Thursday, April 4, 2013

The Nuclear-Powered Swimsuit

And I'm not talking about the bikini.

In the mid to late 60s, the AEC studied the use of Pu-238 radioisotope heaters to provide heat for a Navy wetsuit:

Figure 1: Pu-238-heated Wetsuit

(Okay, yes, technically it's a wetsuit, not a swimsuit.   I thought swimsuit made a better title.)

That aluminum can holds about 800 grams of plutonium-238.   This is not the stuff used in bombs; wrong isotope.   It's still intensely radioactive, however - even more than regular plutonium.   It's so radioactive, in fact, that it will get hot enough to actually glow orange-red from the heat generated by radioactive decay.   These days it's mainly used to power space probes that are going to places where solar power is inadequate - the Mars Curiosity rover's nuclear battery holds 4.8 kilograms of Pu-238.   This is because, in addition to a long half-life and generating a lot of heat, almost all of Pu-238's radioactive emissions are alpha particles, a form of radiation that is very easy to shield against, which is why that man is wearing almost a kilogram of the stuff.

The plutonium generates about 400 watts of heat.   This is used to heat water, which is circulated by a battery-powered pump through tiny plastic tubes woven into the wet suit, keeping the diver nice and warm in the briny deep, up to 600 feet below the surface.

Figure 2: Suit Heater Schematic

Figure 3: Radiothermal Unit Schematic
(Click to enlarge)

The AEC evidently hoped that similar devices could eventually be developed for "pilots, astronauts, and Arctic researchers."   The Pu-238 cell could provide power essentially indefinitely, unlike conventional heated suits whose batteries would run out in a few hours.   However, while the suit would never run out of heat, the batteries for the pump would eventually be used up; the documents I have don't say how long that would take.

Figure 4: Man Wearing Suit

The suit was developed by the Atomic Energy Commission's Mounds Laboratory, and patented in June of 1967.   (Oddly, two almost identical patents were filed.   No idea why.)   Mounds is better known for its work on weapons, but the AEC's weapons labs have a long history of getting involved in other fields.   Mounds didn't go as far as Los Alamos or Lawrence Livermore in this regard, but they did become involved in the SNAP program (Systems for Nuclear Auxiliary Power), developing RTGs and miniaturized reactors for satellites and the like, which this seems to be a spinoff of.   The suit was delivered to the Navy's Medical Research Institute at Bethesda, Maryland, in late 1967, for evaluation prior to its use in Sealab III.   I haven't been able to get hold of the official evaluation, but I did find an abstract:
The present state of the thermonuclear (i.e., radioisotope heat source) diver heating system is discussed and the results of the only dive to date using this system are reported. The inlet and outlet temperatures of the system are recorded together with the time-course of skin and rectal temperatures. It is concluded that the system in its present state is incapable of maintaining thermal balance in a diver at depth, and its use under SEALAB III conditions would entail a grave risk of hypothermia.
As far as I know, that was the end of the project.

Another document, of which I again only have the abstract, appears to suggest the problem was an inadequate supply of Pu-238.   The stuff is not easy to make, or cheap.   The Department of Energy is currently trying to get Congress to allocate money to resume production of the stuff; for $75 to $90 million they'll be able to produce 1.5 to 2 kg per year - enough to fuel 2 to 2.5 wetsuits per year.   It would have been a lot cheaper back in the 60s - I believe a lot of that money is going to one-time expenses to rebuild dismantled facilities - but it would still be pretty darn expensive.


Annual Report to Congress of the Atomic Energy Commission for 1967.   Atomic Energy Commission, 1968.

"Wet Suits Heated by Radioisotopes."   The Bulletin Board, Volume 21, Number 40, October 21, 1967.
"Key Uses Seen for Actinide Isotopes."   Chemical and Engineering News, Volume 46, Number 44, October 14 1968.
Grega, M. G., for US Atomic Energy Commission.   "Suit Heater."   US Patent No. 3,402,708.   Filed June 27, 1967, Granted Sept. 24, 1968.
Shivers, R. W., et al, for US Atomic Energy Commission.   "Suit Heater."   US Patent No. 3,402,709.   Filed June 27, 1967, Granted Sept. 24, 1968.


Figure 1: From Annual Report to Congress of the AEC for 1967.   US Government.
Figure 2: From US Patent No. 3,402,708.   US Government, modified by author for clarity.
Figure 3: From US Patent No. 3,402,708.   US Government, modified by author for clarity.
Figure 4: From "Key Uses Seen for Actinide Isotopes."   US Government (assumed).

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