Nuclear Powered Spacecraft – Fact or Fantasy

Picture credit USNC-Tech

With US, Chinese and UAE probes arriving at Mars within the space of a few days and Elon Musk regularly launching and crash-landing his prototype Starship as part of his dream to colonise the planet, long distance space travel is very much in the news. NASA thinks if you want to get astronauts to Mars or further afield and back, you need to go faster, and conventional rockets cannot achieve that. Nuclear Thermal Propulsion could.

The concept of a thermal nuclear rocket, or TNR, is both simple and old. Indeed, the US spent $1.5Bn on Project Rover between 1955 and 1973. They built several prototype engines and proved the concept, but none ever made it off the ground. In the end NASA prioritised the Space Shuttle.

The simple principle of a TNR is that you pass a gas, any gas, but normally hydrogen, through a nuclear core, heating it, then allow the hot gas to expand through a rocket nozzle creating thrust. The advantage of this type of engine is that besides the nuclear fuel, you only have one very light fuel (hydrogen) as opposed to two (hydrogen and oxygen) so you save the weight of all that oxygen and therefore you can build an engine with a much-improved power to weight ratio, or ISP as rocket scientists like to call it. The higher the temperature to which you can heat the gas, the more thrust (and higher ISP) you get.

Picture Credit NASA

The drawbacks of a TNR are equally simple: The exhaust is likely to be radioactive and the consequence of any launch accident ‘unfortunate’ to say the least. TNRs then are never going to be seen launching spacecraft from the surface of the earth. They come into their own however for long-distance space travel, to Mars and beyond, and therefore interest in TNRs is reviving, on both sides of the Atlantic.

NASA, through AMA (Analytical Mechanics Associates) is funding a $125m study and working with industry partners, including General Atomics, Ultra Safe Nuclear Technologies (USNC-Tech) and BWX Technologies.

UKSA recently announced a study and is working with Rolls Royce. Their approach will be slightly different as they will use a nuclear reactor to produce electricity which will then be used to provide the thrust. I am guessing this could either be thermally or possibly an ion drive, something the UK has good experience of already.

When it comes to rocketry, hotter is better. That does not sit too well with conventional nuclear reactors where cooling is critical, and when cooling fails the consequences can be catastrophic, as evidenced by Three Mile Island, Chernobyl and Fukushima to name but three.

To build an TNR then you need to develop a heat resistance fuel. Fortunately, a lot of work has gone into making melt-down proof nuclear fuel, the best known is Tristructural Isotropic Particle Fuel, known as TRISO fuel. TRISO fuel particles typically consist of a 0.5mm dia kernel of Uranium OxyCarbide encapsulated by 4 layers of porous carbon, an inner layer or pyrocarbon, Silicon carbide and an outer layer of pyrocarbon. These layers are deposited by CVD in a specialised reactor known as a Fluidised Bed Reactor or FBR. This combination of coatings serves to both protect the kernels and contain the highly radioactive fission products.

Picture Credit US DoE

There are still problems though. The silicon carbide layer in the TRISO particle needs to be upgraded, the leading candidate to replace it is zirconium carbide, ZrC. ZrC is one of a family of materials know as Ultra High Temperature Ceramics or UHTCs. ZrC has a melting point of 3540°C, that’s 840°C higher than where SiC turns to a vapour!

Once you have the TRISO particles you still must form them into fuel elements, either spherical or cylindrical. Historically the matrix for this has been carbon powder and a binder or graphite, but for space flight something more robust is required. USNC-Tech employ a Silicon Carbide matrix in their FCM fuel (Fully Ceramic Micro-encapsulated).

Long cylindrical fuel elements are made by inserting a stack of cylinders of fuel into a jacket. In a typical nuclear reactor, this jacket is a zirconium alloy, but this will not do for a TNR. The leading candidate material to replace the Zircalloy is a Ceramic Matrix Composite, CMC, made up of a silicon carbide matrix reinforced by silicon carbide fibres (SiC/SiC). This material has the required properties in terms of temperature, radiation resistance and the short cylinders of fuel are packed end on end into SiC/SiC tubes, the end sealed making a fuel element that can run at very high temperature, is melt-down proof, does not leak fission product, and can last up to 30 years without replacement. All these things are essential for a TNR but they are also very useful for new generations of power generating reactors back on earth too, especially with Small and Micro Modular Reactors (SMR/MMRs) being all the rage at the moment. 

If you’ve read this far, congratulations. So where does ATL come in? In several ways:

ATL is currently designing and building a CVD FBR reactor specifically for the development of TRISO fuel particles.

The development of CVD and CVI processes, and the manufacture of equipment for, the deposition of interlayers and matrices for CMCs has been one of ATL’s major activities for many years. We are now able to offer equipment that can produce SiC/SiC CMC tubes for fuel rods up to 5m long.

ATL is currently ramping up its activities in the field of UHTCs, particularly those with a melting point above, 3,200degC, specifically HfC, TaC, NbC, ZrC, HfN, HfB2, ZrB2. ATL has carried out research programmes on TaC and ZrB2 and work on the other UHTCs particularly ZrC is in progress or planned.

Should you wish to learn more, please contact the author or take a look at the ATL website www.cvd.co.uk

Tantalum carbide

Over the last 10 years or so LEDs have transformed our everyday lives. The advent of high power blue & UV LEDs has enabled full colour 4K LED TV displays and white LED automotive and domestic lighting. These are based on Gallium Nitride which is deposited by the MOCVD process on substrate wafers supported on CVD SiC coated graphite furniture. During processing the SiC coating is attacked by the ammonia gas employed in the process, reducing lifetime. A new more inert coating is required to enhance coating lifetime and reduce overall cost. Tantalum carbide is seen by the industry as a replacement which can greatly enhance resistance to this aggressive atmosphere and provide a solution which is not yet available. ATL is developing the TaC CVD coating which will enable penetration into the large and growing market for GaN based LEDs and power semiconductors. ATL’s aim is to sell CVD TaC coated graphite parts and industrial scale coating equipment to suppliers of the semiconductor industry.

brand new all metal composite material

It’s not often you can say that you are part of a team that has developed a brand new material, but so far as I am aware we have done just that, in partnership with the Max Planck Institute for Plasma Physics (IPP) at Garching near Munich. The new material in question is a tungsten fibre reinforced tungsten metal. What is so wonderful about that? Well if you carefully tailor the interface between the fibres and the matrix, you can turn an brittle material into a material with considerable toughness. This is particularly useful in high temperature environments where high thermal stresses cause solid tungsten to crack up. An example being a fusion power plant such as ITER currently under construction on Caderache in France.

The tungsten fibre is the same as that you find in old fashioned filament light bulbs and the matrix material is deposited using ATL’s specialism chemical vapour infiltration or CVI for short.

You can read more about it here: http://phys.org/news/2013-05-brittle-material-toughened-tungsten-fibre-reinforced-tungsten.html

A bit more rocket science

So the 6-month Pathfinder Project has wound up. The work done, the reports written, the grant money paid: Thank you very much Astrium/UKSA!

And the results?

Well, like most research, as many new questions asked as old ones answered. Here is the summary of the public report:

The project made significant steps in coating iridium onto C103 niobium alloy to provide a high temperature thruster material.

Initial coating trials with a refractory metal interlayer were successful, but coatings of iridium onto the substrate were problematic. However, the experiments and analysis showed that a duplex layer of refractory metal and iridium could be deposited on a modified substrate. Due to the scientific potential of the material combination and its commercial interest, work will continue on the coatings under the ESA HTAE project until May 2013.

Summary of the summary: There are a few problems, but some results are promising and as the prize is big, we continue to work towards it.

If you want to know more, please get in contact.

More Rocket Science

After successfully completing the first phase of the ESA High Thrust Apogee Engine project, we now have the green light for the next phase.

ATL has also been successful in securing a UK Space Agency Funded Pathfinder Project. This is a 6-month project entitled ‘Development of a High Temperature Anti-Oxidation Coating for next Generation Rocket Thrusters’ 

Some Rocket Science

We have just started a new project. It’s for the European Space Agency and we are part of a group designing a new engine for missions to Mars and exciting stuff like that. Interesting stuff.

Our particular brief is to look at high-temperature materials an amazing number of which are made by CVD or CVI: For instance, W, Re, Ir, Nb, Ta, SiC, HfC & TaC

Tantalum

TaCl5 Generator

Tantalum is an extraordinary metal. It has a melting point of 3017degC which makes it the third highest metal behind Tungsten and Rhenium. This is a useful property, however it is its extreme chemical resistance that makes it most interesting. Below 150degC it is inert to aqua regia (mixture of concentrated hydrochloric and nitric acids). Aqua regia dissolves other metals, such as gold and platinum,  normally thought of as inert, in a trice.

Tantalum has a variety of uses, the biggest of which is in capacitors, but for coatings, the interest in mainly in putting an extremely corrosion resistant layer onto fasteners, crucibles, pipe bores etc.  In the last year the price of tantalum metal has rocketed and this is expected to stimulate interest in coatings where solid tantalum components could be replaced by cheaper metals coated with tantalum.

Tantalum is fairly easy to deposit by CVD, normally from TaCl5 & hydrogen. The main difficulty is that TaCl5 is practically impossible to evaporate in a controlled manner. TaCl5 melts at 216degC and evaporates at around 240degC  The change from liquid to vapour involves dissociation of the dimer Ta2Cl10 to the monomer, so this complicates things. It is also hygroscopic so that any exposure to air completely  changes the evaporation properties. ATL overcomes this problem by generating the tantalum in situ through the reaction of chlorine gas with tantalum metal. Obviously handling a gas as toxic as chlorine is not to be taken lightly, but it does make Ta CVD a reliable, repeatable and scalable process. Chlorine is cheap and tantalum scrap is relatively easy to buy, although prices are currently eye-watering (I blame the Chinese).

ATL has developed and sell its own design of industrial scale TaCl5 generator. Transport of up to 1Kg per hour of Ta metal is possible with the current design. These are available for sale either as part of a complete turn-key coating system or as a bolt on to customers’ existing CVD equipment. Control can be stand alone or incorporated into the main system. The same generators can be used for making TaC coatings which are of increasing interest in the semi-conductor market.

Silicon Carbide

Silicon Carbide deposited by CVD is a wonderfully versatile material with a myriad of applications and excellent physical properties.

Its high hardness means it has abrasive and protective applications.

Its 100% dense structure combined with excellent chemical resistance gives it corrosion resistance applications.

Its high temperature strength combined with low density (3.2 g/cc) makes it an ideal space, rocketry and aerospace material.

These properties plus its low neutron activation cross-section makes it an ideal material for fission and fusion applications.

All those tiles on the underside, leading wing edges and nose cone of the Space Shuttle may have got bad press, but they are SiC and there’s no better material.

All silicon chips have passed across CVD SiC Coated furniture during their manufacture. The SiC is dense, pure and inert enough to keep the required  incredibly high purity levels. For the most critical semiconductor applications, the furniture is made from solid CVD SiC. This is possible because SiC is one of those materials (like tungsten) that can be deposited several cm thick by CVD.

I could go on an on, but then I really would be rambling!

So you want to know about ATL?

ATL Presentation
So now you know all about CVD: It’s the process for you and you are looking for a CVD specialist. Look no further!
Make sure you’ve got your sound switched on.
Apologies for the blatant self-publicity, but it is a pretty nifty presentation.

So you want to know more about CVD?

CVD Presentation

A rather nifty presentation on CVD, how it works, its applications and advantages.

Make sure you’ve got your sound turned on!