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Radioisotope (1)

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An atom with an unstable nucleus Excess energy is emitted; radiation is created from the nucleus It is also called radioactive decay

Radioactive decay Particles (4He nuclei, electrons, photons ) are emitted

, , and decay respectively

Each particular isotope has its own decay mechanism

Mass is not conserved slightly; E = mc2 (particles kinetic energy)

radiation

radiation

radiation

Penetration capability

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Radioisotope (2)

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Half-life The time required for the quantity to decay to half of its initial Each particular isotope has its own half-life

Ex 1) A radioisotope has 30-year half-life. After 1 year its outputheat decreases by 2.28%

Ex 2) Let say a mission duration is 10 year and output should beat least 80% of the initial state.

0228.05.0130/1=

yearx

x

063.312.05.01

/10

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Radioisotope (3)

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Suitable for space power systems various power levels for more than several months Operations independent of distance and orientation w.r.t sun

Inherent limitation of solar powerBest candidates for maximizing specific power

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Radioisotope (4)

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Pros Useful for long duration mission Suitable for remote system

Any care or control is not needed

Cons Risk of radioactive contamination

In some cases, heavy shielding must be needed

Characteristics required for using as a fuel Long half-life; typically several decades

decay is preferable

Large energy per mass and volume

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Radioisotope Power System (RPS)

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Application Satellite and Space probe Surface rover

Various remote station

Pace maker

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Radioisotope power conversion

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From thermal energy of decay to electric energy Directly static conversion Indirectly dynamic conversion

Electric

Kinetic

Thermal

Brayton cycle

Rankine cycle Stirling cycle

Thermoelectric (TE)

Thermionic (TI)

Thermo-photovoltaic (TPV)

Decay

Absorbing radiation

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Static conversion: TE (1)

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Thermoelectric Has the longest history; so called RTG

238Pu was used in US and 90Sr was used in USSR

Seebeck effect

Lowest conversion efficiency (~ 7%)

Specific power: ~ 5 We/kg

MMRTG in US Power decrease of MMRTG

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Static conversion: TE (2)

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Preferable TE material characteristics Low thermal conductivity (high temperature difference) Low electrical resistivity

Large Seebeck coefficient

Ongoing research Si-Ge nanocomposite

Superlattice BiTe

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Static conversion: TI

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Thermionic (TI) Thermionic emission Can achieve conversion efficiency 10~20%

Need high temperature

Nuclear reactor can be useful

Need high radioactive fuel like 210Po

High radioactive fuel has very short half-life Infeasible to use radioisotope

Thermionic emission The heat-induced flow of charge carriers from a surface

Thermal energy can make charge carriers move away Charge carrier can be electrons or ions

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Static conversion: TPV

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Thermo-photovoltaic Infrared photons emitted by a hot surface electricity Spectrally tuned to optimize the conversion

Conversion efficiency about 20%

Have not been used in practice

Schematic diagram of TPV TPV cell of EDTEK.inc

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Dynamic conversion: Brayton cycle

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Brayton cycle Converters degradation is negligible Conversion efficiency : ~ 30%

High efficiency and specific power

Key technology Moving parts long life reliability

Heat exchanger for increase thermal efficiency

Scalable to various power range

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Dynamic conversion: Stirling cycle

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Stirling cycle Conversion efficiency ~ 30%

Under development in NASA

Specific power: ~ 7 We/kg

System reliability is most important

Stirling converter in NASASRG in NASA

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Conclusion

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Various conversion methods are studied.

Adequate research topic TI is not considered

dynamic conversion seems to be very hard

TE or TPV More study is required

Use radioisotope in Lab Need a license

Before take the license, safety education is essential So far, dont know whether we can use it or not

Even if we can, it takes about at least 1 year to take a license