Self-induced electrostatic-boosted radioisotope heat sources

Highlights

• A power enhancement for radioisotope heat sources based on a self-induced electrostatic field is proposed.

• Analysis shows enhancements of up to 10% could be attainable using beta sources.

• More heat is released per unit volume of radioisotope used.

Abstract

In this paper the possibility of stimulated self-induced electrostatic fields in radioisotope heat sources for power enhancement is discussed. Because electrons have higher mobility than the positively charged fragments from radioactive decay, a build-up of positive charges can be promoted, leading to an internal induced electrostatic field. This, in turn, results in a repulsive force acting on positive charges, endowing them with additional kinetic energy, and heat release after these charged particles are stopped by inelastic collisions with the boundary wall. Utilizing a simplified geometrical model, an analytical expression for the attainable power enhancement is derived. It is shown that the proposed concept could result in power enhancements of 5–10% for beta sources but enhancements are negligible for alpha sources.

Introduction

Radioisotope Heat Sources (RHSs) are small devices that provide heat through radioactive decay. The heat produced by these devices is released continuously for several decades and, theoretically, depending on the half-lives of the isotopes used, for up to a century or more. The heat produced from RHSs can be used directly to keep structures, systems, and instruments warm enough to operate effectively; such devices are called Radioisotope Heater Units (RHUs). Alternatively, the heat released by RHSs can be converted into electricity by the Seebeck effect; in this case, such devices are called Radioisotope Thermoelectric Generators (RTGs).

The objective of this work was to analyse a new approach based on a self-induced electrostatic field. However, the electrostatic field is not desired for extracting direct electrical energy (as in the case of nuclear batteries), but instead to transform the gained electrostatic potential of the charged particles into heat by increasing their kinetic energy and thus making more vigorous their collisions with their surroundings. In what follows, we will outline the essential ideas behind this concept, and we will derive an analytical expression for the attainable energetic gain using this mechanism.

Although, in rigorous terms, all radioisotopic generators are inevitably heat sources because a fraction of the energy from the radiative decay will always become heat, in this paper we define RHSs to be devices the ultimate goal of which is to maximize the fraction of energy released as heat. This definition establishes a clear distinction from conventional nuclear batteries based on non-thermal converters, the ultimate goal of which is to harness the motion of charged particles to produce an electrical potential.

Fig. 1 shows the range of radioisotope-based generators used for energy production. In this figure, we can see two clear divisions in the use of radioisotope generators. On one hand, there is their use as nuclear batteries, the ultimate goal of which is the production of electricity. These can be sub-classified based on the way in which the electrical conversion is performed: namely as non-thermal converters (taking advantage of the direct motion of charged particles), or thermal converters as in the case of RTGs. On the other hand, radioisotope generators can be used as heater units, the ultimate goal of which is the production of heat. There are many applications for RHSs in terrestrial and airborne systems and, more significantly, in space applications, because of their intrinsic high reliability not only in the production of heat but also in the design of energy extraction systems. Suffice it to say that the simplicity of RHSs allows easy heat extraction by using, for example, an optimal spacing between parallel and flat plates (Bejan and Sciubba, 1992, Hajmohammadi et al., 2013a) or tubular geometries (Hajmohammadi et al., 2013b) and temperature control can be performed by simple wall-thickness and material control (Hajmohammadi et al., 2012, Hajmohammadi et al., 2013c, Hajmohammadi et al., 2015).

In this paper we are interested only in RHSs, which include RHUs as well as RTGs. However, we are only concerned with enhancement of the heat released from radiative decay, so for the particular case of RTGs we do not consider the process of thermal conversion into electricity, which will be essentially the same as for classical RTGs but now with an enhanced heat source, i.e. at a higher temperature, giving better Seebeck effect performance.

The performance of non-thermal-type nuclear batteries is governed by the length scales of the system, namely the range of ionizing radiation and the size of the transducer, and can therefore be improved by geometrical design (Prelas et al., 2014). In contrast, the performance of thermal-type nuclear generators, such as RHUs, basically depends on the kind of radioisotope(s) used. Once the choice of the kind of radioisotope to be used in the RHU has been made, the heat per unit mass is fixed. This feature of RHUs, while giving them their recognised reliability, also limits the scope for improving their performance or that of the heat source in RTGs. As a result, research on RHUs was for many years limited to the selection of the best radioisotope. Most recently, there have been attempts to improve the efficiency of RHU and RTG sources by the use of hybrid designs stimulating parallel fission reactions in subcritical radioisotope heat sources (Wang and He, 2014, Arias, 2011). For the interested reader, an up-to-date, extensive review of the state of the art for nuclear batteries can be found in Prelas et al. (2014) and Radioisotope Power Systems Committee and National Research Council (2009), and the basic principles and parameters of radionuclide generators are well described in Lazarenko et al., 1988, Anderson et al., 2005 and Corliss and Harvey (1964).

The physical concept proposed in the present work is conceptually very intuitive. An electrostatic-boosted radioisotope heat source can be summarised as a classical radioisotope heat source, which provides heat through radioactive decay, but which is further enhanced by a self-stimulated electrostatic field. The reader can visualise and acquire a quick understanding of the proposed concept through its similarity to the well-known phenomenon of fuel fleas, because they are, in essence, based on exactly the same principle, i.e. the asymmetric distribution of charges leading to the creation of an electrostatic field and a repulsive force.

Fig. 2 depicts the phenomenon known as fuel fleas. In summary, fuel fleas are microscopic hot particles of new or spent nuclear fuel. Although small, they tend to be intensely radioactive. The fuel particles, the size of which is about 10 μm, are a strong source of beta and gamma radiation and a weaker source of alpha radiation. The disparity between alpha and beta radiation levels (alpha activity is typically 100–1000 times weaker than beta, so the fuel particle loses many more negatively charged particles than positively charged ones) leads to a build-up of positive electrostatic charge on the particle, causing the particle to “jump” from surface to surface and easily become airborne. Another mechanism associated with the build-up of positive electrostatic charge due to the higher mobility of negative charges is the so-called aggregate-recoil phenomenon (Trenn, 1980). This phenomenon has been suggested as the possible cause of the anomalous volatility and ability easily to become airborne exhibited by some alpha sources, such as polonium-210. If polonium-210 is heated in air to 55° C, 50% of the sample vaporizes in 45 h to form diatomic Po₂ molecules, even though the melting point of polonium is 254° C and its boiling point is 962° C. One suggestion of how polonium does this is that small clusters of polonium atoms are spalled off by the build-up of positive electrostatic charges.

Now, we will explain how the usually undesired phenomenon of fuel fleas can be harnessed to boost a nuclear heat source.