Radiological Surveys: Techniques and Documentation

What Is a Radiological Survey?

A radiological survey is a systematic measurement of radiation levels or surface contamination in a defined area. Surveys are required by plant procedures before workers enter a radiologically controlled area (RCA), after work is completed, and whenever conditions change. Two types are covered here: area radiation surveys, which measure dose rates in air (typically in mrem/hr or rem/hr), and surface contamination surveys, which measure radioactive material deposited on surfaces (typically in disintegrations per minute per 100 cm², or dpm/100 cm²).

Accurate surveys protect workers from unexpected dose, prevent the spread of contamination, and fulfill regulatory requirements under 10 CFR Part 20 and plant Technical Specifications. Falsifying or skipping surveys is a serious safety and legal violation.

Instruments Used

Geiger-Müller (GM) meters with thin-window probes are the primary tool for surface contamination surveys. The thin mica or mylar end-window allows low-energy beta and alpha particles to reach the detector. Always verify that the window is intact before use—a cracked window changes the detector's response and can render readings unreliable.

Ion chamber meters are used for area radiation surveys where dose rate in mrem/hr or rem/hr is needed. Ion chambers respond accurately across a wide dose-rate range and are less prone to the "saturation" effect that can cause GM meters to read artificially low in very high fields.

Before any survey, perform an instrument operability check: verify calibration sticker is current, perform a battery check, and use an approved check source to confirm the instrument responds. Never begin a survey with an instrument whose calibration has expired or whose check-source response is out of tolerance.

Area Radiation Survey: Step-by-Step

1. Review the procedure before entering. Know the expected dose-rate ranges, the posting limits your plant uses, and any special radiological work permit (RWP) conditions.

2. Don required PPE: at minimum, lab coat, safety glasses, and gloves. Follow RWP requirements; respirators or additional protective clothing may be required depending on airborne or contamination hazards.

3. Select and check your instrument: for area surveys, use the ion chamber on the appropriate range. Start on the highest range and step down only after confirming the reading is within scale.

4. Establish your survey grid: use the map template to define a systematic path—typically a grid pattern—so no area is missed. Mark your starting point.

5. Maintain proper geometry: hold the probe approximately 30 cm (about 12 inches) from surfaces unless the procedure specifies otherwise. Consistent distance is essential for reproducible results.

6. Control scan speed: move the probe slowly enough for the instrument to respond—approximately 2 inches per second for GM meters on contamination surveys. Too fast a scan can cause you to pass over a hot spot before the meter deflects.

7. Record every reading on your survey map as you go. Note the highest reading in each zone. If a reading exceeds a posting threshold, stop and notify your supervisor before proceeding.

8. Identify the highest-dose location and mark it clearly on the map.

Surface Contamination Survey: Key Differences

For surface contamination surveys with a GM thin-window probe, maintain the probe close to but not touching the surface—typically within 1 cm—to detect the weakly penetrating radiation from beta and alpha emitters. Rotate the probe end slightly outward (end-window facing the surface, not parallel). Scan in overlapping parallel passes to ensure full coverage. If a reading spikes, slow down and circle the area to define the boundary of contamination.

Radiological Posting

Plant procedures and 10 CFR Part 20 define boundary dose rates that require specific postings:

• Radiation Area: dose rate at 30 cm from accessible surfaces exceeds a defined threshold (check your plant procedure—do not rely on memory alone).
• High Radiation Area: higher dose rate requiring additional controls.
• Contamination Area / High Contamination Area: based on removable and fixed contamination levels per plant limits.

Post signs at all access points to the controlled zone, not merely at one entrance. The sign must face outward so approaching workers see it before crossing the boundary.

Safety and LOTO Considerations

Radiological surveys are often performed near energized equipment. If the survey requires working on or near electrical hazards, confined spaces, or pressurized systems, lockout/tagout (LOTO) must be applied before entry, in accordance with OSHA 29 CFR 1910.147 and plant procedures. Radiation protection and industrial safety requirements apply simultaneously—one does not override the other.

Never use damaged instruments. Never handle sealed demonstration sources without RSO authorization and never attempt to open or modify them.

Survey Map Completion and Documentation

After the survey, transfer all readings to the area diagram neatly and legibly. Use standard plant symbols for dose-rate contours and contamination boundaries. Record: date, time, surveyor name, instrument ID and calibration date, range used, and check-source result. The completed map becomes a legal record. Sign and date it. Submit it to the radiation protection (RP) supervisor for review and filing.

A well-documented survey map is evidence that the area was safe for the work performed. Gaps in documentation create regulatory findings and, more importantly, leave workers without the protection they deserve.