Updated November 27, 2023
Iran and Nuclear Weapons Production
Background
the key nuclear weapons-development activities that would
Iran’s nuclear program has generated widespread concern
be necessary to produce a testable nuclear device.”
that Tehran is pursuing nuclear weapons. According to U.S.
The JCPOA-mandated restrictions on Iran’s nuclear
intelligence assessments, Tehran has the capacity to
program and Iran-specific monitoring and reporting
produce nuclear weapons at some point, but has halted its
requirements both supplement Tehran’s obligations
nuclear weapons program and has not mastered all of the
pursuant to the government’s comprehensive IAEA
necessary technologies for building such weapons. (For
safeguards agreement. Such agreements empower the
additional information, see CRS Report RL34544, Iran’s
agency to detect the diversion of nuclear material from
Nuclear Program: Status, by Paul K. Kerr.)
peaceful purposes, as well as to detect undeclared nuclear
Since the early 2000s, Tehran’s construction of gas
activities and material. These agreements also require
centrifuge uranium enrichment facilities has been the main
governments to declare their entire inventory of certain
source of proliferation concern. Gas centrifuges enrich
nuclear materials, as well as related facilities. Safeguards
uranium by spinning uranium hexafluoride (UF6) gas at
include agency inspections and monitoring of declared
high speeds to increase the concentration of the uranium-
nuclear facilities. (For more information, see CRS Report
235 (u-235) isotope. Such centrifuges can produce both
R40094, Iran’s Nuclear Program: Tehran’s Compliance
low-enriched uranium (LEU), which can be used in nuclear
with International Obligations, by Paul K. Kerr.)
power reactors, and highly enriched uranium (HEU), which
Prior and subsequent to the JCPOA’s January 2016
is one of the two types of fissile material used in nuclear
implementation, IAEA and U.S. officials expressed
weapons. Tehran asserts that its enrichment program is
confidence in the ability of both the IAEA and the U.S.
meant to produce fuel for peaceful nuclear reactors.
intelligence community to detect an Iranian breakout
The 2015 Joint Comprehensive Plan of Action (JCPOA)
attempt using either Tehran’s IAEA-monitored facilities or
requires Iran to implement various restrictions on its
clandestine facilities. (See CRS Report R43333.)
nuclear program, as well as to accept specific monitoring
Estimated Nuclear Weapons
and reporting requirements. (For additional information, see
Development Timelines
CRS Report R43333, Iran Nuclear Agreement and U.S.
U.S. estimates concerning Iranian nuclear weapon
Exit, by Paul K. Kerr and Kenneth Katzman.)
development account for the time necessary to produce a
Then-President Donald Trump announced in May 2018 that
sufficient amount of weapons-grade HEU and also
the United States was ending U.S. participation in the
complete the remaining steps necessary for an implosion-
JCPOA. Following this decision, Iran stopped
style nuclear device suitable for explosive testing.
implementing much of this agreement, as well as JCPOA-
Fissile Material Production
required International Atomic Energy Agency (IAEA)
The time needed to produce enough weapons-grade HEU
monitoring. Beginning in July 2019, the IAEA verified that
some of Iran’s nuclear activities were
for a nuclear weapon is a function of a nuclear program’s
exceeding JCPOA-
enrichment capacity, as well as the mass and u-235 content
mandated limits. Tehran’s subsequent expansion of the
country’s
of the UF6 stockpile fed into the enrichment process. LEU
enrichment program has decreased the amount of
used in nuclear power reactors typically contains less than
time needed for Iran to produce enough weapons-grade
5% u-235; research reactor fuel can be made using enriched
HEU for a nuclear weapon—an action frequently termed
“breakout.”
uranium containing 20% u-235; HEU used in nuclear

weapons typically contains about 90% u-235.
According to official U.S. assessments, Iran halted its
The JCPOA mandates restrictions on Iran’s declared
nuclear weapons program in late 2003 and has not resumed
enrichment capacity and requires that Iran’s enriched
it. This program’s goal, according to U.S. officials and the
uranium stockpile must not exceed 300 kilograms of UF6
IAEA, was to develop an implosion-style nuclear weapon
for Iran’s Shahab
containing 3.67% u-235 “or the equivalent in other
-3 ballistic missile. Iran has not made a
chemical forms.” This quantity of uranium hexafluoride
decision to develop nuclear weapons, according to February
“corresponds to 202.8 kg of uranium,” according to the
and March public U.S. intelligence assessments.
IAEA.
The U.S. government assessed prior to the JCPOA that
The aforementioned JCPOA restrictions constrained Iran’s
Tehran had not mastered all of the necessary technologies
nuclear program so that Tehran, using its declared
for building a nuclear weapon. Apparently confirming
persisting gaps in Iran’s nuclear weapons knowledge, the
enrichment facilities, would, for at least 10 years, have
needed a minimum of one year to produce enough
2023 U.S. Intelligence Community Annual Threat
weapons-grade HEU for one nuclear weapon. The
Assessment observes that “Iran is not currently undertaking
agreement does not explicitly mandate such a timeline. The
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Iran and Nuclear Weapons Production
timeline would begin to decrease after JCPOA restrictions
in March 2023 that Iran would need “several months to
on Iran’s enrichment capacity, as well as the mass and u-
produce an actual nuclear weapon.” The assumptions
235 content of the UF6 stockpile, begin to expire in January
underlying this estimate are unclear.
2026.
An implosion-style nuclear explosive device, according to
Iran’s number of installed centrifuges, the mass and u-235
the Office of Technology Assessment, uses “a shell of
concentration of Tehran’s enriched uranium stockpile, and
chemical high-explosive surrounding the nuclear material ...
number of enrichment locations currently exceed JCPOA-
designed (for example, by being detonated nearly
mandated limits. Tehran is also conducting JCPOA-
simultaneously at multiple points) to rapidly and uniformly
prohibited research and development, illicit uranium metal
compress the nuclear material to form a supercritical mass”
production, and centrifuge manufacturing and installation.
necessary for a sustained nuclear chain reaction.
Numerous IAEA reports note that Iran has produced
IAEA reports suggest that Iran does not yet have a viable
batches of enriched uranium containing up to 2% u-235, up
nuclear weapon design or a suitable explosive detonation
to 5% u-235, up to 20% u-235, and up to 60% u-235,
system. Tehran may also need additional experience in
respectively. Iran has enough fissile material that, if further
producing uranium metal; weapons-grade HEU metal for
enriched, would be sufficient for several nuclear weapons,
use in a nuclear weapon is first “cast and machined into
according to U.S. officials.
suitable components for a nuclear core.”
According to an April 2021 State Department report,
Discussion
“Iran’s expansion of uranium enrichment activities ... allow
The aforementioned one-year fissile-material breakout
[sic] Iran to enrich more uranium more quickly and to
estimate assumes that Iran would use its declared nuclear
higher levels.” The U.S. government estimates that Iran
facilities to produce fissile material for a weapon. But the
would need as little as one week to produce enough
breakout concept does not accurately measure Tehran’s
weapons-grade HEU for one nuclear weapon, according to
nuclear weapons capability.
a State Department official in March 2022. During a March
The U.S. government continues to assess that Iran is more
23, 2023, House Appropriations Subcommittee on Defense
likely to use covert, rather than declared, facilities to
hearing, Chairman of the Joint Chiefs of Staff Mark Milley
testified that Iran could produce this amount of HEU “in
produce the requisite fissile material. Neither the U.S.
government nor the IAEA have publicly described any
approximately 10-15 days.”
evidence that Iran is conducting such activities. Former
If Tehran were to resume implementing its current JCPOA
National Nuclear Security Administration official Corey
obligations, this fissile material production timeline would
Hinderstein, who was involved in JCPOA implementation,
increase, but would be less than one year, according to State
wrote in a January 2020 Defense One article that producing
Department officials. This estimate reflects Iran’s recent
fissile material in such a manner would require more time
accumulation of knowledge gained by operating centrifuges
than executing a breakout scenario.
that are more sophisticated. Former National Intelligence
During JCPOA negotiations, the breakout timeline was an
Council official Eric Brewer noted in an October 2021
unclassified proxy measure of Iranian nuclear weapons
Center for Strategic and International Studies publication
capabilities. A State Department official described the
that, absent this experience, Iran would probably have used
breakout “concept” in a September 2021 email as “a useful
less efficient, first-generation centrifuges for a breakout
metric to help quantify” U.S. negotiating goals and as “a
attempt.
useful analytic framework to structure the negotiation of
Even with a breakout timeline of less than one year, Iran
technical measures related to enrichment.” The timeline
would be “unlikely” to make such an attempt, Brewer
was also “helpful in explaining the deal and selling it
wrote, arguing that the JCPOA monitoring provisions
politically,” the official noted, adding that the timeline has
“would almost certainly” enable the United States to detect
“become an important political yardstick” for evaluating the
such a move.
agreement’s merits. In a February 2022 Bulletin of the
Weaponization
Atomic Scientists article, Jon Wolfsthal, a National Security
Council official during the Obama Administration,
At the time when the JCPOA negotiations concluded, the
explained that the one-year breakout goal was meant to
U.S. intelligence community assessed that Iran would have
provide enough time “to generate an international response
needed one year to complete the necessary steps for
to any Iranian move to build weapons.”
producing a nuclear weapon that do not involve fissile
material production. This estimate assumed that Iran could
Former State Department official Robert Einhorn discussed
complete fissile material production and weaponization in
this point in a 2021 United Nations Institute for
parallel, which meant that Iran would have needed about
Disarmament Research report. The Obama Administration,
one year to produce a nuclear weapon.
according to Einhorn, argued that stopping Iran from
developing nuclear weapons required preventing Tehran
The U.S. intelligence community assesses that Iran has not
“from having the fissile material production infrastructure”
resumed work on its weaponization research. A State
to break out “in less time than it would take the
Department official told CRS in an April 2022 email that
international community to intervene to block it.”
Iran would need approximately one year to complete the
necessary weaponization steps. This timeline “takes into
Paul K. Kerr, Specialist in Nonproliferation
consideration assessed knowledge gaps and reflects” the
intelligence community’s “view of Iran’
IF12106
s fastest reasonable
path to overcome them,” the official added. Milley testified
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Iran and Nuclear Weapons Production


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https://crsreports.congress.gov | IF12106 · VERSION 10 · UPDATED

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