Sealed Source Leak Testing & Inventory

Sealed source leak testing and physical inventory are two of the most routine, and most commonly cited, obligations in a radioactive materials license. Leak testing confirms that a source's encapsulation is intact by wiping it and checking for removable contamination at or above the 185 becquerel (0.005 microcurie) limit, while the semiannual inventory confirms that every source is physically present and accounted for.¹

These are small tasks with outsized consequences. A missed leak test, a lost source, or incomplete records are among the findings inspectors cite most often, and a genuinely leaking source is both a contamination hazard and a regulatory event. This guide explains the 10 CFR 35.67 requirements, the exemptions, the recordkeeping rules, the underlying measurement physics, and how a radiation safety officer builds a defensible program.

Introduction

Sealed sources are everywhere in medical and research facilities: dose calibrator reference sources, flood sources for gamma camera uniformity, brachytherapy sources, calibration check sources, and instrument sources for survey meters. Each is a small package of radioactive material encapsulated so that, under normal conditions, none of the material is accessible. The entire premise of a sealed source is that the encapsulation stays intact.¹⁶

Leak testing exists to verify that premise. Encapsulation can degrade through corrosion, mechanical damage, thermal stress, or simple age. A leaking source can spread removable contamination to hands, surfaces, instruments, and personnel. Because the failure is invisible, regulators require periodic wipe testing rather than relying on visual inspection.¹

Inventory serves a parallel purpose: confirming that every source the license authorizes is still where it should be. A source that cannot be located is a potential safety hazard and, for larger quantities, a security concern subject to additional physical-protection rules.⁵ Together, leak testing and inventory form the backbone of sealed source accountability. DRPS supports both as part of its radiation safety officer and radioactive material license support services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

What is a leak test?

A leak test is a periodic check that the encapsulation of a sealed source is intact and not releasing radioactive material into the environment. The licensee, or an authorized service provider, takes a wipe (smear) sample from the most accessible surface of the source or its nearest accessible point, analyzes that wipe for removable radioactivity, and compares the result to the regulatory limit.¹

The test measures removable contamination, not the source's own radiation field. A source can read high on a survey meter and still pass a leak test, because the question is not "how radioactive is it?" but "is any material coming off of it?"

For related contamination-control practices on incoming radioactive packages, which use a similar wipe-test methodology but for a different purpose, see package receipt and wipe testing.

What is the semiannual inventory?

The physical inventory is a documented confirmation that every sealed source the licensee possesses is physically present and accounted for. It is performed at least every six months and recorded with enough detail, model and serial numbers, radionuclide, activity, and location, to demonstrate complete accountability.¹²

Leak testing and inventory are often performed together as a single periodic task, but they are distinct requirements with distinct records.

Key Technical Principles

The 185 Bq (0.005 µCi) limit

The central number in 10 CFR 35.67 is 185 becquerels (0.005 microcurie) of removable radioactive material. This value appears twice in the regulation: the licensee must analyze the wipe sample so that the test can detect 185 Bq, and if the test reveals 185 Bq or more of removable contamination the source is considered to be leaking.¹

This same 0.005 µCi limit appears in the analogous leak-test provisions for industrial radiography (10 CFR 34.27), well logging (10 CFR 39.35), and irradiators (10 CFR 36.59), making it the universal removable-contamination threshold across NRC byproduct-material programs.⁹

Measurement physics of a wipe test

A leak test converts a measured count rate into an estimate of removable activity. The activity on the wipe is:

A = \frac{R _{net}}{ε}

where $A$ is the activity in becquerels, $R_{net}$ is the net count rate (gross minus background) in counts per second, and $ε$ is the detector counting efficiency (counts per disintegration) for the radionuclide and geometry. To express a counting result in counts per minute, recall:

R [cps] = \frac{R [ cpm ]}{60}

The instrument and method must be sensitive enough to detect the limit. To confirm this, compute the count rate the limit would produce:

R_{limit} = ε \cdot A_{limit} = 0.10 \times 185 Bq = 18.5 cps \approx 1110 cpm

assuming a counting efficiency of $ε = 0.10$ . As long as the system's minimum detectable activity is below 185 Bq, the method satisfies the detection requirement.¹

Worked leak-test example

Suppose a wipe of a Cs-137 check source is counted in a well counter:

Assumptions:

Gross count: 200 cpm.
Background: 110 cpm.
Net count rate: $R_{net} = 200 - 110 = 90 cpm = 1.5 cps$ .
Counting efficiency for Cs-137 in this geometry: $ε = 0.12$ .

The estimated removable activity is:

A = \frac{1.5}{0.12} = 12.5 Bq

This is well below the 185 Bq limit, so the source passes. If instead the net rate had been 3000 cpm (50 cps), the activity would be:

A = \frac{50}{0.12} \approx 417 Bq

which exceeds 185 Bq. The source would be considered leaking, immediately withdrawn from use, stored to prevent the spread of contamination, and reported to the regulator within 5 days of the leak test.¹

Summary of the 10 CFR 35.67 requirements

Parameter	Requirement	Citation
Removable contamination limit / detection sensitivity	185 Bq (0.005 µCi)	10 CFR 35.67(c), (d)
Leak test interval	Not to exceed 6 months (or per SS&D Registry-approved interval); before first use unless supplier certificate is ≤ 6 months old	10 CFR 35.67(b)
Action on a positive test	Withdraw from use immediately; report within 5 days	10 CFR 35.67(d)
Physical inventory interval	Semiannual (every 6 months); GSR sources excepted	10 CFR 35.67(g)
Leak test record retention	3 years	10 CFR 35.2067(a)
Inventory record retention	3 years	10 CFR 35.2067(b)

Clinical Impact

Why this matters in a medical facility

In a nuclear medicine department, sealed sources support daily operations: dose calibrator reference sources verify instrument constancy, sealed flood sources check gamma camera uniformity, and brachytherapy sources deliver therapy. If a reference source is leaking, contamination can spread to the instrument, the hot lab bench, and staff hands, and can confound the very QC measurements the source is supposed to support.⁶

A reliable program protects three things at once: staff and patients from unplanned contamination, data integrity by keeping QC sources clean, and the license by demonstrating compliance during inspection. Many facilities fold leak testing and inventory into the same periodic radiation safety routine that includes survey meter performance checks and area surveys.

Inventory as the first line against loss

A source that drifts out of inventory, misplaced during a room renovation, transferred without documentation, or simply never reconciled, is the beginning of a lost-source event. Lost and stolen sources draw intense regulatory and sometimes public attention. The semiannual inventory is deliberately frequent enough to catch a discrepancy before it becomes an incident, and the records create an auditable chain of custody for every source on the license.¹⁵

Practical Optimization Tips

A defensible sealed source program is built on routine and documentation.

1. Maintain a master source list

Keep a single authoritative list of every sealed source: radionuclide, model and serial number, nominal activity, purchase and receipt dates, location, and leak-test and inventory due dates. This list drives everything else.

2. Calendar the intervals conservatively

Schedule leak tests and inventory before the 6-month deadline, not on it, to absorb scheduling slips, staff absences, and shipping delays for sources sent out for testing.

3. Verify your counting method detects the limit

Confirm that the counter, efficiency, and geometry can detect 185 Bq for each radionuclide tested. Document the efficiency and the minimum detectable activity.¹

4. Wipe the right location

Sample the most accessible surface near the source consistent with ALARA and the manufacturer's instructions; never disassemble a source. For high-activity sources, wipe the nearest accessible surface of the housing or device.

5. Apply the exemptions correctly

Do not leak test sources that are genuinely exempt (short half-life, gaseous, low-activity, Ir-192 nylon-ribbon seeds), but document why each exempt source qualifies. Remember that sources in storage must still be tested before being placed back into use if they have not been tested in the prior 6 months.¹

6. Keep the records for 3 years

Retain both leak test and inventory records for at least 3 years with all required content. Missing or incomplete records are a frequent inspection finding even when the underlying tests were performed.²

Common pitfalls to avoid

Confusing leak testing with package wipe testing. They use similar wipes but answer different questions and satisfy different rules.
Measuring the source's field instead of removable activity. A high dose rate is not a failed leak test.
Letting the interval slip past 6 months, which is itself a violation regardless of the result.
Assuming a source in storage never needs testing. It must be tested before any return to use.
Incomplete records. Omitting the serial number, activity, location, or tester name undermines an otherwise valid test.
Forgetting the 5-day report when a source actually fails.

Regulatory Considerations

Sealed source leak testing and inventory are governed by the medical-use regulations at 10 CFR Part 35 and supported by the broader byproduct-material framework. The records and procedures should be written into the facility's radiation protection program so they are defensible during inspection.

Key frameworks:

10 CFR 35.67 — Requirements for possession of sealed sources and brachytherapy sources, including the leak test limit, interval, exemptions, action on a positive test, and the semiannual physical inventory.¹
10 CFR 35.2067 — Records of leak tests and physical inventory, each retained for 3 years with specified content.²
10 CFR Part 20 — Standards for Protection Against Radiation, the dose-limit and survey framework that contextualizes contamination control.³
10 CFR 30.53 ("Tests") — the Commission's general authority to require testing of byproduct material and equipment; note this is a general testing-authority provision and is not itself the source of the 0.005 µCi limit.⁴
10 CFR Part 37 — Physical Protection of Category 1 and Category 2 Quantities of Radioactive Material, which adds security and accountability requirements for aggregated high-activity sources.⁵
NRC NUREG-1556, Volume 9, Revision 3 (2019) — program-specific guidance for medical-use licenses, including expectations for leak testing, inventory, and recordkeeping.⁶
The NRC Sealed Source and Device (SS&D) Registry — the national database of registration certificates that may specify a leak-test interval other than 6 months for a particular source design.⁷

Agreement States administer their own equivalent programs. Of the states DRPS serves, Florida, Maryland, Virginia, California, and Nevada are NRC Agreement States that license medical use under their own radiation-control rules, while Washington, DC and Delaware are regulated directly by the NRC for radioactive material. A licensee must verify which authority issues its license and confirm any source-specific interval before relying on a default. For broader licensing context, see our NRC license guide and the related Part 37 source security overview. To prepare the program for review, see preparing for an NRC inspection.

The international risk framework behind source security, the IAEA categorization of sealed sources into Categories 1 through 5, underlies the Category 1/2 thresholds referenced in Part 37 and is useful context when prioritizing accountability effort by hazard.⁸

Frequently Asked Questions (FAQs)

What is sealed source leak testing?

Sealed source leak testing is a periodic check that the encapsulation of a sealed radioactive source is intact and not releasing radioactive material. The licensee wipes the accessible surface of the source or its container, analyzes the wipe for removable radioactivity, and confirms it is below the regulatory limit. A positive test means the source may be leaking and must be removed from use.

What is the leak test limit under 10 CFR 35.67?

Under 10 CFR 35.67, the leak test must be able to detect 185 becquerels (0.005 microcurie) of radioactive material on the sample, and that same value is the action limit. If a leak test reveals 185 Bq (0.005 microcurie) or more of removable contamination, the source must be immediately withdrawn from use, and a report must be filed within 5 days of the leak test.

How often must sealed sources be leak tested?

Sealed sources must be leak tested at intervals not to exceed 6 months, or at other intervals approved by the NRC or an Agreement State and specified in the Sealed Source and Device Registry. A source must also be tested before its first use unless the licensee has a supplier certificate showing it was tested within the previous 6 months.

Which sealed sources are exempt from leak testing?

Under 10 CFR 35.67, leak testing is not required for sources containing only byproduct material with a half-life of less than 30 days, sources containing only gaseous byproduct material, sources containing 3.7 MBq (100 microcurie) or less of beta- or gamma-emitting material or 0.37 MBq (10 microcurie) or less of alpha-emitting material, and iridium-192 seeds encased in nylon ribbon. Sources in storage and not in use are also exempt while stored, but must be tested before use or transfer if not tested in the previous 6 months.

What is the physical inventory requirement for sealed sources?

A licensee possessing sealed or brachytherapy sources must conduct a semiannual physical inventory of all such sources, except gamma stereotactic radiosurgery sources. The inventory verifies that every source is present and accounted for and is documented with the model and serial numbers, radionuclide, nominal activity, location, and the name of the person performing it.

How long must leak test and inventory records be kept?

Under 10 CFR 35.2067, records of sealed source leak tests and the semiannual physical inventory must each be retained for 3 years. Leak test records must include the source identity, activity, test result, date, and tester; inventory records must include source identity, nominal activity, location, and the person performing the inventory.

Why does source accountability matter for security, not just safety?

Lost or stolen sealed sources are both a radiation safety hazard and a security concern. For aggregated Category 1 and Category 2 quantities, 10 CFR Part 37 imposes physical protection requirements against theft and diversion. A reliable inventory and leak testing program is the first line of defense, ensuring sources are tracked, intact, and secured throughout their lifecycle.

Key Takeaways

The leak test limit is 185 Bq (0.005 µCi) of removable contamination, which is both the required detection sensitivity and the action limit under 10 CFR 35.67.
Test at intervals not exceeding 6 months, and before first use unless a supplier certificate covers the prior 6 months.
A positive test triggers immediate withdrawal and a 5-day report.
A semiannual physical inventory confirms every source is accounted for, with GSR sources excepted.
Records for both must be kept for 3 years with the required content under 10 CFR 35.2067.
Know the exemptions, but document why each exempt source qualifies, and remember stored sources must be tested before returning to use.

Conclusion

Sealed source leak testing and inventory are unglamorous, repetitive tasks, and that is precisely why they are so often cited. The physics is simple, the limits are fixed, and the intervals are short, so failures are almost always failures of process: a missed deadline, an incomplete record, a stored source returned to use without testing, or a source that quietly dropped off the inventory.

A radiation safety officer who treats these obligations as a managed routine, with a master source list, conservative scheduling, a verified counting method, and disciplined recordkeeping, turns a common source of citations into a quiet, defensible part of the radiation protection program. The result is not just inspection readiness; it is confidence that every source is intact, located, and accounted for.

How DRPS Can Help

Diagnostic Radiation Physics Services helps licensees build and maintain sealed source accountability programs. This includes establishing master source inventories, verifying that leak-test counting methods can detect the 185 Bq limit, setting conservative testing and inventory schedules, drafting procedures and record templates aligned with 10 CFR 35.67 and 35.2067, and supporting RSO duties and inspection readiness.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware. To discuss your sealed source program, contact our team.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR 35.67: Requirements for possession of sealed sources and brachytherapy sources. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 35.2067: Records of leak tests and inventory of sealed sources and brachytherapy sources. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 30.53: Tests. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 37: Physical Protection of Category 1 and Category 2 Quantities of Radioactive Material. ecfr.gov
U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Consolidated Guidance About Materials Licenses — Program-Specific Guidance About Medical Use Licenses. 2019. nrc.gov
U.S. Nuclear Regulatory Commission. National Sealed Source and Device (SS&D) Registry. nrc.gov
International Atomic Energy Agency. Categorization of Radioactive Sources (Safety Guide RS-G-1.9). 2005. iaea.org
U.S. Nuclear Regulatory Commission. 10 CFR 34.27: Leak testing and replacement of sealed sources (industrial radiography). ecfr.gov
U.S. Nuclear Regulatory Commission. NUREG-2155, Revision 2: Implementation Guidance for 10 CFR Part 37. nrc.gov