selenium-75 radiography overexposure · selenium-75 iridium-192 half-life 120 days 74 days gamma...

Megan Shober, Nuclear EngineerAugust 16, 2018

Selenium-75 Radiography Overexposure

Abbreviations

• Millimeter (mm)• Curie (Ci)• Roengten (R)• Iridium-192 (Ir-192)• Selenium-75 (Se-75)• Half-value layer (HVL)• QSA (not an abbreviation)

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Outline

• Overview• Timeline• Causes• Lessons learned

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Overview

Selenium-75 Iridium-192Half-life 120 days 74 daysGamma energy 66-401 keV 206-612 keVSteel thickness 3-29 mm 12-63 mmHalf-value layer (HVL)(tungsten)

0.8 mm 3.3 mm

Gamma constant(at 1 m per Ci)

0.203 R/hr 0.48 R/hr

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*Information from QSA Global 880 Series Operating Manual

Overview

• Digital radiography on multi-story boiler during mill outage

• Failed to retract source• Did not use survey meter• Did not hear alarming ratemeter• Held collimator in hands

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Work Site

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Work Site

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4 HVLs (Ir-192)

17 HVLs (Se-75)

Timeline

• Performed radiography• Noticed 100 mR on direct-reading

dosimeter, suspected extremity overexposure

• Notified shift supervisor and RSO• Began dose assessment• Notified State

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7:45

8:15

8:30

9:00

10:00

Calculate exposure

• Estimated 10 second exposure• 96 Curie selenium-75 source

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• Initial estimate of 650 rads to hands• Final estimate of 176 rem to each hand

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Investigation

• All inspectors away at training− No steel-toed boots− No inspection supplies

• Limited access to:− License information− Phone numbers− Procedures− Printer

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Investigation

• Interview radiographers• Visit jobsite• Refine dose estimates• Determine root cause

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Root Cause (Licensee)

Used Eight Disciplines analysis and Five Whys.• Crew unfamiliarity• Low readings on survey meter• Radio distraction• Lock indicator

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Failure to use survey meter*Image from QSA Global 880 Series Operating Manual

Root Cause (State)

In addition, we evaluated:• Lack of knowledge about selenium-75• Digital radiography• Alarming ratemeter

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Root Cause (State)

• We rejected the licensee’s root cause.• Multiple contributing causes related to the

use of Se-75. • Survey meter provided same information as

lock indicator.

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Poor technique for moving collimator

Lessons Learned

• Surveys of cameras containing Se-75 mostly measure radiation from the depleted uranium shield.

• Alarming ratemeters only alarm in the uncollimated beam.

• Safety-significance of lock indicator is underappreciated by users and regulators.

• There are no training requirements for new sources or devices.

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Lessons Learned

• Keep incident procedure and event reporting contacts in easily accessed location.

• Additional training for on-call staff.− Reporting timeframes for materials incidents− Information gathering (re-enactments)

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Questions?

Megan Shobermegan.shober@wisconsin.gov

(920) 448-5346

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