In the nuclearreactor we use heavy water as the moderater. Is there any alternative for the moderaters???

Regular water and carbon are two common moderators. Heavy water is used for natural uranium reactors, owing to its low absorption cross-section. Once you've enriched the uranium a few percent, you can tolerate some neutron loss and don't need the added expense of using heavy water.

Out of curiosity, I know that certain "poisons" build up in the water used in reactors... Does heavy water have a greater resistance to poison build up?

It's a matter of deffinition I think.

The "poisons" are nuclei with high netron capture cross sections. They do that whether they are dissolved inlight or heavy water or, for that matter , if they are in the gas phase or anything else.

On the other hand, because the capture cross section for heavy water is less than that for light water, you can get away with a slightly higher concentration of "poisons" in heavy water.

It's a matter of deffinition I think.

The "poisons" are nuclei with high netron capture cross sections. They do that whether they are dissolved inlight or heavy water or, for that matter , if they are in the gas phase or anything else.

On the other hand, because the capture cross section for heavy water is less than that for light water, you can get away with a slightly higher concentration of "poisons" in heavy water.

 

 

Right. You have a smaller loss due to capture in water, so you can tolerate a larger loss elsewhere. Normally, this is capture in U-238. Which is why heavy water is used in reactors trying to produce Pu-239, which is two short beta decays away once the U-238 has captured a neutron.

 

Out of curiosity, I know that certain "poisons" build up in the water used in reactors... Does heavy water have a greater resistance to poison build up?

 

Not particularly. Poisons as produced by fission are going to be essentially the same, AFAIK. And the poisons are often in the fuel elements, since they are produced from the fission.. "Burnable" poisons are sometimes put in place to suppress the neutron flux and spatially even out the fission rate, which extends the life of the core.

Right on. Thanks for the clarification guys.