CANDU

--- Bill wrote:
> >
> > Silicon moderator probably wouldn't be very good as the best
> moderators are
> > elements with a low atomic mass. Silicon would probably be just too
> heavy to
> > work well. Even if it does work the reactor is going to be very
> large.
>
> Actually silicon is surprisingly a very good moderator. The problem is
> that uranium silicide is a liquid at about 600 C. Most reactors a
> running at about a 1000C.

That is interesting. I wonder why it would be like that. As for liquid
reactors, I've been thinking about having Uranium metal in liquid form with a
graphite moderator. The uranium would pass through pipes made of Boron or
Hafnium when they are going to the heat exchangers. The Uranium would of
cousre stay liquid. I wonder if such a setup would work (would probably use W
walls).

>
> > I have heard that it may be possible to use Magnesium Flouride as a
> moderator
>
> I was actually testing MgF2 as a moderator at University a lifetime
> ago. It was a project that we had. It didn't do very well. Alpha rays
> made it brittle.

In which case we shouldn't be using it.

> could use
> > Zirconium Deutride instead to allow Natural U to be used.
> >
> Interesting idea but the main problem in CANDUs at the moment is
> zircomium-hydride cracking of the tubes. The uranium is contained in
> Zr tubes. The heavy water at high pressure and temperature produces
> zircomium hydride that crack the tubes.

I've never heard of the ZrH2 problem with CANDUs and would also expect it to
appear in LWR reactors (most of them use Zr as the cladding material). Anyway
I'm thinking about using the ZrD2 as a solid lump of moderator (they
investigated ZrH2 for use as an NTR moderator).

> > I have also been thinking about using a *gasp* plastic moderator
> (would still
> > require the enrichment), but the low melting point and radiation
> damage
> > problems plastics have would make that a bad idea.
>
> I think the key hear is to use it as a moderator not as a support
> structure. Imagine plastic pellets packed with uranium pellets. Maybe
> it would work.

It might work, although there is likely to be quite a bit of outgassing and
decomposition while the reactor is operating.