Question on mobile nuclear reactors (subs and aircraft carriers)

In the midst of all the questions raised about existing commercial nuclear power plants stemming from the Fukushima disaster, I've started wondering about all of the reactors on board nuclear-powered submarines and aircraft carriers. How safe are they? Is it possible for one of them to suffer a meltdown, and if so, what would be the consequences to the surrounding area? I'm also wondering how they remove and transport the spent fuel, since that is one of the reasons there are growing stockpiles in the spent fuel pools at the commercial plants. I know that there was a high-profile incident of a U.S. nuclear sub sinking (the Thresher, I think) and there was the Russian sub that suffered an accident that was portrayed in a movie starring Harrison Ford and Liam Neeson (something, "The Windowmaker").

The description of what went wrong with the K-19 has some eerie Fukushima parallels.

"On 4 July 1961, under the command of Captain First Rank Nikolai Vladimirovich Zateyev, K-19 was conducting exercises in the North Atlantic close to Southern Greenland when it developed a major leak in its reactor coolant system, causing the water pressure in the aft reactor to drop to zero and causing failure of the coolant pumps. A separate accident had disabled the long-range radio system, so they could not contact Moscow. The reactor temperature rose uncontrollably, reaching 800 °C (1,470 °F) — almost the melting point of the fuel rods — and the chain reactions continued despite the control rods being inserted via a SCRAM mechanism. The reactor continued to heat up as coolant is still required during shutdown until the reactions decrease. Despite Zateyev's and others' earlier requests, no backup cooling system had been installed.

As a cooling back-up system had not been installed, Zateyev made a drastic decision; a team of seven engineering officers and crew worked for extended periods in high-radiation areas to implement a new coolant system by cutting off an air vent valve and welding a water-supplying pipe into it. Since the ship carried chemical suits, instead of radiation suits (not available at the time and developed after accidents like this), they were certain to be lethally contaminated, but the repair team was unaware of the degree of risk, believing the suits they wore would protect them from contamination. The released radioactive steam, containing fission products, was drawn into the ventilation system and spread to other sections of the ship. The cooling water pumped from the reactor section worked well."

because there is no profit motive, there is no real motivation to save money by cutting corners. They are refueled about every 15 years (new design submarine reactors every 30 years) - it is a long (2-3 yrs) process done in a shipyard by specialized workers. The spent rods are sent for storage to Hanford WA.

is presumed to be sunk due to bad welds in one of her water systems.

But that doesn't answer the question: what happens to all that nuclear material when an accident like this happens. To be honest, not sure...the Thresher is in something like 8000 ft of water 200+ miles off the US Coast.

There have been a few radioactive discharges but none caused by a failure of the reactor systems.

Other nations not so much (more a function of how few mobile nuclear reactors they have). The track record has been very good. Partially that is attributed to the design (reactors are much smaller output hence they can be designed to refuel only once a lifetime) simplifying operation. The large factor is lack of profit motive. There is no incentive to squeeze as much performance out of the reactor as possible, keep than longer than originally planned, or push to maximize uptime.

As far as a meltdown it is much less likely but not impossible. In a reactor there are two dangers.
a) uncontrolled fission - massive amount of heat. Which is why fission is halted in an emergency.
b) decay heat. much less about 7% declining to <1% within days of peak power output but still a danger.

Subs/ships are surrounded by water hence the ability to conduct cooling ops ( b ) is much easier. Even in a completely loss of containment like sub sinking water would rush in to all compartments including reactor providing cooling. The danger still exists of an uncontrolled fission but cooling is much easier.

"I'm also wondering how they remove and transport the spent fuel, since that is one of the reasons there are growing stockpiles in the spent fuel pools at the commercial plants."
They remove and transport the fuel because the DOE allows it. There is no technical reason we can't store spent fuel in governmental facility. No such facility has been built and only DOE can authorize one. While we likely won't agree on a deep geological repository anytime soon (like Finland is building now) we could at a minimum agree on regional interim spent fuel storage facilities despite to store dry casked spent fuel for decades until a deep geological repository is built.

Sure, it's a nuclear reactor. However, it is also surrounded by water, which would act as a coolent in the case of loss of cooling water. So the chances are far less then on land.



Good question. Personally, it's not how safe, but but happens when they fail. As we have seen by TMI, Chernobyl, and now Fukushima, the failure of nuclear generation is catastophic. Not so much at TMI, but Chernobyl and Fukushima are, and will be, uninhabitable.



Acually, it was called K-19 The Widowmaker. Here is a link to some of it's history,
http://www.nationalgeographic.com/k19/index.html

And as far as the Thresher goes, here is some more info,
http://www.history.navy.mil/danfs/t/thresher.htm

...Fukushima will be uninhabitable, at least not from the stuff that has leaked so far. You need large quantities of Strontium 90 and Cesium 137 or other long life radioactives in great quantities to make a place uninhabitable in the long run. That what made the Mayak waste storage explosion so devastating, it was almost all long half life isotopes that were scattered. Iodine 131, no matter how much you release will not hang around for long.

My friend was a reactor operator on an aircraft carrier. They have multiple reactors on each surface vessel on the theory that if one gets knocked out in combat there are others to continue to power the ship. He said that flooding the reactor with sea water was the next to last resort (just before the core melting through the bottom of the ship and going to the sea floor) and needed the captain present with a failsafe key to hit the seawater switch. They drill every day on reactor emergencies of every nature. He felt the designs were reasonably safe, but I don't know. I share your concerns. That's why I asked him about it.

1) Uncle sam has no profit motive and DEEP pockets. Safety is everything on a nuclear vessel and expenses are generally not spared. In the private sector, there is a direct balance betwen profits and saftey budgets.

2) Current naval reactor designs operate differently. They use highly enriched uranium fuel up to 95% (weapons grade, much more expensive) whereas commercial reactors use uranium or MOX fuel enriched to about 3%-8%. This means a naval reactor can operate basically it's whole life (25-30 years) never needing refueled. Naval reactors use Halfnium control rods. Much better than commercial Boron control rods... and disproportoinately more expen$ive.

3) They operate in the ocean. If there ever is a major problem they reactor itself basically sits in an infinatly large cooling pool.

...about four meters of water blocks the radiation so the immediate radiation hazard even from a lost and exposed reactor is fairly small and local. Naturally the radioactives can dissolve and spread but then it will be hard to get any really dangerous concentrations of radiation.

but the most important is that there is no profit margin. We spare no expense when it comes to technology and training when it comes to Navy Nuclear Power.

Commercial plants would lose money if they had to operate at the standards imposed on Navy Nuclear Power. We could do it, but folks electric bills would probably have to double to make it profitable.