Why GRAPHALLOY bushing are used in pumps of Nuclear Power plant? Is it because of high temperature or what?

Answer 1

Graphalloy is a self lubricating material used in many pumps where there is the potential of contact between rotating parts and stationary parts usually due to lack of lubrication. Since Graphalloy will not gall, designers can also use Graphalloy to reduce operating clearances. This can reduce vibration.

Examples are pumps which might have insufficient lubrication are pumps that run out of pumpage (tank unloading pumps), pumps that pump liquids that don't have much lubricating ability (light liquid gases or hot water), and pumps where the pumpage is close to the vapor pressure. In all these cases, a pump properly fitted with Graphalloy can survive a loss of pumpage with minimal damage and continue operating while a pump with standard construction might seize or self-destruct.

Nuclear plants employ Graphalloy for those pumps that they fear might have metal to metal contact as a result of loss of pumpage or due to vibration (seismic for example) but which must be able to survive and continue pumping when liquid is returned or vibration stops.

Use of Graphalloy material gives the designer a comfort level that the pump can survive and do its job even major upsets occur. Seemanufacturere web site for more info www.graphalloy.com\html\pump.html .

Answer 2

Reliability, for one. Ball bearings require maintenance and periodic lubrication.

Another guess would be the radiation itself breaking down some materials in grease or in bearing seals.

It could be high temperatures. Typically, a coolant (molten salt of some kind) is run through a heat exchanger to actually generate steam.