TABLE D.1 Primary Emissions Produced by Radioisotopes with Half-lives of 15 to 100 Years

Isotope

Half-Life (years)

Type of Primary Emissions

Promethium-145 (Pm-145)

18

gamma

Halfnium-178m (Hf-178m)

31

gamma

Bismuth-207 (Bi-207)

33

gamma

Europium-150 (Eu-150)

37

gamma

Titanium-44 (Ti-44)

47

gamma

Platinum-193 (Pt-193)

50

gamma

Terbium-157 (Tb-157)

99

gamma

Actinium-227 (Ac-227)

22

beta, some alpha

Niobium-93m (Nb-93m)

16

beta, gamma

Lead-210 (Pb-210)

22

beta, some alpha

Strontium-90 (Sr-90)

29

beta

Cesium-137 (Cs-137)

30

beta, gamma

Argon-42 (Ar-42)

33

beta

Tin-121m (Sn-121m)

55

beta

Samarium-151 (Sm-151)

90

beta

Nickel-63 (Ni-63)

100

beta

Curium-244 (Cm-244)

18

alpha, spontaneous fission

Curium-243 (Cm-243)

29

alpha, gamma

Uranium-232 (U-232)

72

alpha, spontaneous fission

Gadolinium-148 (Gd-148)

75

alpha

Plutonium-238 (Pu-238)

88

alpha, spontaneous fission

SOURCE: Department of Energy, information memorandum and associated transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, Office of Nuclear Energy, Science, and Technology, Washington, D.C., Table 1, updated.

thorium-228) emit a significant level of gamma radiation, resulting in dose rates that are higher than either 244Cm or 238Pu heat sources of comparable size. This leaves 238Pu and 244Cm as the only isotopes worthy of further consideration.3

Table D.2 compares the characteristics of 238Pu and 244Cm. Both produce gamma radiation (although the amount produced is much smaller than the amount from isotopes that produce gamma radiation as a primary emission). As shown, 244Cm produces much more gamma radiation than 238Pu. Also, the fast neutron radiation level from 244Cm is nearly 450 times that of 238Pu. These high gamma and neutron radiation levels would require shielding during handling and use of the 244Cm heat sources to protect personnel and sensitive components. The shield weights would most likely be too heavy for deep-space applications.

Nearly all of the gamma dose from 238Pu is attributable to the decay chain of the 236Pu isotope impurity in the fuel, which is limited to very small amounts by 238Pu fuel quality specifications.

TABLE D.2 Characteristics of 238Pu and 244Cm Isotope Fuels

Isotope

Plutonium-238

Curium-244

Half-life

87

18.1

Type of emission

Alpha

Alpha

Activity (curies/watt)

30.73

29.12

Fuel form

PuO2

Cm2O3

Melting point (°C)

2,150

1,950

Specific power (watt/g)

0.40

2.42

Power density (watt/cc)

4.0

26.1

Radiation levels

 

 

Gamma dose rate (mR/hr @ 1m)

~5

~900

Gamma shield thicknessa (cm of uranium)

0

5.6

Fast neutron flux @ 1m (n/cm2sec)

260

116,000

NOTE: mR, milliroentgen.

a Gamma shielding to reduce dose rates to ~5 mR/hr @ 1m (equivalent to Pu-238)

SOURCE: Department of Energy, information memorandum and associated transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, Office of Nuclear Energy, Science, and Technology, Washington, D.C., Table 2.

POWER DENSITY/SPECIFIC POWER CONSIDERATIONS

The power density (watts/cubic centimeter) and specific power (watts/gram) of radioisotope fuel is directly proportional to the energy absorbed per disintegration and is inversely proportional to half-life. (As shown in Table D.2, 244Cm has a higher specific power and power density than 238Pu, because the former has a shorter half-life, but the selection of RPSs powered by 238Pu to power many important missions has demonstrated that its specific power and power density are acceptable.) Higher power density leads to smaller volume heat sources for comparable power levels and higher specific power leads to lighter weight heat sources. Both characteristics are highly significant for space power heat sources. For radioisotope fuels with comparable half-lives, a beta emitting heat source will be larger and heavier than an alpha emitter.

FUEL FORM CONSIDERATIONS

The radioisotope fuel must be used in a fuel form that has a high melting point and remains stable during credible launch accidents and accidental reentries into Earth’s atmosphere. The fuel form must also be noncorrosive and chemically compatible with its containment material (metallic cladding) over the operating lifetime of the power system. It is desirable that the fuel form have a low solubility rate in the human body and the natural environment. Daughter products and the decay process must not affect the integrity of the fuel form. All of the alpha emitting isotopes listed in

3

Four additional alpha emitters have half-lives between 100 and 500 years (polonium-209, americium-242m, californium-249, and americium-241). In addition to the problem of low specific power (caused by their long half-life), all four also emit significant amounts of gamma rays.



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