Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Neutron Radiography and Tomography Facilities at NIST to Analyze In-Situ PEM Fuel

Cell Performance

Facility StaffDavid L. JacobsonDaniel S. Hussey

Elias BalticMuhammad ArifPhysics Laboratory

Project Design EngineerJames LaRock

Center for Neutron Research

National Institute of Standards and Technology Technology AdministrationU.S. Department of Commerce

Fuel Cell PartnersJon Owejan

Jeffrey GagliardoThomas Trabold

General Motor Fuel Cell Activities

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Old Facility

• Began operation in 2003• Located at BT-6• Facility was small

– Volume 3 m3

• Basically no support for fuel cell experiments other than– Hydrogen gas 1.2 slpm

– Nitrogen from bottle

– Air from bottle

• Ceased operation in December of 2005

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

New Facility

• First users February 27, 2006• Located at BT-2• Much bigger 30 m3

• House gases/fluids– Hydrogen (18.8 slpm)– Oxygen (11.1 slpm)– Nitrogen– Air– Deionized water– Chilled water

• Freeze chamber for low temperature testing (-40 oC) available early 2007

• Fuel cell test stands available and supported by NIST staff

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Hydrogen Safety

• Computational Fluid Dynamics modeling

– Free software

– NIST Fire Dynamics Simulator (FDS)

– http://www.fire.nist.gov/fds/

• Release point in reactor confinement building

• Extremely high buoyancy turbulently mixes hydrogen resulting in low concentrations throughout room

Diffusion coefficient at STP in air = 0.61 cm2 s-1

Diffusion velocity at STP in air 2 cm/s

Buoyant velocity at STP in air = (1.9 m/s to 9 m/s)

Explosive equivalent at 28% H2 in air 1 g H2

= 24 g TNT

Lower explosive limit by volume fraction 4%

Upper explosive limit by volume fraction 77%

Hydrogen Plume 22.4 lpm

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Modeling the Release Point

Lower flammability limit

Upper flammability limit

• Maximum release modeled for 22.4 liters per minute (2 g H2).

• Very seldom is this release rate actually required.• Above release point a maximum of 68 mg of hydrogen is

expected to be within the range of 77% to 4% and so an unlikely detonation of such a mixture is expected to have a maximum explosive yield similar to a few firecrackers.

• Conclusion: for our system it is safe to release hydrogen to the room.

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Modeling Hydrogen Release in Reactor Confinement Building

Mass (kg/kg) x 10-

4

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Schematic of New Facility

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Design of Neutron Collimation

• Primary collimator tapers beam

• Bismuth filter– 15 cm long

– Liquid nitrogen cooled

• Apertures– 5 positions

• Local shutter– Heavy concrete filled

– 60 cm in length

– 3 through tubes for additional collimation

• Fast exposure control– Allows < 1 second exposure

control

Primary collimator

Apertures

Bismuth filter

Fast exposure

Local shutter

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Primary Collimator

• Hollow steel frame• Filled with heavy concrete• 10 cm long steel rings taper

beam down to 2 cm maximum aperture size

• Borated aluminum discs are used throughout to reduce long term activation

Steel rings taper beam from reactor

Steel caseHeavy concrete filled

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Bismuth Filter

• Bismuth crystal– Filters gammas and high

energy neutrons

– Ideally single crystal

– H ere we have several large single crystals

• 5 cm reduces thermal neutrons by 57 %

• 15 cm reduces neutron fluence to 19 %

• Banjo Dewar– Provides insulated liquid

nitrogen jacket

– Sealed and evacuated during operation

Banjo Dewar (named after the musical instrument)

Super insulation/liquid nitrogen jacket

10 cm dia. hole for bismuth to sit

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Aperture Assembly

• Can be any material that fits• 5 positions• Largest aperture diameter is 2 cm

due to primary collimation• Easily changed without major

shielding manipulations

5 apertures (2 cm, 1.5 cm, 1.0 cm, 0.5 cm, 0.1 cm)

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Rotating Drum

• Rotates to 1 of 4 positions– Position 0 beam is blocked

– Position 1 beam is collimated for 1 cm effective aperture

– Position 2 beam is collimated for 2 cm effective aperture

– Position 3 no collimation currently

• Filled with heavy concrete• 60 cm long

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Fast Exposure

• Designed to ensure uniform fluence.– Beam is opened and closed in the

same direction

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Fast Exposure

• Designed to ensure uniform fluence.– Beam is opened and closed in the

same direction

• Time for each motion is about 0.1 seconds.

• Can be set for fixed times and manually operated

• Can be operated by computer control.

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Shielding

• Steel shot and wax external to the reactor.

• Inside reactor only heavy concrete and steel is used.

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Schematic of New Facility

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Flight Path and Sample Position

• 6 meters from aperture to sample position.

• Aluminum flight tube evacuated.

• Short sections can be made into a shorter tube for closer positions.

• Closest position is 1 meter.

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Real-Time Detector Technology• Amorphous silicon

• Varian Paxscan 2520 high energy version– Cost is ~$100,000.00 US– No longer produced as of 2006– However, if you are willing to void the warranty

you could convert a low energy detector (still produced) to a high energy detector.

• Radiation hard

• High frame rate (30 fps)

• 127 micron spatial resolution

• No optics – scintillator directly couples to the sensor to optimize light input efficiency

– Standard green Li6ZnS scintillator 0.3 mm thick

• We experienced a failure of the readout electronics in July of 2006

– Failure is believed to be due to radiation damage– We were able to quickly fix by swapping the guts

of a spare low energy panel with this detector frame.

• Data rate is 42 Megabytes per second (160 gigabytes per hour)

Neutron beam

scintillator

aSi sensor

Side view

Readout electronics

Scintillator aSi sensor

Front view

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

New High Resolution Imaging Device

• 25 micrometer resolution available this fall.• An order of magnitude improvement in spatial resolution.• 10 micrometer resolution expected in 2007.• Less than 10 micrometer???

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Beam Properties

L

(m)

D

(cm)

L/d Beam Dia.

(cm)

Fluence Rate

(s-1 cm-2)

Fluence Rate No Bismuth

(s-1 cm-2)

2 2 100 8 5.1x107 3.0 x108

3 2 150 13 3.4 x107 2.0 x108

4 2 200 17 2.5 x107 1.5 x108

6 2 300 26 1.7x107 1.0 x108

6 1.5 400 26 1.0x107 5.9 x107

6 1.0 600 26 4.3x106 2.5 x107

6 0.5 1200 26 1.0x106 5.9 x106

6 0.1 6000 26 4.3x104 2.5 x105

At 6 m

• A factor of 2 in beam fluence rate can be gained by removing 5 cm of bismuth

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Schematic of New Facility

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Beam Stop

• With most intense beam the field is less than 0.2 mrem hr-1 or 2 Sv

• Magnesium is used instead of aluminum to avoid harsh 7 MeV gamma from aluminum

• Box of boron carbide 15 cm thick absorbs majority of beam

• The rest is wax and steel shot

90 cm

45 cm

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Hydrogen Systems• State of the art, custom-built, PEFC test stand • Flow control over H2, Air, N2, He, O2 with accuracy of 1 % full scale:

– H2: 0-500 and 0-3000 sccm – N2: 0-2000 sccm – Air: 0-100, 0-500, 0-2000, 0-8000 sccm – O2: 0-500, 0-5000 sccm – He: 0-600, 0-6000 sccm

• Users can create custom gas mixtures for anode and cathode in the stand

• Measurement of high current densities with boost power supply allowing voltage control of the cell to a minimum of 0.01 V

• Heated Inlet gas lines • Built-in humidification of anode and cathode gas streams for all flow

rates • Graphical User Interface • Logs and stores files of all cell parameters during operation • Multiple thermocouple inputs • Interfaced with facility hydrogen safety system • All users of the NIST NIF have access to the stand

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Hydrogen Systems Continued

• Piping manifold appears in back.

• Nitrogen gas supplied from liquid nitrogen dewar.

• Hydrogen generator provides 18.8 liters per minute.

• Deionized water for the hydrogen generator and test stand humidifiers.

Nitrogen Hydrogen Deionized waterHydrogen and

Oxygen Sensor readout

Neutron Imaging Facility: http://physics.nist.gov/MajResFac/NIF/index.html

Final Remarks• Facility is accepting proposals

through the NIST user proposal system as well as proprietary requests directly to NIST staff.

• Users from both industry, national laboratories and academia use the facility for both proprietary and non-proprietary research.

• Reactor cycle is 290 days per year currently all have been utilized.