Sealed source leak testing: requirements, frequency, and choosing a lab

Leak testing is one of the most routine, recurring compliance obligations for any sealed source license, and also one of the most commonly under-documented when inspectors go looking. This guide covers what leak testing actually verifies, how often it's required, and how to think about doing it in-house versus outsourcing it to a lab.

Leak testing verifies source integrity, not activity or exposure rate. A leak test is specifically designed to detect removable (not fixed) radioactive contamination on the surface of a sealed source, which would indicate the source's containment has failed — a crack in the encapsulation, corrosion, or manufacturing defect allowing radioactive material to migrate to the accessible surface. It is not a measurement of the source's current activity (that's tracked separately via decay calculation from the calibration certificate) and not a general area radiation survey (that's a separate, also-required, routine task). Confusing these three distinct compliance activities — leak testing, activity tracking, and area surveys — is a common source of program gaps.

The standard method is a wipe test, and the regulatory action level is well-defined. The typical leak test method involves wiping the source's accessible surface (or, for sources not directly accessible, the surface of the device housing it) with a swab or filter paper, then counting that wipe on a sensitive instrument (typically a liquid scintillation counter or gas-flow proportional counter) capable of detecting activity well below the regulatory action level. The standard action level under 10 CFR and most Agreement State equivalents is 0.005 microcuries (185 Bq) of removable contamination — a wipe result at or above this level requires immediate source withdrawal from use and notification to your regulatory authority.

Frequency depends on source type and use category, generally every 6 months as the default, with real exceptions. Most sealed sources in active use require leak testing at intervals not to exceed 6 months. Some categories have different intervals: sources in storage and not in use may qualify for a reduced testing frequency (sometimes annual) under specific conditions defined in your license. Very low-activity sources (below specific thresholds defined in the regulations) may be exempt from routine leak testing altogether. Check your specific license conditions rather than assuming a blanket 6-month rule applies to every source in your inventory — license-specific conditions can be more or less stringent than the general regulatory default.

New sources get tested on receipt, and existing sources get retested after certain events regardless of the routine schedule. A newly received source is leak tested (or a valid recent leak test certificate from the manufacturer is accepted, depending on your license conditions) before it's placed into use. Outside of the routine schedule, a source should also be leak tested after any event that could plausibly compromise its integrity — dropping the source or its housing, a fire or significant heat exposure, evidence of physical damage to the device, or any incident report that raises a reasonable question about containment.

In-house leak testing is common and permitted, provided you have the right instrumentation and a documented procedure. Many license holders with sealed sources perform leak tests in-house rather than sending sources out, using either a liquid scintillation counter (common in institutions that already have one for other research purposes) or a wipe-and-send-to-a-commercial-lab hybrid approach where the wipe is taken on-site but counted by an outside lab. In-house testing requires your license to specifically authorize it, a documented procedure, and access to counting equipment with adequate sensitivity to reliably detect activity at the regulatory action level — a general-purpose survey meter is not an adequate substitute for proper wipe-counting instrumentation.

Outsourced leak testing services are widely available and often the more practical choice for smaller programs. For license holders without in-house counting capability, or with a small enough source inventory that building in-house capacity doesn't make economic sense, commercial leak testing kits are available from multiple vendors: a pre-made wipe kit is shipped to you, you perform the physical wipe per the included instructions, and ship the wipe back for laboratory counting, typically receiving a certificate of results within days. This is the most common approach for small-to-mid-size license holders with modest sealed source inventories (a handful to a few dozen sources).

Evaluating a leak testing service or lab. Confirm the lab's counting instrumentation and stated minimum detectable activity (MDA) — it should be comfortably below the 0.005 microcurie action level, with margin, not just barely under it. Confirm turnaround time, particularly if you're leak testing sources that are actively needed for ongoing work and can't sit idle awaiting results. Confirm the certificate format meets what your license and your regulator expect — source identification, wipe date, count date, result, and MDA should all be documented, not just a pass/fail statement. Ask about their track record with your specific isotope and source type, since counting efficiency and appropriate methodology can vary by isotope.

Recordkeeping is where leak testing programs most commonly fail inspection, not the testing itself. The physical act of leak testing is usually done correctly by license holders. The more common inspection finding is incomplete or disorganized records — a leak test performed but not filed against the correct source serial number, a gap in the testing interval that wasn't caught because no one was tracking due dates systematically, or a certificate that doesn't clearly document the actual numeric result and MDA rather than just a checkbox. A simple tracking system — even a spreadsheet listing every source, its testing interval, last test date, and next due date — closes most of this gap and is exactly what an inspector will ask to see.

Sources in devices (gauges, irradiators) versus bare sources add complexity. A sealed source permanently installed in a nuclear gauge or other device is typically leak tested through the device's designated access port or by wiping the device's exterior surface per the manufacturer's specified procedure, rather than by direct access to the source itself. Follow the device manufacturer's specific leak testing procedure rather than improvising a generic approach, since device geometry significantly affects where a leak test wipe will actually be representative of the source's condition.

The procurement playbook. Confirm your specific license's leak testing interval for each source category rather than assuming a blanket default. Decide in-house versus outsourced based on your source inventory size and whether you already have adequate counting instrumentation. If outsourcing, confirm MDA, turnaround, and certificate format before committing to a vendor. Build a simple tracking system for testing due dates regardless of which approach you use — this is the single highest-leverage step to avoid the most common inspection finding in this area.