The benefit of building a Nuke-powered destroyer or crusier.

The reactor itself won't blow up. The potential problem lies in the radiated water being used to cool the reactor core and acts as the primary steam circuit. This goes into a condenser where the heat is transferred to a secondary steam circuit. The steam on the secondary circuit is safe. Its on the first circuit that is always the concern about.

Despite these concerns, the statistical and empirical record says PWR is very safe. You would sooner die being hit by lightning than by a nuclear reactor accident.

Next generation, non water using nuclear reactors is even more safe. For example using Helium for HTGR. Helium is extremely unlikely to form a radioactive isotope. If Helium that is in contact with a reactor core was released into the atmosphere, it would still be completely safe. The Russians experimented with using liquid lead as a nuclear reactor coolant in their Alfa class submarines. Despite the notorious safety record of Russian reactors, the conditions they are in, and being a first generation technology, all those reactors, used in five Alfa class nuclear submarines, never killed anyone. After all, if liquid lead leaked, what's going to happen to it? It will only solidify and sink to the floor.

The problem remains the sheer high cost of fielding a nuclear powered ship. Did I remember a figure that it cost 64 million a year to operate the Long Beach, compared to 45 million for a Burke?

Add to that cost is the sheer cost of dismantling the nuclear powered ship. To junk a nuclear powered ship cost a lot more than a conventional powered one. The Russians are still having problems trying to dispose of all those obsolete nuclear subs, and the problem of scrapping is that the steel (and in the case of the Russians, sometimes Titanium) is so high a tensile strength its very difficult to cut them. Do you ever wonder why some ships end up as museums, others as missile or torpedo target practice? Couple that with the problem of corrosion that can happen on the primary cooling circuit, you still have a mess to be concerned about.

if i remember correctly the walls surrounding the reactors on carriers are about 2 foot thick (~40cm) you need some serious firepower or velocity penetration to get through that hunk of metal surrounding it. then you got the actuall hull around that too.

Even if destroyed or penetrated, the reactor core won't blow. As a matter of fact, fragmenting the reactor core stops fissioning; fissioning requires you must sufficient mass and density present, and fragmenting the core is the opposite of that, it destroys the conditions for nuclear fission. The core simply won't nuke. However, you are going to have a big mess of radioactivity to clean up.

The only way a core could go uncontrolled critical is when all the control rods are pulled out. But chances during battle damage is that the control rods would drop completely in by gravity, stopping fission completely.

I think HTG reactors for shipboard use are a fantasy. The power density is simply not there. Non naval types often are not aware of the space and weight limitations of shipboard power plants. Water cooled reactors are the only type that pack enough heat into a small enough enclosure to be practical for a ship.
Nuclear power allows a ship to be freed of the necessity of refueling every few days at sea, and allows some very high speeds for extended periods of time. The obvious drawback is cost. A few years ago the USN ran a quick and dirty study of how high the price of oil would have to rise to make certain classes of conventionally powered warship practical to build with nuclear power. For an LHA or LPD sized ship oil needed to stay above about $85/bbl for the lifecycle cost of the nuclear ship to be lower than for it's conventional counterpart. For a destroyer or cruiser to be less costly over it's lifecycle oil had to rise significantly above $200/bbl.
The US Congress has been debating the value of having all future combat ships nuclear powered. I believe the next LHA class will be nuclear powered, and the CGX is being debated now.

crobato said:

Even if destroyed or penetrated, the reactor core won't blow. As a matter of fact, fragmenting the reactor core stops fissioning; fissioning requires you must sufficient mass and density present, and fragmenting the core is the opposite of that, it destroys the conditions for nuclear fission. The core simply won't nuke. However, you are going to have a big mess of radioactivity to clean up.

The only way a core could go uncontrolled critical is when all the control rods are pulled out. But chances during battle damage is that the control rods would drop completely in by gravity, stopping fission completely.

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Unlike commercial reactors, the USN uses high enriched uranium in it's shipboard reactors. Bomb grade stuff. This why reactor cores can now last the life of the ship without refueling.
I think HTG reactors for shipboard use are a fantasy. If you have not been in a navy you might not understand just how crammed full of gear a warship is. Space and weight are at a severe premium. So far, only water cooled reactors have the power density to be practical for shipboard use. None of the current or proposed HTG schemes can match a water cooled reactor for power density.
A few years ago the USN did a quick study of the total ownership costs of conventional and nuclear powered ships. For a nuclear powered LHA to be less costly than it's conventional counterpart, oil had to remain above $85/bbl. For a nuclear powered destroyer to be less costly oil had to rise significantly above $200/bbl. Congress asked the Navy to further refine these numbers and I have not seen anything in the press on the results.
Meanwhile the US Congress has been debating legislation requiring all new surface combatants be nuclear powered. This is not for cost reasons but to lighten the logistics foot print. Keep in mind a nuclear powered ship can use space previously used to carry the ship's fuel as bunkerage for it's aviation assets. Double bonus, the ship can fight more days before running out of aviation fuel.

If you like sheer power density, nothing really beats a diesel engine. Nuclear power, ala PBR, is really nothing more than steam powered, needs water tanks, needs boilers, need steam turbines, associative plumbing, and so on. As a matter of fact, it currently requires two pressure steam circuits as opposed to one using conventionally fired setups. It might have power density, but energy transmission is not as efficient. Which means in HTGR, even if less power is generated for the same amount of space, less energy is wasted in the transmission and conversion of that energy into electricity, even more if a closed circuit gas turbine is used without the use of a steam circuit. That means the heated helium gas blows into the turbine like a gas turbine.

What is interesting about nuclear power is that it uses low pressure, around 124 psi wet steam. These plants accept huge heat increases without protest and respond immediately to power increases. The high pressure superheated steam plants that are mostly history now, are famously touchy. Too much fuel too quickly and bad things happen. They are super finicky about water chemistry, and can in some cases run the ship out of distilled feed water if run at high power for extended periods of time. Nuclear powered ships have no such problems.
HTG plants, as of the current technology, simply do not have the horsepower per cubic meter that shipboard water cooled reactor plants do. Keep in mind the USN is running high enriched fuel. HTG reactors are a great idea for a land based powerplant where load is more or less constant over time and weight and space are not so constrained, but unlike merchant ships where big diesels rule ( but big diesels do not respond as quickly as a nuke and must be shut down to reverse them, then restarted ), combat ships have to be able to accept huge increases and decreases in speed at a moment's notice along with being able to sustain hard maneuvers. Nuclear carriers are typically faster than any escort, and they accelerate faster too. Firewall an old Charles F. Adams class can alongside a CVN and the CVN will be over the horizon in minutes. Gone. Those high pressure steam plants ( 1300 psi ) were little time bombs and had to be handled very deftly at high power settings or bad things happened very quickly.

Current HTG technology requires the primary gas cooling circuit to exchange heat against a secondary steam circuit which in turn drives a steam turbine. That loses efficiency and requires space---anything that has two circuits do.

What HTG allows and this is not with current technology yet, is a closed circuit turbine design where the heated gas can propel a gas turbine directly. No boilers needed, no pumps, and none of the weight of water. Without a secondary circuit, that saves on both space and energy efficiency, and it delivers the advantages of a gas turbine over a steam turbine. In addition its a lot more safe, because if leaked, helium is not going to be radioactive unlike water. Furthermore in the future, the holy grail of nuclear reactor research is MHD, meaning obtaining electricity directly from the fluid motion without the use of turbines and generators. MHD electricity generation is best achieved using gas, as it is easier to separate electrons on fluid that is on a gaseous rather than liquid state and make the fluid magnetic. (Not to be confused with MHD propulsion drive considered for submarines; same principle but on the opposite end). Considering we are talking of direct energy weapons and railguns, I'm looking at the future context.

The future warship acts more like a power plant than anything else. Turbines replaced by electric motors, making the reactors decoupled from the transportation process, and the power plant is more dedicated towards the generation of electricity more than anything else. Electricity to drive rail guns and direct energy weapons. If we use condensers that will store electricity and release huge amounts of them in bursts, necessary for short dashes or for the energy weapons, the nuclear reactor and generators can afford to be running in a constant speed.

As hydrogen can now be easily extracted from water/seawater is there any reason that you could not have a hydrogen powered sub or surface ship. Since they could use seawater for fuel they could stay at sea as long as the food supply lasts. Any navies experimenting with this?

eh, hydrogen is not easily extracted from seawater. Doing so with current technology requires vast amounts of electricity. Where do you find the fuel to create all this electricity? Nulcear power? Ok, but there are far more efficient ways to use that nuclear power than to use it to extract hydrogen from sea water.
One other thing to keep in mind is the weight of a shipboard power plant. The USN found that 1300 psi steam was simply too difficult to keep confined in a shipboard installation. It would leak constantly ( and 1300 psi anything is enough to cut an arm off, so leaks in those plants were sources of hideous injuries ) and proved tough to maintain. Yet, shore based gas fired steam plants routinely run 3000 psi. WTF? Well, ashore you can use heavy pipes with the wall thickness required to reliably constrain such high pressures. Such an installation would sink any ship, it'sjust too heavy. So exercise care when postulating the use of shore power schemes aboard combat ships. Weight and space are always at a premium at sea.