Radioisotope sources

Radioisotope thermoelectric generators (RTGs, Figure 9.2) are attractive power sources - reliable, compact and with only modest sensitivity to orientation and environmental conditions. They are, however, rather heavy and costly: much of the cost is due to the burdensome safety regulations associated with radioactive materials.

It is usually considered that a radioisotope power source quietly provides power at a well-determined rate that falls off exponentially with time due to the decay of radioactivity. In fact, as always, reality is much more complicated.

10 cm

RTG mounting flange Cooling tubes Radioactive heat source

10 cm

RTG mounting flange Cooling tubes Radioactive heat source

Figure 9.2. Radioisotope thermoelectric generator, as flown on Galileo, Ulysses, Cassini and New Horizons.

First, while the heat produced by the radioactive decay of a fixed amount of a pure element does indeed follow an exponential curve, it must be remembered that the daughter products of radioactive decay are themselves radioactive, and these daughter products have different activities and half-lives from the primary element (almost invariably 238Pu in the form of PuO2; this material has a half-life of 86 years and a specific power via alpha-decay of 410 Wkg~x).

Secondly, while the heat being produced continuously by the source follows a deterministic (close to but not quite a pure single exponential curve) decay with time, the conversion of that heat into electrical power is less deterministic. The conversion, usually by a set of thermoelectric converters (usually a 'thermopile' of semiconductor slabs) depends on both the intrinsic performance of the converters, and on the environment.

The intrinsic performance depends on the thermal design of the converter, i.e. how much heat flows through the thermopile, and how much just leaks con-ductively through the inert housing material. Notionally, one would want as little heat to be wasted as possible.

The performance also depends on the solid-state physics of the converter material. The latter's properties are uniquely related to the semiconductor

300 r

300 r

Figure 9.3. Electrical output power of the radioisotope thermoelectric generator on the Ulysses solar probe, versus time. The power produced (solid line) declines markedly with time, and more steeply than a simple exponential decay with a half-life of 86 years (dashed line) would suggest.

material, and may degrade with time through thermal or radiation effects (Figure 9.3). (As an aside, it may be noted that the thermoelectric coolers used on some modern microprocessors are essentially the same as the converters used on spacecraft - just used in reverse.)

The overriding concern with radioisotope power sources is safety, not just from the purely radiological standpoint, but also from the severe chemical toxicity of plutonium. This element appears to generate a level of concern far out of proportion to the probability of release, and the legal costs to a project of confronting objections by protesters can be formidable. Launch and disposal of RTGs in the USA requires a launch order signed by the President.

For outer Solar System exploration where solar fluxes are simply too low to be practicable, radioisotope power sources are mandated.

Although the specific thermal power of the source material is high, the electrical specific power is rather low - a little over 2WkgT 1 for the small 35W RTG used on Viking. The larger designs with more efficient converters used on Galileo and Ulysses provided 285 W at about 5WkgT 1. This specific power is some 80 times lower than the fuel itself - the conversion efficiency is of the order of 5%, and only a fraction of the RTG is fuel - a large mass fraction must be devoted to the thermoelectric converters, the radiators for heat rejection, and especially to shielding to prevent release of material in the event of an accident. A new generation of RTG, the multimission RTG (MMRTG) is being developed (Schmidt et al., 2005), nominally a 42 kg unit providing 126 W at beginning of mission under Mars surface conditions (128 W in deep space, where the radiators can operate more efficiently), i.e. 3WkgT 1.

Higher specific powers may be obtained through energy-conversion techniques more efficient than thermoelectric devices. Alkali metal thermionic emission technology (AMTEC) is one possible technique, although at a modest technology readiness level at present. Perhaps more promising in the long term is a Stirling engine, a reciprocating heat engine.

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