Norsk Hydro and heavy water - what was the perceived threat? Through various raids and acts of sabotage during WWII, the Allies succeeded in preventing Germany from coming into possession of large quantities of heavy water produced by the Norsk Hydro plant in Vemork Norway.  These actions were carried out at considerable expense in terms of Allied troops and civilian casualties, presumably because the heavy water was perceived as a providing Germany with a substantial "leg up" in producing an atomic bomb.
But from my (admittedly limited) knowledge of the construction of Fat Man and Little Boy, heavy water was not a factor (at least not in the final weapons).  Why was it believed that heavy water would make a German atomic bomb much more likely?
(The reason I ask is that I recently saw an old (1965) movie ("The Heroes of Telemark") about the sabotage operations.  While a bit too "Hollywood" (with Kirk Douglas and Richard Harris), it was apparently reasonably accurate.  And several years back I saw something on PBS about the sinking of the ferry carrying the heavy water.)
 A: Heavy water is an effective moderator for the production of $^{239}$Pu, which is a possible active ingredient of a fission bomb. The heavy water itself is not used in the final weapon.
The allies realized that this might work - and they decided to set back any attempt by the Nazis to create a plutonium based atom bomb by depriving them of the moderator material needed.
Starting with a mixture of $^{235}$U and $^{238}$U, the reactions are:


*

*fission of $^{235}$U generates neutrons

*Neutrons are slowed down ("moderated") by collisions with deuterium

*The slower neutrons are captured by $^{238}$U, creating $^{239}$U

*The $^{239}$U decays into $^{239}$Pu

*Lots of $^{239}$Pu becomes a key ingredient of an atom bomb. When it absorbs a neutron, it splits - in the process emitting more than one neutron on average. Result - chain reaction. Boom. Nazis win the war. Final solution. Bad news for lots of people.


But a series of attacks on the plant (and on the materials that were shipped out once the Germans realized that (a) the Allies cared way too much, so they must be on to something, and (b) they needed this capability in a more protected place) stopped this train from leaving the station. Good thing really.
See http://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage for more details.
A: I am not an expert on nuclear technology or weapons but the Wikipedia page on the plant and it's destruction provides some clues. 
Ultimately there were many potential methods that might be used to design a weapon. It was known at the start of the war that bombarding Uranium with neutrons resulted in nuclear fission which could be chained together. Heavy water was one way to slow, or moderate, the free neutrons (graphite rods are another option, which is what is used currently in nuclear reactors). 
So while the heavy water would not be used in the final weapon, it was to be used to refine the Plutonium that went into the final weapon. The water could be used as a neutron moderator which would enable the production of weapons-grade Plutonium, which would then be turned into bombs. 
