Inverted maritime nuclear reactors

April 20, 2009 4:43 PM

What design features, if any, allow nuclear reactors on submarines to survive being inverted?

Most nuclear reactors don't need to be designed with the possibility of turning upside down in mind, but I suppose those on nuclear submarines must be. While rolling over wouldn't be pleasant for the crew, I imagine it's something that could conceivably happen in battle or as the result of an accident. As such, it might be a possibility that the designers kept in mind when designing these reactors. How would they be different from conventional reactors? Is any information about this publicly available?

posted by sindark to technology (16 comments total)

Well, for one thing, you couldn't count on gravity to help you SCRAM the pile. So you'd have to ensure you could do it mechanically, with a spring or geared device.
posted by Cool Papa Bell at 5:14 PM on April 20

Another concern is the moderator. If it is liquid (like heavy water) and there is any open space in the reactor vessel, an inversion could leave the bottom of the fuel rods exposed.
posted by sindark at 5:28 PM on April 20

Why does it follow that they must be designed for turning upside down? I don't think submarines are designed for being upside down any more than surface ships are.
posted by craven_morhead at 6:27 PM on April 20

I don't think submarines are designed for being upside down any more than surface ships are.
they're designed as weapons and thus a target themselves. do you need a calculator for that equation?
posted by krautland at 6:31 PM on April 20 [1 favorite]

Another concern is the moderator. If it is liquid (like heavy water) and there is any open space in the reactor vessel, an inversion could leave the bottom of the fuel rods exposed.

Naval reactors are pressurized water reactors, which use regular water as the coolant, but the water is kept at about 275ºC and 15MPa (150atm). Consequently, there is no open space or air in the reactor vessel. Because everything is kept under pressure and driven by pumps, the reactor design doesn't inherently rely on a particular orientation.

In any case, nuclear submarines routinely roll 5 to 10 degrees during violent storms, so it must have been taken into consideration to that extent.

More evidence that the design does take it into account comes from early experiments with a 'natural' (i.e., non-pumped) circulation design (ignore the political cartoon at the top and scroll down). Basically, the Navy investigated a natural circulation design, which does depend on gravity, and to test it built a plant that floated in a large pool of water and could be tilted to simulate hard turns and waves.
posted by jedicus at 6:33 PM on April 20

The sub that had the collision in the Gulf recently rolled more than 70 degrees.

The design of the reactor in submarines is highly classified. Most of the crew of the sub are not permitted into the reactor room; the "nukes" are the inner priesthood of the dolphins, who collectively are known as the "silent service" because they don't tend to talk about what they do.

Anyone who knows the answer to your question won't talk.
posted by Chocolate Pickle at 7:06 PM on April 20 [2 favorites]

I'm not sure what you mean by "survive being inverted". Like the sub is doing crazy maneuvers like barrel rolls and loops as evasive action?

Because that can't happen. Inverting a submarine is not a survivable event.

Modern submarines are much more like a blimp than an airplane, in that they maintain buoyancy by having chambers of air that are open at the bottom to let a high density fluid inside them (air in blimps, water in subs), while keeping a certain amount of low density fluid trapped in the chamber (helium in blimps, air in subs). If you flip the sub upside down, the air that is keeping it afloat is released and the sub sinks. A simple demonstration of this is to put an inverted plastic cup in the sink, then let the air out.

Also, all of the nuclear subs I know about use steam turbines to generate a) their electrical power, and b) the means of propulsion. This steam comes out of a large heat exchanger with a space for steam at the top. Turn the sub upside down, and you get water in your turbines, which is not a Good Thing, and will cause them to explode all over the engine room, so you couldn't drive yourself to the surface even if you wanted to.

The Reactor is set up to SCRAM when the electrical generators fail, because without the main pumps, there would not be adequate cooling to protect the core from damage.

Also, the Pressurized Water Reactors used by the US are designed such that the amount of power output by the reactor is directly related to the amount of steam that is drawn out of their steam generators. So, since you aren't drawing any more steam out of the system, the reactor will reduce its power output, until it shuts itself down.

google USS Scorpion and USS Thresher, to learn a little more about this stuff.
posted by ArgentCorvid at 7:33 PM on April 20 [3 favorites]

What the silver bird said.
posted by furtive at 8:07 PM on April 20

I don't think submarines are designed for being upside down any more than surface ships are.

Recreational offshore sailboats are often (but not always) designed to survive a roll. I would not be surprised to find that some of the smaller (less than 100') naval surface ships are designed to survive a roll in heavy weather.

Modern submarines are much more like a blimp than an airplane, in that they maintain buoyancy by having chambers of air that are open at the bottom to let a high density fluid inside them (air in blimps, water in subs), while keeping a certain amount of low density fluid trapped in the chamber (helium in blimps, air in subs). If you flip the sub upside down, the air that is keeping it afloat is released and the sub sinks. A simple demonstration of this is to put an inverted plastic cup in the sink, then let the air out.

That's an interesting theory, but I don't know why anyone would build a military submarine like that. If submarines had open air cavities on their underside, you would just need to "burp" out the air to sink them. If you seal the cavity and use pumps with outlets around the sub, the attack is much harder.

Also, what the pickle said. At best, we can make guesses based on known civilian technology. Fun game but not one you can really win, unless you are trying to get a job with a Chinese defense contractor.
posted by b1tr0t at 8:25 PM on April 20

That's an interesting theory, but I don't know why anyone would build a military submarine like that. If submarines had open air cavities on their underside, you would just need to "burp" out the air to sink them.

Oh, believe me, that's the way they work. There's a valve on top of the buoyancy chamber (called the Main Ballast Tank) that lets the air out so the boat can submerge. The air used to surface in an emergency is kept in what are basically extremely large welding gas bottles, under high pressure. Keep the valve shut, and add air (and therefore reduce the amount of water), and you increase how buoyant the ship is.

It seems kind of retarded to do it that way, but it's really simple, and except in the case of the sub flipping over, extremely unlikely to fail. Even so, there are an enormous amount of safety protocols that are followed on submarines to ensure that they work correctly. These protocols, collectively known as the SUBSAFE program, came about because of the Thresher and Scorpion incidents.

This is one of the several reasons why submarines need to be good at hiding, by the way.
posted by ArgentCorvid at 9:07 PM on April 20 [4 favorites]

Submariner here. US Navy, 20 years. None of the following is classified.

ArgentCorvid is mostly right and mostly wrong. Heh. A submarine's main ballast tanks are open on the bottom like an inverted cup. However, they only have air in them when the ship is surfaced. That's the "make damn sure the boat is on top" volume. To submerge, the MBTs are vented and water fills them. Completely. That's important, because with an air bubble in there compressing and expanding at different depths, buoyancy and depth control would be a pain in the butt.

The MBTs are sized so that, fully flooded, the ship is nearly neutrally buoyant, so it can just be "flown" like a plane with the control surfaces. In reality it's still too light to sink.

Inside the pressure hull, there are a number of auxiliary tanks that are then fully or partially filled to make the ship neutrally buoyant. Some of those tanks have secondary purposes, like being a source of flushing water for toilets, draining torpedo tubes, etc.

As the ship passes through different depths, salinity of water, temperature of water, etc, the buoyancy changes, requiring water to be pumped off or let in. Also, consumable stores like food or missiles weigh something and have to be compensated for.

The ship can also get out of trim fore/aft, requiring water to be pumped forward to aft or aft to forward. The forward-most and aft-most tanks are called trim tanks and are used for this purpose. The chief of the watch is responsible for keeping the boat neutrally buoyant and trimmed fore/aft. It sometimes is not an easy job, especially in very dynamic environments like under polar ice.

Anyway - the boat isn't designed to be inverted. There are max design pitch/roll angles which I won't tell you. Those angles are limited by a number of things, not the least of which being the crew - most people are walking around, not strapped into chairs. Water may get into the steam headers and cause some turbine damage, but they have ways of protecting themselves which I also will not tell you.

The second biggest deal would be that every tank inboard, from aux ballast tanks to lube oil, to sewage, to what-have-you would overflow. Overall ballast weight wouldn't change, since it would still all be in-hull, but trim would get all jacked up. Imagine one of those Toblerone packages with a bit of water in the bottom. It's inherently unstable - if you tip it to the right, all the water sloshes to the right, making the tilt worse. (Free surface effect). There are measures to minimize that normally, but that would be too much.

The biggest deal, I won't tell you. An inverted sub might survive, but it would be a nail biter.

Finally, a question I know something about.
posted by ctmf at 12:28 AM on April 21 [18 favorites]

Hah, jedicus, that prototype is where I first qualified. S5G in Idaho Falls, Idaho. Now decommissioned.

This historical site about the INEL, where S5G was, is super-interesting reading (if you're into nuclear power.)
posted by ctmf at 12:46 AM on April 21 [1 favorite]

Oh, I guess I should also say (original question and all) - if an inverted sub didn't make it, the reactor would not be the reason. It would stay safe. That's as much detail on that topic I can really talk about, sorry.
posted by ctmf at 1:06 AM on April 21 [1 favorite]

Yeah, as I was falling asleep, I kept thinking of all the stuff I got wrong. But then I knew some smart-ass bubblehead would come along and correct me.
posted by ArgentCorvid at 4:47 AM on April 21

I'm not sure what you mean by "survive being inverted". Like the sub is doing crazy maneuvers like barrel rolls and loops as evasive action?

Because that can't happen. Inverting a submarine is not a survivable event.

Even if everyone on board has died, you probably still don't want a major failure of the reactor - especially if the sub is in your own waters or those of a friendly state.
posted by sindark at 5:55 AM on April 21 [1 favorite]

The biggest deal, I won't tell you.

All of the trident missiles would fall out of their silos.
posted by furtive at 9:13 AM on April 27 [1 favorite]