Decay-in-Storage of Radioactive Waste (35.92)

Decay-in-storage (DIS) is the simplest and most economical way to dispose of short-lived medical radioactive waste: hold it until it decays, survey it to background, remove the labels, and discard it as ordinary trash. Under NRC 10 CFR 35.92, a licensee may hold byproduct material with a physical half-life of 120 days or less for decay and then dispose of it without regard to its original activity — provided a surface survey with an appropriate meter on its most sensitive scale, no interposed shielding, cannot distinguish the waste from background, and all radiation labels are removed or obliterated. ¹

DIS is deceptively simple, which is exactly why it generates citations. Facilities get the half-life eligibility wrong, survey with the wrong instrument or on the wrong scale, forget to deface labels, keep incomplete records, or let a decay room become a disorganized radiation-safety liability. This guide explains the physics, the current rule, and the operational discipline that turns DIS into a clean, defensible waste pathway. DRPS builds these programs as part of radiation safety officer and radioactive material license support services.

Introduction

Nuclear medicine runs on short-lived radionuclides. That is a clinical advantage — dose to the patient falls quickly — and it is also the foundation of the cheapest disposal method in the field. Because activity decreases exponentially, most diagnostic waste becomes indistinguishable from background within days, and even many therapy nuclides do so within weeks. Rather than pay to ship this material off-site as radioactive waste, a licensee can simply let physics do the work and hold it until it decays.

That is the entire idea behind decay-in-storage. It is authorized specifically for short-lived byproduct material, it eliminates the cost and paperwork of transferring waste to a licensed disposal facility, and it is used every day in nuclear medicine departments. But "hold it until it decays" is a regulatory action with defined criteria, not an informal habit. The licensee must know which material qualifies, must survey correctly before release, must remove labels, and must keep records that will survive an inspection.

Decay-in-storage is one of several disposal pathways a comprehensive program uses; it sits alongside transfer to authorized recipients, and the separate rules for sewer disposal of soluble excreta and for certain liquid scintillation and animal-tissue wastes. For the broader picture, see our guides to radioactive waste management in nuclear medicine and sewer disposal of radioactive material.

Topic Explanation

What is decay-in-storage?

Decay-in-storage is the practice of holding radioactive waste in a controlled location until radioactive decay has reduced it to a level indistinguishable from background, after which it may be disposed of as ordinary or medical waste. The legal basis for medical licensees is 10 CFR 35.92, "Decay-in-storage," or the equivalent Agreement State regulation. ¹

The rule has two operative requirements before disposal:

Monitor at the surface. The licensee monitors the byproduct material at its surface before disposal and determines that its radioactivity cannot be distinguished from the background radiation level, using an appropriate radiation detection survey meter set on its most sensitive scale and with no interposed shielding. ¹
Remove the labels. All radiation labels are removed or obliterated (other than on containers that will continue to be used for licensed material), so that discarded material is not mistaken for — or handled as — radioactive. ¹

Meeting both requirements allows disposal without regard to the material's original radioactivity: the survey, not the starting activity, governs.

What qualifies — the 120-day limit

Only byproduct material with a physical half-life of 120 days or less is eligible for decay-in-storage under 10 CFR 35.92. ¹ That threshold captures essentially all routine nuclear medicine waste:

Diagnostic: Tc-99m (~6 hours), F-18 (~110 minutes), I-123 (~13 hours), In-111 (~2.8 days), Tl-201 (~3 days), Ga-67 (~3.3 days).
Therapy and other: I-131 (~8 days), Lu-177 (~6.6 days), I-125 (~59 days).

It does not cover longer-lived material such as Co-57 (~272 days), Cs-137 (~30 years), or Ge-68/Ga-68 generator components governed by the generator's own return program. Long-lived waste must be transferred to an authorized recipient or disposed of by another NRC- or state-approved route. The first question in any DIS decision is therefore always the half-life.

Key Technical Principles

The decay math

Radioactive decay is exponential. For an initial activity $A_{0}$ and physical half-life $T_{1/2}$ , the activity after time $t$ is:

A (t) = A_{0} (\frac{1}{2})^{t / T_{1/2}} = A_{0} e^{- λ t}, λ = \frac{ln 2}{T _{1/2}}

After $n$ half-lives, the remaining fraction is simply $(1/2)^{n}$ . The familiar "ten half-lives" rule of thumb comes directly from this:

(\frac{1}{2})^{10} = \frac{1}{1024} \approx 0.098% \approx 0.1%

So after ten half-lives, less than about 0.1% of the original activity remains — usually indistinguishable from background for typical medical activities. This is a scheduling convenience, not the legal test: the rule authorizes disposal based on the survey result, and highly active starting material may still read above background at ten half-lives, while low-activity material may reach background sooner.

Hold times for common nuclides

The table below converts half-lives into a practical ten-half-life "rule-of-thumb" hold time and flags DIS eligibility under the 120-day limit. Actual release always depends on the surface survey, not the calendar.

Radionuclide	Physical half-life	~10 half-lives (rule-of-thumb hold)	DIS-eligible (≤120 d)?
F-18	~110 min	~18 hours	Yes (often decays same day)
Tc-99m	~6.0 hours	~2.5 days	Yes
I-123	~13.2 hours	~5.5 days	Yes
In-111	~2.8 days	~28 days	Yes
Lu-177	~6.6 days	~66 days	Yes
I-131	~8.0 days	~80 days	Yes
I-125	~59 days	~1.6 years	Yes (long hold)
Co-57	~272 days	—	No (>120 days)
Cs-137	~30 years	—	No

A short worked example: for Tc-99m ( $T_{1/2} \approx 6.0$ h), ten half-lives is about $10 \times 6.0 = 60$ hours, or roughly 2.5 days. For I-131 ( $T_{1/2} \approx 8.0$ d), ten half-lives is about 80 days. This is why a decay room needs enough space and organization to hold weeks of I-131 and Lu-177 waste while Tc-99m turns over in days.

The survey is the release criterion

Because the regulation ties disposal to a background comparison, the survey technique is where most DIS programs succeed or fail. Three details are explicit in the rule and non-negotiable: an appropriate instrument (typically a properly calibrated GM survey meter with a thin-window/pancake probe suited to the emissions), the most sensitive scale, and no interposed shielding between the probe and the waste surface. ¹ The reading is then compared to the background in the same area. Instrument selection and calibration matter here; see our guidance on choosing the right radiation survey meter.

Clinical Impact

Decay-in-storage is where radiation safety, operations, and cost intersect in a nuclear medicine department. Done well, it is nearly free and invisible; done poorly, it is a recurring source of inspection findings and wasted staff time.

The operational stakes include:

Cost avoidance. Shipping waste to a licensed low-level radioactive waste facility is expensive. DIS eliminates that cost for the large majority of medical waste, which is short-lived.
Space and workflow. A decay room must physically hold the volume generated over the longest relevant hold time (driven by I-131, Lu-177, and I-125). Under-sized or disorganized decay storage leads to overflow, mislabeling, and mixing of waste streams.
Contamination control. Decayed material is only "clean" if the survey confirms it; a bag with a hidden hot syringe or a mislabeled long-lived source can send activity into the regular trash — a reportable problem.
Inspection exposure. Decay-in-storage records and the decay room itself are routine inspection targets. Incomplete records or defaced-label failures are common, avoidable citations, as we discuss in common radiation safety violations.

Because it is so routine, DIS is easy to treat casually — which is precisely why disciplined procedures, training, and records pay off.

Practical Optimization Tips

Segregate by decay time at the point of generation

Separate waste by nuclide and half-life when it is generated, not later. A common scheme uses distinct, dated containers for short-lived (Tc-99m/F-18), medium (I-123/In-111), and longer-lived (I-131/Lu-177/I-125) waste so nothing is released before the slowest constituent has decayed. Date every container when sealed.

Survey correctly, every time

Use a calibrated, appropriate meter for the emissions present, on the most sensitive scale, with no interposed shielding. ¹
Establish and record the background in the same location immediately before surveying the waste, since background can vary near the hot lab.
Survey the surface of each container, and open bags to survey contents where a sealed bag could shield a hot item.
If anything reads above background, return it to decay and re-survey later.

Remove or deface every label

Remove or obliterate all radiation labels (radioactive symbols, warning tags, and generator/unit-dose markings) before the material leaves as ordinary or medical waste. ¹ This is one of the most frequently cited DIS failures and one of the easiest to prevent with a checklist.

Keep complete records

Record, for each disposal, the date of disposal, the survey instrument used, the background radiation level, the surface radiation level of each container, and the name of the individual who performed the survey, per 10 CFR 35.2092. ² Retain the records as required and keep them retrievable for inspection.

Route the decayed waste to the correct stream

Decay-in-storage removes the radioactive character; it does not change other hazards. Decayed sharps still go out as medical/biohazard waste, and certain liquid scintillation fluids and animal tissues have their own provisions under 10 CFR 20.2005. ⁴ Mixed waste (radioactive plus hazardous chemical) requires additional controls and cannot simply be trashed after decay.

Common pitfalls to avoid

Assuming eligibility. Confirm the 120-day half-life limit before placing anything in decay for disposal. ¹
Surveying on the wrong scale or through shielding. Both violate the rule and can pass hot waste. ¹
Skipping label removal. A frequent, avoidable citation. ¹
Thin records. Missing background readings or surveyor names are common findings. ²
Treating "ten half-lives" as the law. It is a scheduling heuristic; the survey is the release criterion.

Regulatory Considerations

Decay-in-storage is authorized by 10 CFR 35.92 and its record requirement 10 CFR 35.2092, within the broader waste-disposal and radiation-protection framework of 10 CFR Part 20. The program must be consistent with the facility's radioactive material license and ALARA obligations.

10 CFR 35.92 — Decay-in-storage. Authorizes holding byproduct material with a half-life ≤120 days for decay and disposal, subject to the surface-survey and label-removal criteria described above. ¹
10 CFR 35.2092 — Records of decay-in-storage. Specifies the required disposal-record contents. ²
10 CFR 20.2001 — General requirements for waste disposal. Establishes the permissible disposal routes for licensed material, of which decay-in-storage (via §35.92) is one; others include transfer to an authorized recipient and disposal as authorized under Part 20. ³
10 CFR 20.2005 — Disposal of specific wastes. Provides separate provisions for certain liquid scintillation media and animal tissue containing small concentrations of H-3 or C-14. ⁴
ALARA and dose limits. While in storage, waste must be managed so that occupational and public doses remain within 10 CFR Part 20 limits and as low as reasonably achievable — a properly located, shielded, secured, and access-controlled decay area. Our ALARA program guide covers the underlying framework.

A key modernization: earlier versions of Part 35 required holding waste for at least ten half-lives before disposal. The revised rule and NRC guidance removed that fixed ten-half-life hold, replacing it with the survey-to-background criterion; the ten-half-life figure now functions only as a practical planning rule of thumb. NRC guidance summarizing this change is captured in Regulatory Issue Summary 2004-17 and program-specific guidance in NUREG-1556, Volume 9. ⁵⁶⁷

Jurisdiction. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that administer their own equivalent decay-in-storage rules, while Washington, DC and Delaware are regulated directly by the NRC. Confirm the exact regulation and any state-specific conditions with the authority that issues your license before relying on this pathway.

Frequently Asked Questions (FAQs)

What is decay-in-storage?

Decay-in-storage (DIS) is a disposal method in which a licensee holds short-lived radioactive waste until it has decayed to background, then disposes of it as ordinary trash. Under NRC 10 CFR 35.92, byproduct material with a physical half-life of 120 days or less may be held for decay and disposed of without regard to its original radioactivity once a surface survey shows it cannot be distinguished from background.

Which radioactive waste qualifies for decay-in-storage?

Only byproduct material with a physical half-life of 120 days or less qualifies under 10 CFR 35.92. That covers most diagnostic and many therapeutic nuclear medicine nuclides, such as Tc-99m, F-18, I-123, In-111, Lu-177, and I-131, but not long-lived nuclides such as Co-57 or Cs-137, which must be transferred to an authorized recipient or disposed of by another approved route.

How do I survey waste before disposing of it after decay?

Monitor the waste at its surface with an appropriate radiation detection survey meter set on its most sensitive scale, with no interposed shielding, and confirm the reading cannot be distinguished from the background radiation level in the same area. All radiation labels must be removed or obliterated before the material is discarded as ordinary or medical waste, as appropriate.

Do I still have to hold waste for ten half-lives?

The current 10 CFR 35.92 does not require a fixed ten-half-life hold; the regulatory criterion is that a surface survey cannot distinguish the material from background. Ten half-lives (which reduces activity to about 0.1 percent of the original) remains a widely used practical rule of thumb for scheduling, but the survey — not the calendar — is what authorizes disposal.

What records must be kept for decay-in-storage disposal?

Under 10 CFR 35.2092, records of decay-in-storage disposal must include the date of disposal, the survey instrument used, the background radiation level, the radiation level measured at the surface of each waste container, and the name of the individual who performed the survey. These records must be retained as required by the rule and be available for inspection.

Can decay-in-storage be used for radioactive sharps and liquids?

Decay-in-storage applies to short-lived byproduct material regardless of physical form as long as the half-life and survey criteria are met, but the eventual disposal must still follow the correct waste stream — for example, decayed sharps as medical/biohazard waste and certain liquid scintillation or animal-tissue wastes under the separate provisions of 10 CFR 20.2005. Mixed and hazardous wastes require additional controls.

Key Takeaways

Decay-in-storage is for short-lived waste only — physical half-life of 120 days or less under 10 CFR 35.92. ¹
The survey, not the calendar, authorizes disposal: an appropriate meter on its most sensitive scale, no interposed shielding, reading indistinguishable from background. ¹
Remove or deface every radiation label before discarding — a top avoidable citation. ¹
Ten half-lives (~0.1% remaining) is a planning rule of thumb, not the legal test.
Keep complete 10 CFR 35.2092 records: date, instrument, background, surface reading, and surveyor name. ²
Decayed waste still needs the right stream — medical/biohazard, chemical, or the special provisions of 10 CFR 20.2005 — and long-lived nuclides need a different disposal route entirely. ³⁴

Conclusion

Decay-in-storage is the workhorse disposal method of nuclear medicine precisely because it lets exponential decay eliminate the hazard at no cost. But the simplicity is on the physics side, not the compliance side. A defensible DIS program knows the 120-day eligibility limit, segregates waste by decay time, surveys correctly to background with the right instrument on the most sensitive scale and no shielding, removes every label, keeps complete records, and routes the decayed material to the correct final waste stream.

Facilities that treat decay-in-storage as a documented radiation-safety procedure — rather than a bin in the back of the hot lab — save money, protect staff and the public, and walk into inspections with clean records. The rule rewards discipline, and the discipline is entirely learnable.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine facilities build and audit decay-in-storage and broader radioactive-waste programs: decay-room layout and shielding review, survey-instrument selection and calibration checks, written procedures, staff training, record templates aligned with 10 CFR 35.2092, and license-condition review. We deliver this through radiation safety officer, radioactive material license support, and radiation safety training services.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware. To review or build a decay-in-storage program, contact our team.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR 35.92: Decay-in-storage. nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR 35.2092: Records of decay-in-storage. nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.2001: General requirements for disposal of licensed material. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.2005: Disposal of specific wastes (liquid scintillation and animal tissue). ecfr.gov
U.S. Nuclear Regulatory Commission. Regulatory Issue Summary 2004-17, Revision 1: Recent Changes to 10 CFR Part 35 and Related Guidance. nrc.gov
U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Consolidated Guidance About Materials Licenses — Program-Specific Guidance About Medical Use Licenses. nrc.gov
U.S. Nuclear Regulatory Commission. Low-Level Radioactive Waste Toolbox: Decay-In-Storage (DIS). nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation (including § 20.1101 radiation protection programs/ALARA and § 20.1301 public dose limits). ecfr.gov