Nuclear Reactor, Moderator confusion?!?

Question

Okay, im doing my Physics assignment on Pressurized Water Reactors and came across the statement:

"The use of water as a moderator is an important safety feature of Pressurized Water Reactors as any increase in temperature causes the water to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hence reducing the reactivity in the reactor. Therefore, if reactivity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat. "

I thought the reduced moderation of neutrons would speed the chain reaction up, since wont the radioactive material react more quickly if the neutrons are moving quicker?

Please Help!!

Answer 1

The reactor needs slow neutrons to work properly. Quick neutrons tend to escape before reacting with Uranium.

Answer 2

In general the faster a neutron is moving the less likely it will cause a nuclear reaction with any particular atom. You can think of it like this, the faster the neutron is traveling the shorter the interaction time between the atom and the neutron, and thus the less likely the this interaction will be able to produce a nuclear reaction.

What really is going on is too complex to described here, but what I said above is a good starting point.

Answer 3

we require slow or thermal neutrons for effective transfer of momentum. Another thing is that it  requires time for efficient collission. Hence fast neutrons are undesirable.

Answer 4

No the neutrons must bounce off of the water molecules to slow down enough to be absorbed by the radioactive material.  This is known as the thermal attenuation factor and is inversely propertional to the reactivity rate.  The ratio of fast to thermal neutrons is controlled by the reactor temperature. As temperature goes up you get less thermal neutrons and reactivity goes down producing less heat which allows more neutrons to slow down. It is a continuous balance.

Answer 5

These are good answers I just want to point out that.  The speed of the neutrons depends on the "fuel" (fissile material) for the fission reactions.

Most reactors are thermal neutrons (thermal in this context means neutrons with speeds of 2.2 km/s) due to the U-235 fuel.  When a fission occurs, the neutrons released are fast neutrons, so we need a moderator (something as light as the neutron to do it in the fewest collisions possible) to slow it down to 2.2 km/s.  

By decreasing the water's density through increasing the temperature, you reduce this moderation and therefore the reduction of thermal fissions.  This is called a negative temperature coefficient.  Some reactors have a positive temperature coefficient such as the former Soviet Union's RBMK reactors, the most famous of which was the one that had a meltdown in Chernobyl.

Some reactors don't have moderation at all, those are called fast reactors, b/c fast neutrons are used to make the fission.  But those reactors are a proliferation risk (meaning it can make weapons grade nuclear materials)