Daily Archives: 16 - Feb - 2009

Nuclear subs collide

I guess Slashdot has finally picked up on the collision between a British and French SSBN. Since submarining is my Real Job ™ I figured I’d try to disspell some of the inaccuracies I saw among the Slashdot comments.

I saw lots of comments regarding how extraordinarily low the probability “must be” for having two SSBNs collide in the Atlantic, therefore it must be some secret exercise, which I wanted to go into.

First off, SSBNs on patrol would not be able to just go anywhere in the Atlantic that they wanted to. Any missile system will have a set range, so depending on what country or countries they suspect they’d have to launch against, they’d at least have to be within some kind of minimum weapons range.

Further, there could be other criteria limiting their possible operating area. Maybe a country wants to keep their SSBNs within range of their maritime patrol aircraft so that there is air cover available to try and detect hostile SSNs looking for the SSBN. Maybe a country wants to keep their SSBN within a certain number of days of transit time to allow for timely changeout of SSBN in the event of mechanical failure or personnel casualty. There’s a lot of variables that could be put into the problem of determining a good operating area.

Given the fact that Britain and France are right next to each other it stands to reason that whatever operating areas they chose could be near each other or even intersecting. Unless the countries corresponded with each other to deconflict their operating areas they could unwittingly be having their SSBNs operate in the same waterspace.

France has been performing submerged strategic deterrance patrols since 1971, and Britain since 1968. That’s almost 40 years of submerged patrols going on and the fact that there’s been 1 collision in that time between countries which do not coordinate waterspace is frankly not that shocking.

The question then becomes how the submarines did not take action to avoid collision, since both were equipped with sonar. The answer lies in the fact that there are two major types of sonar, active and passive. Active sonar is the type of sonar you see in the Hollywood movies, where the submarine crew looks anxiously at the hull while it is being “pinged”. It’s actually counter-productive for a SSBN to use though, as it draws attention toward itself. Imagine a completely dark room with two people, and one turns on a flashlight. The person holding the flashlight is brilliantly obvious to the other person, but the person without a flashlight can still try to run out of range of the beam. Active sonar is more like an omni-directional light than a beam but in the context of avoiding detection at all costs it is still a no-go. So neither SSBN would have been using active sonar.

Passive sonar is the fallback, and is simply “putting an ear into the water” and listening for sounds. There are all manners of noise producers in the ocean. There is wave action, weather, biological activity, merchant traffic, normal background noise, etc. etc. When you keep in mind that both France and Britain have probably spent substantial sums of money designing, building, and maintaining a SSBN fleet that is nearly undetectable it doesn’t seem so surprising that passive sonar would have been ineffective at detection, not to mention tracking. There are other ways of trying to detect a submerged submarine but the common thread is you’d need to be close no matter what you do.

Given that there have been collisions between submerged submarines before (for instance, USS Tautog), even back when they were much noisier it’s not surprising per se that it would happen again.

The other major comment I read was the usual uninformed commentary over nuclear weaponry and/or propulsion. Many were concerned with the effects on either the warheads or the propulsion plant. I can’t speak to warhead design but the engineering problem of preventing an inadvertent detonation of an unarmed warhead is not exactly rocket science. The hard part was getting it to blow up in the first place! In addition the reactor plant (if it’s anything like ours at least) is probably going to be the most likely thing to work after a collision. Keep in mind they are designed to sustain battle damage from things like torpedoes and depth charges. The real contamination people are worried about is embedded inside the fuel plates in the reactor core. The reactor coolant itself is not nearly as large a concern. In fact, American nuclear-powered ships maintain a coolant discharge log (section 25.2.4) to record coolant discharges at sea.

Even if the nuclear plant were to have sprung a leak, it has to get through the submarine hull itself before it escapes to the environment (and in that situation the submarine is likely sunk anyways).

What would have happened in the event of the real catastrophe, that one or both submarines would have sunk? Same thing that happened to USS Thresher and USS Scorpion, the reactor plant would scram, and the decay heat cooling would be provided by the seawater flooding into the ship and reactor compartment as the submarine broke up while it was sinking. If contamination did leak it would be at the bottom of the ocean (the phrase “like peeing into the ocean” springs to mind).

The warheads themselves would sit inert in the ocean, undoutedly to be recovered at great expense (if feasible, at least). A US Air Force nuclear bomb was dropped into the Mediterranean Sea without blowing up in 1966 (three other bombs fell on Spain, two actually blew up without detonating the warhead). So the consequences would be severe but at the same time, a magic detonation just from going bump isn’t a plausible failure mode for the warheads.

So to sum up, this is a big deal, I’ll be interested to see what is publically released after the inquiry but there is no reason to be up in arms about warheads blowing up or reactor cores melting down or weird conspiracies between French and British SSBNs gone awry. (In fact, if the British and French had been doing an exercise it’s highly unlikely they would have collided, it’s very easy to say “you stay above 100m and I’ll stay below 200m” and thereby prevent the very possibility of collision.)

As an aside, if you think this collision was bad, then you must not be familiar with USS San Francisco, which ran into a mountain underwater at flank speed (and survived). And the reactor plant not only survived unscathed, but she managed to drag herself home under her own power.