h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Interesting - metals would be pretty poor coolants I would have thought as they tend to have very low heat capactities (i.e it takes little energy to heat them up unit temperature).

add to sodium it's ..erm.. well known properties on exposure to water (also air - especially when molten!).

This would seem to make sodium partiucarly unsuitable -maintenance of the coolant would be an interesting technical problem I would say.

However there is on big plus side I can see. The coolant inevitable gets massively irradiated and will no doubt get very radioactive on its own account. Consider the removal of a liquid coolant such as water - slops about, vapourises, generally quite a messy clean up procedure. A metal however, just let it cool down, solid waste that is easily handled.
I wonder if that is thinking behind this...?

I believe they use Sodium as a coolant in SSNs and SSBNs - the reason being - no need for coolant pumps running - meaning less noise.

Metals presumably have the advantage that they don't need huge pressure to contain them at high temperature?

I know lead is good at blocking radiation, but I wonder if it is an effective neutron absorber or moderator?

Yes the pressure thing is of course a bonus.

I would be surprised if noise was an important factor, safety would be much higher priority surely - I can imagine no pumps means lesser expense...

"I would be surprised if noise was an important factor,"

SSN/SSBN = Submarine = invisible = quiet

The only way to detect them is the noise they give off... too much noise and they're all dead!

----

"I know lead is good at blocking radiation, but I wonder if it is an effective neutron absorber or moderator?"

As far as I could see it was being used as a reactor coolant/heat tranfer fluid rather than as a moderator.

I'm not suggesting it has to be a neutron absorber or a moderator, I'm just wondering if it is.

Lead is a very poor neutron absorber...

http://www.leadinfo.com/ARCH/rad.html

I thought that might be the case as it is the stable decay product for lots of isotopes. Stable nuclei tend to have less tendecy to absorb neutrons as far as I am aware.

Yes but we're not talking about submarines here are we. A nuclear reactor is in a big concrete containment building miles from anywhere - noise in this instance is less of a factor than it is for a stealth submarine.

Orcus - what upset you today?

I was merely responding to SoRBs comment about sodium cooled reactors - which, in general, don't happen to be stuck in big concrete containment buildings miles from anywhere.

Surely we were talking about reactors, whereever they may be?

I thought nuclear reactors were nearly always miles from anywhere, if not I stand corrected.
I still am on the sceptical side of noise being even in the top 10 of priorites when reactor design is on the drawing board. After all, one still has to turn turbines and that must be quite noisy. Just as noisy as a cooling pump I would say.

Sorry if my post came across as agressive, it wasn't intended to be

drops

Sorry, you're talking about nuclear powered subs aren't you...

Forgot about the existence of them,

The RN only uses pressurised water reactors, i.e. water cooled. Not sure about anyone else though.

**Looks through a few books**

Actually, I was wrong anyway... Sodium seems to be used in Fast Reactors... What I was actually thinking about was the old Soviet Alfa class subs - that _did_ use lead as a coolant. The only downside was that if the reactor was shut down for any length of time the whole thing would have solidified. (Alongside in a harbour they used to feed superheated steam through them to stop them 'setting'.

Firstly I'll thank everyone that help me with my project, it went very well and I'm hoping for good results.

Secondly, liqued metals (normally sodium, lead or NaK) are used in fast breeder reactors. Fast breeder reactors perform fission of Uranium-238 by fast Neutrons. Uranium-238 is not normally a suitable fuel for a nuclear feactor as i isn't normally a fissile material, however in the Fast breeder reactor the Uranium-238 is bombarded by fast, high energy neutrons. The Uranium-238 absorbs the neutrons, decays and becomes fissle Plutonium-239. This process would not work if the neutrons were low energy like used in a normal reactor. Liqued metals are used in these reactors as they do not impede the movement of the fast neutrons in the way that a moderator would, they also act as an efficient transfer of heat and do not need to be pressurized.

Hope that Helps,

_LAWE_

Though even in a non-breeder reactor, a deal of the energy (about a third) actually comes from burning produced plutonium.
However, a fast breeder reactor *can* burn plutonium isotopes that other reactors can't, and can also produce plutonium from U238 faster than it burns it.

Well, that is really cool that liquid metals are used!

Tsk. You'd think no-one had ever seen Terminator 2.