Re: [DIYbio] Re: I had idea on biospheres.

On Sat, Oct 27, 2012 at 12:47 AM, Mega <masterstorm123@gmail.com> wrote:
>> Since [the radioisotope battery]
>>
>> it's a spacecraft component, it's super-optimized for low weight.
>
>
> Think so?

Yes.

> It's shielded with tons (figuratively) of graphite

"Figuratively?" Why speak figuratively when there are ... actual figures?

http://en.wikipedia.org/wiki/File:MMRTG_schematic_-_english_labels.png

Hard to see the "tons" of graphite in this one.

> in case the rocket would
> have failed, the plutonium wouldn't have been set free. It would have
> fallen into the ocean as one block.

Yes, and they used graphite because it's light. Which is part of
optimizing it to be as light as possible. For space applications. As I
said.

What would you have preferred? That these things be launched with no
protection for human populations at all?

> And, they surely haven't used 100% radioactive isotopes. I haven't looked it
> up,

Of course not. Looking stuff up requires work or something.

> but it may be 30% radioactive plutonium isotopes out of total plutonium.

83.5% Pu-238 in an oxide, according to this source:

http://www.mdcampbell.com/Bennett0606.pdf

Since plutonium has a very heavy nucleus, and oxygen does not, PuO2's
weight is about 2/3rds of its Pu part. There is little or no shielding
required (millimeters of lead will do; in some applications they forgo
it.) The Iridium cladding is also millimeters. The graphite jacket
will be quite lightweight, of course, because ... well, it's graphite.

Much of the remaining weight appears to go into the problem of making
the thing useful at all as an electric power source. Not into "tons of
graphite."

> If you take near 100%, of course, it means less weight per Watt. (Just make
> sure not to reach the critical mass to create a chain reaction)

Not reached with pure Pu-238 until you have about 9kg in a sphere.
(Not sure it's reached at all in its oxide form.) That's about 4500 W
of heat output. Now, starting with ice at around -160 deg K, .... or
am I the only one who's going to look up numbers and do arithmetic?

Regards,
Michael Turner
Project Persephone
1-25-33 Takadanobaba
Shinjuku-ku Tokyo 169-0075
(+81) 90-5203-8682
turner@projectpersephone.org
http://www.projectpersephone.org/

"Love does not consist in gazing at each other, but in looking outward
together in the same direction." -- Antoine de Saint-Exupéry


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