Dr. Zoomie – it seems I keep reading and hearing stories about some Russian super-nuke-torpedo. They’re saying it will travel thousands of miles, operate all on its own, and will create a radioactive tsunami that will make tens of thousands of square miles uninhabitable for hundreds of miles inland. Being honest with you, I live about 50 miles from the ocean and I don’t know if I should be scared, if I should sell my house, or if it’s just a load of BS. Can you help me understand how worried I should be? Thanks!
Ah yes – the Poseidon torpedo. This one dates back to about 2015 when, during a televised news piece, a Russian TV crew “accidentally” filmed part of what looked like an engineering drawing showing a “secret” Russian super-weapon. Here’s the thing – given the degree to which Russian TV is controlled by the state the possibility that a camera crew accidentally caught a glimpse of a genuine top-secret drawing of a new secret weapon that just happened to have not been covered up, and neglected to catch this before broadcast strains credulity. That, plus a Russian tendency to overhype their new weapons (e.g. the “unstoppable” hypersonic Kinzhal missile), makes me dubious about Russian claims that a single Poseidon warhead would create a 500-meter tsunami that would “plunge Britain into the depths of the sea.”
But let’s take a look at some of the specific claims to see how they hold up.
The Poseidon is said to be nuclear-powered. This is challenging, but possible. Russia built miniature nuclear reactors that were launched into orbit in the 1960s and 1970, as did the US. These were a far cry from something that could push a huge torpedo long distances through the ocean at high speed…but the US Navy developed a compact nuclear propulsion plant in the 1960s for their small research submarine, the NR-1. So this part is plausible. And a nuclear propulsion plant makes a 10,000 km (6200 mile) range plausible as well, although navigating this distance autonomously would be difficult to do undetected if we assume that the torpedo would require the occasional GPS location fix as well as the chance to receive communications from Russia – it would be a shame (for example) to launch a totally autonomous weapon and to have no way to recall it, change its targeting, or cancel its mission if peace happened to break out during the days or weeks it was in transit to the target. And every time a submerged vessel goes shallow – especially if it sticks a radio mast above the surface – it increases the odds of being detected. The bottom line is that a nuclear-powered super-torpedo with an exceptionally long range is technologically feasible, but it has some limitations that might limit its effectiveness as a weapon.
The very high speed (speeds as high as 100 knots have been mentioned in various accounts, although 50 knots is more in keeping with current torpedo performance) are a bit more dubious – water is a fairly dense medium and it take a lot of energy to move through it at high speeds; the precise amount of energy depends on the speed and the hydrodynamics of the hull along with the weight of the vessel. On the one hand, the Poseidon is likely to be much smaller than a full-sized submarine and it wouldn’t have to waste energy on things like air conditioning, ventilation, distilling fresh water, atmosphere controls, lights, and all the other things needed to keep the crew alive. On the other hand, the amount of energy expended on these “hotel loads” is only a small fraction of the amount of energy that goes into propulsion. If we say, for the sake of argument, that the Poseidon is about 10% the cross-sectional area, and a quarter the length of one of our older attack submarines, running at 100 knots would require somewhere between 10%-50% the reactor power output used to run the 1960s-era nuclear attack submarines (this is a very rough estimate). The thing is, those reactors ran on weapons-grade uranium and the reactor, steam, and propulsion plant machinery, while compact compared to a commercial reactor, were huge compared to the available space in Poseidon. I honestly don’t know if Poseidon has the room to house an engineering space large enough to produce the power needed to reach such high speeds – and even 50 knots would be challenging. So on this one I have to say that I’m dubious.
As far as the warhead goes, there’s certainly no theoretical impossibility to making a 500 megaton (MT) thermonuclear warhead, but that’s about 10 times the yield of the largest test ever conducted (that was the USSR’s 50 MT “Tsar Bomba” detonated in 1961), so this one has to be considered plausible.
On the other hand, how likely is it that even so large a warhead could produce a 500-meter radioactive tsunami that could wash over entire nations? And here I’m very dubious.
Take a moment and fill your kitchen sink with water to a few inches or so below the rim (we don’t want to make too much of a mess). Put both hands in the water and make like you’re pushing the water to the other side of the sink – it’s not very hard to make a wave that overflows the sink and wets the counter…and you don’t even have to push very hard. Now put both hands together in the center of the sink and pull them apart abruptly. No matter how hard you do so there won’t be as large a save – the reason is because the water flows in around your hands to fill in behind them. And (I hear you wonder), what in the world does this have to do with setting off a huge thermonuclear weapon underwater? And it’s not a perfect analogy – but it’s instructive, and it’s something you can do at home without even upsetting an HOA or your neighborhood watch!
Here’s the thing – to make a tsunami you need to push the water. Earthquakes can do this quite nicely; the earthquake that triggered the 2011 tsunami that killed so many people in Japan accomplished this by shoving a chunk of seafloor up to 50 meters high along a 70-80 mile front and releasing about the same amount of energy as a 50 MT nuclear device to produce a tsunami 40 meters high (https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/thumbnails/image/Mag-Energy-Freq-sm.gif). So with this as a baseline, can’t we just say “10 times the energy means 10 times the height”? Well…no. Because a save that’s 10 times as high will have much more than 10 times the volume of water and much more than 10 times the mass of the smaller wave because the base will have to be thicker to keep the wave from collapsing. Increasing the volume of water that’s moved by a factor of 10 will increase the height by maybe a factor of 3 or so, and maybe less. But there’s more to it than that. This is where your experiment in the sink comes into play.
See, a nuclear explosion pushes water in all directions while the earthquake pushed water in only one direction. And even though a huge explosion is going to boil a lot of seawater and will push a lot of water away from the site of the explosion, as the water cools the steam will condense and the water will flow back to fill the void. We can see this (albeit at a smaller scale) in video footage of subsurface explosions – even the largest generated a wave that was only a meter or so high. To generate a tsunami hundreds of meters high might be possible with a warhead of this size, but it would require finding a way to focus the energy in only one direction, which would be a challenge. And on top of that, it seems likely that the actual warhead is far smaller than the 500 MT figure bandied about – a yield of a few to several MT seems much more likely, with an even smaller wave. So a tsunami hundreds of meters high is unlikely.
Having said all that, let me inject a practical note into this discussion. Just because the Poseidon is not likely to cause the worst things we’ve been reading about does not make it the equivalent of tossing a firecracker into a pond. Depending on how close to shore it was detonated, the depth of the water and the depth of the detonation, the layout of the harbor and city that were attacked, and other factors, a lot of people could die, many more might be injured, billions of dollars of infrastructure and property could be damaged or destroyed, and much more. This would be a horrible day in the history of any city; the fact that it’s not quite as horrible or as far-reaching as advertised won’t change that in the slightest. But, that being said, the impact would be far lower than what our imaginations might lead us to believe.