CFEET (Compact Fuel Element Environmental Test)

The CERMET Fuel Element Environmental Test (CFEET) stand was originally proposed in 2012 by David Bradley at NASA’s Marshall Spaceflight Center as a lower-cost alternative to the Nuclear Thermal Rocket Environmental Effects System (NTREES). One of the consistent problems in engineering is that to make something more flexible the complexity must increase. This increases the cost to both build and maintain the test stand, which results in a higher cost per test. Also, the larger the volume the test stand uses, the more supplies are needed (in the case of NTREES, GH2 and GN2, plus water for the cooling system), which also increases cost.

Diagram, Gomez

CFEET model, NASA MSFC

CFEET is a low-cost, small scale test stand for NTR fuel elements. It also exposes a test article to temperatures and hydrogen environment that they would experience in the core of an NTR, but again the radiation effects aren’t accounted for since this is purely an inductively heated test stand.

Rather than have the extensive piping, effluent cleanup, and exhaust systems that NTREES uses to model thermal transfer and erosive patterns in hydrogen moving at the same velocity as an NTR fuel element would experience, CFEET uses a simple vacuum chamber with a single RF coil for induction heating to test thermal properties and general reactions with the hydrogen (The hydrogen is pumped through the FE during testing, but I can’t find any information about flow rate of the gas).

This means that the majority of CFEET fits on a (large) desktop. The vacuum chamber is only 16.9” tall and 10” in diameter, and it’s the largest component of the system. Rated to 10^-6 Torr, the chamber has a vacuum-rated RF feed-through port one one side, and opposite that port another, sapphire one for pyrometer readings. Additional ports connect the turbopumps and other equipment to the chamber.

CFEET Dimensions, Bradley

CFEET Elevation Diagram, NASA MSFC

The induction heating equipment used for CFEET is a Flexitune water-cooled unit rated to 15 kW, with an output frequency of 20-60 kHz. While significantly lower output than NTREES, CFEET is still able to get test articles to reach temperatures over 2400 K. An insulating sleeve (with a hole formed in it to allow pyrometer readings) of various materials is used to minimize heat loss through radiation.

Additional precautions are taken with the test article mounting system to further thermally isolate the sample from the environment – and therefore reach higher temperatures for the tests.

Fuel Sample Pedestal detail, Bradley

Fuel Element Thermal Isolation Pedestal,

While CFEET is not able to simulate gas flow, as NTREES is, it is able to assess thermal, chemical, and mechanical properties of materials at temperature and in a pure-hydrogen atmosphere. Because the system is far simpler, and takes far fewer consumables to operate, it is far cheaper to use as a test bed.

 

Additional Reading and Resources:

Low Cost Nuclear Thermal Rocket CERMET Fuel Element Environmental Testing (CFEET), Slideshow, Bradley Yetispace/NASA MSFC 2012

Low Cost Nuclear Thermal Rocket CERMET Fuel Element Environment Testing, Bradley et al, NASA MSFC 2012

Induction Heating Model of CERMET Fuel Element Environmental Testing (CFEET), Conference paper, Gomez et al, NASA MSFC 2014

Hot Hydrogen Testing of Tungsten-Uranium Dioxide (W-UO2) CERMET Fuel Materials for Nuclear Thermal Propulsion, Hickman et al, NASA MSFC 2014