WAO 2003

WAO 2003
Radiation Safety issues associated with operating
and maintaining intense beam accelerators
Straight 1 replacement and upgrade on the ISIS
800MeV accelerator
Presented by:Tom Noone
Assistant Duty OfficerRutherford Appleton Labs

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A brief description of ISISISIS is currently the world’s most powerful spallation neutron
source.
Acceleration begins with a Penning H- ion source at apotential of -665 kV feeding into a 70MeV Alvarez type linac.
On entering the synchrotron, the -H beam is passed through astripping foil, thus removing both electrons, converting thebeam to protons..
In the ring, six RF cavities trap the beam into two bunches
and accelerates them to 800 MeV in 10 milliseconds.
This proton beam is deflected, during a single turn, into theextracted proton beam line (EPB). This is accomplished using
three fast kicker magnets.
From the EPB the beam strikes a heavy metal target toproduce the intense neutron flux.
The entire acceleration process is repeated 50 times a second

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Beam Management a key to success

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From the beginning we have run ISIS within very
tight operating parameters.

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The design philosophy of “hands on” maintenance,
provided a key focus on minimising beam loss
throughout the machine.

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This has been accomplished by continuous
monitoring of 39 Brookhaven type ionisationchambers (B.L.M.’s) placed round the ringw

Each B.L.M. has it own preset warning and trip levelsw

The crew can access individual B.L.M.’s to see theloss patterns during the machine cycle, a very usefuldiagnostic tool.

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Beam Management a key to success5w

Additionally we monitor beam efficiency during
trapping and acceleration in the “Time Domain”

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In the ring beam torroids are sampled at injection,
just after beam bunching (trapping), acceleration
and at extraction.

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Efficiency monitoring provides warnings if limits arebeing approached or trip the machine if a limits isexceeded.

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This system works on individual cycles and by taking
an average over several cycles.

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THE CASE FOR REPLACEMENT

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Straight 1 is one of the most active areas in the ring,w

Over the last 18 operating years, components like IONpumps and Septum magnet connections have failed.

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The original design for collectors did not catch an
unexpected mid cycle loss, resulting in higher thanexpected activity levels downstream of the collectors.

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The lack of personnel radiation shielding restricted
general access, making “hands on” working in this area
more hazardous than was anticipated in the designstage.

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A business case increasing the machine’s beam intensity
had been agreed resulting in the need for a new extract
septum magnet and collector system.

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THE OLD STRAIGHT 1

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8ISIS STRAIGHT 1

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ISSUES RASIED DURING PLANNINGw
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9No one has worked on this area of the machine for15 years.
Activity levels were high, experienced practical,people with best knowledge available had to be used
Planning and preparation had to be excellent
The original sources of machine build information
had to be studied in detail.
Where information was unclear, or missing practicalmethods such as detailed photography and manual
measurements, were employed to gain additionalknowledge.

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Radiation Issues during Decommissioning

The conducted radiation survey revealed high levelsof activity in all of the components concerned
w A 6 month cooling down process prior to any “handson” work commencing was needed.
w The degree of hazard was high, staff would beworking in dose rate fields of up to 1000µSv/h
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w Safe handling and storage of removed components
was essential.
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Issues during Decommissioning

In order to conduct work safely the ISIS Health
Physicist produced a detailed Safety Protocol.
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allotted, estimated dose rate, total dose expected
and protective measures to be used.
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Pre-Buildw

The planning stage identified that a “pre-build” of
all components in a clean and safe area would
provide the following benefits:Allow staff to experience hands onfamiliarisation with the tasks to be conducted.
2. Check mechanical alignment and fit ofcomponents and modules.
3. Allow vacuum and electrical testing prior toinstallation.
4. Work would not be held up whilst activity in thering decayed.
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Pre-build Vacuum Testing

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14Pre-build, Extract Septum Magnet

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Installation
This process worked well, the six month build andtest of the new components, reduced installation
time down to two weeks.

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Summary
In a high energy accelerator it is possible to run
the machine within levels of beam loss required fora hands on maintenance regime.
When faced with a major mechanical changes inradioactive areas, it takes many hours of planning
involving several different departments working
together.
Preparation is key to conducting such an exercise
within the limits of Radiation Regulations
Staff have to be experienced, knowledgeable,
motivated and committed to team planning andimplementation