Decommissioning and Dismantling of the Moata Reactor-
A Radiation Protection Perspective
Prashant Maharaj
Australian Nuclear Science & Technology Organisation
A History of Moata
• Graphite moderator / reflector
• Cooled by light water
• Shielding - high density and low density concrete
• Constructed at ANSTO in 1961• ARGONAUT type reactor • Built as a 10 kW reactor, but modified
in 1972 to 100 kW
The Moata Reactor
From 1961 to 1995:– Neutron
radiography– Soil analysis– Cancer
treatment research
– Quality Control for HIFAR
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.
1995 1996 1997 1998 2009
Shutdown after 4,519 start-ups
Fuel unloaded
Decomm
issioning Project Team
established
Primary coolant (light water)
drained
Reactor Control System rem
oved
Planning comm
enced for
dismantling
Preliminary Dism
antling began
Preliminary Dism
antling complete
Containment tent installed
2010
Biological Shield Dismantling began
Biological Shield Dismantling Com
plete
Structural (Biological Shield) Dismantling
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
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
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 7-4000 Sv/hr
Dose Estimation and Constraints
TASK SET 2
Main Bioshield (Heavy Density Concrete) Active Zone
Estimated time to perform task
Est. Exp Time (hours) for most exposed individual
Estimated Doserate (µSv/hr) for Worker 1,2,3 or 4
Estimated Dose Received (µSv) Worker 1-Cardinal
Estimated Dose Received (µSv) Worker 2-Cardinal
Estimated Dose Received (µSv) for Worker 3-Cardinal
Estimated Dose Received (µSv) for Worker 4-Cardinal
Estimated Doserate (µSv/hr) for HPS 1
Estimated Dose Received (µSv) HPS 1
Estimated Collective dose man-Sv
Machine demolition-Remote operation 24 12 70 840 840HPS Monitoring surveys 8 8 70 560Bobcat waste removal 8 4 130 520 520Material removal with 5tonne excavator 16 8 70 560Manual filling of waste containers 1 0.5 300 150Total For Cutting and Removal Of Main Bioshield (Active concrete) 57 840 840 520 1230 560
Summary Dose = 840 840 520 1230 560 3990
Dose Estimates:• Collective – 10,400 man-μSv• Av. Individual – 1156μSv (assuming 9 workers)• Max. Individual – 1444μSv
Dose Constraints:• Individual – 1500μSv• Daily – 50μSv
Radiation Management & Monitoring
Controlling exposure using distance
Radiation Management & MonitoringControlling exposure using shielding
Steel Plates for shielding
Radiation Management & MonitoringControlling exposure using shielding
Shielded containers for activated waste:
– Specially designed 6mm steel boxes for active concrete
Radiation Management & Monitoring
Dose Management:– Radiological
surveys
Dose Monitoring:– EPD System– TLDs– Whole Body
Monitoring
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.
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.
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
Outcomes & Lessons Learnt– Estimated doses vs. actual doses:
Dose ANSTO Estimated
ARPANSA Constraint
ANSTO Constraint Actual
Collective 10,400 person- μSv
13,000 person- μSv
10,400 person- μSv
1,679 person- μSv
Max. Individual 1444 μSv 1750 μSv 1500 μSv 252 μSv
Daily - - 50μSv 46μSv (max)
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