4.4 Radioactive Waste Disposal and Regulatory Records
Key Takeaways
- Decay-in-storage under 10 CFR 35.92 applies only to byproduct material with a physical half-life under 120 days: hold at least 10 half-lives, survey with no interposed shielding on the most sensitive scale until indistinguishable from background, deface all radiation labels, and monitor each generator column separately.
- Sanitary sewer release under 10 CFR 20.2003 requires readily soluble or dispersible biological material, monthly average concentrations within Appendix B Table 3, and annual totals no greater than 5 Ci hydrogen-3, 1 Ci carbon-14, and 1 Ci of all other radionuclides combined.
- Xe-133 and radioaerosol effluents are controlled by charcoal traps, exhaust engineering, and Appendix B Table 2 airborne release limits—trap saturation is a release-limit problem, not a housekeeping detail.
- Long-lived and mixed waste leaves through a licensed broker or authorized disposal site; nothing is released to ordinary trash without a documented survey and label removal.
- Retention periods differ by record type: many operational records (surveys, leak tests, dose-calibrator QC, written directives, decay-in-storage, patient release) are kept 3 years, while individual monitoring and disposal records are kept until the license is terminated.
4.4 Radioactive Waste Disposal and Regulatory Records
Quick Answer: Four legal exits exist for licensed material: decay-in-storage (half-life < 120 days, hold ≥ 10 half-lives, survey to background, deface labels), sanitary sewer (soluble/dispersible, within concentration and annual quantity limits), effluent release (trapped/monitored gases and aerosols), and transfer to an authorized recipient (licensed broker). Every exit requires a record, and retention runs 3 years for most operational records but until license termination for monitoring and disposal records.
Section 4.1 introduced waste and records in outline. Domain II items go further: they hand you a nuclide, a half-life, a survey reading, and a container and ask which disposal pathway is lawful. Memorize the numbers.
Storage Before Disposal
Waste is segregated at the point of generation, because mixing defeats every downstream option.
| Segregation axis | Practical bins |
|---|---|
| Physical form | Sharps, dry solids, absorbed liquids, bulk liquids, gases, biological/pathological |
| Half-life | Short-lived (Tc-99m, F-18, I-123), intermediate (In-111, Ga-67, Tl-201, I-131), long-lived (Cs-137, Co-57 sources) |
| Hazard overlap | Radioactive-only vs mixed waste (radioactive + RCRA hazardous) vs radioactive + infectious |
Each container is labeled with the nuclide, date sealed, and responsible area, stored in a posted, secured location, shielded so that dose rates at the boundary satisfy restricted/unrestricted-area limits. Mixed waste cannot be decayed away as a shortcut—the chemical hazard survives the radioactivity, so it goes to a permitted mixed-waste pathway.
Decay-in-Storage (10 CFR 35.92)
Decay-in-storage (DIS) is the workhorse for diagnostic nuclear medicine because most of what a hot lab generates is short-lived.
| Requirement | Rule |
|---|---|
| Eligible material | Byproduct material with a physical half-life of less than 120 days |
| Hold time | At least 10 half-lives (10 half-lives leaves < 0.1% of the original activity) |
| Survey before release | Survey the surface of the waste with an appropriate instrument set on its most sensitive scale, with no interposed shielding, and confirm the reading cannot be distinguished from background |
| Labels | Remove or deface all radiation labels before disposal in ordinary trash |
| Generators | Each generator column is monitored individually, not as part of a bulk bag |
| Records | Date sealed, nuclides, survey date/result, instrument used, technologist |
Worked example. A bag holds Tc-99m sharps sealed at noon Monday. Ten half-lives at 6.02 h is about 60 hours, so the earliest lawful survey is Wednesday around midnight—departments simply hold Tc-99m waste 72 hours for margin. An I-131 bag (T½ 8.02 d) needs about 80 days, still under the 120-day eligibility ceiling. A Cs-137 check source (T½ 30 y) is never DIS-eligible.
Trap: the survey requirement is not "read background on the ratemeter across the room." It is a surface survey, most sensitive scale, no shielding—including no lead pig left around the vial.
Liquid Disposal and the Sanitary Sewer
Under 10 CFR 20.2003, release to the sanitary sewer is allowed only when the material is readily soluble or readily dispersible biological material in water and the licensee stays within:
| Limit type | Value |
|---|---|
| Concentration | Monthly average release concentration at or below Appendix B, Table 3 values |
| Annual quantity | 5 Ci of hydrogen-3, 1 Ci of carbon-14, and 1 Ci of all other radionuclides combined |
Patient excreta are explicitly exempt from these sewer limits—therapy patients use the toilet. Bulk stock vials, unused therapy doses, and laboratory liquids are not exempt and are usually held for decay instead. Never pour an unknown, an organic solvent, or a scintillation cocktail down a drain.
Gases and Aerosols
| Effluent | Control |
|---|---|
| Xe-133 | Closed spirometer/delivery system with an activated charcoal trap or direct exhaust to a monitored stack; track cumulative trap loading and change on schedule |
| Radioaerosol (Tc-99m DTPA) | Shielded, vented nebulizer with a filtered exhaust path; contain the exhaled fraction |
| Room air | Negative pressure and non-recirculating exhaust per license commitments |
Airborne releases must satisfy Appendix B, Table 2 effluent concentration limits and keep public dose within 0.1 rem (1 mSv)/year. A saturated xenon trap is a regulatory release event, not a maintenance annoyance—document the change-out.
Transfer to an Authorized Recipient
Long-lived waste, sealed sources, mixed waste, and anything failing DIS eligibility goes to a licensed broker or authorized disposal site on a manifest that names the licensee, nuclides, activities, dates, and package type. Shipping follows the DOT rules in §4.2. Sealed sources are frequently returned to the manufacturer under the original purchase agreement.
Record Maintenance and Retention
Inspection reality: an activity you cannot document did not happen.
| Record | Common retention |
|---|---|
| Receipt, transfer, and disposal of licensed material | Disposal records until license termination; transfer/receipt per license conditions |
| Area surveys and wipe tests | 3 years |
| Instrument calibration and dose-calibrator QC (constancy, accuracy, linearity, geometry) | 3 years |
| Sealed-source leak tests and semiannual inventory | 3 years |
| Decay-in-storage disposal logs | 3 years |
| Written directives and administration records | 3 years |
| Patient release records and instructions (§35.75) | 3 years |
| Individual (occupational) monitoring records and declared-pregnancy files | Until the license is terminated |
Records must identify the nuclide, activity, date, location, instrument, and the individual who performed the action. Electronic records are acceptable when they are secure, retrievable, and attributable.
Exam Hooks
- "Tc-99m sharps sealed 24 hours ago, meter reads twice background" → not yet releasable; hold to 10 half-lives and resurvey.
- "Cs-137 flood source no longer needed" → not DIS; return to manufacturer or licensed broker.
- "Unused I-131 therapy capsule" → decay or transfer, never the sewer.
- "Charcoal trap differential pressure rising after xenon studies" → change and document; effluent limits apply.
- "How long do we keep the wipe-test log?" → 3 years; the dosimetry file, by contrast, stays until license termination.
A sealed bag of Tc-99m contaminated sharps has been held for 72 hours. Which combination of steps makes disposal in ordinary trash lawful under 10 CFR 35.92?
Which material is NOT eligible for decay-in-storage disposal?
Under 10 CFR 20.2003, which statement about sanitary sewer disposal of licensed material is correct?