Jim Nix wrote:
>
> Joe,
>
> I've read about RTG's but would like more info on how they work, ie schematics. ... How much U-238 is required ...
No, they do'nt use uranium, but plutonium (Pu) in the form of plutonium
dioxide pellets. This oxide is prepared by sintering as a sort of
ceramics, practically insoluble in water. Therefore in the case of
return to the Earth the contamination of the environment is rather
small, if the radioactive material escapes the protective cover.
Obviously the metallic Pu is highly poisonous, carcinogenic (due to
alpha radiation) and therefore the general public is fearing of those
devices.
The construction of RTG (Radioisotope Thermionic Generator) is very
simple: It consists of a steel tube, filled with pellets of PuO2. This
tube is heated to hundreds of degrees of Celsium by the radioactive
decay (alpha decay) of Pu. Steel walls do'nt allow the radiation to
escape outside the tube (but some of decay products are also producing
gamma rays, which can penetrate very thick shielding, which is not used
in RTG, therefore after some lifetime, the RTG might be dangerous not
only to humans, but to the electronics too).
Back to the construction: There are thermionic converters (TC ie. stack
of solid state "thermocouples") whose are by its one side (AKA "hot
side") in contact with steel tube containing PuO2, the other side ("cold
side") of converters are in contact with the outer hull of RTG, exposed
to the cold outer space. This outer hull is usually equipped with
cooling fins, enlarging the radiative surface of RTG. The voltage
produced is dependent on temperature difference between the hot and cold
side of
TCs, power level on the total heat flow (amount of heat per second). For
hundreds of Watts of electrical output (BOL = begin of life) the
necessary amount of PuO2 is few pounds (1 - 3 lbs).
RTGs are used now almost exclusively on the deep space missions to the
outer planets, where the solar radiation is not powerfull enough to
supply the electric energy by the solar (photovoltaic) cells (remember
the reciprocal square law).
Protest against RTG is common in the anti-nuclear community, but in my
opinion, the RTGs are safe, realiable (no moving parts at all), with
usefull lifetime in the range of tens of years (remember Pioneer 10 and
11, lanched more than a quarter of century ago and working yet).
Precautionary measures for the case of launch failure are sufficient to
protect the environment against radiation contamination.
The power output of RTG is diminishing with the time, not due to decay
of Pu (I am unable at the moment to remeber exact figure for decay
halftime of Pu, but it is in the range of tens of thousands years), but
due to the damage of crystal structure of TCs by the gamma radiation of
decay products.
RTG are usually placed on booms outside of spacecraft body for three
reasons: 1) to allow the waste heat to escape freely in surrounding
space, 2) to diminish the influence of gamma radiation (in later stages
of the mission) on spacecraft electronics, 3) to diminish influence of
magnetic field created by rather high electrical current drained from
TC.
Sorry for another OT message.
--
Mgr. Antonin Vitek, CSc.
Office: Main Library, Academy of Sciences of the Czech Republic
Narodni 3, CZ-11522 Praha 1 - Phone: +420(2)21403255, fax
+420(2)24240611
Home: Kytin 127, CZ-25210 Mnisek p. B., Czech Republic
Phone: +420(305)592865 - Coord.: 14.2194 deg E, 49.8488 deg N, 442 m ASL
My satellite home page: http://www.lib.cas.cz/www/space.40/index.html
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