In detail what is a half-life



A Radioisotope generator, also Isotope battery or Nuclear battery called, generates electrical energy from the energy of the spontaneous nuclear decay of a radioisotope.

In isotope generators, their energy is obtained through radioactive decay and not through nuclear fission with a chain reaction. They are therefore to be distinguished from nuclear reactors.

The essential characteristics of isotope batteries are that they supply autonomous, maintenance-free and very long (years to decades) electrical energy and - apart from the piezoelectric isotope generator - work without moving parts.

Overview of various radioisotope generators

Several principles come into question or have been tested for energy conversion:

  • Thermoelectric isotope generator; a radionuclide as a heat source operates a thermoelectric generator, similar to a Peltier element (Seebeck effect or inverse Peltier effect). This type of isotope generator is the most common and is described in detail below.

There are also other types:

  • Thermionic isotope generator; it uses the glow emission of electrons from a glow cathode heated by the radionuclide
  • Thermophotovoltaic isotope generator; it uses the infrared radiation of the radionuclide, which heats up to the point of embers, and converts it into electricity with photodiodes, similar to solar cells
  • Direct charging generators; they use the electrical charge generated by the emission of charged particles (beta or alpha radiation)
  • Optoelectric isotope generatorIn it, the radioactive radiation stimulates gases to glow, the radiation of which is converted into electricity with photodiodes.
  • so-called Betavoltaics, They convert beta radiation in a semiconductor, similar to a photodiode, directly into electrical current.
  • Piezoelectric isotope generator; Here a piezoelectric body is periodically deformed by absorbing charge from a beta emitter and releasing it into an electrical contact when it deforms, which it closes through its deformation.
  • Isotope batteries with alkali metal-thermal-electrical converter alkali-metal thermal to electric converter, short AMTEC), they use the electrochemical system of the sodium-sulfur battery similar to a fuel cell, in that the heat of the radionuclide presses sulfur evaporated through a separator or solid electrolyte made of aluminum oxide ceramic.

Thermoelectric isotope generator

It is also named after the English name radioisotope thermoelectric generator also briefly RTG called. It generates electricity from heat that arises from the natural decay of radioactive isotopes and consists of a radioactive heating element. radioisotope heating unit (RHU) or general purpose heat source (GPHS)), and a thermoelectric generator.

principle

The spontaneous radioactive decay of an artificially produced radioisotope generates heat that is generated directly by a thermoelectric generator, i. H. without moving parts, is converted into electricity. The efficiency is only 3 to 8%.
 

Radionuclides

So that the RTG does not lose too much of its performance during the period of use, the radioisotope used should have a half-life that is significantly longer than the planned operating time. In practice, this usually means a half-life of several decades. In space travel, the radioisotope has to emit energy-rich radiation in order to achieve a large amount of heat for its weight and volume. The radiation must also be stopped by a thin shield so that the RTG does not become too heavy. Therefore beta emitters are not well suited because of the release of bremsstrahlung, gamma emitters and isotopes with a high spontaneous fission rate due to the release of gamma rays and neutrons[1]. For applications on earth, the weight of the shielding and the power density are often less important, but the price of the radionuclide is. This is why beta emitters are also used in RTGs on Earth. The decay products (in the entire decay series) of the selected isotope must also not be extremely strong emitters, since even small amounts of them are formed almost immediately.

In practice, therefore, only radionuclides with a half-life of over 10 years are usually considered:

plutonium238Pu that has to be incubated in nuclear reactors is used as a radionuclide in most RTGs, especially in space travel. It has a high energy density that is more than 100 times that of gasoline. The thermal output generated by spontaneous decay is around 450 watts per kilogram. plutonium238Pu is an alpha emitter with a low spontaneous fission rate and thus low neutron and gamma emissions with a half-life of 87.7 years, ie after 87.7 years it is 225 W, after 175.4 years about 112 W, etc. The relatively long half-life (= several decades of use of the RTG) and low emissions of radiation that are difficult to shield mean that only the thinnest radiation shielding of the isotopes mentioned here is required. A quantity of 300 g plutonium-238 delivers after thermoelectric conversion with approx. 8% efficiency e.g. approx. 11 watts of electrical power and thus approx. 933 kilowatt hours of electrical energy within 10 years.

strontium90Sr is a fission product in nuclear reactors and is a beta emitter with a half-life of 28.78 years. During braking, this beta radiation releases bremsstrahlung in the surrounding material. The decay product yttrium 90Y releases even harder beta radiation, which leads to even stronger bremsstrahlung. That's why you need strontium 90Sr a much thicker shield than alpha emitters. It can be considered an advantage that it can only be obtained via the intermediate stage mentioned (yttrium90Y with a half-life of 64.10 hours) to stable zirconium90Zr decays and so the radiation can be considered largely insignificant after about 10 half-lives (287.8 years)[2]. strontium 90Sr can be recovered in abundance from reprocessing and is used in RTGs on Earth, where the weight of the shield is not as critical as it is in space travel.

Cesium137Cs occurs as a fission product in nuclear reactors and has a half-life of 30.17 years. It needs, because it emits beta radiation and the decay product barium 137mBa is a strong gamma emitter[3], a more complex shielding for the radiation than alpha emitters. It can be considered an advantage that it can only be obtained via the aforementioned intermediate stage (barium137mBa with a half-life of 2.55 minutes) to stable barium137Ba decays and so the radiation can be considered largely insignificant after about 10 half-lives (301.7 years)[4]. Cesium 137Cs can be recovered in large quantities from reprocessing.

Curium244Cm has to be incubated in nuclear reactors and has a half-life of 18.1 years. As an alpha emitter, it only needs a thinner shield than the beta emitter, but its spontaneous fission rate and thus the neutron and gamma radiation is higher than that of plutonium 238Pu, which means that the required shielding must also be significantly thicker. Its half-life is also much shorter, so that an RTG with it would have a much shorter period of use.

Americium241Am is created by the decay of the plutonium, which is produced in small quantities in nuclear reactors 241Pooh With a half-life of 432.2 years, it would be suitable for RTGs that can supply electrical energy for centuries rather than just a few decades. Due to the longer half-life, the energy release is distributed over a longer period of time than with plutonium 238Pu, so that at the beginning the radiation power is only about 1/4. However, americium is not a pure alpha emitter, but emits large amounts of relatively soft gamma radiation when it decays, because only about 0.35% of all 241At the atom, give the entire decay energy to the alpha particle[5]. Therefore, these RTGs need a slightly thicker shield than those with 238Pu filling.[6][7]

construction

A radioisotope generator contains one or more radioactive heating elements, which are either inserted directly into the radioisotope generator or, in modern types, are first hermetically encapsulated in several layers of resistant materials to increase safety. The radioisotope generator consists of a metal cylinder in the wall of which the thermocouples are embedded. It has cooling fins on its outer wall in order to give off the heat generated by the heating element and thus create the temperature difference necessary for the operation of the thermocouples. If the radioactive heating elements are not individually packaged against external influences, the inside of the housing of the radioisotope generator must consist of various protective layers in order to prevent the release of radioactive material.

application

Due to their simplicity and long service life, RTGs are used where there is no power grid, no electricity can be generated in any other way (e.g. with solar cells) and where maintenance and refilling of a generator is seldom or never possible. Very powerful RTGs were used in the USSR 90Strontium fill used to operate lighthouses and radio beacons in the Arctic Circle [1].
In [2] there is a table from the Soviet Union 1976 to 1990 for the terrestrial use of manufactured RTGs. From this, efficiencies of approx. 2.5… 6% can be calculated. The use of isotope batteries in pacemakers of the first generation that were operated with Pu-RTGs is considered problematic today [3]. Due to the longevity, unnecessary interventions to change the battery should be avoided. Today (2005) there are still around 100 carriers of such implants worldwide. For a long time, however, pacemakers have only been operated with chemically working lithium batteries and thus achieve an operating time of up to 10 years.

The best known and only application of RTGs today is space travel, where they are used to power probes to the outer planets. Beyond the Mars orbit, the radiation from the distant sun is no longer sufficient to cover the energy requirements of the probes with solar cells of practicable size. RTGs are currently the only generators that are light and reliable enough to be integrated into a probe and that can deliver power for a long enough period. All space probes that fly to the planet Jupiter or further, such as Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini and New Horizons, were equipped with isotope batteries.

The automatic measuring stations (ALSEP) set up on the moon by the Apollo astronauts in the early 1970s also obtained their energy from isotope batteries.

Typical generators for space probes are made with ceramic plutonium dioxide (PuO2) filled in the form of solid blocks - it is chemically more stable, almost insoluble in water, does not atomize and has a higher melting point than metallic plutonium.

Heating elements are used in space travel (RHU, R.adioisotope Heater Unit) to enable the operation of electronic circuits in cold rooms far from the sun. As in the RTGs, plutonium oxide is used for this. One RHU contains 2.7 g PuO2, enclosed in a 3.2 cm x 2.6 cm capsule, and provides a thermal output of approx. 1 W with a total weight of approx. 40 g [4].

Radioactivity problem

The radioisotopes in an isotope battery decay according to the half-lives specific to the respective isotope and not through neutron-induced nuclear fission. Therefore there is no risk of a chain reaction.
However, if the shielding is damaged, destroyed or defective, the isotopes can contaminate their surroundings. There is also a risk of theft. Nuclear weapons cannot be produced with it, since this would require isotopes such as plutonium-239 that are easily fissionable by neutrons, but the radioisotopes could be used by terrorists in "dirty bombs", in which radioactive material is deliberately scattered in order to spread terror.

An extensive debate on the radioactivity of RTGs in space took place in 1997 when NASA's Cassini-Huygens mission to Saturn started. Residents of the spaceport and environmental groups protested, as they feared serious environmental damage in the event of a false start. Another risk was seen with regard to a fly-by maneuver of the probe on Earth in August 1999, which accelerated it to Saturn.

The topic came to the public again when NASA launched the New Horizons space probe with an isotope battery on board to the dwarf planet Pluto in January 2006.

However, the housing of the probe batteries is designed in such a way that they can withstand an explosion by the launch vehicle or an uncontrolled reentry without the radioisotopes being released into the atmosphere.

Because of the large amount of isotope material, its use in the successor states of the USSR is also seen as problematic. There, lighthouses and military radio stations, some of which are remote, have been and are being supplied with isotope batteries. Because of the high power requirements, large amounts of radioactive material have to be used.

It is feared that some of these systems are inadequately secured, which can lead to theft or release due to corrosion. It was reported from Georgia that scrap collectors found the abandoned components of the isotope batteries of former mobile military radio systems in forests.

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  1. http://saturn.jpl.nasa.gov/spacecraft/safety/eisss2.pdf Section 4
  2. http://de.wikipedia.org/wiki/Halbwertszeit#Radioaktiv_Halbwertszeit
  3. http://en.wikipedia.org/wiki/Caesium-137
  4. http://de.wikipedia.org/wiki/Halbwertszeit#Radioaktiv_Halbwertszeit
  5. http://87.139.25.178:81/ger/theory.htm
  6. en: Radioisotope_thermoelectric_generator # Fuels
  7. Nuclides for RTGs (PDF) last page

Category: Nuclear Technology