Non-proliferation and safeguards

Two IAEA inspectors are visiting a US fuel fabrication plant that has been selected for inspection under the country's voluntary offer agreement, and Hana is assigned to accompany the plant's material control and accounting manager. The inspectors verify the plant's uranium inventory, take samples for analysis and check the records of receipts and shipments. "The United States does not have to accept inspections at all," the manager tells her, "but it does, to show that civil nuclear facilities are held to the same standard everywhere. And the NRC holds us to our own accounting rules every day."

The non-proliferation regime

The peaceful use of nuclear energy depends on confidence that civil nuclear materials and facilities are not being used to make nuclear weapons. The international non-proliferation regime provides that confidence through treaties, verification, export controls and physical protection. Its centerpiece is the Treaty on the Non-Proliferation of Nuclear Weapons, the NPT, which opened for signature in 1968 and entered into force in 1970. Almost every country in the world is a party. The treaty recognizes five nuclear-weapon states, the United States, Russia, the United Kingdom, France and China, which undertake not to help other states acquire nuclear weapons and to pursue disarmament. Non-nuclear-weapon states undertake not to acquire nuclear weapons and to accept IAEA safeguards on all their nuclear material. All parties have the right to peaceful uses of nuclear energy.

The United States is a nuclear-weapon state under the NPT. It has no treaty obligation to accept IAEA safeguards, but it has offered its civil facilities for safeguards under a voluntary offer agreement, so that US industry does not have a commercial advantage over industries in countries that must accept full inspections.

IAEA safeguards

Safeguards are the technical measures the IAEA uses to verify that states are meeting their commitments. Under a comprehensive safeguards agreement, based on the IAEA document INFCIRC/153, a non-nuclear-weapon state declares all its nuclear material and facilities, keeps accounting records and reports to the IAEA, and allows inspectors to verify the declarations. The IAEA's objective is the timely detection of any diversion of significant quantities of nuclear material from peaceful activities, and deterrence of diversion by the risk of early detection.

After it was discovered in 1991 that Iraq had run a secret weapons program at undeclared sites despite inspections at its declared facilities, the IAEA strengthened the system. The Model Additional Protocol, adopted in 1997 as INFCIRC/540, requires states to provide much broader information about their nuclear fuel cycle activities, including mining, research and manufacturing of related equipment, and gives inspectors complementary access to sites at short notice. Together, a comprehensive agreement and an additional protocol allow the IAEA to give assurance not only that declared material has not been diverted but also that there is no undeclared material or activity in the state.

The US voluntary offer agreement entered into force in 1980, and its additional protocol in 2009. The IAEA chooses facilities from a list offered by the United States, and selects a few for inspection, often new facilities or those of special interest, such as the plants that have downblended surplus highly enriched uranium. Domestically, the NRC enforces material control and accounting requirements for licensees in 10 CFR Part 74 and implements the safeguards agreements through 10 CFR Part 75, while the DOE and its National Nuclear Security Administration have equivalent requirements for their own sites.

Nuclear material accountancy

Nuclear material accountancy is the foundation of safeguards. Each facility is divided into material balance areas, and every transfer of nuclear material into or out of an area is measured and recorded. Periodically, the operator takes a physical inventory, measuring all the material present. The difference between the book inventory, which is the beginning inventory plus receipts minus shipments, and the measured physical inventory is called material unaccounted for, or MUF. Because every measurement has some uncertainty, MUF is rarely exactly zero. The question is whether it is consistent with the known measurement uncertainty.

Here I_begin and I_end are the beginning and ending physical inventories, R is the receipts and S is the shipments during the period. The operator and the IAEA each estimate the standard deviation of MUF, written σ_MUF, from the uncertainties of the measurements that went into it. A MUF within about two or three standard deviations is normal; a larger one must be investigated and explained.

# Material balance accounting: inventory difference and its statistical test (illustrative numbers).
# MUF (material unaccounted for) = beginning inventory + receipts - shipments - ending inventory.
begin, receipts, shipments, end = 12000.0, 3500.0, 3400.0, 12095.0    # kg uranium in a fabrication plant
muf = begin + receipts - shipments - end
sigma = 4.0                                                           # kg, combined measurement uncertainty
print(f"MUF = {muf:.1f} kg U   sigma = {sigma:.1f} kg")
z = muf / sigma
print(f"MUF / sigma = {z:.2f}")
print("within normal measurement uncertainty" if abs(z) < 3 else "investigate: larger than expected")
for kg in (5.0, 15.0):
    print(f"if MUF were {kg:4.1f} kg the ratio would be {kg/sigma:.2f}")

MUF = 5.0 kg U   sigma = 4.0 kg
MUF / sigma = 1.25
within normal measurement uncertainty
if MUF were  5.0 kg the ratio would be 1.25
if MUF were 15.0 kg the ratio would be 3.75

The output shows a MUF of 5 kg of uranium with a combined uncertainty of 4 kg, a ratio of 1.25, which is well within normal measurement variation. A MUF of 15 kg with the same uncertainty would be nearly four standard deviations and would trigger an investigation, which usually finds a measurement error, a recording mistake or material held up in equipment, such as powder in ducts or filters. Plants work to reduce holdup and improve measurements so that their MUF stays small and well understood.

Containment, surveillance and verification

Accountancy is supported by containment and surveillance. Seals placed by the IAEA on containers, casks and equipment show whether they have been opened. Cameras record movements of material at key points. Inspectors verify the operator's records, measure items with portable instruments, take samples for analysis at IAEA laboratories and compare their results with the declarations. Environmental sampling, in which inspectors take swipes from surfaces to look for traces of undeclared activities, is a powerful tool introduced in the 1990s.

Light water reactors are refueled during planned outages, so fuel movements are concentrated in a few weeks every 18 to 24 months. Safeguards at such reactors, where they are applied, rely on item counting of assemblies, seals on the reactor vessel and pool, and cameras. Enrichment and fuel fabrication plants, which handle bulk material in the form of UF₆ or powder, rely more on measurement and material balance accounting.

Export controls and physical protection

Safeguards are complemented by export controls. The Nuclear Suppliers Group, formed in 1974 after India's nuclear test, coordinates national export controls on nuclear materials, equipment and technology, and on dual-use items, so that exports go only to states with safeguards in place. Physical protection is a separate obligation: under the Convention on the Physical Protection of Nuclear Material, strengthened by an amendment that entered into force in 2016, states must protect nuclear material and facilities against theft and sabotage, with stronger protection for more sensitive material.

Section 123 of the Atomic Energy Act requires an agreement for cooperation, known as a 123 agreement, before significant US nuclear exports to another country, and such agreements must meet non-proliferation criteria set by Congress, including safeguards and limits on enrichment and reprocessing of US-origin material without US consent. The NRC licenses exports of nuclear materials and equipment, and the Department of Energy authorizes transfers of nuclear technology under 10 CFR Part 810.

Safeguards and the future fuel cycle

New technologies bring new safeguards tasks. Advanced reactors using HALEU, small modular reactors deployed in many places, and possible recycling facilities will all need safeguards designed in from the start, an approach called safeguards by design. A deep geological repository raises its own questions, because the fuel inside will become inaccessible: the IAEA and national authorities are developing methods to verify the fuel before emplacement and to monitor the site so that no undeclared excavation takes place. Finland's ONKALO and Sweden's Forsmark repository are the first to apply these methods.

The role of the national regulator

States implement safeguards through a national system of accounting for and control of nuclear material. The national authority keeps a central record of all nuclear material in the country, receives reports from facilities, carries out its own inspections, and sends the required reports and declarations to the IAEA. It also coordinates the IAEA's inspections and complementary access visits, and ensures that inspectors have the access, visas and support they need. A strong national system makes the IAEA's work more efficient and gives the state credibility.

In the United States, the NRC and the DOE share this role for civil and government facilities respectively, with the NRC's material control and accounting rules applied to commercial licensees. For facilities selected under the voluntary offer agreement, licensees provide reports and access in the same way as facilities in non-nuclear-weapon states. The United States also supports the IAEA through funding, training of inspectors, development of safeguards equipment at national laboratories and analysis of environmental samples.

Nuclear security

Safeguards address diversion by states; nuclear security addresses theft and sabotage by individuals or groups. The two are related but separate. Nuclear security includes physical protection of facilities and materials in use, storage and transport, protection of sensitive information and computer systems, detection of nuclear material out of regulatory control, and response to incidents. Physical protection is graded: the most sensitive materials receive the strongest protection, with layers of barriers, detection systems, armed response forces and strict personnel screening. The IAEA's Nuclear Security Series provides guidance, and states report trafficking incidents to its Incident and Trafficking Database.

In the United States, the NRC's physical protection rules in 10 CFR Part 73 set requirements for power reactors, fuel facilities and transport, and the NRC tests plant security through force-on-force exercises. The DOE and NNSA also support programs to secure and remove vulnerable nuclear material worldwide.

Together, safeguards, export controls and nuclear security underpin public and international confidence in the peaceful use of nuclear energy. For engineers working on the fuel cycle, they are part of everyday practice: designing facilities so that material can be measured and verified, keeping accurate records, and supporting the inspectors whose independent checks make that confidence possible.

Safeguards and the repository

Disposal creates a new safeguards question: once used fuel is placed underground and the tunnels are backfilled, it can no longer be inspected. The IAEA's approach is to verify the fuel thoroughly before it is encapsulated, using instruments that measure gamma and neutron signals to confirm that each bundle or assembly is what the records say it is, and to maintain continuity of knowledge, using seals and surveillance, from the encapsulation plant to the placement room. After closure, the site will be monitored, for example by satellite imagery and seismic sensors, to detect any undeclared excavation. Finland's ONKALO is the first repository to apply this approach in practice.