Radioactivity Release at the Holifield Radioactive Ion Beam Facility

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Radioactivity Release at the Holifield
Radioactive Ion Beam Facility

• B. Alan Tatum

2009 DOE Accelerator Safety Workshop August
18-20, 2009 Brookhaven National Laboratory
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Topics

• Description of HRIBF
• Overview of the July 2008 Radioactivity Release
• Response to the Event
• Key Corrective Actions
• Lessons Learned
• Summary Comments

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Holifield Radioactive Ion Beam Facility

• The principal mission of the HRIBF, commissioned
  in 1996, is the production of high quality beams
  of short-lived radioactive isotopes to support
  research in nuclear structure physics and nuclear
  astrophysics.
• HRIBF is currently unique worldwide in its
  ability to provide neutron-rich fission fragment
  beams post-accelerated to energies above the
  Coulomb barrier for reactions.
• HRIBF is based on two accelerators, one for
  production of radioactive species (ORIC) and one
  for post-acceleration (25 MV Tandem), that were
  part of a pre-existing complex (HHIRF) at ORNL
• Oak Ridge Isochronous Cyclotron (ORIC)
• commissioned in 1963, but extensively renovated
• first used as a stand-alone light ion accelerator
  
• served as a booster accelerator for the Holifield
  Heavy Ion Research Facility (HHIRF)
• now serves as the driver accelerator for RIB
  production
• 25 MV Tandem Electrostatic Accelerator
• commissioned in 1982 for the HHIRF
• largest accelerator of its type in the world
• now serves as the RIB post-accelerator
• HRIBF is funded by the DOE Office of Nuclear
  Physics.

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Radioactive Ion Beams (RIBs) are Produced by the
Isotope Separator On-Line (ISOL) Technique
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HRIBF Post-accelerated Beams
175 RIB species available (26 more
unaccelerated) 32 proton-rich species 143
neutron-rich species
Post-accelerated Intensity
Beam list increased by 50 since 2003
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25MV Tandem Electrostatic Accelerator
Injector for Radioactive Ion Species 1 (IRIS1)
Injector for Stable Ion Species (ISIS)
Oak Ridge Isochronous Cyclotron (ORIC)
Enge Spectrograph
Daresbury Recoil Separator (DRS) nuclear
astrophysics endstation
High Power Target Laboratory (HPTL) IRIS2
Recoil Mass Spectrometer (RMS) nuclear structure
endstation
On-Line Test Facility (OLTF)
HRIBF ST Review 2008
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HRIBF Accelerator Specifications

• ORlC Light-Ion Beam Parameters
• Protons 55 MeV 50uA
• Deuterons 50 MeV 25uA
• 3He 133MeV 10uA
• 4He 100 MeV 10uA
• Tandem Accelerator Operating Parameters
• Ion mass 1 amu through 250 amu
• Maximum beam power 175 Watts
• Injected ion energy 150 keV-300 keV
• Terminal operating potential 1 MV - 25.0 MV

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Injectors for Radioactive Ion Species 12

• High voltage platform systems biased to /- 200kV
• Provides necessary energy and negative ions for
  injecting into the 25MV tandem accelerator
• Target/ion source assembly resides on the
  platform and is biased to /-60kV
• Targets include hafnium oxide and pressed powder
  uranium carbide

IRIS1
IRIS2
UC Target
IRIS2
Target/Ion Source
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HRIBF Safety Documentation

• Although the concept of hazard classification is
  no longer required by the Accelerator Safety
  Order DOE 420.2B, HRIBF was approved by DOE as a
  Low Hazard facility as a result of the hazard
  screening documented in HS/6000/F/1/R1.
• The 46-year range of commissioning dates is
  reflective of the dynamic nature of the facility.
• The HRIBF ASE
• Establishes the envelope for safe operations
• Compliant w/ Order 420.2B and consistent w/
  Implementation Guide
• Addresses Credited Controls identified in SAD
• Current revision date is July 2005. Presently
  being updated to incorporate IRIS2 and event
  corrective actions.

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Overview of the July 2008 Radioactivity Release

• Monday morning, July 28, 2008
• Experiment in progress ORIC providing 12?A of 50
  MeV protons to an IRIS1 UCx target for production
  of neutron-rich 81Zn delivered to new LeRIBSS
  facility
• Elevated radiation levels were detected outside
  the IRIS1 RIB production vault
• Maximum dose rate 4 mrem/h
• Transferable contamination found in same area
• Building 6000 was evacuated as a precaution
• Operational Emergency declared by ORNL
• Electronic dosimeters of experimenters collected
• TLDs of all 71 people who entered Bldg 6000 July
  25-28 collected/read
• Six individuals sent for whole body count (all
  negative)
• No evidence of any measurable exposure was found

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Response to the Radioactivity Release

• A Management Investigation was chartered by ORNL
• HRIBF and other ORNL staff designated as Recovery
  Team
• Throughout the investigation the Recovery Team
• Provided information to investigation team
• Carried out physical examination of hardware
  involved
• Report of investigation team released in late
  November
• Based on Judgments of Need in the investigation
  report, a Corrective Action Plan was developed in
  December by HRIBF staff in consultation with ORNL
  management and the investigation team
• HRIBF developed a phased restart plan and mapped
  it to Corrective Actions

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Phased Restart

• Stable beam operation resumed September 2008.
• On-line Test Facility (OLTF) operation with
  non-uranium targets (nA scale production)
  January 2009. (OLTF is a facility for testing
  target and ion source systems with low intensity
  driver beams.)
• Batch mode operation at IRIS1 February 2009.
  (IRIS1 is the RIB production facility in C111S).
• High power target lab (HPTL) operation with
  non-uranium targets May 2009. (HPTL is
  high-intensity driver counterpart of OLTF).
• Proton-rich RIB production at IRIS1 or testing at
  HPTL May 2009
• OLTF operation with uranium targets June 2009.
• Full operation of HRIBF including neutron-rich
  beam delivery (uranium targets) June 2009.

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What happened?

• Two-fold failure
• Leak in off-gas system
• Pin-hole leak in roughing pump oil-fill plug.
• Resulted from corrosion of plug
• Stamped carbon steel 1mm thick, threaded
• Failure of shielded vault HVAC system
• Belt driving 13,000 cfm exhaust fan failed
• Interlock was based on motor operation, not on
  fan itself
• Resulted in slight ( 3x10-4 atmosphere) positive
  pressure in shielded vaults
• Consequent leakage of hot off-gas out of vault
  (2 liter/s leak rate)
• Subsequent analysis determined 100 of released
  activity accounted for by noble gases (Xe and Kr
  isotopes)
• Total noble gas inventory
• Concentration of activity in C111S 2.2x10-3
  mCi/ml (512 DAC)
• Concentration of activity outside door 1.2x10-5
  mCi/ml (3.2 DAC)

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IRIS1 Vacuum System

• No loss of high vacuum (10-7 Torr during event)
• Leak on exhaust side of RP1 (at atmospheric
  pressure)

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Roughing Pump Oil-fill Plugs
RP1 Oil-fill plug after event, before cleaning
RP1 (right) and RP2 Oil-fill plugs after
cleaning
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IRIS1 Ventilation
Supply

• Supply is single 17,000 cfm fan
• Distribution by duct sizing
• Exhausts are 3 separate fans

Exhaust
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IRIS1 Ventilation
Supply

• Supply is single 17,000 cfm fan
• Distribution by duct sizing
• Exhausts are 3 separate fans

Exhaust
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Key Correctives Implemented

• Differential pressure interlocks on HVAC system
• Important new engineered control - but does not
  rise to the level of a credited control (as per
  Accelerator Safety Order 420.2B)
• Enhanced surveillance program by Radiological
  Protection staff
• Enhanced maintenance program
• More frequent inspection and regular replacement
  of pumps and other critical components
• Developed revised and expanded Safety Assessment
  Document
• Corrective action, ASRC review, incorporation of
  IRIS2

Additional Actions

• Will proactively institute monitoring of HVAC
  exhaust stacks for the RIB production vaults
  (IRIS1, IRIS2) for airborne radioactivity
• Intended as early warning, not safety system or a
  quantitative monitor of releases.

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Thorough response recognized by DOE Office of
Enforcement

• Letter received from Office of Enforcement May
  13th
• The Office has elected not to pursue
  investigation recognizing the comprehensive
  scope of your investigation and corrective
  actions
• Extent of Condition Review has been completed.

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Lessons Learned

• Maintenance periodically review PM schedules to
  ensure that
• all equipment is on the list
• maintenance frequency is appropriate
• instructions are clear and complete
• PM results are recorded
• Radiological surveillance
• Ensure that a rigorous plan is in place
• Review the plan regularly to ensure that it will
  effectively identify off-normal conditions
• Expand SAD to include all plausible accident
  scenarios
• Ensure that lab emergency response personnel have
  access to appropriate facility information

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Summary Comments

• The July 2008 Operational Emergency dominated our
  effort and our attention in FY2009.
• The most important fact concerning this event was
  that nobody received a measurable radiological
  dose.
• The phased restart that we were able to execute
  allowed us to continue to produce exciting
  science while waiting for neutron-rich operation,
  but nevertheless, events such as this are always
  detrimental to research programs.
• We believe we have learned a great deal from this
  regrettable occurrence, and have taken measures
  to reduce the likelihood of recurrence.
• I hope that this information will be of
  assistance to you as we all strive for continuous
  improvement in safe operation of our facilities.

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