Nuclear Material Control and Accounting

The U.S. Nuclear Regulatory Commission's (NRC's) regulations require the licensee to maintain a nuclear material control and accounting (MC&A) program that tracks and verifies special nuclear material (SNM) that is on site. The MC&A regulations ensure that the information collected by the licensee about SNM is accurate, authentic, and sufficiently detailed to enable a licensee to 1) maintain current knowledge of its SNM and 2) manage its program for securing and protecting SNM. The MC&A program, together with physical protection of facilities and information security requirements, make up the primary elements of the NRC's SNM safeguards program. The MC&A component of the larger safeguards program helps ensure that SNM within a licensed facility is not stolen or otherwise diverted from the facility.

The NRC places great importance on the licensed facility's MC&A program. The proper control and accounting of nuclear materials provides confidence that the use of licensed nuclear materials are consistent with the NRC's Strategic Goal. The MC&A program when properly implemented by the licensee ensures adequate protection in the secure use and management of radioactive materials. By ensuring the security of these materials, the MC&A program protects public health and safety, protects the environment, and promotes the common defense and security.

Nuclear material accounting is the component of a safeguards program that pertains to systems and procedures to prepare and maintain accounting records, perform measurements, and to analyze the information for confirming the presence of nuclear materials and for detecting potential theft, loss, or diversion of nuclear materials that trigger an appropriate response. The central strength of material accounting is the power to detect anomalies and, conversely, to provide assurance for the operating system that nuclear materials are present in the absence of significant anomalies and in the context of prevailing adversarial threat assumptions. Moreover, performing physical inventories and drawing material balances provide an exacting cross-check on the overall effectiveness of a facility's material controls and on the absence of unidentified loss mechanisms that could include theft or diversion.

The NRC oversight program is responsive and comprehensive to ensure that the licensed facilities maintain effective and efficient safeguards programs. An important oversight aspect of the NRC-licensed facility's MC&A program is performing routine and reactive inspections. The routine inspections are established to determine the performance of the facility's MC&A program in order to ensure the program is functioning in accordance with the NRC's regulations and the facility's plans and procedures. Routine inspections typically are performed on a semiannual to an annual basis. If the need arises because of reported events or other events of interest, the NRC can conduct reactive inspections (e.g., special inspection team or augmented inspection team). All inspections are performed by certified inspectors with specialized training and experience in the material control and accounting discipline.

The NRC maintains MC&A regulations in 10 CFR Part 74. For a status of rulemaking efforts in 10 CFR Part 74, please see the supplement to the Fuel Cycle Program Integrated Schedule maintained for the Cumulative Effects of Regulation. The status of regulatory guides pertaining to MC&A are also provided there, if appropriate.

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Records and Reporting

All licensees are required to maintain records showing the receipt, inventory, acquisition, transfer, and disposal of all SNM in its possession regardless of its origin or method of acquisition. The regulations in 10 CFR 74.13 require material status reports (Form 742) and physical inventory listing reports (Form 742C), and 10 CFR 74.15 requires nuclear material transaction reports (Form 741). NUREG/BR-0006 and -0007 provide guidance for submission of transaction reports and of material balance reports and physical inventory listings, respectively, to the national database designated as the Nuclear Materials Management and Safeguards System (NMMSS).1Regulations in 10 CFR 74.17 specifically require facilities with SNM also to submit Form 327, "Special Nuclear Material Physical Inventory Summary Report." These report forms should be completed in accordance with the instructions provided in the NUREG/BR-0096 guidance document.

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Physical Inventory

The basic principle of material balance accounting is maintaining the records of the material amounts that should be on site (i.e. the book inventory) and comparing them with a measured physical inventory. This comparison ensures that the licensee has not lost material. These periodic material balances—accomplished  by physical inventories of all nuclear materials—provide one of the primary means for detecting theft or diversion.

Performing periodic physical inventories is important for any licensed facility that possesses nuclear materials, whether the facility is a processing facility or simply a storage facility. The principal purpose of conducting a physical inventory is to confirm that nuclear materials are present in their recorded locations. Such physical inventories also serve the purposes of confirming the accuracy and reliability of the facility's accounting records, detecting any unmeasured material losses, and detecting any diversion or theft of nuclear materials that could go undetected without a defined physical inventory program. Physical inventories play a complementary role of confirming that the other more timely loss detection systems (e.g., process monitoring, item monitoring) are performing properly. In addition, physical inventories serve as the most reliable method for detecting protracted material losses.

Performing physical inventories for closing material balances generally consists of counting and measuring all material receipts, shipments and discards, and maintaining records of these transactions. Statistical methods and tests are further used to evaluate indicated imbalances including measurement uncertainties and accountability anomalies in the amount of nuclear materials that are received, produced, shipped, discarded, added to or removed from a facility's inventory.

A material balance is closed by such inventories and is defined as the determination of an inventory difference2. As defined in 10 CFR 74.4, "Definitions," inventory difference (ID) is the arithmetic difference obtained by subtracting the quantity of nuclear materials determined by the physical inventory from the book inventory quantity. Book inventory quantity is equivalent to the beginning inventory (BI) plus additions to inventory (A) minus removals from inventory (R), while the physical inventory is the ending inventory (EI) for the material balance period in question as physically determined. Thus mathematically,

  • ID = Book inventory – Ending Inventory or
  • ID = (BI + A – R) – EI.

In theory, in the absence of any theft or diversion, the inventory difference should always equal zero. For a facility that possesses only items, the inventory difference should be zero. An inventory difference different from zero is a serious problem because either the missing item has been lost, diverted, or misplaced, or the book keeping practice is inadequate. For a process facility, a non-zero inventory difference is to be expected, and can be either positive or negative due to many contributing factors, such as measurement uncertainties, measurement mistakes, recording errors, or unmeasured material holdup and losses. Referring to the inventory difference equation above, a positive inventory difference implies a loss of material, while a negative inventory difference indicates a gain in material. If the inventory difference is positive, it is possible that material may have been stolen.

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Inventory Differences

After a licensee performs a physical inventory and calculates the inventory difference, the licensee must determine if the inventory difference is statistically significant. (The licensee would not expect a “zero” inventory difference due to measurement uncertainties.) For this evaluation, the licensee must calculate the standard error of the inventory difference (SEID). The SEID is defined as the standard deviation of an inventory difference that takes into account all measurement error contributions to the components of the inventory difference.

The SEID is used to establish control limits for the inventory difference. The regulations in 10 CFR Part 74 contain limits for the SEID and inventory difference based on the risk significance of the material.

  • Category I:
    • SEID ≥ 0.1% of active inventory3
    • U235 inventory difference ≥ 3 SEID and 200 grams of Pu/U233 or 300 grams of U235 (HEU)
  • Category II:
    • SEID ≥ 0.125% of active inventory
    • U235 inventory difference ≥ 3 SEID and 200 grams of Pu/U233 or 300 grams of U235 (HEU) or 9,000 grams of U235 (LEU)
  • Category III:
    • Warning-level: U235 inventory difference ≥ 1.7 SEID + 500 grams U235 or U inventory difference ≥ 1.7 SEID + 10 kilograms of U
    • Significant inventory difference: U or U235 inventory difference ≥ 3 SEID
    • Major inventory difference: U235 inventory difference ≥ detection quantity – 1.3 SEID

Any inventory difference, either positive or negative, that is greater than its calculated control limits should trigger a facility's investigation to determine the cause of the excessive inventory difference. For example, although an inventory difference larger than its overall measurement uncertainty may be a good indicator of an anomaly, the fact that an inventory difference falls within its measurement uncertainty, even a zero inventory difference value, does not provide evidence that a loss or theft of nuclear materials has not occurred. An excessive positive inventory difference does not necessarily indicate that a loss has occurred, but that the hypothesis that no loss has occurred should not be accepted from a statistical viewpoint. An excessive inventory difference can also be an indication that the measurement and accounting programs may not be functioning adequately. If an inventory difference is considerably greater than its SEID, prompt response actions in addition to an investigation are needed. To be effective, inventory difference evaluations must be combined with other elements of the MC&A system to ensure timely detection of loss, theft, or diversion of nuclear materials. Therefore, each licensed facility should have a well-defined program for evaluating total plant inventory differences and taking actions when inventory differences exceed certain predetermined limits.

In addition to requirements for notifying the NRC, response actions to inventory differences that exceed regulatory limits should be established, maintained, and implemented. When an inventory difference exceeds a warning level limit, the minimum response should be a documented investigation conducted by the responsible safeguards organization. Such investigation should provide a conclusion for the probable causes of the excessive inventory difference and provide recommendations for avoiding recurrences, including activities such as reviews of the inventory listing, inventory documentation and records for human errors and anomalies, measurement results for unidentified biases, etc.

When an inventory difference exceeds the limit designated as a significant problem, a more extensive investigation should be conducted. If a significant inventory difference problem cannot be satisfactorily explained, a re-inventory will normally be deemed necessary. When an inventory difference exceeds the limit designated as a major problem, and remains unresolved following an intensive and thorough investigation, a complete shutdown, cleanout, and considerable re-measurements will be required, as well as a re-inventory.

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Routinely, all NRC inspection results are evaluated by the NRC enforcement policy. Depending on the severity of the noncompliance, the NRC may issue enforcement actions to licensees who violate regulatory requirements. These enforcement actions can include notices of violation, monetary fines, or the issuance of an order to restrict, suspend, or revoke a license.

1 Additional guidance instructions are provided in NMMSS Report D-24, "Personal Computer Data Input for NRC Licensees".

2 Inventory difference is termed "material unaccounted for", or MUF, by the International Atomic Energy Agency (IAEA), the European Union and most other countries.

3 Active inventory = BI + A + R + EI with all unchanged items excluded.

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Page Last Reviewed/Updated Friday, December 10, 2021