NuMI Tritium Experience and Personal Observations

Rob Plunkett

Fermilab DUSEL Beam Working Group

October 27, 2008

Brief Historical Summary

• Radiology/groundwater was always of first importance in NuMI beamline design.

• Tritium at low levels was detected in site effluent in November 2005 and a strong reaction was mounted.

• Proximate cause was broken pipe. But root cause was NuMI sump water in ICW system.

• Characterization and mitigation began immediately – chiller condensate water identified as source and mitigated already in December 2005.

• Effort ongoing since then. Peaked with installation of shielding tunnel dehumidifiers.

• Disclaimer – this talk is summary of the mighty efforts of others.

Tritium Characterization Efforts

• As soon as problem was detected, a large network of tritium measurements established.– Still going on – sump measurements every 3 days. Other water

measurements sitewide.• There was a large program of measurements of flow in

grates and humidity profiles using mostly homemade equipment

• Core samples in tunnel – a large job• Core samples in target hall• Condensate measurements and other target hall

measurments• Effect of operating conditions. Delayed accesses to

target hall components to see how sump would react.• Effluent measurements by ESH.• We owe great thanks to the many people involved.

Original Tritium Mitigation

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EAV2-3 fans ON/OFF

Chase SEALED/OPEN

Re-circ fan ON/OFF

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beam POT e17/day

Collect chiller condensate

Dry air to upstream decay tunnel

Graphic from Jim Hylen, as are all similar.

Summary of Mitigation Projects

• Much work by FESS on surface water flow.• Collection of tritiated condensate off chase

cooling coil.• Dehumidification of air to decay tunnel.• Dehumidification of chase cooling air.• Correction of partial blockage of flow under

target hall, which caused extra humidity• Dehumidification of absorber area (work in

progress).

Chase dehumidification reduces sump levels significantly.

Schematic of Dehumidifier System (Hylen)

There is a reservoir of stored tritium as well as instantaneous production

A year’s worth of Tritium This factor of 0.6 due to chase dehumdifiers

Summary of Our Understanding

• Outlines of water flow semi-understood.– Construction measurements– Grates and underdrains + geometry

• Air transport dominant mechanism– Probably via H20 -> HTO exchange on solid surfaces,

both to load the air and to dump the HTO into drains.– Shown by effectiveness of dehumidification efforts as

well as modeling.

• Reservoir of tritium likely to be in steel.– Spallation cross-sections, mass at correct location,

diffusion of hydrogen (recall embrittlement incidents).

Lessons learned - target hall area

• Materials selection and positioning critical.– An example – diffusion coefficients in steels vary by orders of

magnitude, and are temperature dependent.– Expect significant cost implications from choices

• Area near hottest beam absorbtion places needs to be extremely dry.

• Isolate air flows in target hall as much as possible and dehumidfy them.

• Design cooling system thinking about radiological interaction with shielding and other surroundings.– Strategy needed since there is a fundamental question – how

fast to get rid of the accumulating tritium. – Cost implications here too.

Long term diffusion efforts

• Eventually, direct production of tritium in decay pipe shielding will diffuse/convect to the outside world

• Tools for study– Coring study done in 2006 (McCluskey, Andrews)– MARS comparisons– LBL consultant numerical modeling

• Result of first phase of modeling was prediction of sump concentration of about 1 pCi/ml for each 1E20 POT, peaking about 7 years after exposure.

• Next phase of modeling is fully 3-dimensional and uses realistic lab proton profile. Underway.

• Fully documented in lab TM 2378 and 2379

Jim Hylen’s simple 1-D target hall diffusion model for various steel diffusion constants

-8 cm2/s

Note lower diffusion constants seem more likely from literature at our temperatures

Comparing Models with Decay Pipe Data (Lundberg)

Two-component model

Diffusion constant about 1E-6 cm**2/sec, not unreasonable

Use MARS to extrapolate closer to pipe

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(a) Mesh

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(b) Pressure (Pa)

One of numerous cross-sections used in LBL model

Continuous grid defined along length of tunnel, with smooth junctions between drawing sections

Concrete parameters will be taken from our own measurements of simulated shielding

Water flow vector diagram

Animation showing water flow diagram as tunnel progresses downhill.

Have form from MARS for z-dependence of source term.

Expect results soon.

Lessons Learned – shielding area

• The decay region is a SCIENTIFIC INSTRUMENT and needs to be treated as one. This means it needs to be quite heavily designed with access and measurement in mind.

• Access to places around the shielding for measurements of flow and/or insertion of samples to measure activation will be needed.

– NOTE – it’s not trivial to interpret any “plugs” of material put in place in the shielding because of the diffusion/microconvection aspect of the tritium flow.

• Materials used in the shielding should be specified with the long term in mind and characterized as they are fabricated.

• Water flows and air flows need even more special study than we gave them for the NuMI beam line. We are succeeding, but only just, and by intense catch-up effort. An understanding of flow helps enormously in modeling tritium transport.

• Transition regions between target hall and absorber need special attention. NuMI has ended up with lots of retrofitting in these places.

• It will take special attention to underground contracting to implement such a design.

• I believe we should assume we will use geotechnical numerical modeling perhaps as early as the design phase.

Input from LBL consultants

• Need to properly “capture and characterize” three elements– Facility design– Facility operation– Natural conditions (rock, fractures, hydraulics)

• Measurements both test hypotheses and allow model calibrations

• Fundamental processes need understanding.• Materials properties, both natural and artificial.

Thoughts for the future• “Un homme averti en vaut deux.”– (A man who is informed is worth two) –

Napoleon.– We now know much more what needs to be done to avoid and mitigate tritium

problems.– We should use it from the beginning.

• Although radiological concerns have always been addressed, they will be even more central at higher intensities.

– Tritium represents a special problem because of its mobility and the difficulty of unambiguously interpreting in situ measurements.

• Our situation put an unprecedented strain on our measurement facilities, who responded brilliantly.

– But in project X era we will have to be sure they are always adequately upgraded and staffed. This is not a luxury!

• It has proven vital to understand the literature of other fields such as metallurgy and nuclear engineering to make good choices.

– We should start reading now!• We are disposing of all tritium by evaporating it into the air. Should be

revisited to be sure of future regulatory and public awareness issues.