Electronics > Power/Renewable Energy/EV's

Diamonds Powered Nuclear Batteries

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LukeW:
It's basically a betavoltaic battery, with a beta-active radionuclide and a semiconductor junction which captures the charge carrier injection and generates electricity. Betavoltaics are an established, existing thing - not snake oil. Good for specialized applications where a tiny amount of power is needed for a very long time with no external energy supply or maintenance, but not something that is ever going to run a light bulb.

http://www.bristol.ac.uk/cabot/research/casestudies/2016/diamond-battery.html

http://www.bristol.ac.uk/media-library/sites/cabot/documents/Diamond_battery_FAQs_Nov_2016.pdf

Carbon-14 is a low-energy beta emitter, like tritium, but it has a far longer half-life. The low-energy beta particles won't escape the betavoltaic structure, so it's safe. They won't penetrate very far, and if the beta particles did get out without being captured, this would be an undesirable waste of energy. Neither the radiation nor the radioactivity is ever going to escape outside the device. The radiation is internally captured and turned into electricity.

Diamond is a hard material, it can be fabricated by established techniques like chemical-vapor deposition, and it keeps the radioactivity tightly bound within the diamond matrix. It sounds like they are talking about fabricating a single structure which contains both the radioactive source and the betavoltaic diode as a single piece, all fabricated together, potentially meaning that fabrication is cheaper, easier and more scalable. They're talking about CVD from methane, and combining layers of 12C diamond and 14C diamond. The scalability seems limited by the feedstock of enriched 14C methane.

They're talking about 1 gram (170 GBq, 4.6 Ci) of 14C (for comparison, there's 74 GBq of tritium in the NanoTritium batteries.) If the average beta energy is 49 keV you get about 115 joules per day, so you need about 13% overall capture efficiency in the betavoltaic to get 15 joules per day, which seems plausible.

15 joules per day at 2 volts is about 87 microamps. A tiny amount of power, but maybe useful for tiny systems, MEMS devices, microscopic low-power sensors, nanotechnology etc.

Just like a photovoltaic cell which is essentially the same thing, for a betavoltaic there is a voltage-current curve with a maximum power point somewhere in the middle, and the quoted voltage is only the open-circuit voltage with no current draw.

The City Labs NanoTritium batteries are a commercial product now, and they claim 50-350 nanoamps maximum current (at zero volts) and an open-circuit voltage of 2.4 volts, for a device with 74 GBq of tritium in it.

http://www.citylabs.net/index.php?option=com_wrapper&view=wrapper&Itemid=25

That's an existing, manufacturable, packaged, licensed commercial product. That's what I think they would have to economically compete with, for the same small niche markets and applications.

It's not clear where they plan on getting the 14C from. They're talking about recycling graphite moderator waste from nuclear power reactors, but graphite-moderated reactors are relatively rare (UK gas-cooled AGR reactors, RBMKs, and the prismatic/HTGR or pebble-bed type systems.) I'm sure one AGR moderator contains lots of carbon, but is it really practical to separate 14C from it?

How much 14C is formed in a graphite moderator? You're talking about two successive thermal neutron captures on 12C (or one neutron capture on relatively rare natural 13C), and the overall cross section therefore doesn't seem like it's going to be that impressive. The 14C is dispersed in a matrix of mostly 12C, it's dilute, so you can't get it with any significant specific activity and you can't chemically separate it. Unless you're talking about isotope separation which is intrinsically expensive and energy intensive.

Carbon is pretty light, so enrichment is somewhat possible by distillation or chemical exchange. For example turning all the carbon into a chemical form such as cold liquid carbon monoxide, followed by repeated distillation. This is easier than something like gas centrifugation for uranium where the mass difference is far smaller, and cheaper, but I don't think it's a practical or economical way of recycling reactor graphite by the ton.

Lord of nothing:
Its possible to separate the radio active particles by smash the graphite blocks and flush everything truth some centrifuge?

Codebird:

--- Quote ---Its possible to separate the radio active particles by smash the graphite blocks and flush everything truth some centrifuge?
--- End quote ---

Yes. Possible, but not efficient - the process needs to be repeated many, many, many times. That's why it's so expensive.

richard.cs:
There are 28 Magnox reactors in various stages of decommissioning, each with over 1100 tonnes of graphite, the prototype AGR has also been decommissioned, and there are 14 AGRs currently operating with around 1300 tonnes of graphite in each, all expected to cease operation by 2030. There are 11 currently operating RMBKs with 5 decomissioned, each with about 2000 tonnes. That is a sizeable supply of irradiated graphite. Irradiated CO2 is also produced by the AGRs, but is (or at least was in the 1990s) vented into the atmosphere after filtering to remove particulates.

This document: http://web.ornl.gov/info/reports/1977/3445605743782.pdf suggests that more 14C is formed from the nitrogen impurities in the graphite than from the 13C in the the graphite, but does not consider the formation of 14C from 12C via 13C at all (possibly the double capture contribution is negligible?). Table 5 gives graphite 14C as 16.27 Curies per Megatherm, so if we take Sizewell A as an example, having operated at 1010 MW thermal for 40 years, we take a stab at it having been online for 80% of that time, then in round numbers that's 160,000 Curies, or about 140 Curies per tonne. I make a kg of pure 14C 4460 Curies so each tonne of graphite must contain about 30 grams of C14.

That's quite a low concentration, but then I don't know what concentration they wanted?

In the reddit ask anything they talk about using laser separation, they're obviously planning on a significant amount of processing. I haven't checked if their numbers and mine match, if they're within a factor of ten I'll be happy.

MagicSmoker:

--- Quote from: LukeW on January 19, 2017, 12:31:37 am ---...
It's not clear where they plan on getting the 14C from. They're talking about recycling graphite moderator waste from nuclear power reactors, but graphite-moderated reactors are relatively rare (UK gas-cooled AGR reactors, RBMKs, and the prismatic/HTGR or pebble-bed type systems.) I'm sure one AGR moderator contains lots of carbon, but is it really practical to separate 14C from it?

--- End quote ---

Well, the Chernobyl reactors were graphite moderated, IIRC...  :scared:

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