Dynamic Cryogenic Seals to Support

Fueling of Fusion Tokomaks U. Naranjo and J. W. Leachman

School of Mechanical and Materials Engineering Introduction / Motivation

Sealing failures cost billions of dollars in damaged products

every year. Sealing at cryogenic temperatures is a

substantially more difficult task than sealing at room

temperature since materials tend to shrink, become brittle,

and crack.

I have constructed a prototype dynamic seal made of

Polychlorotrifluoroethylene (PCTFE) and a system to test the

seal immersed in liquid nitrogen, along with plans and

drawings to incorporate the seal into the prototype solid

hydrogen extruder in development at the WSU HYPER

laboratory. The purpose of a cryogenic dynamic polymer seal

is to move a gate back and forth which will vary the area of a

nozzle opening. This will be attached to the solid hydrogen

extruder using the mechanism shown in figure 3B.

Knowledge from this extruder will be used to develop fueling

systems for fusion reactors such as the ITER tokomak.

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REFERENCES

Ekin, J. W. Experimental Techniques for Low-temperature Measurements: Cryostat Design, Material

Properties, and Superconductor Critical-current Testing. Oxford: Oxford UP, 2006. Print

Hablanian, M. H. High-vacuum Technology: A Practical Guide. New York: M. Dekker, 1990. Print ACKNOWLEDGEMENTS This work was supported by the National Science Foundation’s REU program under grant number EEC 1157094

Uriel Naranjo uriel.naranjo@email.wsu.edu

hydrogen.wsu.edu

Jacob Leachman jacob.leachman@wsu.edu

509-335-7711

Figure 3: A) Cross section of the placement of

PCTFE interference seal on model. The seal will be

placed around a rod to see if the seal can be

maintained while the rod is turned. B) the

mechanism which will be used to turn the rod placed

through the seal.

Summary The dynamic polymer seal was not able to maintain a

vacuum after being heated, immersed in liquid

nitrogen, and heated once again. The most promising

experiment, where vacuum grease was used, was able

to hold a seal at room temperature and cryogenic

temperatures but not while being rotated at cryogenic

temperatures. The estimated leak rate agrees with the

bubble test as there came a point where very few

bubbles escaped from the pressurized chamber and

leaks larger than 10-1 atm*cm3/s can be spotted

visually (Hablanian 368). The leak rate of the

dynamic polymer seal is currently too high to use in a

high vacuum cryogenic chamber.

Future Recommendations I recommend applying a cryogenic lubricant such as

spray on graphite (Ekin 533) between the dynamic

polymer seal and bolt, and running the experiment

once more to ensure an adequate seal.

In the future this test will be run once again using a

seal with minimum epoxy to ensure the PCTFE is

shrinking around the bolt as intended. A helium leak

detector will be used to accurately measure the leak

rate up to 10-12 atm*cm3/s /

Observations

Application to Extruder

Acceptable leak rates

Experimental Technique

How a polymer seal works

Figure 1: Cross sections of A) PCTFE interference seal at

room temperature and B) A PCTFE interference seal at

cryogenic temperature

Figures 2A-2E:

2A. Conflat chamber at room temperature

2B. Outgassing the conflat chamber by heating outside to 50°C

2C. Dipping conflat chamber in liquid nitrogen for 3-5 minutes

2D. Heating conflat chamber to 50 °C once again

2E. Checking for leaks in the seal by pressurizing chamber with nitrogen

gas and dipping the seal in water

Product/System Leak Rate Specification

(atm cm3) / s

Chemical Process Equipment 10-1 to 1

Torque Converter 10-3 to 10-4

Beverage Can End 10-5 to 10-7

IC Package 10-7 to 10-8

Pacemaker 10-9 to 10-10

*PCTFE seal estimated leak

rate at cryogenic temperature 1.6*10-1

Table 1: The acceptable leak rates for various products

(Hablanian 362)

2A 2B 2C

2D 2E

Table 2: The vacuum inside the conflat chamber at different steps and for

different trials

A B

A B

Epoxy Brass Vacuum

Sealing

points

Sealing

points

Hydrogen

PCTFE

SEAL Steel Bolt

Procedure Trial 10 Trial 11 Trial 12 Vacuum (in Hg) Heating and pulling vacuum 22.8 22.8 22.8

Immersing seal in Nitrogen for 3-5 Minutes 25.2 24.2 24.5

Heating conflat chamber for 3-5 minutes 22.8 21 21.2

Bolt removed and vacuum grease added X X Bolt removed and Apiezon cryogenic vacuum grease added X Rotation X No rotation X X