Re: satellites carrying atomic reactors

From: Vitek, Antonin (
Date: Sat Aug 25 2001 - 16:58:48 PDT

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    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
    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
    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
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