Room For Alternative Naval Nuclear Technologies

Pete Comment

Debate is continuing, on Submarine Matters, concerning Alternative Nuclear Propulsion Science/Engineering uses. While I am a firm supporter of conventional Pressurised Water Reactors (PWRs) that can enable an SSN to move at 30+ knots, there is room for discussion of alternative nuclear technologies. 

This is also noting Operation Ivy Bells used a large radioisotope thermoelectric generator long life “battery”. Such generators, sitting on the seafloor, may find usage  covertly recharging the Lithium-ion batteries of visiting AUVs, up to the size of 10 tonne Orca XLUUVs.

ARTICLE BY ANONYMOUS

Much of what Anonymous writes below I agree with. So I've bolded those bits.

In late January 2022, Anonymous commented:

"This just an expansion of my views on Lex’s brilliant ideas for alternative nuclear propulsion. To be clear, I agree with the vast majority of his points and insights, but ultimately my view is that it’s just a little too innovative for the Aussie navy. The only people who really seem to be experimenting with funky reactors are the Russians – and whatever you may think of their [lack of] Quality Assurance, they are masters of the physics and with arguably the deepest practical nuclear experience of any country.

The KRUSTY / Kilopower designs are fascinating – I read up on them some years ago. Arguably the most innovative part of their design is not in the reactor or Stirling mechanisms, both of which are well-executed “first-principles” examples of established tech, but the heat-pipe system, which is breaking ground in new materials science and thermodynamic properties thereof.

The key issue with these systems and their numerous historical brethren, including https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generators, is they are designed from the outset for space usage, with no human complement. This meant instead of shielding the reactor itself, only critical electronic systems on the relevant vehicle need protection, and this is often achieved by simply distancing the power-unit from the rest of the vehicle (think RTG-on-a-boom, ala Voyager probes). Ultimately their usage profile provides advantages (minimal shielding requirements) and disadvantages (mass restrictions and non-serviceability).

Inversely, to meet crew safety requirements, sub reactors need full shielding, they do also have volume and, to a lesser extent, mass, constraints, but also have the advantages of ready cooling supplies and serviceability. There are many more aspects to this of course, my point being that, although there is an attractive superficial applicability of space-based nuclear plants to Submarines, it doesn’t really hold unless there is a complete redesign anyway.

That’s not to say a higher power (100kw-600Kw) broad equivalent-tech systems could not be successfully created and deployed on a submarine, I believe it could, but the advantages wither next to the downsides: development and, shall we call it “nuclear innovation”, costs, which are with very few exceptions, pernicious in the extreme. For a country like Australia, with no nuclear experience…those details would make the devil blush. Ultimately you’d be creating a whole new fabulously expensive AIP system with capabilities not much in excess of current Fuel Cell AIP systems.

Moving up the scale to more mid-range systems: the 5MW type plant. Although the referenced new designs are indeed very different to existing PWR’s, there is a step-change greater commonality with current in-use nuclear powers systems than with the “for-space” developed platforms, and a proportionally lower risk profile. Such a plant is not far off the power output profile of some early nuclear subs: the US Skate-class SSNs and most particularly, USS Tullibee (SSN-597)  . Using a plant like this combined with energy storage stack (IE LIB’s) has a likely somewhat lower integration risk – if nothing else there are a lot of commercial operators actively designing similar style plants for the ever-popular micro-nuclear-power-station-in-your-backyard market, which just never seems to take off for some reason...

Irony aside, the engineering issues associated with these types of plants is more widely understood, albeit clearly not so much in Australia, and there are very few actively running examples. NuScale Power is probably the leader in this field, but there are others. Like the “for-space” designs, I certainly think such a plant could conceivably be made for a sub, and would be operationally more viable. Such a boat could probably extract 1MWe+ of net power from a 5MWt plant, which would be enough to sustain transit speeds of c. 10-12 knots. This would be on par with current diesel boats, but crucially would (largely) eliminate the snorkeling requirement, and probably be much quieter too. The discrete energy storage stack could provide power top-up for high-energy maneuvers, but this would unlikely match full power nuclear boats. On the whole, however, such a system is really just a “small normal” system. With most of the same issues as a full size plant, and with some serious extra complexities and performance compromises. The idea of forgoing energy storage and using multiple such plants, ala Russian style, would somewhat address the shortcomings around relative power density, but this would result in a very much full-size boat with sub-par performance in its weight class. 

Ultimately, both classes of system will bump into the tyranny of shielding issue which afflicts any fission based nuclear reactor system, and which is particularly acute on subs.

Most western countries have adopted ALARA (“As Low As Reasonably Achievable”) as the fundamental principal for shielding nuclear systems. This was present from day one in [Father of the nuclear US Navy] Admiral Rickover’s programs. 

Although the specific designs in use on current subs is classified, the basic science is well understood within civil nuclear industry and is obviously much the same. Shielding is such a big heavy beast that, as far as I understand, its actually somewhat variable on boats, with usually forward and aft bulkheads and one side (port or starboard) fully shielded and the other sides (top, bottom and the other side) somewhat less so. Additionally, I read somewhere that US designers do things like put diesel storage tanks just forward of the reactor to give an extra layer. When the reactor is at higher powers, crew have restrictions on how close they can get to it, varying by locality and its associated shielding. The older S5W series boats had different shielding and accessibility arrangements (over the top).

The chief problem with almost any nuclear fission system is gamma radiation. The best historic material for stopping this is lead (there are some funky composite materials which are a bit better but are mind-bendingly expensive, these are typically used in, aha, space craft). Other typical materials are concrete, water and good old steel/iron. Some of the links below give relative effectiveness of these materials and explain the scattering and inverse-squaring reduction applicability. The best is just…space. Gamma will attenuate with space very effectively.

There are also other radiation types to deal with of course, Neutrons particularly, but gamma is the swine because unlike all the others, its not a decaying particle.

Clearly the quantum of radiation that has to be accommodated varies by reactor size and fuel density, as well as the nature of the reactors design operation.

Nonetheless, gamma radiation distance travel doesn’t become less, just because a given reactor is running at lower power or is smaller: gamma radiation IS gamma radiation, you may have less of it emitted per unit volume at low powers, but its travel remains the same, and therefore has to be shielded against.

Rickover and his Merry Men discovered this with the S4W on the Skates: 

So, the long and short of it is that whilst you could put a mini-reactor onboard, to safely do so means a degree of shielding that ultimately renders it pointless, IF your objective is to produce a small boat.

Finally, I do see and acknowledge the point of quick/easy reactor cycling [with a hull section that can be opened] being the primary advantage of these designs. Clearly the shielding could be wrapped around such a compact plant and the whole plant extracted through and out of it with a minimum of fuss. This was also my conclusion some years ago on first thought – and it would likely be necessary because burn-up rates would probably be high too. If you’re not concerned with the other issues, which ultimately are factors against compactness, then this is a very valid idea.

The only remaining issue then is really innovation risk – but that is ultimately less of a technical point as a project management issue and by extension in this usage case, a political one. I know if I asked your typical Politician, "Would you like to try

(a) this funky new idea which no one has ever done before and the risks and costs of which are completely unknown,

OR

(b) this guaranteed expensive, pork-barrel optimised, tried and tested tech, and which will require masses of international shuttle diplomacy and general hob-nobbing, on your flagship new defence project".....I KNOW which one they'll opt for - every time!

Sources From Anonymous

I draw your attention to this rather neat and abridged study into submarine shielding design and construction constraints that emerged from HMS Vanguard SSBN [still in deep maintenance since 2015] investigations. 

Another very brief but concise explanation on gamma scattering and absorption profiles of various materials is here.

…and the definitive guide, which I have NOT read (only skimmed) and is not recommended light bedtime reading: [800 pages of “Reactor Shielding for Nuclear Engineers, dated 1973 https://www.nrc.gov/docs/ML1000/ML100070680.pdf.