From: Paul Dietz <dietz@interaccess.com>
Newsgroups: sci.space.policy
Subject: Re: Moon base
Date: Sun, 28 Jun 1998 13:27:33 -0500

Michael Pelletier wrote:

> It accomplishes this by using heavy water in the primary coolant loop,
> ie, di-deuterium monoxide.  Heavy water is a far, far better reflector
> of neutrons -- normal water is apt to absorb or steal significant
> momentum from a neutron, which prevents that neutron from initiating
> further fission events.  However, neutrons bounce off heavy water
> molecules without losing very much momentum, and so can fission more
> atoms of uranium before petering out or escaping from the core.


Whoa.  Losing momentum does not prevent a neutron from causing
further fission events.  There is no neutron energy threshold
for fission in U235.  Even ultracold neutrons (cooled to liquid
helium temperature) will cause fission.

Heavy water moderated reactors, like most thermal reactors,
moderate most of the neutrons to below the U238 absorption resonance
before the neutrons diffuse back into the fuel elements.  "Fast
fission" is a minor effect in thermal reactors, mostly involving
neutrons that have not scattered at all, interacting in the fuel
element where they were produced.

The actual reason for heavy water's advantage is its much lower
absorption cross section.  The thermal neutron absorption mean free
path is heavy water is 134 meters, vs. 0.52 meters in light water.
So fewer neutrons are lost in the moderator, and the lattice
spacing can be made larger to soften the neutron spectrum and
reduce resonance absorption.

	Paul