Moderators, Neutrons, and Enrichment, oh my!

I want to discuss a few other reactor designs besides the typical US power reactors, the PWR and BWR types.  Before I do that, it's important to understand a few things that force us down certain paths in designing reactor cores. 

We need to understand the relative abundances of U-235 and U-238, why we use neutrons to split atoms, and also how a moderator works, in a physical sense.

First off, about Uranium.  According to Wikipedia, naturally occurring Uranium consists of three major isotopes: U-238 (99.28%), U-235 (0.71%), and U-234 (0.0054%).  The stuff we need for fission (U-235) is less than 1% of naturally occurring Uranium.

There is an entire industry based around the enrichment of Uranium.  The process of enrichment is as fascinating as it is tedious.  I might do a post on that process at some point, but for now it will suffice to point out that it is difficult to sustain a fission chain reaction using only naturally occurring Uranium.  For this reason, in most reactors, the concentration of U-235 in U-238 is increased, or enriched.

A fission chain reaction must be accomplished by freeing up large numbers of neutrons from the nucleus of a parent atom.  Why neutrons?  A couple of reasons.  Since neutrons have no charge, they are not repelled by the positive charge of a nucleus.  Thus they can wander at will through any part of an atom.  Neutrons can also pass through many materials other than Uranium, as if that material weren't there.  Lastly, since fission produces additional neutrons, it can become self-sustaining.

When a fission occurs, 2-3 neutrons are ejected, but they are moving at 2MeV, which is about 20% of light speed.  At this speed, they CAN interact with another Uranium nucleus and cause a fission, but this is unlikely.  In order to improve the likelihood of a fission, there are two things we can do:  Increase the number of U-235 atoms in the core, or moderate (slow down) the neutrons. 

In practice, both methods (moderation and enrichment) are usually used.  Reactors using no moderator, and containing a core of highly enriched U-235 have been built and operated.  Since there is no moderator slowing the neutrons down, these type of reactors are called "Fast Neutron Reactors". 

The vast majority of reactors are "Thermal Neutron Reactors".  The reasons most reactors use thermal neutrons are two-fold.  Fast Reactors require fuel enriched above 20%, which can potentially be diverted and used for a nuclear weapon.  Additionally Fast Reactors have poor safety characteristics from a reactor physics standpoint, and not surprisingly, have an abysmal safety record.  Of the handful of Fast Reactors built, several have suffered meltdowns.

More on moderators though.  The purpose of moderation is to slow neutrons down to the point where they are at thermal equilibrium with the surrounding material.  The way to accomplish this is by allowing them to impact (or "scatter") against atoms with low mass, and low neutron absorption.  If a neutron is absorbed by the moderator, then it is no longer available to split a Uranium atom.

Think of the neutron as a ping-pong ball fired from a cannon.  To remove the most energy by collision, ideally it should run into another ping-pong ball, which will recoil and remove some speed.  If it collides with a heavy nucleus, like steel, it would be like our ping pong ball bounced off a boulder.  It would not lose as much energy to the boulder.  

Water is a decent, but not great, moderator.  It has two hydrogen atoms for neutrons to scatter against, and it is reasonably dense.  The oxygen molecule has the ability to scatter a neutron, but it due to its mass (16) it would take many more oxygen atom collisions to accomplish the task.  Water is also plentiful and cheap.  This is why in most applications, water is used as a moderator.

Water though, is not an ideal neutron moderator, and here is why:  Both the hydrogen and oxygen atoms have a small, but noticeable possibility of absorbing a neutron, thus removing it from the process and making it unavailable to cause a fission. 

There are other moderators that are superior to light water from the standpoint of neutron absorption.  Even though a Fast neutron may require a larger number of collisions to reach thermal equilibrium, the lower likelihood of being absorbed make other materials superior moderators.

Moderator         Number of collisions      Likelihood of neutron absorption %
Hydrogen                   18                                             0.3326
Dueterium                  25                                             0.000519
Beryllium                   86                                             0.0076
Carbon                       114                                           0.0035
Oxygen                      150                                           0.00019
Uranium                    2152                                         7.57

Dueterium (in the form of heavy water) is probably the best moderator.  This is water with hydrogen atoms that already have one neutron and one proton.  Normal hydrogen atoms have just one proton, and occasionally capture one, taking it out of use for the reactor.  Dueterium hydrogen atoms have already absorbed a neutron, and are highly unlikely to absorb a second one.

Dueterium is found in nature, but like U-235, is not abundant.  It also needs to be enriched at great difficulty and expense to be used as a moderator.  However, it is such an efficient moderator that a reactor can be operated with Uranium fuel that has not been enriched.

Beryllium is another excellent moderator, with very little likelihood of absorbing a neutron.  Its disadvantages are expense and high toxicity.

Carbon is an excellent moderator, even with the large number of collisions required, its low neutron absorption, and low cost make it a practical moderator, even if it is not a good material to remove heat.  A graphite-moderated reactor can also sustain a chain reaction with Uranium that has not been enriched.

The other two items on the list, Oxygen and Uranium are not moderators at all.  They just show how increasing the mass of the target atom increases the number of collisions required to moderate a neutron.  However on the right hand column, notice the likelihood of absorption for Naturally Occurring Uranium.  It does love to vacuum up neutrons :)

In the next post, using what we have learned here, I will describe a few unusual reactors that have been built, either for testing purposes, or for breeding additional nuclear fuel, for nuclear weapons production, and how a nuclear weapon itself works.