Nationally Tracked Sources and the NSTS

A nationally tracked source is a sealed source at or above the Category 1 or Category 2 threshold quantities of radioactive material, and the National Source Tracking System (NSTS) is the NRC's cradle-to-grave database that follows each one from manufacture to disposal. For an RSO, the practical obligations are specific: report every transaction to the NSTS by the close of the next business day on NRC Form 748, reconcile the physical inventory against the national database each January, and understand how source-level NSTS tracking differs from the aggregate-based physical security of 10 CFR Part 37.¹²³

The system exists because the most dangerous sealed sources are also the most consequential to lose. A high-activity Co-60, Cs-137, or Ir-192 source that goes missing is not just a licensing problem — it is a public-safety and security event, and history has the casualties to prove it.⁴ This guide explains what makes a source nationally tracked, the reporting and reconciliation mechanics, the Category 1/2 threshold values and the aggregation rule, and the common misconceptions that trip up licensees. DRPS supports these obligations through its radioactive material license support and Radiation Safety Officer consulting services.

Introduction

Most of what a medical or industrial RSO tracks is unsealed, short-lived, and low-consequence — the technetium eluted this morning, the iodine capsule administered this afternoon, both effectively gone within days. Nationally tracked sources are the opposite category: sealed, long-lived, high-activity sources whose loss or theft could cause acute injury, contaminate a facility, or be used to build a radiological dispersal device. The regulatory response is proportionate to that consequence — a dedicated national database and a reporting cadence far tighter than routine license recordkeeping.²⁴

The National Source Tracking System was built to answer a simple, hard question: at any moment, who holds every high-risk sealed source in the country, and can we account for all of them? To answer it, the NRC requires licensees to report the life events of each nationally tracked source — who made it, who has it, where it went, and when it was disposed of — into a secure national ledger, and to periodically confirm that the ledger matches physical reality.¹²

This post treats NSTS as an operational RSO responsibility, not an abstraction: what to report, when, on which form, and how it fits alongside the security rules that share its thresholds but not its logic.

Topic Explanation

What makes a source "nationally tracked"

A nationally tracked source is defined as a sealed source containing radioactive material at or above the Category 1 or Category 2 threshold quantities. The definition lives in the NRC's radiation-protection regulations, and the threshold values are the internationally harmonized Category 1 and 2 quantities reproduced in NRC regulation.² Two words in that definition do a lot of work:

Sealed — the material must be in a sealed source. Unsealed radiopharmaceuticals, however active, are not nationally tracked.
At or above the threshold — evaluated against current activity. A source that has decayed below the Category 2 threshold is no longer a nationally tracked source.

Explicitly excluded from the definition are material encapsulated solely for disposal and special nuclear material in fuel assemblies, subassemblies, fuel rods, or fuel pellets.²

The sources that actually populate the NSTS are the familiar high-activity sealed sources of medicine and industry: Co-60 teletherapy and stereotactic (gamma) units, Cs-137 and Co-60 self-shielded blood/research irradiators, Ir-192 and Co-60 industrial radiography and HDR brachytherapy sources, and sealed Am-241, Cf-252, and Se-75 sources used in gauging and radiography.

Category 1 and Category 2 thresholds

The Category 1 and 2 thresholds come from the IAEA Code of Conduct framework and are reproduced in NRC regulation (10 CFR Part 37, Appendix A, and the corresponding Part 20 nationally-tracked-source thresholds). Category 1 is the more dangerous tier; Category 2 is a factor of 100 lower in activity. A representative selection:

Radionuclide	Category 1 (TBq)	Category 1 (Ci)	Category 2 (TBq)	Category 2 (Ci)
Co-60	30	810	0.3	8.1
Cs-137	100	2,700	1	27
Ir-192	80	2,200	0.8	22
Am-241	60	1,600	0.6	16

Two points of interpretation matter. First, terabecquerels (TBq) are the legal standard; the curie column is a rounded convenience conversion, so calculations should be done in TBq. Second, the Category 2 threshold is uniformly the Category 1 value divided by 100, which makes the tiers easy to reason about once the Category 1 value is known.³

What NSTS is — and what it is not

The NSTS is a tracking and reporting system. It is a secure national database that records the existence and custody of every Category 1 and 2 sealed source and follows it across its life. It is not the physical-security rule, and it is not the old "Increased Controls" orders. Those functions belong to 10 CFR Part 37, which shares the same thresholds but does an entirely different job (discussed under Regulatory Considerations). Keeping the two straight is the single most common source of confusion for licensees.³

Key Technical Principles

Per-source tracking vs. aggregated security

The deepest conceptual point in this area is that NSTS applies per source, while Part 37 applies to the aggregate at a location. A source is a nationally tracked source only if that individual source meets or exceeds the Category 2 threshold. Physical-security applicability under Part 37, by contrast, is determined by summing across everything at the location using a sum-of-fractions (sum-of-ratios) rule:

i \sum \frac{A _{i}}{T _{i}} \geq 1

where A_i is the total activity of radionuclide i at the location and T_i is that radionuclide's Category threshold. If the sum reaches 1 against the Category 2 column, the aggregate is a Category 2 quantity and Part 37 security applies — even if no single source is individually a nationally tracked source.

Worked example. An industrial radiography vault holds three sealed sources, none individually at its Category 2 threshold:

\frac{0.4}{0.8}_{Ir-192} + \frac{0.8}{2.0}_{Se-75} + \frac{0.05}{0.3}_{Co-60} = 0.500 + 0.400 + 0.167 = 1.067

The sum exceeds 1, so the aggregate is a Category 2 quantity and Part 37 physical protection applies — yet because each source is below its own Category 2 threshold, none of them is individually a nationally tracked source reportable to the NSTS. This is the aggregation-versus-per-source distinction in a single calculation.

Decay-corrected activity and the threshold

Because thresholds are evaluated against current activity, a source can cross the Category 2 boundary as it decays. The time for a source to decay from A_0 to a threshold activity A is:

t = \frac{T _{1/2}}{ln 2} ln (\frac{A _{0}}{A})

Co-60 example (T_{1/2} = 5.27 yr). A 1.85 TBq (50 Ci) Co-60 source decaying to the 0.3 TBq Category 2 threshold:

t = \frac{5.27}{0.693} ln (\frac{1.85}{0.3}) = 7.60 \times ln (6.167) \approx 13.8 years

For contrast, Cs-137 (T_{1/2} = 30.08 yr), a 3 TBq source decaying to its 1 TBq threshold takes:

t = \frac{30.08}{0.693} ln (\frac{3}{1}) = 43.4 \times ln (3) \approx 47.7 years

The long-lived Cs-137 stays trackable and secured for nearly half a century, while the Co-60 self-declassifies in well under two decades — a practical driver of end-of-life and disposal planning for sealed-source programs.¹

Clinical Impact

For a medical facility, the sources most likely to make it a nationally tracked source holder are the gamma stereotactic (Co-60) units, Co-60 teletherapy sources where they remain in service, Cs-137 or Co-60 blood/research irradiators, and HDR brachytherapy Ir-192 at the high end of activity. Each of these is a Category 1 or 2 quantity for much or all of its service life, and each carries the full weight of NSTS reporting plus Part 37 security.⁴

The stakes are not theoretical. The canonical case is the Samut Prakan accident, in which an orphaned Co-60 teletherapy source of roughly 15.7 TBq (425 Ci) was scrapped, breached, and handled by people who did not know what it was, causing three deaths and dozens of injuries — precisely the scenario that source tracking exists to prevent.⁴ More recent health-physics work continues to map the risk landscape of high-activity sources in hospitals and universities and the programs — such as disused-source recovery efforts — that reduce it.³⁷⁹

The clinical-operations reality is that NSTS obligations attach to equipment the therapy and blood-bank teams think of as fixtures, not "sources." An irradiator in the blood bank or a gamma unit in radiation oncology is, to the RSO, a nationally tracked source with next-business-day reporting on installation, transfer, and disposal, and an annual reconciliation obligation. The RSO's job is to make sure those events actually get reported when the clinical team simply sees a machine being delivered or removed.

Practical Optimization Tips

Build the reporting into the workflow

Report transactions by the close of the next business day. Manufacture, transfer, receipt, disassembly, import, export, and disposal of a nationally tracked source all trigger a report on NRC Form 748 (OMB control no. 3150-0202). Build a checklist so a source arriving or leaving the dock generates the report automatically.¹
Correct errors within five business days of discovery. A missed or erroneous transaction must be corrected promptly; do not wait for the annual reconciliation to catch it.¹
Do the annual inventory reconciliation in January. Reconcile the physical inventory against the NSTS records each year by January 31. Treat it as a real physical inventory, not a paperwork sign-off.²

Track the right number, and the right rule

Use decay-corrected activity. Evaluate both the NSTS threshold and the Part 37 aggregation against current activity, not the certificate value. A source near end of life may have quietly crossed a threshold.
Run the aggregation separately. Compute the sum-of-ratios for Part 37 applicability independent of the per-source NSTS question; the two answers can diverge.
Do not over-report. NSTS covers Category 1 and 2 sources only. Sources below the Category 2 threshold, Category 3 and lower quantities, exempt material, material encapsulated solely for disposal, and nuclear fuel are outside the tracked-source definition. Reporting things that are not nationally tracked wastes effort and muddies the record.²

Coordinate tracking, security, and disposal

Because the same high-activity sources drive NSTS reporting, Part 37 security, and eventual decommissioning, coordinate them as one program. Sealed sources must also be leak-tested on a schedule, secured against unauthorized access, and dispositioned through a legitimate pathway at end of life. See our related guides on radioactive source security under 10 CFR Part 37, sealed source leak testing, and securing licensed material under 10 CFR 20.1801–1802.

Regulatory Considerations

The NSTS obligations sit within a stack of NRC (or Agreement State) rules that share the Category 1/2 thresholds but govern different things — tracking, security, and reporting — and a defensible program addresses all three.

Key frameworks to reference:

10 CFR 20.2207 — Reports of transactions involving nationally tracked sources. Establishes the transaction reporting obligation, the close-of-next-business-day deadline, the five-business-day correction window, and the use of NRC Form 748.¹
10 CFR 20.1003 and the nationally-tracked-source thresholds. Provide the definition of a nationally tracked source and the Category 1/2 threshold quantities against which it is measured.²
10 CFR Part 37 — Physical Protection of Category 1 and Category 2 Quantities of Radioactive Material. The security rule: access authorization (including background investigations), physical protection during use and storage, monitoring and response, and transportation security. Part 37 became effective in 2013–2014 and superseded the earlier Increased Controls and fingerprinting orders, which should no longer be cited as current. Its applicability uses the aggregation rule above.³
NRC Form 748 (OMB 3150-0202) and NUREG/BR-0472. The transaction report form and the NRC's program overview documentation for the NSTS.¹⁵

An important scope point: the NSTS covers both NRC and Agreement State licensees — a source in an Agreement State is tracked in the same national database. Among the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States, while Washington, DC and Delaware are regulated directly by the NRC; in every case, nationally tracked sources are reported into the same national system. For how these obligations connect to license maintenance, see our guides on radioactive material license renewal and decommissioning and license termination.

Frequently Asked Questions (FAQs)

What is a nationally tracked source?

A nationally tracked source is a sealed source containing radioactive material at or above the Category 1 or Category 2 threshold quantities defined in NRC regulations. These are the sources that could cause serious harm if lost, stolen, or misused — for example, Co-60 teletherapy and irradiator sources, Cs-137 blood irradiators, and Ir-192 industrial radiography sources. Sources below the Category 2 threshold are not nationally tracked.

What is the National Source Tracking System (NSTS)?

The NSTS is the NRC's secure national database that tracks Category 1 and Category 2 sealed sources from manufacture through transfer, receipt, and final disposal or decay. Licensees report each transaction so the regulator maintains a cradle-to-grave record of who holds every high-risk source, supporting security, loss prevention, and rapid response if a source goes missing.

How quickly must a nationally tracked source transaction be reported?

Transactions — manufacture, transfer, receipt, disassembly, import, export, or disposal of a nationally tracked source — must be reported to the NSTS by the close of the next business day after the transaction, using NRC Form 748. If a licensee later discovers a reporting error or a missed transaction, it must be corrected or filed within five business days of discovery.

What is the annual inventory reconciliation?

Separate from transaction reporting, each licensee that possesses nationally tracked sources must reconcile its physical inventory against the NSTS records once a year, in January, with a deadline of January 31. The reconciliation confirms that the sources the licensee actually holds match what the national database says it holds, catching discrepancies that transaction reporting alone might miss.

How is NSTS tracking different from 10 CFR Part 37 security?

They share the same Category 1 and 2 thresholds but do different jobs. NSTS is an inventory-tracking and reporting system built around individual sources. Part 37 is the physical-security rule — access control, background checks, monitoring, and transport security — and it applies based on the aggregate quantity of material at a location using a sum-of-ratios calculation, so Part 37 can apply even when no single source is individually a nationally tracked source.

Are short-lived medical radionuclides like Tc-99m or Lu-177 nationally tracked?

Generally no. Nationally tracked sources are sealed sources at or above the Category 1 or 2 thresholds. Unsealed diagnostic and therapeutic radiopharmaceuticals such as Tc-99m, I-131, F-18, and Lu-177 are not sealed sources and are not nationally tracked. NSTS tracking centers on long-lived, high-activity sealed sources like Co-60, Cs-137, and Ir-192.

Does a source stop being nationally tracked as it decays?

Yes. The thresholds are evaluated against current, decay-corrected activity, not the original certificate activity. As a source decays below the Category 2 threshold it ceases to be a nationally tracked source, and it may also drop out of the Part 37 aggregation. This is why licensees should track decay-corrected activity rather than nameplate values.

Key Takeaways

Nationally tracked sources are the highest-risk sealed sources — Category 1 and 2 quantities such as Co-60, Cs-137, and Ir-192 — tracked by the NRC cradle-to-grave in the NSTS.²
Report transactions by the close of the next business day on NRC Form 748, and correct errors within five business days of discovery.¹
Reconcile the physical inventory each January (by January 31) against the NSTS records — a separate obligation from transaction reporting.²
NSTS ≠ Part 37. NSTS tracks individual sources; Part 37 secures the aggregate at a location using a sum-of-ratios rule, so Part 37 can apply when no single source is nationally tracked.³
Evaluate against decay-corrected activity. A source can cross a threshold as it decays — Co-60 self-declassifies far sooner than long-lived Cs-137.¹
Don't over-report. Sub-threshold sources, exempt material, disposal-only encapsulation, and nuclear fuel are outside the tracked-source definition.²

Conclusion

The National Source Tracking System turns a security principle — always know where the dangerous sources are — into a set of concrete RSO tasks: report each transaction by the next business day, reconcile the inventory every January, and evaluate thresholds against current activity. The conceptual key is to keep NSTS and Part 37 distinct: one tracks individual sources, the other secures the aggregate, and they can reach different conclusions about the same facility. Programs that build the reporting into their receiving and disposal workflows, run the aggregation calculation deliberately, and track decay-corrected activity will stay both compliant and defensible — and, more importantly, will not be the facility that loses track of a source the way the cautionary cases did.

How DRPS Can Help

Diagnostic Radiation Physics Services helps medical and research facilities translate sealed-source obligations into workable programs. That includes radioactive material license support, NSTS reporting and reconciliation procedures, Part 37 security-plan review, aggregation and decay-correction calculations, sealed-source leak-test and inventory programs, and end-of-life disposition planning, coordinated with Radiation Safety Officer consulting and radiation safety training — all supported by board-certified medical physicists.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A source-tracking program is not just a reporting chore — it is the difference between always knowing where your most dangerous sources are and finding out the hard way that you don't.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR 20.2207: Reports of transactions involving nationally tracked sources. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1003: Definitions (nationally tracked source; Category 1 and Category 2). ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 37: Physical Protection of Category 1 and Category 2 Quantities of Radioactive Material (including Appendix A thresholds). ecfr.gov
Thongpraparn T, Chaudakshetrin P, Buranapong P. Lesson learned from Co-60 accident in Thailand. Australas Phys Eng Sci Med. 2002;25(4):172-174. doi:10.1007/BF03178291. PubMed
U.S. Nuclear Regulatory Commission. NUREG/BR-0472: National Source Tracking System. nrc.gov
U.S. Nuclear Regulatory Commission. NRC Form 748: National Source Tracking Transaction Report (OMB 3150-0202). nrc.gov
Rane S, Harris J. Development of a potential facility risk index for radiological security. Risk Anal. 2021;41(8):1257-1273. doi:10.1111/risa.13625. PubMed
Pham MH, Yu C, Rusch M, et al. Evolving societal risks and necessary precautions in the age of nuclear power and therapeutic radiation: an American perspective. World Neurosurg. 2014;82(6):1060-1070. doi:10.1016/j.wneu.2014.09.007. PubMed
McBurney RE, Steves K. Comprehensive review of CRCPD efforts in the disposition of disused radioactive sources. Health Phys. 2025;128(3):233-239. doi:10.1097/HP.0000000000001870. PubMed
Nisbet A, Li C, Berkovskyy V, et al. Radiation accidents and malicious events — scenarios and scope of the work of ICRP Task Group 120. Disaster Med Public Health Prep. 2025;19:e183. doi:10.1017/dmp.2025.10094. PubMed