A series of
underground tests was conducted
from 1963–1968 at the Nevada Test
Site to certify the bomb’s yield and
confirm its military characteristics.
“Shot Halfbeak,” one of six B61-
associated tests conducted in 1966,
is suspected of being fired on June
30 at full yield—about 350 kilotons.
Nuclear testing resumed in the mid-
1970s to perfect the Mod 3 and 4
versions, which entered the stockpile
in 1979.
The bomb can be delivered as a
free-fall airburst, a retarded airburst,
a free-fall surface burst, or in “laydown”
mode from aircraft flying as
low as 50 feet. In laydown mode, the
bomb must survive ground impact;
to do this, a parachute quickly slows
the bomb’s descent and controls its
trajectory. Originally, a 17-foot
diameter nylon parachute was used.
Later models switched to a 24-foot
diameter nylon/Kevlar version.
The B61 has been deployed on a
THE B61 BOMB IS PERHAPS THE MOST
versatile and abundant nuclear
weapon in the U.S. stockpile. Close
study of its complex history reveals
something that the nuclear weapon
labs may not want to admit: After
mastering the basics of sub-megaton
nuclear bomb design several decades
ago, the only subsequent innovations
have been marginal improvements
to B61 safety and security
features. The fact is that many of
the original plutonium pits, some
more than 30 years old, are still in
service—calling into question the
need for much of the $5 billion-ayear
Stockpile Stewardship Program
and the future $2 billion–$4 billion
Modern Pit Facility.
We estimate that the total stockpile
of intact B61 bombs is approximately
1,925, of which 1,265 are
considered operational. All B61
models are scheduled to undergo life
extension and retrofit programs over
the next decade, and approximately
400 bombs are scheduled to be
“consumed” in quality and reliability
testing through 2025.
The basic B61 bomb weighs
approximately 700 pounds, is slightly
over 13 inches in diameter, and is
11.8 feet long from nose to fin-tip.
The earth-penetrating version, the
B61-11, weighs an additional 450
pounds.
B61 background. The first B61
production unit began in October
1966. Problems stalled the program,
and in January 1967 the bomb was
withdrawn and changed slightly.
Full-scale production started in
January 1968. The bomb has been
manufactured in six basic modifica-
The B61 family of bombs
Nuclear
Notebook
NRDC
November 22, 1996: A B-2 Stealth bomber test drops a B61-11 earth-penetrating
weapon at the Tonopah Test Range in Nevada.
SANDIA NATIONAL LABORATORIES
Vol. 59, No. 1, pp. 74-76
DOI: 10.2968/059001020
wide variety of tactical and strategic
aircraft. Strategic versions have been
carried on B-52, FB-111, B-1, and B-
2 bombers. Tactical versions, with
lower yield options, have been
deployed on a variety of U.S. and
NATO air force aircraft, including
the F-100, F-104, F-4, F-105, F-15E,
F-16, F-111, F-117, and Tornado.
The U.S. Navy and Marines have
used the B61-2/5s on A-4, A-6, A-7,
and F/A-18 aircraft. After the navy
terminated the nuclear strike mission
from U.S. aircraft carriers in the
early 1990s, the bombs were retired
and disassembled. According to the
Bush administration’s recent Nuclear
Posture Review (NPR), some
future Lockheed Martin F-35 Joint
Strike Fighters may be nuclear capable.
They would most likely use the
B61 bomb.
The B61 has also served as the
basic design for three other warheads:
the W80-0 sea-launched
cruise missile warhead; the W80-1
warhead for the air-launched cruise
missile and the advanced cruise
missile; and the W85 warhead for
the Pershing II missile. The
Intermediate-Range Nuclear Forces
(INF) Treaty, signed on December 8,
1987, marked the Pershing II missile
(among others) for elimination.
Although the missiles and launchers
were destroyed by mid-1991, as the
treaty called for, the warheads were
retained, converted, and probably
returned to European air bases as
B61 bombs. The “physics package”
(the guts of the nuclear explosive)
was removed from the W85 warhead,
repackaged in a bomb casing,
and re-designated the B61-10. While
not technically illegal under the INF
Treaty, it can be argued that this violated
its spirit (see the November
1990 Bulletin, pp. 14–16).
Strategic B61s. There are currently
two strategic versions of the
B61. The B61-7, produced from
1985–1990, is a variable-yield gravity
bomb for the B-52 and B-2. The
B61-11 is an earth-penetrating
weapon (EPW) for the B-2 with a
“single yield,” according to the
NPR. Full-scale drop tests of the
B61-11 were conducted in Nevada
and Alaska, initially from F-16, B-1,
and B-52 aircraft. After the B-2
Stealth bomber became operational
in the Single Integrated Operational
Plan (SIOP) in October 1997, it was
chosen as the designated carrier of
the B61-11. Of its three drop tests
conducted in 1998, one involved
two unarmed bombs dropped at an
air force test range in the Yukon in
Alaska. With its hardened steel case
and nose cone, the B61-11s penetrated
the frozen tundra to a depth
of only two to three meters. Its conventional
cousin, the 5,000-pound
GBU-28, is said to penetrate about
six meters of concrete.
Development of the B61-11 was
initially proposed by U.S. Strategic
Command, endorsed by the 1994
Nuclear Posture Review, and directed
by Presidential Decision Directive
30. The first four production units
were delivered to the air force in
December 1996. It is estimated that
in 1997 some 50 B61-7s were converted
to B61-11s and deployed to
Whiteman Air Force Base (AFB),
Missouri, home of the Stealth
bomber wing. B61-7 bombs are
stored at four other bases: Barksdale
AFB in Louisiana, Minot AFB in
North Dakota, Nellis AFB in
Nevada, and Kirtland AFB in New
Mexico.
The B61-7 “laydown” bomb also
served as the basis for the W61 program
in the late 1980s and early
1990s, which was an effort to equip
the small Midgetman intercontinental
ballistic missile with a strategic
January/February 2003 Bulletin of the Atomic Scientists 75
B61 safety and control features
Bomb type Control feature IHE Type FRP ENDS
B61-0 Cat B PAL/AMAC No PBX-9404 No No
B61-1 No PAL No PBX-9404 No No
B61-2 Cat D PAL/AMAC No PBX-9404 No No
B61-3 Cat F PAL/AMAC Yes PBX-9502 No No
B61-4 Cat F PAL/AMAC Yes PBX-9502 No No
B61-5 Cat D PAL/AMAC No PBX-9404 No Yes
B61-6 (modified B61-0) Cat D PAL/AMAC Yes PBX-9502 No Yes
B61-7 (modified B61-1) Cat D PAL/AMAC Yes PBX-9502 No Yes
B61-8 (modified B61-2,-5) Cat D PAL/AMAC Yes PBX-9502 No Yes
B61-9 (modified B61-0) Cat F PAL/AMAC Yes PBX-9502 No Yes
B61-10 (converted W85) Cat F PAL/AMAC Yes PBX-9502 No Yes
B61-11 (converted Mod 7) Cat D PAL/AMAC Yes PBX-9502 No Yes
PAL (Permissive Action Link): A system included in or attached to a nuclear weapon system to
preclude arming or launching until the insertion of a prescribed discrete code or combination.
The earliest versions were 5-digit mechanical combination locks. The most modern electrical
versions are the six-digit Cat D and the 12-digit Cat F, both with a “limited try” feature that permits
a specific number of attempts to enter the correct code, after which the electrical circuits
self-destruct, disabling the weapon. Cat B is an earlier electrical version.
AMAC (Aircraft Monitoring and Control): Equipment installed in an aircraft to permit monitoring
and control of the safing, arming, and fuzing functions of a nuclear bomb or missile delivered by
the aircraft. It is the avenue for transmission of PALs.
IHE (Insensitive High Explosive): An improved conventional high explosive designed to be more
resistant to shock than earlier types, thus lessening the risk of a detonation and the dispersal of
plutonium in an accident.
PBX (Plastic-Bonded Explosive): Since 1979, the Energy Department has used the Los Alamosdeveloped
PBX.
FRP (Fire-Resistant Pit): In an FRP, the plutonium is encased in a metal shell with a high melting
point designed to withstand exposure to a jet fuel fire of 1,000 degrees Celsius.
ENDS (Enhanced Nuclear Detonation Safety): The ENDS system, developed at Sandia National
Laboratories in 1972, isolates the electrical elements critical to detonation to prevent premature
arming of a nuclear weapon subjected to abnormal environments like extreme heat or radiation.
It was first used on the B61-5.
76 Bulletin of the Atomic Scientists January/February 2003
warhead, was developed in the 1970s
but canceled in September 1980.
Tactical B61s. The current tactical
versions of the B61 are the Mods
3, 4, and 10. Most of these are
stored at Nellis and Kirtland; some
may be deployed with the 4th
Fighter Wing at Seymour Johnson
AFB in North Carolina and the 27th
Fighter Wing at Cannon AFB in
New Mexico. Approximately 150
B61s are deployed with U.S. Air
Force units in Britain, Germany, and
Turkey, and held in U.S. custody for
use by NATO allied air force wings
and squadrons in Belgium, Germany,
Italy, the Netherlands, and
Turkey. Greece has apparently
ended its nuclear role in NATO.
The B61 bomb has the unique distinction
of being the only remaining
nuclear weapon deployed outside
U.S. borders (excluding the missile
warheads on patrolling nuclearpowered
ballistic-missile subs).
Nuclear Notebook is prepared by
Robert S. Norris of the Natural
Resources Defense Council, Hans M.
Kristensen, and Joshua Handler. Direct
inquiries to NRDC, 1200 New York
Avenue, N.W., Suite 400, Washington,
D.C., 20005; 202-289-6868.
earth-penetrating warhead. When
the Midgetman program was canceled
by the first Bush administration,
so was authorization for the
W61.
The Robust Nuclear Earth
Penetrator program, recommended
by the latest NPR, could use the B61
(or B83) in an effort to build an
earth-penetrating weapon that
would be more effective than the
B61-11. But a serious flaw in the
concept of nuclear earth-penetrating
weapons, even those with relatively
low yields, is that they cannot penetrate
deeply enough to contain a
nuclear explosion and its deadly
radioactive fallout. If used in an
urban environment, such a weapon
would cause thousands of casualties
(see Robert W. Nelson, Science and
Global Security, Vol. 10: pp. 1–20,
2002).
The United States fielded two
earth-penetrating weapons in the
1950s, the Mark 8 and Mark 11
bombs. The uranium gun-type Mark
8 bomb (nicknamed “Elsie” for LC,
or light case) was almost 10 feet
long, 14 inches in diameter, 3,250
pounds, and had a yield of approximately
25 kilotons. It was developed
by the navy for targeting underground
facilities, enemy submarines
located in sheltered pens, and
armored ship decks. It was in service
from 1952 to 1957. The Mark 11
was an improved version of the
Mark 8, slightly heavier, and according
to the National Atomic Museum,
“able to penetrate up to 22 feet of
reinforced concrete, 90 feet of hard
sand, 120 feet of clay, or five inches
of armor plate,” and fuzed to detonate
90–120 seconds after penetration.
The W86, an earth-penetrating
alternative to the W85 Pershing II
2002 B61 stockpile/yields
B61-3 520 .3, 1.5, 60, or 170 kilotons
B61-4 680 .3, 1.5, 10, or 45 kilotons
B61-7 470 four yields to 350 kilotons
B61-10 205 .3, 5, 10, or 80 kilotons
B61-11 50 “single yield” (according
to the NPR)
Total 1,925
Original builds (estimates)
B61-0/1 1,200 January 1968–April 1971
B61-2 235 March 1975–January 1977
B61-3 545 May 1979–?? 1989
B61-4 695 May 1979–?? 1989
B61-5 265 June 1977–September 1979
Subtotal 2,940
B61-10 215 circa 1990–1991
Total 3,155
Conversions
B61-7 A Mod 1 with CAT D PAL and
IHE; about 700 converted from
June 27, 1985, to April 9, 1990
B61-10 A W85 Pershing II warhead with
CAT F PAL and IHE
B61-11 A Mod 7, an EPW, weighing
about 1,200 pounds; about 50
converted in 1997
Canceled programs
W61 A converted Mod 7 with CAT D
PAL and AMAC
B61-6 A converted Mod 0 with CAT D
PAL and IHE
B61-8 A converted Mod 2 with its CAT D
PAL and IHE
B61-8 A converted Mod 2 and 5 (?) with
new CAT F PAL, IHE
B61-9 A converted Mod 0 with new
CAT F PAL and IHE
Retirements, dismantlements
B61-0 500 August 10, 1995, to June
17, 1996
B61-2 215 June 1, 1996, to March
13, 1997
B61-5 236 March 13, 1997, to August
22, 1997
The “laydown” version of the B61 bomb, slowed by a parachute.
LOS ALAMOS NATIONAL LABORATORY
