|Fig. 1: Image of non nuclear Pacemaker battery commonly used today. (Source: Wikimedia Commons)|
Radioactive waste is a harmful byproduct of nuclear reactions. Nuclear fuel, after being spent, produces nuclear waste which is extremely radioactive and can cause radioactive poisoning, cancer, or even death in large enough dosages. This waste is produced generally through the back end fuel cycle; nuclear fuel reactions occur in two stages, front end, which contains the process from mining uranium ore to using the enriched uranium in fuel reactors, and back end, which encompasses the removal of used fuel from the reactor and its subsequent treatment and disposal. This waste is particularly difficult to dispose of due to environmental and health effects of radioactivity. This hazardous material is destined to be stored underground in containment units approximately 500 meters deep in geologically stable formations. In the nearer term, the material is stored in man-made ponds to allow it to release its radioactive heat safely.
Bristol University's Professor Tom Scott from the material science department alongside a team of physicists and chemists claim to have discovered a way to turn nuclear waste into diamonds that function as batteries. Through encapsulating radioactive artificially made diamond batteries inside of non radioactive man made diamonds, they suggest they've solved the long term problem of nuclear waste and found a way to convert it to not only a harmless byproduct but a repurposed useful energy source. Diamonds represent the extremes in every characteristic measurable for materials. For batteries, the most important characteristics are electron mobility, hole mobility, and carrier diffusion and drift velocities. These characteristics determine ultimately how well this material will be able to conduct current and provide power to whatever loads it must drive. The Electron mobility coefficient is 2200 cm2/(V · s), while the hole mobility is 1600 cm2/(V · s).  The electron saturated velocity is 2.7 × 107 cm/s while the hole saturated velocity is 1.0 × 107cm/s.  To put these numbers in perspective; in terms of the electron velocities, the speed of light is 3x108m/s. Even with the conversion of the above listed values from centimeters to meters, the velocities approach the speed of light rapidly which is very good for the flow of current. With regards to the mobility coefficients, silicon, the material used in processors everywhere today, have an Electron mobility of 1500 cm2/(V · s) and a Hole mobility of 475 cm2/(V · s). Processors have never been faster than they are today and this is largely due to mobility coefficients and the transistors' doped regions. So to have almost double the electron mobility and 4 times the hole mobility shows the strength of diamonds as a conductive material. While there is a mathematical correlation for mobility and velocity to conductivity, for the sake of this overview, you can assume the correlation is linearly related.
The teams from Bristol University's Cabot Institute have developed this technology through a process called Chemical Vapor Deposition. To produce these radioactive diamonds, these researchers took graphite blocks used in nuclear reactors that were used to aid the nuclear enrichment process that were then discarded after the uranium fuel was spent. Researchers found that the blocks were very radioactive but less so in the bulk interior. They heated up the block of graphite and captured the radioactive gas that was emitted. After the gas was collected, they used artificially low pressure and high temperatures to create man made diamonds through Chemical Vapor Deposition.  These diamonds, when placed in a radioactive field, generate a current. However, since these are formed from radioactive materials, they inherently produce low-level radioactivity which results in a current being generated. This radioactive diamond is further encased in a non-radioactive diamond through CVD. This outer case absorbs the inner diamond's radioactivity, making the device safe for medical use. This research team claims to have produced a device that has a near 100% efficiency, where a one gram graphite block would only produce around 15 joules per gram; an AA Alkaline battery of 20g produces 700 Joules/gram.
With such a low energy density, the true strength of such a device comes from the longevity of the battery life. Because the diamond is radioactive, the time it will take the battery to discharge to half power is the half life of the radioactive carbon in the graphite blocks. This is approximately 5,730 years.
Nuclear battery pacemakers were once used for a time; however due to major concerns about the radioactivity, these devices are no longer in use. Practical nuclear batteries use plutonium (Pu-238). It has a half-life of 87 years so the output degrades only by 11% in 10 years. However it is highly toxic and 1g in the blood stream could be fatal.  Another issue with these types of batteries were that they were too large and bulky. As seen in Fig. 1, the pacemaker batteries that are commonly used today are highly compact and are meant to be replaced every two years or so. For nuclear pacemaker batteries however, once the user passed away, the nuclear battery would have disposal issues no different from those of nuclear waste material from a reactor.
The nuclear diamond battery technology created by Bristol University is hypothetical at this point. I can find no technical paper published about it in a refereed journal. However should a prototype of this beta battery exist, exploring additional types of nuclear waste (beyond just C-14 graphite blocks) could lead to even greater efficiencies, greater reduction in nuclear waste globally, and improved energy storage capabilities. These types of batteries hold the key for low power, implantable biomedical devices where replacement of the energy source simply isn't possible or too invasive to be done often. With radioactive half lives (battery longevity), this advancement has the possibility of changing the medical device market on a global scale.
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