8/18/2010

8/18/2010 Robert Fairchild

LBNL Bevatron Characterization of Activation

Products

Robert FairchildLawrence Berkeley National Lab

Radiation Protection GroupASW-2010 Lesson Learned Breakout Session


Bevatron History

1. Operated from 1954 to 19932. Accelerated primarily protons (6.2

GeV) in the early years and switched to heavy ions in later years (mid-1960s) including 238U (2.1 GeV/amu).

3. Lots of Depleted Uranium used as shielding, beam stops, etc.

4. Activation of concrete shielding and metal


Modeling the Activation ProductionInduced Radioactivity in Bevatron Concrete Radiation Shield Blocks, Moeller and Donahue, July 19941. Modeled the neutron production and depth of maximum thermal

flux using LAHET Code System (Monte-Carlo based).2. Neutron production and transport modeled 10 GeV to thermal

energies3. Neutron production was modeled by simulating 4.0 GeV protons

on iron target (magnets)4. Considered materials/construction:

o Magnet Steel 2.5 to 4.0 feet thicko Concrete wall shielding 5 to 10 feet thick and stacked up to 16 feet high.

No roof shield initially o In 1962 upgraded to 10 feet thick at all wall locations and added 7-feet

thick roof shield.o Used heavy concrete (225 lbs/ft3) for center course of walls and normal

density (150 lbs/ft3) on roof.o Target locations (in ring until ~1963, then External Proton Beam (EPB)

hall)


Activation

• Primarily from incident beam on magnets, targets, beam dumps, plunging devices, and beam line apparatus creating prompt radiation.

• Produced by spallation and low energy neutron absorption.• Emit primarily gamma-rays, but ~ 75% emit beta. Due to self

shielding, survey method is by detecting gamma-rays.• Most activation produced during proton runs and prior to 1974,

thus 36 years have passed.• Only long-lived isotopes are of concern.


Isotopes of Concern

• 15 long-lived isotopes could have been produced by spallation. Only 4 emit gamma-rays, but production rate is about the same (~ 1 mb), thus the gamma-emitters can be used as surrogates for the others.

1.26Al (1.809, 0.511, …MeV) 7.3E5 years2.22Na (1.275, 0.511, …MeV) 2.6 years3.54Mn (0.835 MeV) 312 days4.44Ti (0.078, 0.068 MeV) 52 years

• 4 long lived isotopes could have been produced through thermal neutron capture.

1.60Co (1.33, 1.17 MeV) 5.3 years2.152Eu (0.121, 1.41, …MeV) 13.5 years3.154Eu (0.123, 1.27, …MeV) 8.6 years4.134Cs (0.658, 0.089, …MeV) 2.1 years


What about 55Fe?

55Fe was not modeled by Moeller and Donahue, but we can determine the 55Fe activity by using surrogates. • Thermal neutron capture cross section

o 151Eu is 5900 barn and 54Fe is 2.32 barn (Warner, et al., 2010), thus ~2500:1 Eu to Fe

• Abundance in concrete (Hampel, et al., 2002)o 54Fe 5.0 mg/kg and 151Eu 0.002 mg/kg, thus ~2500:1 Fe to Eu

• Half-Lifeo 55Fe (2.7 years) and 152Eu (13.5 years), thus 5:1 activity ratio 55Fe to 152Eu

• Potential production stopped in 1974• 300 times less 55Fe in the concrete today than 152Eu


Conclusion of Moeller and Donahue paper

1. Peak activation production is at depth of 6 cm, but essentially uniform from surface of concrete to 16 cm (6.5 inches).

2. This showed good correlation to results from Princeton-Pennsylvania Accelerator concrete analysis.


Early Block Removal Process (EPB Hall Deconstruction)

1. Over 1000 blocks from the EPB hall were characterized2. Survey each face with NaI to locate hottest spot3. Performed in-situ gamma-ray spectroscopy on hottest spot on

each block4. Process took approximately 2 years5. Approximately 1/3 were determined to be activated above the 1

pCi/g waste determination MDC and disposed of as radioactive waste.


Phase 2 Removal of ~75 upper-tier roof blocks (early 2004)

1. Early process was too long and labor intensive.2. Performed measurements to correlate a 3x3 NaI to HPGe

measurements3. Established procedure to survey the blocks using 3x3 NaI and

ensure 1 pCi/g MDC.4. If a block failed the NaI survey process, then an in-situ HPGe

analysis would be made.5. Approximately a dozen blocks failed the NaI survey, but were

released following further HPGe analysis.


Reconnaissance Level Characterization (late 2006)

Due to earlier surveys and sampling of shield blocks, characterization of the shield blocks was not included in the scope of the Reconnaissance Level Characterization.


Phase 3 Demolition (RFP 2008)

Survey remaining shield blocks comprising lower tier roof blocks, original inner layer wall blocks and outer layer added in 1962. Approximately 800 blocks in all.• Naturally occurring isotope concentrations are not as homogenous as in

upper tier roof block, so determined new MDC for 3x3 NaI to be ~2 pCi/g based on many measurements of non-impacted concrete.

• Obtained site office concurrence for new measurement sensitivity requirement

• Planned to use same survey protocol as upper-tier roof blocks.• Early project planners estimated the number of blocks that would be activated

based on percentage of blocks from EPB hall that were disposed of as radioactive waste.


Lower-Tier Roof Blocks

• Reference block selected from area that was least likely to contain activation.

• Characterized by HPGe and verified to be free of activation down to a measurement sensitivity of approximately 1 pCi/g, which was below the agreed upon 2 pCi/g sensitivity level.

• Began surveys of roof blocks, failed a few and cleared a few.• Subcontractor decided to core the elevated area of a failed block

and send it to an offsite lab for analysis. The subcontractor also decided to core the reference block.


Reference Block Coring

• Below MDC for all suspect isotopes except 152Eu• 152Eu at 0.235 pCi/g with an error of 0.085 pCi/g and MDC of

0.14 pCi/g• The cleanest remaining lower tier roof block contained activation

products.


Path Forward

1. Pursue Authorized Release Limit or Not?2. How would California Executive Order D-62-02 affect decision?3. If granted authorize release, could we rubbelize and reuse in

CA?4. Cost vs. benefit analysis


Final Decision

The Bevatron Project Management determined that there was no cost benefit to pursuing an authorized release. All blocks and concrete that were within direct line-of-sight to the beam line would be disposed of as radioactive waste.


Outer Wall Blocks

• A reference block for the outer wall blocks was selected and analyzed by coring sample to a sample sensitivity level of 0.2 pCi/g and verified to be free of activation.

• All remaining outer wall blocks were then surveyed using the 3x3 NaI process.

• Like the upper tier roof blocks, all outer wall block were determined to be free of activation.


Lessons Learned

• Don’t assume anything during your planning phase (samples are cheap).

• Ensure sub-contractors clearly communicate with Site.• In today’s climate, consider pursuing authorized release limits as

soon as characterization results are available.• No disposal or reuse path for known rad-added in CA.





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