Nuclear energy in militay submarines

[Dav333]

How does 4KG of uranium last powering for over 20years in the uss Pennsylvania?

The documentary i watched made it sound like it was the same 4kg of material being used over that long period. But in power plants don't they go through a lot of fuel?

thanks.

 

[Astronuc]

The reference was probably 4 kg of U consumed or fissioned. There's a lot more than kg's of U metal in a reactor.

In a commercial LWR power plant, the fuel is enriched up to a maximum allowed 5% U-235 and the rest is U-238 (with traces of U-234). About half of the U-235 is fissioned by the end of operation (useful lifetime of the fuel), and some of the U-238 is converted to Pu-239 through neutron capture and two successive beta-decays. Some of that Pu-239 is also fissioned.

 

[Hologram0110]

Basically the reactors are designed to do different things.

The military reactor is designed to provide a large amount of power relative to it's small form factor for many years. To achieve this, they use highly enriched fuel which can sustain the fission reaction for a longer period of time. It cost a fair amount of money and energy to enrich fuel, plus highly enriched fuel is considered a proliferation risk.

Civilian reactors are designed around economics. The fuel is enriched to a level which provides maximum profit. In this case the you use more lower enrichment fuel because you are not concerned with keeping things small.

Military reactors are also generally not run at as high of a capacity factor therefore they consume less fuel.

 

[Dav333]

thanks for replies.

 

[rod_worth]

I used to be in the nuclear navy, hence the name. Hologram0110 has a really good answer to this, and I can pretty much only repeat him. The biggest key to running a reactor for long periods of time are the 'highly-highly' enriched uranium (I can't say how much). Normal commercial power plants provide cities with power non-stop, so they are constantly 'at power', whereas a submarine might do a six month deployment, usually at relatively low powers, then come back and be in the shipyard for a few months shutdown; then rinse and repeat year after year. These two factors alone are enough to give you your 20 years. For a cost comparison, I can tell you that a nuclear aircraft carrier costs nearly $5 billion US to construct; it has two (2), $2 billion reactors which means out of the $5B to construct the entire boat, 80% of that is just to purchase the reactors. Commercial power at this price would not be economical as Hologram said.

 

[Morbius]

The military reactor is designed to provide a large amount of power relative to it's small form factor for many years. To achieve this, they use highly enriched fuel which can sustain the fission reaction for a longer period of time.

Hologram0110,

Another big reason for using highly enriched uranium is so the reactor can override a
Xenon transient.

A naval vessel has to be able to move when the skipper says so. One can't have the
engineer telling the skipper - "Sorry - we just shut down the reactor and won't be able
to restart until tomorrow" The Navy would never accept such a power plant.

The Professor that I did my doctoral dissertation with at MIT used to be head
of the calculation methods development at Bettis - one of the labs that designed
the naval reactors.

Dr. Gregory Greenman