[Simon Gunson]

Beryllium is a neutron reflector... not an absorber. Beryllium is used to reflect neutrons back into the core of an H-bomb so it might be a scary thing to have inside a reactor.

Also Flinguist Boron carbide pellets are spaced inside zirconium alloy tubes between pellets of uranium in modern reactors. As the uranium gets too hot the Boron pellets expand pushing the uranium pellets apart and thus shutting down reaction.

This is a new fail safe aspect of fuel rods which was not available in the 70s when Three Mile Island had it's problems.

[jdurg]

Beryllium is a neutron reflector... not an absorber. Beryllium is used to reflect neutrons back into the core of an H-bomb so it might be a scary thing to have inside a reactor.

Also Flinguist Boron carbide pellets are spaced inside zirconium alloy tubes between pellets of uranium in modern reactors. As the uranium gets too hot the Boron pellets expand pushing the uranium pellets apart and thus shutting down reaction.

This is a new fail safe aspect of fuel rods which was not available in the 70s when Three Mile Island had it's problems.

You are not quite correct there.  The presence of Beryllium inside nuclear weapons is to generate a massive flux of neutrons for when the subcritical masses become critical.  Be absorbs ALPHA PARTICLES and emits neutrons.  It is NOT a neutron reflector.  It is present inside the core of nuclear weapons, along with Polonium which is a powerful alpha emitter, so that when the subcritical masses become supercritical there will be a massive number of neutrons available to start the chain reaction going.

U-238 is used as a neutron reflector and not beryllium.  The U-238 tamper that surrounds the core will reflect neutrons back into the "reaction chamber".

[pantone159]

According to John Emsley, "Nature's Building Blocks",

"Beryllium has a rather unusual property: it does not absorb neutrons and even reflects them, and for this reason it is used in nuclear weapons and the nuclear energy industry.  In a nuclear warhead, which relies on neutron bombardment releasing energy from uranium, a casing of beryllium ensures a higher neutron flux within the bomb."

[jdurg]

According to John Emsley, "Nature's Building Blocks",

"Beryllium has a rather unusual property: it does not absorb neutrons and even reflects them, and for this reason it is used in nuclear weapons and the nuclear energy industry.  In a nuclear warhead, which relies on neutron bombardment releasing energy from uranium, a casing of beryllium ensures a higher neutron flux within the bomb."

Hmmm.  Well I'll be damned.  Learn something new every day.    I had only heard of Be being used in a nuclear weapon as a source of neutrons due to the cost of using it for any other material.  

[ddaubert]

It was touched upon in several replys, so I thought I would sum it up.  Hafnium is main material in control rods.  Boron carbide pellets are interspersed in the reactor tubes to help moderate the reaction, especially in areas of high flux.

does anybody know the composition of the zirconium alloy use for radioactivity shielding in nuclear reactors?

The only materials I have experienced as being radioactivity shielding are lead, Polypropylene, water and distance  the further away you are the better!

As for the zirconium alloy composition, I have never really heard of it referred to as anything but zirconium alloy.  Sorry I couldn't be of more help.

Don

[AndyHolland]

Material for control rods for thermal reactors (LWRs) consists of B4C and Hafmium. Both are thermal absorbers - that is they absorb low energy neutrons in reactors that maintain criticality using thermal neutrons. Hafnium control rods have had swelling problems in commercial reactors.

Berillyium is a moderating material and makes the nuclear reaction go faster. In an LWR, it would displace the water and slow down the reaction owing to its high atomic mass - however, it would not be used as a control rod.

I believe stainless steel was used for fast reactor control rods which use fast neutrons (thermal neutrons are slowed down with a moderator, like Be, C, Heavy Water D2O, ZrD2 ZrH2 and water H2o).

BeO is an excellent moderator and has been used in India for example. It was studied extensively in the old days in the US.

Alpha emission in Be is actually a good thing because alpha-N neutrons allow for a safer startup, providing a better source of neutrons to detect approach to critical. It is bad however from a transportation standpoint if you load it close to Plutonium for example - however, alphas are easily shielded.

Zirconium is a cladding material used to protect fuel. It can be mixed with fuel to in Uranium - Zirconium allows, or Zirconium Hydride or Zirconium Deuteride can be used as solid moderators (spaceship applications). ZrH2 is lower density than H2O so is normally not used otherwise (why not just use water).

[jdurg]

Beryllium is most definitely NOT a heavy atomic mass substance.  Be's atomic mass is about 9 grams per mole which is HALF that of water.

Zirconium is the metal used in a reactor because it is completey transparent to neutrons and is able to withstand the conditions within the reactor.  The problem is trying to separate the completely transparent Zr from the neutron blocking Hf which is almost chemically identical.

[AndyHolland]

Certainly Be is not massive, however for a moderator - compared to Hydrogen, it most certainly is.

Be Atomic mass of 9 is 9 times higher than that of Hydrogen - which is why its moderating power is much lower. As a moderator, it is massive compared to Hydrogen and Deuterium. Hydrogen has a high thermal absorption cross section so that enriched cores are required.

A Be moderated reactor is therefore physically larger and neutronically smaller than a water moderated core. As a result, inserting Be or BeO in an H2O moderated core actually slows down the nuclear reaction.*

Removal of Hafmium from Zr is done on a large scale in a Westinghouse facility in Utah, and Zr is used to clad nuclear fuel in PWRs and BWRs.

--------------------------------------------
*When a neutron of atomic mass 1 hits a hydrogen proton of atomic mass 1, it can loose all of its energy in a single collision, thermallizing with just one collision. When a neurtron of atomic mass 1 hits a Be nucleus of atomic mass 9, it looses far less energy than a collision with hydrogen. So a neutron must go through more scatters, and traverse a larger physical distance to slow down. This makes the core neutronically larger than a water moderated core.

[jdurg]

Okay, that's a better explanation.  In your original post you talked about Beryllium displacing water and being more massive.  In comparison to hydrogen, yes it is more massive, but your original post was comparing it to water for which it is not more massive.