A Radiation Protection Perspective-

Decommissioning of the Moata Reactor

Biological Shield Dismantling

Australian Nuclear Science &

Technology Organisation

IAEA Workshop (R2D2) – Sydney Australia

7th-11th May 2012

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 2010

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 70 840 840

HPS Monitoring surveys 8 8 70 560

Bobcat waste removal 8 4 130 520 520

Material removal with 5tonne excavator 16 8 70 560

Manual filling of waste containers 1 0.5 300 150

Total 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 & Monitoring Controlling exposure using shielding

Steel Plates for shielding

Radiation Management & Monitoring Controlling 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