A Radiation Protection Perspective- Decommissioning of the Moata Reactor. Biological Shield Dismantling

Transcription

1 A Radiation Protection Perspective- Decommissioning of the Moata Reactor Biological Shield Dismantling Australian Nuclear Science & Technology Organisation IAEA Workshop (R2D2) Sydney Australia 7 th -11 th May 2012

2 Shutdown & Decommissioning Two phases of dismantling: Preliminary Dismantling the removal of the internal components of the reactor, including the steel core structures, graphite moderator and beam line facilities Structural (Biological Shield) Dismantling the cutting and removal of the concrete shielding and the floor area below the shielding.

3 Structural (Biological Shield) Dismantling

4 Structural (Biological Shield) Dismantling Reactor housed in a building used for other operational task A containment tent with HEPA filtered extraction system built around the reactor

5 Getting There The steps involved in planning, completing and evaluating the success of the Biological shield Dismantling: Radiation Protection Plan Characterisation Radiation Management & Monitoring Dose Estimation & Constraints Personal Protective Equipment Final Status Surveys Outcomes & Lessons Learnt

6 Characterisation Survey A radiological survey was carried out to determine the dose rates within the empty cavity areas. The data from this exercise was used to prepare a dose estimate for the Biological shield dismantling. This helped us estimate the maximum individual and collective dose for this part of the project. Measured dose rates ranged from Sv/hr

7 Dose Estimation and Constraints TASK SET 2 Est. Exp Time (hours) for most exposed individual Estimated Doserate Estimated Dose Estimated Dose Estimated Dose Estimated Dose Main Bioshield (Heavy Density Concrete) Estimated time to (µsv/hr) for Worker Received (µsv) Worker Received (µsv) Worker Received (µsv) for Received (µsv) for Estimated Doserate Active Zone perform task 1,2,3 or 4 1-Cardinal 2-Cardinal Worker 3-Cardinal Worker 4-Cardinal (µsv/hr) for HPS 1 Machine demolition-remote operation HPS Monitoring surveys Bobcat waste removal Dose Estimates: Material removal with 5tonne excavator 16 Collective ,400 man-μsv 560 Manual filling of waste containers Av. Individual 1156μSv (assuming 9 workers) Estimated Dose Received (µsv) HPS 1 Total For Cutting and Removal Of Main Max. Individual 1444μSv Bioshield (Active concrete) Dose Constraints: Individual 1500μSv Daily 50μSv Summary Dose = Estimated Collective dose man- Sv

8 Radiation Management & Monitoring Controlling exposure using distance

9 Radiation Management & Monitoring Controlling exposure using shielding Steel Plates for shielding

10 Radiation Management & Monitoring Controlling exposure using shielding Shielded containers for activated waste: Specially designed 6mm steel boxes for active concrete

11 Radiation Management & Monitoring Dose Management: Radiological surveys Dose Monitoring: EPD System TLDs Whole Body Monitoring

12 Radiation Management & Monitoring External gamma monitors and air samplers were placed close to work areas. Barrier and exit monitoring equipment were used to ensure that no contamination was taken out of the classified areas.

13 Personal Protective Equipment Since there was a high potential of generating dust while cutting and demolishing the active concrete it was decided to start from the highest level of PPE which included ; Respiratory protection with particulate cartridge) Tyveks Overshoes and gloves This was than downgraded as we gained operational understanding of the project risks.

14 Final Status Survey Dose rate surveys were carried out on marked survey points (grid survey) Acceptance criteria of 0.5uSv/hr was met to ensure a brown-field condition was met Samples were taken in and around the foundations of the reactor and analysed for activity. The following guides were also used to determine the exemption and clearance criteria for the site IAEA No. RS-G.1.7 ARPANSA Reg 1999, Schedule 2, Part 2

15 Outcomes & Lessons Learnt Estimated doses vs. actual doses: Dose Collective Max. Individual ANSTO Estimated 10,400 personμsv ARPANSA Constraint 13,000 personμsv ANSTO Constraint 10,400 personμsv Actual 1,679 personμsv 1444 μsv 1750 μsv 1500 μsv 252 μsv Daily μSv 46μSv (max)

16 Outcomes & Lessons Learnt From a radiation protection perspective the Biological Shield Dismantling was completed safely and without incident. Dose estimates were higher than those recorded by staff. This was attributed to being overly conservative and some of the contributing factors were; Self shielding by the dismantled concrete Time taken to dismantle and remove the active concrete was less than anticipated. No surface or airborne contamination was detected throughout the project There were no personal contamination events

17

18