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 ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
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