1、- I AIR 1213 REP0 RT Revised the last subsystem is necessary only when the missiQn duration is a substantial fraction of the half-life of the radioisotope fuel selected for the heat source. Operation of the system is as follows: The thermal energy generated in the heat source by the decay of the rad
2、ioisotope is introduced into the energy conversion subsystem from the heat source. in the energy conversion subsystem, a portion of the input heat is converted to electrical energy by either a direct conversion process (i. e. , thermoelectric and thermionic systems) or dynamic conversion process (i.
3、 e. , Rankine and Brayton cycle systems). The unconverted heat is transported from the energy conversion subsystem to the space radiator and then rejected to the environment. if a short half-life radioisotope is utilized in the heat source, excess fuel must be provided at the beginning of the missio
4、n to assure that sufficient thermal energy is available at the end of the mission. For example, when polonium-210 (138-day half-life) is employed as the radioisotope, radioactive decay reduces the initial thermal output by one-half after 138 days of the mission. Some means of controlling the thermal
5、 in- put to the energy conversion system must be provided to prevent an overtemperature of the energy conver- sion system and heat source at the beginning of the mission. To date, this has been accomplished most are controlled by a sensor at a critical temp,erature location. efiectively by allowing
6、the heat source to radiate this heat directly to space through movable shutters Currently, only thermoelectric systems have been extensively developed for use with radioisotope heat sources and practically applied to space missions. These efforts have culminated in numerous terrestrial and space-ori
7、ented systems. Development of radioisotope-powered thermionic systems has been embodied primarily in the SNAP 13 program which is currently inactive. Development of Brayton and Rankine cycle systems for use with radioisotope heat sources is currently in progress. Generally, the efficiency of radiois
8、otope power systems range from 4 to 8 per cent for a thermoelectric system to potentially 20 to 25 per cent for Brayton and Rankine cycle systems with thermionic systems in the intermediate range of potentially 10 to 15 per cent. (The term “potentially“ is used since no extensive system data are ava
9、ilable for these latter systems. ) Typical current 9light-qualifiedtt radioisotope thermo electric space power systems have specific powers of approximately 1-watt per pound and system efficien- cies near 5 per cent. Radioisotope Heat Source: The primary requirements for the radioisotope heat source
10、 are that: (1) It provide a surface for heat input to the power conversion subsystem (2) The half-life of the radioisotope be compatible with the intended mission length (3) Mating components be chemically compatible at the anti- cipated operational temperatures (4) The launch and aerospace nuclear
11、saety requirements are satisfied. The radioisotope heat source contains a naturally decaying radioisotope fuel. This heat source is complete- ly static and requires no control mechanism Bince the thermal output of the fuel is self-regulating, decreas- ing exponentially with time, Thus, he output at
12、any time can be precisely determined f the output at a former time ti is known using the relation COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesSAE AIR*1213 71 8357340 0003777 O W -3- - Xt = P(ti)e where: p (t) = power at time t P(ti) x =
13、disintegration constant = O. 693 T 1/2 = power at some former time T 1/2 = half-life of radioisotope = time since P(t.) was determined. 1 t From this equation it is evident that the power available (and quantity of radioisotope present) with time de- pends on the half-life of the radioisotope chosen
14、 as the fuel. Ideally, it is desirable to select a radioisotope with a half-life of such length that during the mission the reduction in fuel thermal power is negligible, thus eliminating the complexity of a thermal control device. However, radioisotope availability, nuclear safety requirements and
15、cost must be considered in fuel selection. Presently, plutonium-238 (T 1/2 = 87 years) has been considered primarily for space-oriented systems with missions of 1 year and greater, while curium-242 (T 1/2 = 163 days) and polonium-210 (T 1/2 = 138 days) have shown feasibility for missions up to appro
16、ximately four months. 1.2.1 Nuclear Radiations: Present emphasis in radioisotope power generation is on utilizing the decay energy in a heat engine. beta particle, or as the energy of gamma photons. The slowing down and/or absorption of these radia- tions causes an increase in the temperature of the
17、 surrounding material and, in some cases, secondary radiations are produced. Any radiation which is not absorbed to produce heat will become unavailable for use in the heat engine. The energy from a radioisotope initially appears as motion of the emitted alpha or Alpha (u ) particles (helium nuclei)
18、 have a comparatively large mass and a high positive electrical chargi This means that a high energy alpha particle (all important alpha emitters produce about 5 Mev per dis- integration) will have a relatively low velocity, and because of its charge, interacts strongly with other matter; it will be
19、 thermalized in a few hundredths of a millimeter of solid material. Thus, for all prac- tical purposes the thermal energy of an alpha emitter is produced within the decaying material. In some cases gamma radiation is also emitted and carries a small portion of the decay energy. The absorption of an
20、alpha particle in a light-element nucleus can produce secondary neutrons, but this usually repre- sents a radiation hazard rather than a large energy loss. Some alpha emitting materials also undergo a small amount of spontaneous fission, but for practical purposes this has no effect on the heat prod
21、uc- tion rate with the radioisotopes usually considered for power production. It can, however, lead to high neutron fluxes that present a biological hazard. Alpha particles are helium nuclei, consequently when they have lost the kinetic energy associated with decay, they will capture free electrons
22、and form helium atoms. This helium appears as a free gas in the fuel container and may either be contained in the fuel encapsulation or vented to prevent pressure buildup. Beta () particles (electrons) of the energy associated with fuels used in radioisotope power systems will penetrate one to three
23、 millimeters of solid material and, hence, for practical purposes will not es- cape the fuel container. However, in the process of slowing down these particles, high energy electro- magnetic radiation lmown as bremsstrahlung is produced. This radiation, identical to gamma radiation, can contribute a
24、bout 1% heat loss since the photons produced can have energies ranging up to that of the beta particle. of more seriousness is the biological shielding problem associated with the bremsstrah- lung. Bremsstrahlung production is approximately proportional to the square of the atomic number of the beta
25、 absorber and, consequently, can be minimized if beta particles are not allowed to interact with high atomic number materials. The isotopes which have low beta energies (i MeV) produce the least- penetrating bremsstrahlung, but have corresponding low power densities. COPYRIGHT SAE International (Soc
26、iety of Automotive Engineers, Inc)Licensed by Information Handling ServicesI_- - SAE AIRa1213 71 W 8357340 0003780 7 -4- 1.2.2 There are no radioisotopes that decay by pure gamma emission that are of interest for space missions due to the biological shielding problem and the weight of the absorbing
27、material required to capture the energy of the gamma for generation of thermal energy. From the above discussion it is seen that alpha emitting radioisotopes are generally the most attractive for space applications considering their relatively low biological shieldingrequirements and their high ener
28、gy density. high-temperature pressure-containment problems unless venting devices are provided. They are unattractive from the standpoint .of helium gas generation, which presents Isotope Selection: A number of criteria must be considered in selecting a radioisotope fuel. These include : Power Densi
29、ty - The power density required for a particular heat engine is dictated in part by the engine characteristics. For example, a thermionic power system is char- acterized by high temperatures and high heat fluxes, thus, a high power density fuel is desirable. For a thermoelectric power system, which
30、operates at a considerably lower heat flux, a lower power density fuel is acceptable. An upper limit on permissible power density exists due to fuel-handling and capsule-area considerations. Half-life - By definition, the half-life of a radioisotope is the time period in which exactly half of the ma
31、terial decays. An electrical power supply is usually required to produce power at a relatively constant rate during its lifetime. To accomplish this with no thermal control technique requires that the heat source decay no more than 10 to 20%. If thermal control techniques, such as shutters, are empl
32、oyed to allow heat rejection directly from the heat source, mission times up to about one half-life are feasible, The mission length requirement consequently dictates the minimum permissible fuel half-life. Fuel Availability - It is sufficient to mention that fuel quantities are limited and this par
33、ameter becomes important in the selection of the radioisotope. External Nuclear Radiation - in general, the lower the external radiation, the more attractive is the particular radioisotope fuel. As discussed previously, this consider- ation makes alpha emitting fuels particularly attractive. Shieldi
34、ng considerations are discussed later in this report. Radioisotope Cost - Generally, the alpha emitting fuels are relatively expensive, and the fuel cost will exceed that of the remainder of the power system. Thus, a high conversion efficiency is imperative. Depending on the particular mission, the
35、over- all program cost (launch vehicle, payload, etc.) may decrease if a heavier power system design results in a sizeable system efficiency advantage. Beta emitters are generally less expensive, and fuel cost in comparison to power system cost is not as critical. Beta emitters have been primarily l
36、imited to terrestrial applications where the heavy radiation shielding required is not as penalizing as for a space application. Radiological Effects - For space missions, the usefulness of a radioisotope is directly influenced by the consequences of fuel release and by the measures that must be tak
37、en so that potential hazards are reduced to acceptable proportions. These effects are determined by detailed nuclear safety studies which determine the hazard associated with a particular fuel. Fuel Forms and Technolo,q - The fuel forms in all presently considered space systems have been limited to
38、the solid state to ,minimize corrosion problems with containment internals. Consequently, the fuel form melting point becomes an important consideration for high temperature power systems. Also, the fuel form directly affects the biological and radiation hazard of the radioisotope since it usually c
39、haracterizes seawater solubility and secondary radiations. COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesSAE AIR*:1213 71 W 8357340 00037BL 9 -5- 1.2.3 Heat Source Design: The specific fuel encapsulation requirements are dependent on the r
40、adioisotope selected and heat source operating temperatures characteristic of the power system. Generally, it is necessary that the isotope be sealed in a noncontaminated container which is structurally adequate to contain the fuel during normal operation and specified credible accidents. Although v
41、arying with speci- fic missions, these credible accident conditions usually encompass shipping, launch pad explosions and mission aborts. Additionally, the encapsulation must be chemically compatible with the isotope fuel and possible abort environments. If an alpha emitting radioisotope is used it
42、may also be necessary to con- tain the helium pressure buildup within the sealed capsule. Generally, in a radioisotope heat source the fuel is contained within a liner that has demonstrated chemical compatibility with it (see Fig. 2). The liner and fuel are contained within a primary capsule which f
43、unctions as the strength member for pressure containment (if required) and land impact. If the primary capsule material is subject to oxidation, it is usually contained within a corrosion barrier. Thi complete capsule assembly is then fitted into a heat accumulator block (typically graphite) which m
44、ay provide protection against reentry heating. z2 1.3 I3nerg-y Conversion Subsystem: a radioisotope heat source, thermoelectric, thermionic, Rankine cycle and Brayton cycle- types are pre- sently considered to be the most attractive for foreseeable space applications. The first two types are conside
45、red to be “direct conversion“ subsystems in that the thermal input to the conversion device is con- verted directly into electrical energy without intermediate conversion steps. The latter two types employ turbo-alternators for power generation and are generally referred to as “dynamic conversion“ s
46、ubsystems. Although many types of energy conversion subsystems can be coupled with At present , thermoelectric subsystems are the most fully developed of the subsystems under consideration, Numerous terrestrial and space applications have demonstrated the feasibility and reliability of the radio- is
47、otope-powered thermoelectric system for unattended, remote operation. The potentially more efficient thermionic and Rankine and Brayton cycle systems are still in the development stages but could be availablc for application in the mid or late 1970 decade. 1.3.1 Thermoelectric Conversion: Thermoelec
48、tric systems operate on the principle of the common temperaturc measurement thermocouple; when two dissimilar materials are connected at two points which are at dif- ferent temperatures, a voltage is produced which causes an electron current flow around the circuit. Suc systems are more efficient in
49、 converting thermal to electrical power when the Seebeck voltage so produce is high, the electrical resistance of the materials is low, their thermal conductivity is low, and the Carnc efficiency is high. Figure 3 is a schematic of a thermoelectric subsystem. The heat input from the radioisotope establishes a temperature differential across the thermoelectric elements. This temperature difference results in a portion of the input heat being converted directly into electrical energy. The unconverted heat is rejected to space from the thermoelectric converter by means of the radiator s
