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A Modular Design for Nuclear Battery Technology

A Senior Project

presented to

the Faculty of the Physics Department

California Polytechnic State University, San Luis Obispo

In Partial Fulfillment

of the Requirements for the Degree

Bachelor of the Arts in Physics

by

Randy Lao

June, 2011

©2011 Randy Lao

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Section 1: Introduction

The search for alternative energy sources has led to research in many different fields

to find a more efficient way to power every day devices such as phones, laptops, and anything

else that requires a battery to function. One modern concept is the idea of a battery that, instead

of utilizing the energy in a chemical gradient like a dry cell battery, uses the energy given off by

natural radioactive decay of an isotope to create an electric current. This type of cell is called a

nuclear battery. In the past, nuclear batteries were used to power electronics for extended

periods of time such as unmanned spacecraft and remote measurement stations. For a short

period of time in the 1970’s cardiac pacemakers used a nuclear battery for power, capable of

producing enough energy to last over the lifetime of the patient. Lithium-ion batteries soon

replaced these, though, as the large size of the battery was considered impractical to implant in

the human body with an average length of 13 cm and cylindrical design1. With the application of

new technology, a new, modular case design can be proposed while drastically reducing size.

In order for a nuclear battery to function, the power source has to be a radioactive

isotope. An isotope is a nuclide of an element with a different mass due to a different number of

neutrons. For example, chlorine (Cl, Z=17) has 76% of nuclei containing N=18 neutrons while

the other 24% has N=20 neutrons. These isotopes may be unstable structures that decay to form

other nuclides by emitting different types of particles, electromagnetic radiation, as well as heat,

which is a process known as radioactivity2. The stability of an isotope depends on the ratio of

protons to neutrons and a Line of Stability that describes the radioactivity of isotopes can be seen

in Figure [1]. The time scale for radioactivity to occur is different among isotopes and their

decay can be measured in terms of how long it takes for half of the original element to decay to

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another element. This value is known as the half-life or

!

t 12

3. A half-life can be calculated with

the equation:

!

t 12

=ln2"

(1)

Generally, a decay rate can be described as:

!

R = "dN(t)dt

= #N(t) (2)

Fig [1] Line of Stability

(ned.caltech.edu)

As seen in the figure above,

!

"# decay is a type of radiation that certain elements may go through

depending on the ratio of protons to neutrons inside the nucleus. For example, hydrogen-1 and

hydrogen-2 (deuterium) are stable isotopes, but hydrogen-3 (tritium) is radioactive. Tritium has

too many neutrons in its nucleus and is not stable. Therefore one or more of its nuclides will

undergo a nuclear reaction that can be represented in the equation:

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neutron

!

" proton + electron (3)

where the mass of the particles in this equation is conserved. When this reaction occurs in

tritium, the product electron is ejected with some kinetic energy from the nucleus and is known

as a

!

"-particle. Since the resulting nucleus has an additional proton, the atom becomes an

element that is one atomic number higher on the periodic table3. In this case, the resulting

product from a tritium nucleus undergoing beta decay is a helium atom and a beta particle, as

illustrated in Figure [2]. This overall reaction can be written as:

!

13H"2

3He+#10e (4)

with the subscript on the element representing the number of protons and the superscript

representing the atomic mass. The negative subscript on the e represents the negative charge on

that electron.

Fig. [2] Tritium Decay

(library.thinkquest.org/3471/radiation_types.html)

This idea becomes vital for the function of the modular nuclear battery because the

natural decay of a

!

"#-emitting particle is the primary source of energy. This electron can be

used to cause a type of chain reaction, or flow of other electrons, that can be harnessed as

electrical energy. The rate of flow of electrons through a medium is known as a current. Another

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variable to identify when examining the power of an electrical source is voltage. Voltage is the

electrical driving force of a charged object or an electric potential difference4. It can also be

thought of as “electrical pressure” of a system. The higher the voltage, the more driving force

there is to move from a high electric potential to a low electric potential. Combined with current,

there is a quantifiable measure of the rate in which a system is capable of doing work. This idea

is known as power. Power, P, is the rate that work is performed or energy is converted, and can

be written in terms of electricity as:

!

P =VI (5)

where V is the voltage and I is the current of the system. This means that the decay of

radioisotope with beta emission can theoretically drive electrons through a medium, and the

output power of a device that harness such a process depends on the potential difference and

current caused by the radioactivity.

The effectiveness of a nuclear battery is also determined by the ability for the

medium that the beta particle travels through to carry an electric current. This is also known as

the conductivity of the material. On the contrary, resistance is the amount that a certain medium

is able to impede the ability for an electric current to travel through it. There are several

different classes of materials that are separated by their ability to carry current. First, there are

insulators or nonconductors. These materials may carry little to no current through them due to

high resistance or low conductivity. Examples of insulators are wood or plastic. The next class

is conductors. These materials have relatively low resistance and/or very high conductivity.

Examples of conductors include aluminum and copper4. The most important type of material

relative to the function of a nuclear battery is the semiconductor. The special properties of

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semiconductors allow the harnessing of radioactive energy from the decay of radioisotopes that

release of

!

"# particles.

Semiconductors are materials that contain properties that fall in between conductors

and insulators. Conduction in semiconductors can occur in multiple ways. First, electrons may

move throughout a semiconductor in opposite direction to conventional flow. Holes have a

positive charge and move in the conventional flow direction. Holes are a property of only

semiconductor devices, and can be thought of as an absence of an electron in a particular orbital

of an atom4. A process called doping allows semiconductors to be used for the construction of

electronic devices. Doping adds impurities that are either donors or acceptors, either resulting in

an addition of negatively charged (electron) carriers or positively charged holes. P-type material

has an excess of positive charge carriers and N-type material has an excess of negative charge

carriers. Layering these two types of materials together will allow movement of charge,

resulting in the generation of an electric current when a potential is created. This is known as a

PN-Junction. A nearby material that emits electrons toward a PN-junction that is oriented a

specific way will cause a potential gradient and induce an electric current as seen in Figure [3].

This process is known as the Voltaic Effect14.

Fig. [3] Voltaic Effect on PN-Junction Semiconductor Material

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Converging all of the aforementioned ideas, a new source of energy can be utilized.

Many types of these batteries have been designed with these same basic ideas but have slight

modification in mechanics. These designs are proprietary and are made for very specific

applications. The next section will explore some of these technologies, but there will an

emphasis on betavoltaics as it pertains to the proposed modular design.

Section 2: Types of Nuclear Batteries

Nuclear batteries can be separated into two main categories. The first type of

nuclear battery is Thermal Converting Batteries. Thermal Converting Batteries have energy

outputs that directly depend on the temperature difference of certain components that constitute

the energy transfer mechanism. An example is NASA’s Radioisotope Thermo-electric Generator

(RTG) shown in Figure [4].

Fig. [4] Radioisotope Thermo-electric Generator Schematic

(Department of Energy—Office of Nuclear Energy)

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RTGs, like other nuclear batteries, essentially have no moving parts. They generate

electricity by utilizing the heat released from radioactive decay. Heat from decay is generated by

many unstable isotopes, including byproducts of nuclear fission capable of radiating heat for

decades. This heat source conducts onto a thermo-electric generator. The heat-to-electricity

conversion occurs when two different conducting metals are joined together to create a closed

circuit and the two junctions are kept at different temperatures. In the case above, the

thermocouple is made of silicon and germanium. The temperature differential is brought about

by utilizing the radioisotope to transfer more heat to one metal conductor while the other is

cooled (in this case, by heat radiation in space). The result of the system is an electrical current

through the closed circuit5. Other Thermal Converting Batteries work in a similar way, with

minor changes in the conversion mechanism. Although Thermal Converting batteries like RTGs

allow the harnessing of heat energy from radioactive decay, their implementation is not practical

in small, everyday devices.

Non-thermal Converting Batteries extract incident energy of radioactive decay and

does not depend on a temperature differential. There are three existing types of non-thermal

converting atomic batteries that have been developed. The notions for these batteries have come

about in recent decades and the technology for each can be considered in their infancy.

The first type is the Optoelectric Battery, which involves turning the beta decay of

radionuclides to light and then the conversion of light into electrical energy (photovoltaics).

Isotopes are suspended in gas that is capable of being excited to a specific excimer line by

emitted electrons. The chosen excimer gases then emit light that is of certain wavelength, which

is then used to excite a PN-junction in a photovoltaic cell to produce an electric current.

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Technetium-99 and Strontium-90 have both been used as the radioisotope and the gases used

may be a mixture of argon, xenon, and krypton for the production of this type of battery6.

The second type is the Radioisotope Piezoelectric Battery (RPB), developed at

Cornell University. Piezoelectricity is the linear accumulation of electric charge due to

mechanical stress or pressure directly due to the Piezoelectric Effect. The reverse Piezoelectric

Effect is what drives the RPB, using an electric field and mechanical energy to produce a current.

A piezoelectric cantilever is suspended over a thin film of the radioactive isotope nickel-63. This

isotope emits beta particles towards the cantilever, which gradually builds an electric charge. At

the same time, the film of nickel isotope builds a positive charge. As the potential gets stronger,

the piezoelectric cantilever bends toward the film until they come into contact. The charge then

jumps the gap and the current flows back to the isotope, equalizing the potential and resetting the

cantilever7.

Fig. [5] Mechanical Action of a Piezoelectric Cantilever With a Radioisotope

(Cornell News, Cornell University)

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This type of battery has been shown to convert the energy of beta decay into mechanical energy

with about 7% efficiency and can vary its frequency from 120 Hz to a cycle every 3 hours. Since

the half-life of nickel-63 is over 100 years, it will be able to keep a reliable conversion of

electrical to mechanical energy in this form for more than half that time. The cantilever may

mechanically actuate a linear device or can move a cam or ratcheting wheel as examples of how

it may be incorporated to produce movement. Also, a magnet may be placed on the cantilever

and surrounded by a coil in order to produce a current as the magnet passes7.

Fig [6] Diagram of Betavoltaic Cell

The third kind of Non-thermal Converting Nuclear Battery is the betavoltaic cell.

Analogous to a photovoltaic cell, the betavoltaic cell operates on the same principle, but

generates energy from a beta-emitting radioisotope instead of light. When the isotope radiates

toward a PN-junction, it creates hole pairs in the semiconductor material, which in turn generates

a current due to the voltaic effect represented in Figure [6]. Beta-emitting isotopes such as

strontium-90 can provide continuous energy for over a decade, with a half-life of 28.8 years. It

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also decays to yttrium-90, which is another

!

"# particle emitter with a half-life of 64 hours. This

means that there is a potential for more energy from one isotope. There are multiple stages to the

energy conversion of the radioisotope into usable electrical energy.

Stage 1: The ground state, E0 is made from a potential difference between two electrodes that is

provided by an electrical load. This could be from any mechanism requiring power to operate.

Although a potential exists there is no current flow because the pn-junction of the system is in

equilibrium, and there is no energy source.

Stage 2: A beta-emitting radioisotope is introduced into the system. The energy Eb of an emitted

beta particle collides with another electron in the PN-junction and generates electron-hole pairs

by transferring kinetic energy. The energy required to strip an electron from a neutral atom in

the junction will be defined as E1.

Stage 3: The beta particle transfers energy in excess to the ionization potential, which raises

electron energy to an excited level. This excited level is defined as E2. The beta particle will

collide multiple times before effectively transferring all of its energy and returning to a ground

state electron. The number of collisions and electron-hole pairs generated depends the junction

material. More efficient doping will generate a larger amount of excited electrons.

Stage 4: Excited electrons are driven out circuit toward Electrode A in Figure [6] due to the

induced electric field present. Excited electrons in the electrode seek to give up their ionized

energy and go back to ground state. This creats a Fermi potential across the Electrodes in the

circuit.

Stage 5: The Fermi Voltage drives electrons from Electrode A down the circuit and into the

load, RL, where they give up their energy E3. The amount of energy able to be removed from the

system and into the load is

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!

E3 = EB " E1 " L1 " L2 (6)

where L1 is the loss in the conversion and L2 is the losses in the electrical circuit.

Stage 6: After passing through RL, the electrons have the energy E4. From there, the electrons

are driven into Electrode B and into the P-end of the junction where it may fill the hole of a

junction ion, releasing E4 in the form of heat. The electron has then returned to its original

ground state and the circuit is completed. The radioactive source continues to supply this

process constantly at a rate that matches the decay (equation 2) above. The total energy equation

of the system can be written as:

!

E0 = EB " E1 " E3 " L1 " L2 (7)

The result is that the potential difference provides a constant voltage, while the radioisotope is

the generator of the work that moves the electrons since the ground state energy is constant8.

Recent research in betavoltaics has also increased the efficiency of the conversion

system. Traditional semiconductors that accept energy from a voltaic effect are manufactured to

have flat, planar surfaces. Modern technology allows the production of “porous” silicon

semiconductors. The pores in the material allow a three-dimensional well to be in place of a flat

surface. This dramatically increases the surface area of the semiconductor material to absorb

!

"#

particles, resulting in higher efficiency. This works because

!

"# decay occurs in all directions

relative to the isotope. Surrounding the radioactive material within close proximity allows the

maximum amount of energy to be converted. The new type of silicon comes about from an

etching technique that creates wells about 1 micron wide and 40 microns deep in random

locations. In order to maximize this kind of design, the radioactive isotope will have to be in the

gaseous form. Tritium gas has been used to show a ten fold increase in efficiency with silicon

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etched in a random fashion and an estimated 160 fold increase in efficiency once manufacturing

techniques can create etched wells in a uniform lattice9.

Another recent breakthrough in betavoltaics is the idea of using a liquid or

amorphous semiconductor. High-energy

!

"# emission from radioactive isotopes can slowly

deteriorate semiconductors in a solid crystalline or lattice structure10. This results in reduced

efficiency over time. To circumvent this outcome, liquid semiconductors have been used.

Having properties of solid semiconductors, their liquid counterparts can be used because of their

higher versatility in the application of betavoltaics. Liquid semiconductors are made by melting

selenium at high temperatures, then compounding the material with sulfur resulting in a

compound such as Se65S35. The radioactive isotope, no matter solid or gas, can be directly added

to this compound11. This allows the absorption of

!

"# particles from all angles, since the isotope

is completely surrounded by the liquid semiconductor while not deteriorating from constant

bombardment of beta particles.

Either of these methods can be employed into the design of a battery. Because

existing electrical devices already employ some degree of modulation for batteries, the proposed

design is an example of how to utilize the aforementioned betavoltaic technologies.

Section 3: Novel Application

When designing a battery case for every day use with this type of technology the

first consideration must be safety. There is an emphasis on shielding in order to contain

radioactive materials as well as stop any unabsorbed or unconverted radiation. The mass of the

radioisotope can vary depending on application, but there is a threshold for this design that

cannot be determined until further testing, either in a lab or a computer simulation such as

Geant4. The design for the components, including calculations for material properties and

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volumes were all performed in SolidWorks 3D CAD12. The design is based on the standard AA

dry cell battery used in everyday electronic devices.

The first component of the battery casing is the housing of the radioactive material

and semiconductor apparatus seen in Figure [7].

Fig. [7] Housing Tube

The material chosen for the housing tube is solid polyurethane foam. High-energy

!

"# particle

shielding requires a low-density material13, and polyurethane is meets this criterion while still

maintaining rigidity. The outer diameter of the Housing Tube is 0.6 cm, and length is 42.9 mm.

The 1 mm thick walls can be changed to meet the adequate amount of initial shielding for the

battery, depending on what type and amount of beta emitter is used (some beta emitters eject

particles at higher energy8). As it is designed, the Housing Tube weighs 0.06 grams.

Aluminum Alloy 1060 is used for the terminal caps because of its high conductivity,

and conversely, low resisitivity of 2.8

!

"# m . The aluminum caps are also thick (2 mm) in order

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to ensure proper shielding. Both caps have a base diameter of 10 mm. The positive and negative

caps can be seen in Figures [8] and [9], respectively.

Fig. [8] AA1060 Positive Cap

Fig. [9] AA 1060 Negative Cap

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The outer case is the most vital structure to ensure the safety of the battery. It is

responsible for the absorption of any stray particles that are not absorbed by the low-density

polyurethane layer. Also, the outer layer is responsible for containment of the inner materials.

This means the material should be able to prevent a leak of radioisotope from compromised

structure by having a high tensile strength and high compressive strength. Industrial epoxy has

these properties while still maintaining a light weight. The tensile strength of unfilled epoxy is

28 million N/m2 and the compressive strength is even larger at a rated value of 104 million N/m2.

A model of a medial cross-section of the outer casing can be seen in Figure [10].

The epoxy case will be molded over the base flange of each cap to further ensure

structural stability of the whole apparatus. Additionally, the epoxy enclosure will provide

additional shielding and prevent removal of the terminal caps. The total weight of the epoxy

outer case is 7.74 grams.

Fig. [10] Epoxy Outer Case

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Fig [11] Tube, Caps, and Cross-Sectioned Epoxy Case

Once the betavoltaic energy conversion system is inserted into the Housing Tube, the

semiconductors are attached to their respective aluminum alloy caps. The terminal caps are then

mated to the ends of the Housing Tube. The epoxy case is then cast over the tube and cap

apparatus to make the complete battery (Fig. [12]).

Altogether, the casing system for the betavoltaic battery weighs only 8.86 grams. The

battery will weigh more with the radioisotope energy conversion system inside the Housing

Tube, but it will be less than the 23 grams of a standard AA dry cell battery. This provides for

more versatility in applications dependent on weight. This technology can be incorporated into

every day technologies by adding a larger mass of beta emitting isotopes and/or putting more

power cells in series. Since there is no mainstream use for radioactive wastes, beta emitters that

are byproducts of nuclear fission such as strontium-90 can be utilized and recycled.

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Figure [12] Complete Betavoltaic Battery with Modular Case

Figure [13] Negative Side of Betavoltaic Battery

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Although this technology looks promising, little research has been done to increase the

efficiency of the conversion system. Betavoltaics has the potential to provide decades of

constant energy, but without a high efficiency or a dangerous amount of radioisotopes, the

battery is still not capable of powering mobile devices such as phones and laptops that require

high-energy output (LED screens, speaker systems, large mechanical systems, etc.). Although,

without investing into the exploration of the new technology, beta emitters will continue to

naturally decay and release energy that can be harnessed and utilized as an alternative energy

source.

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References

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[14] W Ehrenberg et al 1951 Proc. Phys. Soc. A 64 424