https://crsreports.congress.gov
Updated October 23, 2023
Iran and Nuclear Weapons Production
Background
Iran’s nuclear program has generated widespread concern
that Tehran is pursuing nuclear weapons. According to U.S.
intelligence assessments, Tehran has the capacity to
produce nuclear weapons at some point, but has halted its
nuclear weapons program and has not mastered all of the
necessary technologies for building such weapons. (For
additional information, see CRS Report RL34544, Iran’s
Nuclear Program: Status, by Paul K. Kerr.)
Since the early 2000s, Tehran’s construction of gas
centrifuge uranium enrichment facilities has been the main
source of proliferation concern. Gas centrifuges enrich
uranium by spinning uranium hexafluoride (UF6) gas at
high speeds to increase the concentration of the uranium-
235 (u-235) isotope. Such centrifuges can produce both
low-enriched uranium (LEU), which can be used in nuclear
power reactors, and highly enriched uranium (HEU), which
is one of the two types of fissile material used in nuclear
weapons. Tehran asserts that its enrichment program is
meant to produce fuel for peaceful nuclear reactors.
The 2015 Joint Comprehensive Plan of Action (JCPOA)
requires Iran to implement various restrictions on its
nuclear program, as well as to accept specific monitoring
and reporting requirements. (For additional information, see
CRS Report R43333, Iran Nuclear Agreement and U.S.
Exit, by Paul K. Kerr and Kenneth Katzman.)
Then-President Donald Trump announced in May 2018 that
the United States was ending U.S. participation in the
JCPOA. Following this decision, Iran stopped
implementing much of this agreement, as well as JCPOA-
required International Atomic Energy Agency (IAEA)
monitoring. Beginning in July 2019, the IAEA verified that
some of Iran’s nuclear activities were exceeding JCPOA-
mandated limits. Tehran’s subsequent expansion of the
country’s enrichment program has decreased the amount of
time needed for Iran to produce enough weapons-grade
HEU for a nuclear weapon—an action frequently termed
“breakout.”
According to official U.S. assessments, Iran halted its
nuclear weapons program in late 2003 and has not resumed
it. This program’s goal, according to U.S. officials and the
IAEA, was to develop an implosion-style nuclear weapon
for Iran’s Shahab-3 ballistic missile. Iran has not made a
decision to develop nuclear weapons, according to February
and March public U.S. intelligence assessments.
The U.S. government assessed prior to the JCPOA that
Tehran had not mastered all of the necessary technologies
for building a nuclear weapon. Apparently confirming
persisting gaps in Iran’s nuclear weapons knowledge, the
2023 U.S. Intelligence Community Annual Threat
Assessment observes that “Iran is not currently undertaking
the key nuclear weapons-development activities that would
be necessary to produce a testable nuclear device.”
The JCPOA-mandated restrictions on Iran’s nuclear
program and Iran-specific monitoring and reporting
requirements both supplement Tehran’s obligations
pursuant to the government’s comprehensive IAEA
safeguards agreement. Such agreements empower the
agency to detect the diversion of nuclear material from
peaceful purposes, as well as to detect undeclared nuclear
activities and material. These agreements also require
governments to declare their entire inventory of certain
nuclear materials, as well as related facilities. Safeguards
include agency inspections and monitoring of declared
nuclear facilities. (For more information, see CRS Report
R40094, Iran’s Nuclear Program: Tehran’s Compliance
with International Obligations, by Paul K. Kerr.)
Prior and subsequent to the JCPOA’s January 2016
implementation, IAEA and U.S. officials expressed
confidence in the ability of both the IAEA and the U.S.
intelligence community to detect an Iranian breakout
attempt using either Tehran’s IAEA-monitored facilities or
clandestine facilities. (See CRS Report R43333.)
Estimated Nuclear Weapons
Development Timelines
U.S. estimates concerning Iranian nuclear weapon
development account for the time necessary to produce a
sufficient amount of weapons-grade HEU and also
complete the remaining steps necessary for an implosion-
style nuclear device suitable for explosive testing.
Fissile Material Production
The time needed to produce enough weapons-grade HEU
for a nuclear weapon is a function of a nuclear program’s
enrichment capacity, as well as the mass and u-235 content
of the UF6 stockpile fed into the enrichment process. LEU
used in nuclear power reactors typically contains less than
5% u-235; research reactor fuel can be made using enriched
uranium containing 20% u-235; HEU used in nuclear
weapons typically contains about 90% u-235.
The JCPOA mandates restrictions on Iran’s declared
enrichment capacity and requires that Iran’s enriched
uranium stockpile must not exceed 300 kilograms of UF6
containing 3.67% u-235 “or the equivalent in other
chemical forms.” This quantity of uranium hexafluoride
“corresponds to 202.8 kg of uranium,” according to the
IAEA.
The aforementioned JCPOA restrictions constrained Iran’s
nuclear program so that Tehran, using its declared
enrichment facilities, would, for at least 10 years, have
needed a minimum of one year to produce enough
weapons-grade HEU for one nuclear weapon. The
agreement does not explicitly mandate such a timeline. The
