Pressurized water reactor

The pressurized water reactor has been the system of choice for the nuclear powered ships of the U.S. Navy, especially submarines. While not all of them have carried this design, it has been the most standard propulsion system in the fleet since the inception of nuclear power.

Such reactors have one natural advantage for use at sea - their primary coolant is available by the gigaton all around. While raw seawater certainly isn't the preferred option, this does mean that the reactor isn't carrying around important components that may be dangerous to mix with the surrounding environment. One example of the latter type is liquid sodium, a coolant used in some reactors; the advantage of the liquid sodium type reactor is that in the event of systems failure, the coolant itself has the heat transfer capacity to restrain the core, whereas water must be circulated to avoid overheating. The problem for naval use, of course, is that when sodium and water are mixed, well, bad things happen.

For submarines, the main disadvantage of the pressurized water design is that given above - they require water circulation at all times to avoid overheating. While the water itself may be plentiful, there are many times when a submarine would like to make as little noise as possible - and circulating water under pressure is very difficult to keep silent. At power levels above minimal, furthermore, maintaining this circulation will require the use of pumps, which have their own problems when it comes to quieting.

Having said all this, it is notable that the U.S. Navy has never (to my knowledge) suffered a reactor failure that has led to the loss of a ship. While most of the responsibility for this can be laid squarely on the personnel who man those reactors, and the culture of safety that trained them, it must be said that the pressurized water design does appear to have been the proper choice, despite whatever operational disadvantages it may have.

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A minor quibble with the estimable smartalix, below:

I don't count the U.S.S. Thresher because the initial casualty was a piping/valve defect (design or manufacture, I don't know for sure). The flooding in the reactor spaces was not, in fact, minor; the submarine (which typically maintains slightly postiive buoyancy) sank despite a surfacing blow and a partial 'emergency blow' of the reserve ballast tankage. Later, when the boat was being raised, it made it almost to the surface before it broke free and sank to the bottom rapidly. None of this would be consistent with anything less than a massive flooding of the reactor spaces. While the second surface attempt (the salvage) might have been due to slow, long-term flooding, if the boat was unable to initially blow enough to achieve positive static buoyancy, then the flooding must have been fairly rapid to offset the primary and reserve ballast systems in between the onset of the leak and the commander's order to emergency blow.

In any case, blowing the ballast (as well as the reserve) requires no electrical power (at least, not in emergency procedures) as it relies on stored compressed air, and the boat wasn't far down enough at the time of the incident for its air systems to have been overcome by outside pressure. The electrical shutdown did, however, mean that the plane controls probably failed, which would mean that the sub would have been unable to use forward motion to angle the boat upwards. This might have saved them, by giving them enough time (on battery propulsion, if available, or even on the boat's momentum) to offset the descent long enough to restore power, or at least reach surface and abandon ship.

So, the Thresher was lost not due to a reactor failure but due to a questionable design choice made in the boat's attitude and propulsion control systems. The reactor performed as expected, and when the short circuit occurred, it shut down (as designed) to prevent the loss of the control systems from causing an uncontrolled run (meltdown). As an indication of the success of this design choice, the reactor vessel from the Thresher (closely monitored by the U.S. Navy since the accident, as well as occasionally by civilian watchdog organizations of various nations) shows no sign of leaking radioactive material or radiation; the remains of the Thresher are radiologically nearly identical to the environment.

I'm not aware of any other U.S. submarines lost due to reactor system problems. The U.S.S. Scorpion was lost (it is presumed) to a hot-running torpedo, and those are the only nuclear U.S. boats to have gone down, unless I'm missing an experimental boat somewhere.

Whew. It's a good point to bring up, though.

An addendum to the great w/u by The Custodian: There were a couple of submarines lost to reactor failure, most notably the USS Thresher. While not actually lost due to reactor failure by-products (explosion, radiation, heat), the sub was lost due to a reactor shutdown.

The Thresher was designed to dive deeper than previous designs, and while on a test voyage, was operating at extreme depth when a leak developed in the piping in the reactor room. This caused a short-circuit, and the reactor shut down automatically. In the Navy's infinite wisdom, no back-up power supply was included in the submarine's design. It was this reactor shutdown that doomed the boat.

While attempting to restart the reactor (the sub was dead in the water due to a lack of power), the Thresher slipped below crush depth (the point where sea pressure exceeds the strength of the sub walls to resist it) and the ship was lost.

A nuclear power plant where heat is drawn from the reactor core via a closed cooling circuit containing water under high pressure (as the name suggests) in order to keep it in a liquid state at high temperatures. The heated primary coolant is itself in turn cooled by passing through a steam generator, exchanging heat with water in a secondary circuit which is then used to drive a steam turbine for power generation or marine motive power. It differs from the earlier water-cooled reactor design, the Boiling Water Reactor, where the coolant water (which, having passed through the core, is fairly radioactive) is used directly to drive the turbines, resulting in significantly worse danger of radioactive emissions. In use from the late 1950s on, the PWR is the most popular (in the broad sense of the word) type of nuclear plant; 57% of nuclear electricity generating plants operating today world wide are PWRs.

Although the PWRs are cleaner than the BWRs and other designs where power generation equipment is driven directly by coolant, they have a design weakness in the steam generator. In the American Westinghouse design (the earliest type of PWR used in the west) the steam generator is basically a big heat exchanger in a vertical cylindrical casing. The reactor cooling water enters at the bottom of the tank and passes up and back down through a bundle of narrow U-shaped tubes (to maximise the surface area for heat transfer) immersed in the secondary stage cooling water, and then the cooled water is returned to the reactor; steam from the heated secondary stage coolant is dried and drawn off at the top of the generator and run through the power-generation turbine and a condenser stage before being returned in liquid form to the steam generator.

Although this may seem a relatively simple process, the mechanical problems of vibration (metal fatigue and fretting of pipes against their mountings) as water is driven though the tube bundle, corrosion and the deposition of various impurities and turbine fragments (turbine blades are very vulnerable to impact damage from water droplets if the steam is not properly dried before exiting the steam generator) from the coolant water in both circuits mean that weaknesses appear and the chances that a tube will blow out increase significantly with age and use. A rupture of a steam tube would probably lead to emissions of radioactive steam and boiling water, but worse still would also depressurize the primary coolant circuit, allowing the core to overheat and melt down. An alternative design by Babcock & Wilcox uses more robust straight through pipes for the primary circuit in the steam generator; the drawback is that it reduces the quantity of secondary coolant held in the steam generator: this was the type of system involved in the Three Mile Island incident, in which the flow in the secondary circuit was accidentally interrupted leading to the core overheating.

The smaller PWRs used to power ships generally use a horizontally aligned steam generator which seems to be a more mechanically satisfactory arrangement.

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Sources

• Stuff in my head from publicly available tender documents translated for a commercial client and related research.
• http://www.wowpage.com/tmi/ (Three Mile Island)
• http://www.icjt.org/an/tech/jesvet/jesvet.htm (ICJT nuclear training centre, Slovenia)
• http://www.nucleartourist.com
• http://www.mtn.org/pic/ The Prairie Island Coalition

Addendum the second to The_Custodian's write-up:

Another huge advantage to using pressurized water concerns water's unique properties as a moderator. A moderator slows down and reflects neutrons back into the fuel, increasing the probability that they will be absorbed by the fuel, thus causing reactor power to increase. This increase in power, in turn, causes temperatures inside the reactor to rise. As temperatures rise, the density of the water decreases and thus reduces its ability to reflect neutrons back into the fuel. This causes reactor power to go down. This cycling makes a pressurized water reactor inherently more stable. This property is known as a negative coefficient of reactivity.

For an RBMK reactor, such as Chernobyl, water was used as a coolant, but graphite was used as the moderator. The process begins the same as in a pressurized water reactor, but when the temperature goes up, the ability of graphite to reflect neutrons does not change significantly, unlike water. This causes power and temperature to go up, which will eventually cause the coolant to boil off, removing the reactor's ability to cool itself. Since steam isn't a moderator, more neutrons reach the graphite, causing power and temperature to continue rising higher and higher in an unending cycle now that there is nothing cooling the reactor.

see a better explanation at http://www.eng-tips.com/gpviewthread.cfm/qid/16663/pid/466/lev2/19/lev3/64