Contamination Surveys, Spill Response & Personnel Decontamination

Contamination Surveys, Spill Response & Personnel Decontamination

Unsealed radiopharmaceuticals can spread wherever hands, carts, and patients go. A contamination control program needs the right detectors, a clear definition of what is "removable," spill thresholds matched to each radionuclide's hazard, and decontamination methods that do not make things worse.

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Contamination Survey Methodology: Direct Monitoring vs. Indirect Wipe Testing

Radiation safety programs require regular surveys to confirm that unsealed activity remains confined to designated compounding and administration zones.

Detector Physics and Operational Selection

1. Thin End-Window Pancake GM Detectors: The standard survey instrument for beta-gamma surface contamination is the halogen-quenched pancake Geiger-Müller probe (e.g., LND 7311), featuring an active circular area of $15.5\text{ cm}^2$ and an ultra-thin mica window ($1.7\text{ to } 2.0\text{ mg/cm}^2$).
   • Performance: Highly sensitive to medium- and high-energy beta particles ($^{177}\text{Lu}$, $^{90}\text{Y}$, $^{131}\text{I}$), achieving $4\pi$ counting efficiencies of $20\%\text{ to } 45\%$. In contrast, its intrinsic detection efficiency for gamma rays is poor ($< 1\%$), interacting primarily via secondary electrons ejected from the stainless steel housing.
   • Operational Rules: Surveyors must hold the probe face within $0.5\text{ to } 1.0\text{ cm}$ of the surface without touching the surface (to prevent probe contamination), translating the detector at a scan speed $\le 2.0\text{ to } 5.0\text{ cm/second}$ to accommodate the analog or digital ratemeter time constant.
2. Zinc Sulfide ZnS(Ag) Scintillation Detectors: Dedicated to alpha surface contamination ($^{223}\text{Ra}$ and daughters). ZnS(Ag) phosphor crystals mounted on a light pipe are protected by an aluminized Mylar window ($< 1.0\text{ mg/cm}^2$). Alpha particles penetrate the thin window and excite dense scintillation flashes. Pulse-height discrimination rejects low-amplitude beta/gamma pulses, achieving $< 1\text{ CPM}$ background with high alpha efficiency ($30\%\text{ to } 45\%$).
   • Physical Constraint: Because a $5$–$6\text{ MeV}$ alpha particle travels only about $4$–$5\text{ cm}$ in air and loses energy along the way, the probe must be swept within about $5\text{ mm}$ of the surface.
3. Sodium Iodide NaI(Tl) Scintillation Probes: Essential for detecting low-energy gamma emitters ($^{125}\text{I}$, $^{177}\text{Lu}$, $^{99\text{m}}\text{Tc}$) where high stopping power is required to locate microcurie-level contamination hidden behind shielding or inside porous benchtop cracks.

Direct vs. Indirect (Smear) Surveys

Direct survey with a GM or scintillator measures total surface contamination (fixed plus removable). In high-background areas (near hot cells, generators, or waste storage), ambient radiation can mask low-level contamination, so licensing commitments call for wipe (smear) tests, counted away from the background, to quantify removable contamination.

• Standard Wipe Protocol: A dry or alcohol-wetted absorbent paper disk (e.g., Whatman 41, $4.25\text{ cm}$ diameter) or cotton swab is rubbed over a representative surface area of $100\text{ cm}^2$ using uniform moderate pressure (approximately $20\text{ Newtons}$ or $2\text{ kgf}$), moving across an S-shaped or concentric perimeter path.
• Wipe Removal Efficiency Factor ($W_f$): Wiping does not pick up all of the removable activity. For package surveys, DOT (49 CFR 173.443) divides the activity on the wipe by the wiped area and by the wipe efficiency, which may be assumed to be $W_f = 0.10$ if the actual efficiency is not determined. Many facility area-survey programs instead compare the activity on the wipe directly with their action levels; the written procedure must say which convention is used.

Quantitative Activity Conversion Formula

After counting the wipe sample in a calibrated NaI(Tl) well counter (for gamma emitters) or a liquid scintillation counter (LSC, for pure beta or alpha emitters), the absolute removable activity in disintegrations per minute per $100\text{ cm}^2$ is calculated as:

$\text{Activity } \left(\frac{\text{DPM}}{100\text{ cm}^2}\right) = \frac{R_{\text{gross}} - R_{\text{bkg}}}{\epsilon_{\text{det}} \times W_f}$

where $R_{\text{gross}}$ is the sample count rate in counts per minute (CPM), $R_{\text{bkg}}$ is the background count rate, $\epsilon_{\text{det}}$ is the counting efficiency (counts per disintegration), and $W_f$ is the wipe efficiency when the program applies one (set $W_f = 1$ to report the activity on the wipe itself).

Worked Contamination Calculation

A technician performs a routine $100\text{ cm}^2$ wipe survey of a radiopharmacy injection cart following a $^{177}\text{Lu}$-Dotatate infusion. Counting the wipe in a calibrated automated NaI(Tl) well counter (photopeak window centered across the $113\text{ keV}$ and $208\text{ keV}$ emissions, $\epsilon_{\text{det}} = 0.65$ or $65\%$) yields a gross count rate $R_{\text{gross}} = 540\text{ CPM}$ against an instrument background $R_{\text{bkg}} = 85\text{ CPM}$. Using the DOT-style default wipe efficiency $W_f = 0.10$:

$\text{Removable Activity} = \frac{540\text{ CPM} - 85\text{ CPM}}{0.65 \times 0.10} = \frac{455\text{ CPM}}{0.065} = 7,000\text{ DPM}/100\text{ cm}^2$

In SI units, dividing by $60\text{ DPM/Bq}$ yields $116.7\text{ Bq}/100\text{ cm}^2 = 1.17\text{ Bq/cm}^2$. Even without the wipe-efficiency correction (700 DPM per 100 cm² on the wipe itself), this is well above the action levels facilities typically set for areas open to the public (often a few hundred DPM per 100 cm² for beta-gamma emitters), so the cart must be decontaminated before it leaves the area.

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Radioactive Spill Response Protocols: Minor vs. Major Incidents

NUREG-1556 Volume 9, Revision 3 (Table N-1, "Relative Hazards of Common Radionuclides") gives radionuclide-specific guidance: spills above these amounts should be considered major, and spills below them minor.

The guidance stresses that the choice also depends on incident-specific factors: the number of people affected, other hazards, the likelihood of spread, the surfaces involved, and the radiotoxicity and volatility of the material.

                                 RADIOACTIVE SPILL DETECTED
                                              |
                     +------------------------+------------------------+
                     |                                                 |
          Activity / Volatility                             Activity / Volatility
         BELOW Threshold Limits                            ABOVE Threshold Limits
        (e.g., <=1 mCi I-131, <=100 mCi Tc-99m)          (e.g., >1 mCi I-131, >100 mCi Tc-99m)
                     |                                                 |
                     v                                                 v
             [ MINOR SPILL ]                                   [ MAJOR SPILL ]
                     |                                                 |
    1. NOTIFY persons in immediate area               1. STOP spill / source immediately
    2. PREVENT spread (absorbent pads)                2. WARN all occupants & EVACUATE room
    3. DON PPE (double gloves, coat, booties)         3. ISOLATE area (lock doors, seal jambs)
    4. CLEAN: perimeter -> center (chelating agent)   4. MINIMIZE exposure (time, distance)
    5. SURVEY: GM pancake + wipe test                 5. SECURE entrance (post warning signs)
    6. REPORT to Radiation Safety Officer             6. NOTIFY Radiation Safety Officer ASAP
                                                                       |
                                                      RSO establishes controlled entry,
                                                      PPE, dosimetry & supervised decontamination

Step-by-Step Spill Response Sequence

Minor Spill Procedure (Standard Technician Action)

1. Notify: Immediately alert all individuals in the room that a spill has occurred so they avoid tracking contamination.
2. Prevent Spread: Cover the liquid pool immediately with absorbent paper pads, orienting the absorbent cellulose side facing downward to blot the liquid and the waterproof polyethylene side upward. Do not wipe or smear.
3. Decontaminate: Don protective equipment (double nitrile gloves, disposable lab coat, protective shoe covers). Using tongs, place contaminated absorbent pads into a radioactive waste bag. Clean the surface starting from the outer perimeter of lowest contamination and moving inward toward the center of highest contamination. Use specialized radiochemical decontaminating detergents containing chelating agents (e.g., Radiacwash, disodium EDTA).
4. Survey: Monitor the area with a pancake GM probe. Perform a $100\text{ cm}^2$ wipe test counted in a well counter. Decontamination continues until removable contamination is below the facility's action levels.
5. Report: Document the incident and notify the Radiation Safety Officer (RSO).

Major Spill Procedure (Immediate Evacuation Protocol)

1. Stop & Warn: If safely possible without delaying evacuation, turn upright the overturned container. Immediately order all personnel to evacuate the room.
2. Isolate the Area: Close and lock all access doors. If the spill involves volatile radioiodine ($^{131}\text{I}$), do not alter dedicated negative-pressure radioisotope fume hood exhaust, but disable any standard recirculating room HVAC supply grilles if accessible to prevent airborne migration into corridors. Seal room door seams with tape if volatile vapor release is suspected.
3. Prevent Tracking: Assemble all evacuees at a designated staging point immediately outside the room boundary. Survey all evacuees' shoes and hands with a pancake GM probe to verify that radioactivity was not tracked into unrestricted hallways.
4. Secure & Post: Post prominent "CAUTION - RADIOACTIVE MATERIAL" and "DO NOT ENTER" warning signs at all entrances. Station a sentry if doors cannot be locked.
5. Notify RSO: Contact the RSO immediately. Do not attempt cleanup of a major spill without RSO supervision. The RSO will direct the entry team, specify respiratory protection (particulate/charcoal cartridge respirators), mandate extremity dosimetry (finger rings), and formulate the formal decontamination and waste disposal plan.

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Personnel Decontamination & Mandatory Bioassay Programs

Skin and Wound Decontamination

Direct skin contamination poses severe risks of localized epidermal dose and percutaneous absorption into the bloodstream.

• Immediate Clothing Removal: Contaminated garments are removed promptly, without pulling fabric over the face or wounds; removing outer clothing typically removes most (often cited as up to about $90\%$) of external contamination.
• Skin Cleansing Rules: Wash affected skin areas under running lukewarm/tepid water using a mild, non-abrasive neutral pH soap for $2\text{ to } 3\text{ minutes}$. Pat dry with disposable towels.
• Critical Pitfalls to Avoid:
  • NEVER use hot water: Hot water stimulates cutaneous vasodilation, opening dermal capillaries and dramatically accelerating the transdermal absorption of radionuclides into the circulatory system.
  • Avoid cold water: Decontamination guidance recommends lukewarm water because cold water closes pores and can trap contamination.
  • NEVER use stiff brushes or abrasive cleansers: Harsh scrubbing abrades the stratum corneum barrier, creating microscopic dermal lacerations that transform an external skin contamination event into an irreversible internal systemic intake.
• Wound Decontamination: Flush open wounds copiously with sterile saline or water, encouraging gentle bleeding from the wound edges to wash out particulates. Never apply chemical solvents or strong acids/bases to human tissue.

Radioiodine Bioassay After Handling or Spills (10 CFR 20.1502 & Regulatory Guide 8.20)

Under 10 CFR 20.1502(b), internal exposure must be monitored when an adult worker is likely to receive an intake above 10% of the applicable ALI. Regulatory Guide 8.20, Revision 2 gives the NRC's acceptable radioiodine program:

• When routine bioassay is needed: when, in any 3-month period, a worker handles more than 1 mCi (37 MBq) of volatile or dispersible radioiodine on an open bench, 10 mCi in a properly working fume hood, or 100 mCi in a glovebox (ten times these amounts if the iodine is bound to a nonvolatile agent). Capsules count as sealed unless one is opened or crushed, and a spill of I-131 that exceeds the capacity of the absorbent pad calls for bioassay.
• Timing: special bioassay is performed within 24 hours of an incident; for routine I-131 work the maximum interval is 2 weeks, and measurements for infrequent work are made no sooner than about 6 hours after handling.
• Detection: a calibrated NaI(Tl) thyroid probe at a reproducible distance from the neck, calibrated with a neck phantom.
• Action levels: above 1.0 µCi (37 kBq) in the thyroid, investigate, correct the cause, and repeat the measurement within 24 hours; above 5.0 µCi (185 kBq), also obtain medical consultation promptly (within 2–3 hours of a known intake if thyroid blocking may help), repeat about weekly until below 1 µCi, and restrict the worker if further work could exceed the dose limits.