DIRECT CONVERSION OF RADIATION INTO ELECTRICITY

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1.1 Introduction

The production of electrical energy can be divided into 3 main branches: chemically produced energy, energy produced by renewable energy sources, and energy produced by nuclear reactions. The first method of generating energy requires a constant supply of fuel for a chemical reaction, so it is ineffective in cases where there is no possibility of a constant supply of reagents. The second method is highly dependent on external conditions – tides for hydroelectric power plants, windiness for wind turbines, and solar illumination for solar panels. Nuclear energy, on the other hand, is entirely dependent only on conditions controlled by humans and does not require fuel renewal at long intervals. The only limitation that can stop the intensive development of this area is huge resources needed for its development. As can be seen from the work of "Strata Policy" group of the University of Utah (Table. 1), nuclear power is currently at about the same level in terms of space use as hydrocarbon energy sources, but at the same time wins in stored energy per kilogram: in comparison, when burning 1 kg of the best quality coal (anthracite), about 4·10⁷ J of energy is released, and when using 1 gram of pure uranium-235, 5,8·10⁸ J is released, that is, to obtain nuclear energy contained in 1 kg of natural uranium, it is necessary to burn more than 10 tons of anthracite [1].

Table 1 [2]

1.2 Nuclear power in relation to space systems

The choice of energy source, both thermal and electrical, on the spacecraft is limited. From renewable sources, only solar panels remain, but the energy they receive decreases squared with increasing the distance to the Sun.  In total, there are chemical power sources, which are not enough for long-range flights and nuclear energy sources. Let's take the amount of energy generated on the International Space Station (ISS) as the minimum energy required to maintain the necessary conditions for life on a manned spacecraft. This is 84–120 kW of energy [3]. This energy is generated on the ISS to maintain life support systems, the heat balance system, and other systems necessary for the operation of the station. Among other things, energy is spent on various experiments in microgravity. And most importantly – in the conditions of manned flights, there are also several options for providing the necessary momentum, but any of them still requires a considerable amount of energy. All this energy on the ISS is generated by solar panels, but as already mentioned above, with increasing distance to the Sun, their efficiency as an energy source tends to zero. At the moment, radioisotope thermoelectric generators (RTGs) are used in deep space missions, but their efficiency is negligible, which, however, is enough for unpiloted space vehicles. The characteristics of some are given in Table 2 (Table 2).

Table 2 [4]

1.3 Nuclear reactors

The principle of RTG operation is based on the use of radioactive substance natural decay to heat the conductor and further use the Seebeck effect: due to the temperature difference at the point of contact of dissimilar series-connected conductors, an electromotive force (EMF) occurs in them. Even though this is a consequence of nuclear processes, RTG is not a nuclear reactor, since it uses natural radiation of radioactive elements. A nuclear reactor causes a controlled process of nuclear decay (fusion).

In particular, it is a chain nuclear reaction, during which, as a result of reaching critical conditions in radioactive substance, the process of releasing part of neutrons begins. The critical conditions are regulated by the mass of starting material, the surface area of neutron reflectors, and the medium in which the reaction takes place. As a result of this reaction, particles with high kinetic energy are released, which is then absorbed by the coolant. In the future, the most common use of this energy is to heat the coolant to the boiling point, in order to rotate the turbine of the electric generator as a result of thermal expansion. In total, we get the cycle "nuclear energy – kinetic energy – internal energy – kinetic energy – electrical energy". The thermal efficiency of such reactors reaches about 33–37% [5]. This means that about 60% of the energy is lost in the transition from the kinetic energy of the fission fragments to the internal energy of the coolant and then to the kinetic energy of the turbine. This occurs due to the limitations imposed by thermodynamics. It follows that the maximum efficiency of the heat machine is equal to the ratio of the difference between the absolute values of the temperature of the heater ( ) and the refrigerator ( ) to the temperature of the heater:

1.4 "Direct" conversion of nuclear energy to electrical energy

In addition to the above-mentioned and similar processes, there are methods of direct conversion that pass the stages of kinetic energy transfer to heat carriers, but before proceeding to them, it is also worth talking about methods that are often called direct, which, however, are not. This is a thermoelectric and thermionic conversion. We have already discussed the first one above. It is based on the temperature difference due to which the EMF is formed. Unfortunately, at the moment, the efficiency of such a conversion lies in the range from 5% to 8% [6]. Thermal emission is a process in which, under the influence of high temperatures from the cathode, the electrons acquire sufficient kinetic energy and, overcoming the energy barrier, are transferred to the anode, creating a current in the circuit. The efficiency of thermal emission converters is up to 20% [7]. However, in order for the electrons to overcome the energy barrier, it is necessary to heat the cathode to high temperatures (up to 1500 K [7]). Under normal conditions, this is not critical, but in conditions with increased requirements for the thermal management system, such as on spacecraft, this can become a problem. In fact, the above-mentioned methods of converting nuclear energy into electrical energy are not direct; on the contrary, they are not feasible without the input of heat obtained during nuclear reactions. However, the absence of an intermediate link in the form of a turbine and an electric generator created a false idea, which happened to be formally fixed. If you combine the methods of thermionic, thermoelectric conversion with traditional nuclear reactors, you can increase the efficiency of converting nuclear energy into electrical energy.

1.5 Introduction conclusion

To summarize the part with a description of the existing and used methods of converting nuclear energy into electrical energy, we would like to explain why it is necessary to search for the analogues for the above-mentioned methods of using nuclear energy:

1. Traditional nuclear reactors require a relatively large area. As for Earth it is a question of optimality, then for the cosmos – space is a question of necessity. The main space is occupied by the coolant and its cooling scheme.
2. Traditional nuclear reactors, as well as thermal emission converters, operate at high temperatures. This destroys ecosystems on Earth and creates an additional problem in space missions.
3. The use of such conversion schemes in view of the input conversion to heat is inefficient, because with such input there is a restriction imposed by thermodynamics, and the maximum efficiency is determined from the formula (1.1).
4. In the course of nuclear reactions, ionized particles or (and) neutrons are formed, which are not directly absorbed, so it is necessary to additionally address the issue of protecting the reactor vessel from them, because of this, the reactor's ionic mass increases many times.

2.1 Electrostatic collectors

The idea of electrostatic collectors is simple – among other things, as a result of nuclear reactions, charged particles are obtained – it is necessary to prevent them from combining and to put pressure on the corresponding electrodes. Thus, we directly use the result of fission (synthesis), capturing it and increasing the electrical voltage. However, with the implementation of such an idea, everything is a little more complicated and the models (in the information found) are considered as an addition to conventional reactors. Let's take the example of the "Venetian blinds" scheme, according to which an article was published in 1974 [8].

The flow of particles coming from a thermonuclear reactor first passes through filtration on the neutrons – a neutron absorber is located along the path of the direct course of the particles. Charged particles under the influence of a magnetic field deviate from a straight trajectory and calmly pass further into the expansion chamber. In it, the particles pass a certain distance, due to which the volume charge is distributed in a larger volume. This was necessary in order to further solve the problem of high heat generation as a result of a large charge. Then, at the end of the expansion chamber, there are charge collection surfaces – a metal tape grid (Fig. 1). First, the stream of charged particles is sifted into electrons. This is necessary to maintain the potential difference - the electron-ion pair should not form a neutral particle. The ions, as more energetically advantageous particles, are collected and they continue to move, meeting the first obstacle in the form of positively charged metal bands. Some of the least charged ions are captured at the first stage, some will pass further, but due to the electric field, they will deviate back along a parabolic trajectory and due to the shape of the collection surfaces, they will not be able to leave, thereby being absorbed. Some of the most energetically charged particles will go to the second metal bands and will be absorbed by them.

Fig. 1 Collector and array of belt gratings [9]     

2.2 Induction circuits

In 1973, the possibility of converting the energy of thermonuclear fusion into electrical energy using an induction conversion circuit was studied [10]. Its principle is similar to that of an internal combustion engine. First, the fuel is injected into the reaction chamber, then the fuel plasma (in this case, deuterium-tritium) is adiabatically compressed by external magnetic coils. When plasma reaches the necessary pressure for the reaction, thermonuclear fusion begins. As a result of the synthesis, a denser element is formed, which, due to the isobaricity of the process, begins the expansion of plasma against the magnetic field of coils. This leads to the fact that the energy spent on compressing the fuel is not only replenished, but also supplemented due to the nature of thermonuclear fusion. The efficiency of the system for 1973 was estimated at 62%, and the ideal efficiency – 75%.

2.3 Nanomaterials

In her paper, Liviu Popa-Simil [11] presented her research on the use of metamaterials (metamaterials – artificial composite nanostructures with unique properties of interaction with electromagnetic waves) and nanostructures in general for the conversion of nuclear energy into electric current. The principle of operation of such a device can be seen in Figure 2.

Fig. 2 Metamaterial energy conversion process [12]

The process is as follows:

1) The nuclear decay reaction begins;

2) Neutrons, as reaction products, having a lot of kinetic energy, pass through a material with an increased electron density and hit the wall-reflector, thereby exciting the atoms around them;

3) The excited atoms begin to emit electrons which are directed towards the material with a reduced electron density;

4) A material with a reduced electron density is negatively polarized.

The calculated efficiency of this method is 85%. At the same time, the power of such a circuit is from 1 MW/cm³. At the same time, the thermal conversion is minimal.

3. Conclusion

In addition to those discussed, there are many other methods to use direct conversion: using the excitation of atoms caused by the decay products for luminescence, circuits based on magnetohydrodynamic installations, and so on. In this article, we have considered the most interesting and more developed methods of direct conversion. All of them are not without their drawbacks; the main one is their technological complexity of design and creation. However, despite this, the use of the most efficient methods of energy conversion will prevent the shortage of electricity, space, as well as disasters associated with the thermal nature of current conversion of nuclear energy into electricity.

In addition to efficiency, it is important to develop this area now, in order to also prevent the construction and use of reactors with a traditional method of energy conversion, because decommissioning and replacement are often more difficult and expensive than creating and building a new one.

About the authors

Denis Igorevich Stefanishin

Author for correspondence.
Email: kosmelf98@gmail.com
Russian Federation

Svetlana Olegovna Davydova

Email: davidova.so@ssau.ru

References

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