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Comment by symplee | Hacker News Reader

A couple extra ideas for these beyond land-based power generation:

1) Put a couple of these on trans-Atlantic cruise ships, designed for speed, to make the journey in 3 days (at 50 MPH) carrying 2,000+ people.

2) Cargo ships that operate on the backbone long haul routes. If there are concerns about these getting too close to shoreline or population, just have them only operate 10 miles from land. Other cargo ships can then load and unload them in open water. (This would require some faster loading/unloading mechanism, maybe something where the front would open up and slide out the entire deck into the other ship.)

Thoughts?

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A ship can not go faster because it has a powerful engine or a near limitless energy source. A ship's speed is limited by design, especially with hull length. You cannot make a cruise ship go with 50kts.

50mph not 50 knots. I took one of these on a 75 minute trip last weekend. They cruise at 47mph and top speed is 55mph. https://en.wikipedia.org/wiki/HSC_Fjord_Cat They use huge amounts of diesel fuel, but that would not be the case for a nuclear version.

Edit: This version does 67mph and is powered by two 25 megawatt engines. https://en.wikipedia.org/wiki/HSC_Francisco

Sorry, when you said cruise ship, I was thinking "royal carribean" type ships. Cats can go much faster obviously.

My point is that the power alone can not make a ship faster. Hull length and shape determines max speed.

Still not quite understanding the point.

You mentioned even with unlimited amount of energy and power the speed is still capped and limited by Hull Length and Shape.

This reads to me if I could get a Nuclear Fusion reactor in my ship that has 10 times the power my Max Speed would still be the same. This doesn't sound right to me.

Drag increases dramatically above the hull speed. Obviously more power means more speed, but if you want more speed for a fixed power then your hull dimensions really matter.

https://en.wikipedia.org/wiki/Hull_speed

> if you want more speed for a fixed power then your hull dimensions really matter.

So in other words there's no max speed since we're not talking about fixed power.

I’m not sure I understand.

Power isn’t what’s important. It’s power per volume/weight. If you have an infinitely dense power source then you can send matter at the speed of light through any medium. Is this really an important point to make?

Edit: Oh I see what’s going on above in this thread. It’s nuanced but I believe the original point was that in the Real World with Real Power Sources you have a Real Top Speed. You can’t just always dump more power into your ship to increase it and the right way to increase the max speed is to control hull dimensions.

It's a well known property of boats. Due to fluid dynamics. This is why boats are big. Longer the waterline the faster it can go. Also why fast boats use multi hull designs.

> This reads to me if I could get a Nuclear Fusion reactor in my ship that has 10 times the power my Max Speed would still be the same.

Hull speed is a pretty simplistic approximation of the actual physics behind it, but generally the rule holds: A displacement hull cannot exceed its hull speed. What will rather happen is that the bow wave will pile up and eat your energy. It’s actually a problem if a very powerful boat pulls a smaller one, the bow wave on the smaller boat piles up, making the smaller boats bow rise up until the stern slips under the water and it sinks. (Or something else gives in under increasing strain before that happens)

Now, other hull forms can be built: glider hulls for example that glide on the water surface (think fast sports boats etc.), but scaling those up will certainly be problematic if you’re trying to aim for high carrying capacity. Cats/Multi hull designs sort of cheat the physics: the determining factor is roughly the length of the water line and multi hull designs obviously have more waterline for a given total length.

One of these ships could maximally transport about 100.000 people a year. 50 million cross the Atlantic per year, so you are looking at having 500 of them, probably more when you take into account that they won't be in service 100% of the time.

OTOH if it took 3 days probably a lot of people would just stay at home.

I personally would love to see the return of viable long distance semi rapid ocean. With the Advent of high speed and low latency satellite internet, a 3 day Atlantic crossing sound like a wonderful break. I already love traveling by train and I've taken numerous 3 day train trips accross the United States.

I'm thinking they're getting close to the scale where you could put wings and electric motors on them.

Except crashes~~

The New Zealand Nuclear Free Zone, Disarmament and Arms Control Act 1987 is arguably the strongest anti-nuclear weapon domestic legislation in the world. It bans nuclear weapons and propulsion from New Zealand's land, sea and airspace out to the country's 12-mile territorial limits.

Perhaps they might begin to care about the environment more than their fear of nuclear science and make exceptions for propulsion.

How is this related? Are you quibbling with the 10 mile figure in GP's comment? Or suggesting that NZ is some substantial portion of the hypothetical target market for GP's hypothetical nuclear cruise ship? Something else?

Nuclear-powered US military vessels (subs and aircraft carriers, at least) already sail all around the world. I guess they don't enter NZ's 12 mile zone, or get some exemption when they do.

The damage from an emergency at sea could be much greater than one on land.

The thing you get in the sea is unlimited free cooling and radiation shielding. You could dump the core onto the seafloor and not really have to worry about anything except the fuel elements eventually corroding and polluting a small area.

There are 11 known reactors dumped into various oceans and nothing terrible has come about from them.

Correct

The major concerns seem to be hijacking/piracy, clearance to dock at ports, and the general umbrella of proliferation concerns.

They don't want people taking these and using the material for bad things.

If there was a way around that, then maybe they would be economical. The cheap fuel costs in the past made the NS Savannah non-competitive, but I doubt that's true now: https://en.wikipedia.org/wiki/NS_Savannah

That makes a lot of sense; probably why we only see nuclear reactors on military vessels.

I've thought about this somewhat. We only trust the military to run nuclear reactors; ok, can we work with that? The US military is already a grift and jobs program; maybe we can turn the Navy into a grift and jobs program that is also an electricity utility.

Make them power all the shipping, oceanic towboats.

Not true. The fleet of Russian icebreakers relies mostly on nuclear propulsion. Wikipedia lists four different cargo ships equipped with a nuclear reactor at some point of time. The regulations and infrastructure are lacking but the interest is there.

Burning oil to move goods around the world is not a sustainable long term solution. Nuclear propulsion might be.

I wonder if we're over-reacting with anti proliferation.

It takes 3 very difficult achievements to build a nuclear bomb. Access to reactor grade uranium is only one of them.

I'm not sure the dangers of potential proliferation outweigh the tangible deaths per year that result from large ship pollution.

Building a dirty bomb isn't difficult, if raw materials is readily available.

Dirty bombs are cold war FUD. You could build a dirty bomb out of some smoke detectors. You could build a dirty bomb from the countless orphan sources scattered around the world with far less fuss than attacking a ship and causing an international incident. You could also build much more potent terror weapons without using any nuclear materials.

https://en.wikipedia.org/wiki/Dirty_bomb

The biggest threat a dirty bomb poses is fear. Meanwhile shipping pollution is estimated to be responsible for hundreds of thousands of deaths every year which we just take as a fact of life.

Reactor-grade uranium isn't sufficient either. It's enriched to a maximum of 20% U235. A bomb requires over 90%, and getting to that requires the same kind of enrichment facilities you'd need if you started from uranium ore.

But fission reactors also produce plutonium. Using that is another possibility, but you need very pure Pu239. Once the reactor has been running for over three months you start getting a lot of Pu240, which is a serious contaminant for bomb fuel. Separating those is not currently feasible.

So one expedient for nuclear ships would be to run the reactors a few months in secure conditions before putting them to sea. Reactor-grade plutonium is still rigorously controlled, because it might be possible to make it explode, but nobody has ever done it.

https://www.world-nuclear.org/information-library/nuclear-fu...

> Reactor-grade plutonium is still rigorously controlled, because it might be possible to make it explode, but nobody has ever done it.

The US tested a bomb made from reactor grade Pu in 1962.

From my source above:

> a 1962 US test using UK plutonium from its Magnox reactors had a relatively a high level of Pu-240 but evidently less than ‘reactor grade’ as subsequently defined

Fair enough. Though to be pedantic, at the time of the test, it was considered reactor grade. The subsequent split of the definition of reactor grade into fuel grade (8-20% Pu240) and reactor grade (>20% Pu-240) occurred in the 1970'ies.

That being said, there seems to be "evidence" (maybe more like conclusions that can be drawn from published articles by people with weapons experience etc.) that Pu with almost any isotopic composition can be used to make a usable (as in, very low probability of pre-detonation, low enough radioactivity to not pose a threat to personnel handling it) weapon.

Now, I don't think that terrorist capturing a nuclear powered cargo ship and then making a Pu bomb is a particularly big worry. Making a Pu bomb from spent fuel (whether with weapons grade Pu or reactor grade) requires reprocessing technology (e.g. PUREX) as well as the engineering to create an implosion device, and is probably out of reach for non-state actors.

The worry AFAICS with reactor grade Pu is that an advanced non-weapon state sitting on a big pile of separated reactor grade Pu (cough Japan cough) could relatively quickly (e.g. weeks/months) create a bomb if they so wanted, compared with building weapons grade Pu producing reactors and running them, which would be a several years long project.

Godzilla?

(It’s a joke of course. HN can be sometimes too serious)

Seems like coastal real estate is fucked either way.

And there are already active civilian nuclear propulsion vessels, mostly russian: half a dozen icebreakers and the icebreaker cargo ship Sevmorput.

A variant of the concept is the floating power plant Akademik Lomonosov, a barge housing 2x35MWe reactors. Though somewhat oddly it's not intended as a mobile power plant, it replaces a "normal" power plant being shut down in a far eastern autonomous district.

> You could dump the core onto the seafloor and not really have to worry about anything except the fuel elements eventually corroding and polluting a small area.

Absolutely not. What you're saying is true for nuclear disposal- if you bury nuclear waste under the ocean floor there are no currents and no water table to disturb it, unlike on land where coolant water will rush towards drinking water, smoke does not diffuse evenly, and sediment does not naturally bury heavy isotopes.

It is absolutely not true for an accident. Radionuclides from Fukushima are measurable anywhere in the Pacific ocean. They're all over the planet. That's key though, because such even distribution means that even very dangerous isotopes are very quickly reduced to safe concentrations, or at least lower than background radiation.

On the other hand it is extremely difficult or impossible to save a priority environment if there is a real disaster. Convection will be driving far more radioactive material into the area than a land accident. You're not gonna ever have a meltdown (they already basically don't happen on land), but even a relatively small breach would have the potential to kill an entire area. Over most of the ocean that might be fine, but spawning areas or corals would be huge losses.

It doesn't matter. Because water is one of the most effective shields against radiation that you can possibly have. The effects are kept localised. Alpha to mm, Beta to cm, Gamma to 10s of m. To put things in perspective, you can take a swim in a spent fuel storage pond and be exposed to less radiation than outside it.

The contamination from fukushima is detectable all over the planet just like all modern steel is detectably contaminated from 1960s nuclear testing[1]. It doesn't make it any kind of threat to the environment. We're talking about the same order of magnitude concentrations that are used in radiocarbon dating.

And the very dangerous isotopes you speak of, they have a very short half-life.

See also, the Oklo natural nuclear reactor complete with all the associated waste/contamination and the fact that it had little to no impact on the surrounding environment[2].

At the end of the day, there are 11 known and god knows how many unknown nuclear reactors dumped into the sea, and things have turned out pretty okay. One of them is even a popular dive site.

[1]: https://en.wikipedia.org/wiki/Low-background_steel

[2]: https://en.wikipedia.org/wiki/Oklo

Shielding is totally irrelevant. Even on land it's insignificant compared to the inverse cube law- spreading the dose reduces it much faster than the shielding, once you aren't right next to the reactor. Radiation exposure is not caused by proximity, it's caused by breathing and eating nuclear material. Water makes that FAR faster.

Again you're describing a sealed or mostly sealed, cold reactor, which is what most spills have been like. That is not what an accident would be like. It's a totally different scenario.

> And the very dangerous isotopes you speak of, they have a very short half-life.

Minutes, hours and days- easily long enough to kill local life outright. My point was that the local effects underwater would be far more disastrous than a similar accident on land, if there is life where the accident happens.

> See also, the Oklo natural nuclear reactor complete with all the associated waste/contamination and the fact that it had little to no impact on the surrounding environment[2].

It was active during the precambrian, and buried underground, which was again the exact assumption of yours that I had an issue with. You are really, fundamentally not understanding what I am saying- a nuclear accident is extremely different from disposal.

In a worst case where all the fuel simultaneously becomes a suspended powder, would kill a bunch of sealife in an area smaller than 100m radius. We do far worse on a regular basis with commercial fishing.

You make a sea reactor dump sound like China Syndrome. The fact that 11+ of them happened with no significant problems makes it pretty clear that you're exaggerating.

Microplastics and plastics in the ocean are a far more serious and far more tangible concern than a hypothetically hijacked ship hypothetically dumping the core which hypothetically turns into powder form.

I'd be more worried about people stealing the reactor. How many armed guards do you want to have on board?

I imagine disconnecting a reactor would trigger an alarm. Not particulary easy to steal a hot two bus sized item with a structure built around it. You'd probably need several big cranes and other logistics involved.

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