Examples of Accident Investigations

1. Examples of Accident Investigations Lecture Module 12

2. Types of accidents • Acute, whole body, homogeneous • Acute, whole body, heterogeneous • Acute, part-body • Protracted, continuous • Protracted, fractionated • Protracted, internal emitter • Immediate discovery • Delayed discovery

3. Examples Two examples of actual accidents will be given: • A lost gamma radiography source picked up by a non-radiation worker and put into a pocket • An inhalation of tritium in an industrial accident

4. Case 1. Orphan source incident Iridium-192 7.4 TBq (200 Ci) Small shiny metal cylinder No warning markings on it Used to carry out gamma radiography in a factory construction site

5. Orphan source

6. Radiography apparatus

7. Sequence of events (1) • It happened on a Friday • It was discovered on the following Monday 1. The radiographers carried out their normal radiography 2. The source fell out from the end of the guide tube 3. It was not noticed by the radiographer because of a faulty monitor

8. Sequence of events (2) 4. Source had fallen out onto the floor 5. Three hours later a construction supervisor picked it up 6. He mistakenly thought it had fallen from a mobile crane 7. He picked it up and put it into his shirt breast pocket

9. Sequence of events (3) 8. He travelled home in small bus with 6 colleagues 9. They got off bus at various places along their route 10. He got home after ~40 min, sat down and watched TV 11. 40 min later he felt ill, undressed, put shirt into cupboard and went to bed

10. Sequence of events (4) 12. Saturday morning source was moved to drawer in bedside cabinet 13. Family went out for day 14. Saturday and Sunday nights the man slept closest to cabinet with 6y son between him and his wife 15. He returned to work on Monday morning

11. Sequence of events (5) 16. Loss of source was discovered on Monday late morning when radiographers resumed work 17. Monitors (in working order) were used to search area - Nothing found 18. Replica source was shown to all workers 19. Man recognised it and source was recovered from his home 20. Regulatory authority was informed

12. Next steps • A health physics investigation started to establish what had happened • The man, his family and colleagues placed under medical supervision • Doses were estimated Two approaches: 1. Physical reconstruction 2. Biodosimetry

13. Physical dose reconstruction Estimated timings and geometry • Source in shirt pocket • Source in bedroom cabinet • Calculated doses for other people –few tens of mGy First estimate: averaged whole body dose 1.33 Gy Later refined estimate: 1.06 Gy

14. Isodose contours through torso

15. Biodosimetry-scoring results Personcellsdicringace Man 1000 86 2 60 Wife 500 2 0 10 Child 500 1 0 3 Coll x 500 0 0 1 Coll y 500 0 0 3 Coll z 500 0 0 5

16. Biodosimetry Averaged whole body dose estimates (Gy) PersonDoseLCLUCL Man 1.01 0.57 1.78 Wife 0.12 <0 0.33 Child 0.06 <0 0.28 Coll x 0 <0 0.047 Coll y 0 <0 0.047 Coll z 0 <0 0.047

17. Biodosimetry: non-uniform exposure Cells with number of dicentrics 0 1234 5 v:mu Observed 932 56 9 1 1 1 1.57 12.74 Poisson 918 79 3 0 0 0 Based on expected Poisson distribution, which would be expected for uniform exposure, there were too many highly damaged and too few undamaged cells or cells with only 1 dicentric observed. This suggests non-uniformity.

18. Biodosimetry: non-uniform exposure Contaminated Poisson method Partial body dose: 2.63 ± 0.50 Gy Irradiated fraction: 23% Qdr method Partial body dose: 2.69 ± 0.52 Gy Irradiated fraction: 25% Good agreement between the two methods

19. Biodosimetry: dose rate The exposure was not acute • 2h 40 min in the pocket • Weekend in bedside cabinet Use the G-function adjustment to the dose response curve; Y = αD + G(β)D2

20. Biodosimetry: dose rate corrected • Averaged whole body increased from 1.01 to 1.19 Gy • Contaminated Poisson estimates increased from 2.62 Gy and 36% to 3.15 Gy and 40% • Qdr estimates increased from 2.69 Gy and 25% to 4.99 Gy and 30%

21. Main points from the biodosimetry • Only the man was seriously exposed • Clearly non-uniform exposure • General exposure to majority of his body was serious but not life threatening Biodosimetry information reported to medical doctors indicated that 1. bone marrow injury would not be problem (they were searching for suitable marrow donor) and 2. there would be local injuries requiring treatment

22. Local reactions- chest on day 17

23. Local reactions- hands on day 24

24. Chest -day 1139 after surgery

25. Hands- day 1139

26. Conclusion (1) • This orphan source type accident involving non-radiation worker is not unique; it is typical of many similar scenarios • This case is good example of how biodosimetry inter-relates with physical methods, calculations and account of events to come to overall view of case

27. Conclusion (2) • This case is good example of how biodosimetry can approach problems of: - non-uniformity of exposure - combined with protracted exposure

28. Case 2. Incorporation of tritium • Accident happened in factory manufacturing glass capillary tubes filled with tritium gas • Patient was 33y female • She inhaled aerosol of tritiated water droplets • Cause of accident was over-pressure in tube filling apparatus

29. Diagram of tube filling system P tap capillaries uranium trap

30. What happened (1) It happened on Thursday morning • Woman melted off the first capillary • It did not seal but instead popped open • She realised that this was unusual and asked senior supervisor for help • He sealed off broken capillary and then tried to melt off capillary from second set. It ruptured too and hissing gas was heard

31. What happened (2) He transferred remaining gas in system back to uranium trap He noticed that area radiation monitors were showing increase of tritium in room However significant release of tritium was not at first suspected Urine samples were collected

32. What happened (3) • Friday evening urine measurement results showed: 1.3 GBq / L (woman) 28 MBq / L (supervisor) • Now realised that it was serious • Woman told to delay starting holiday and drink lot of extra fluid

33. What happened (4) Further delay because company management could not be contacted until Saturday evening Sunday morning conference At noon regulatory authority were informed Authority contacted a medical doctor Monday morning woman admitted to hospital for forced diuresis

34. Investigation- What went wrong? (1) Pressure in the system was 1600 mb It should have been 600 mb Manometer dial was confusing; needle had gone twice round dial There was no separate mechanism to warn of over-pressure About 3.4 TBq had leaked from 2 sets of ruptured capillaries

35. What went wrong? (2) Woman had removed part of the containment Her face was close to escaping tritium gas Gas passed through flame where it was oxidised to tritiated water droplets She inhaled about 35 GBq

36. ICRP recommendation Annual limits of intake • 3 GBq for standard man • This converts to about 2.2 GBq for this 53.5 kg woman Committed dose equivalents: 0.8 Sv (woman) 0.025 Sv (supervisor)

37. Tritium excretion in victim’s urine 2000 T½ = 6.4 d, enhanced fluid intake 1000 Tritium conc., MBq l-1 T½ = 2.7 d, forced diuresis 500 200 T½ = 10 d Hospital 100 0 0 10 20 30 Days 20054

38. Dosimetry from urine measurements • Standard formula using parameters: • conc. of H3 in urineintegrated over time • mean energy per disintegration • ratio of total body water / soft tissue for females • Resultant dose to soft tissue = 0.47 Sv

39. Committed Doses Actual dose 0.47 Sv Dose if untreated 0.80 Sv (T½ = 10d) Dose if increased drinking 0.55 Sv (T½ = 6.4d) Dose if earlier diuresis 0.41 Sv Lesson learned: It was probably not worth psychological stress of forced diuresis for amount of dose saved

40. Dose build-up from urine measurements 0.5 Dose to soft tissue, Gy 0.3 0.1 Hospital 40 0 10 20 30 50 180 Days after accident 21318

41. Biodosimetry Blood sampled for the dicentric assay • On days 4, 18, 39, 50, and 178 • 1000 metaphases scored per sample • Dicentric yields referred to calibration curve • Need to consider water content of lymphocytes • Need to consider ratio of body water to soft tissue

42. In vitro dose response for tritium

43. Urine and biodosimetry dose estimates 0.5 Dose to soft tissue, Gy 0.3 0.1 Hospital 40 0 10 20 30 50 180 Days after accident 21317

44. Final dose estimates Urine assay 0.47 Sv Dicentrics 0.37 Gy • Patient remained for several years under periodic medical surveillance and blood was again taken ~ 5, 6 and 11 years • Analysed again for dicentrics and also for FISH translocations

45. Follow-up blood sampling

46. Overall conclusion Two quite different accidents have been described 1. External partial body exposure 2. Internal whole body exposure Confounding factors of dose protraction It was possible to compare information from biodosimetry with that from physics Good agreement was obtained