I think one is to take away waste heat, the other im not sure about.

for the technicians to drink and to throw in the face of the safety inspecter when hes taking too long of a nap

water under high pressure is used to cool the reactor, then the water is pumped to an area under lower pressure where it can form steam and power a turbine

It's heated up and used to turn the turbines, hence creating electricity, and it slows down the neutrons (they are neutrons, right?) so they don't pass straight through the next uranium rod

"Conventional thermal power plants all have a fuel source to provide heat. Examples are gas, coal, or oil. For a nuclear power plant, this heat is provided by nuclear fission inside the nuclear reactor. When a relatively large fissile atomic nucleus is struck by a neutron it forms two or more smaller nuclei as fission products, releasing energy and neutrons in a process called nuclear fission. The neutrons then trigger further fission, and so on. When this nuclear chain reaction is controlled, the energy released can be used to heat water, produce steam and drive a turbine that generates electricity." "The chain reaction is controlled through the use of materials that absorb and moderate neutrons. In uranium-fueled reactors, neutrons must be moderated (slowed down) because slow neutrons are more likely to cause fission when colliding with a uranium-235 nucleus. Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperatures if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate." "As with most power plants, two-thirds of the energy produced by a nuclear power plant goes into waste heat (see Carnot cycle), and that heat is carried away from the plant in the water (which remains uncontaminated by radioactivity). The emitted water either is sent into cooling towers where it goes up and is emitted as water droplets (literally a cloud) or is discharged into large bodies of water - cooling ponds, lakes, rivers, or oceans." Source and further information: http://en.wikipedia.org/wiki/Nuclear_power_plant

Two roles? How about SIX? 1) Water is used to moderate the uranium decay reaction in a Light Water Reactor. 2) (D2O) Water is used to boost the reaction in a Heavy Water Reactor. 3) Water is used to maintain temperature control of the reactor (cooling). 4) Water is used to transfer heat from the reaction to the electricity generating turbines (in the form of steam). 5) Sea water, also used for cooling, can be desalinated by the heat of the reactor, and delivered in the form of clean, fresh water for human consumption. 6) Water is used for cooling, and to contain radiation in pools of spent fuel rods.

Water performs a variety of roles in a nuclear power plant, depending on the design of the reactor and the system using the water. Water is an effective shielding agent. Therefore it is used in shield tanks around reactor vessels to minimize neutron radiation exposure and minimize neutron activation of structural components in the reactor compartment. Water is a cooling agent for the core, used to remove decay heat when the reactor is shut down. It also transports heat produced at power from the core to the steam generators. Water may be a moderating agent for neutrons, acting to reduce neutron energies low enough to allow the neutrons to be absorbed by thermal fuels in the core. Water is NOT always used as a moderator, however. Some reactors, for example, are graphite moderated. Borated water may also be used to chemically control the fission rate of a reactor, even chemically shut down the reactor under certain conditions. Water may be used to produce steam to drive turbines in one of two ways: 1. Directly, as with a Boiling Water Reactor (BWR) which used primary coolant itself to produce the steam to drive the turbines. 2. Indirectly, as with a Pressurized Water Reactor (PWR), which uses primary coolant water to transfer heat to a steam generator, which in turn heats the secondary water in the steam generator to produce steam to power the turbines. Water is also used to cool the low energy steam exhausted from the turbines to condense it back into water so it can be pumped back into the steam generators. Water is used in a two-phase pressurizing system to generate the high pressure required to keep the primary coolant water from boiling in PWR's. Water is used in the charging systems to maintain proper water levels in the primary system. Water is used in the emergency core cooling and injection systems to keep the core covered and remove decay heat in the event of a loss of coolant casualty. Water is used to cool various components in the plant, like reactor coolant pumps, generators, lube oil coolers, and so forth. Water is used to lubricate the bearings of sealed reactor coolant pumps. Water is used to mix chemicals for injection into the primary system. Water is used to mix chemicals for addition into the steam generators. Water is used in the storage pools for shielding and cooling of spent fuel assemblies. There are more uses as well. But I suspect the PRIMARY uses for water that you may have been asking for were with respect to the reactor itself. This is to transfer heat from the core and, for water moderated cores, to moderate the neutrons as described above.