what are PWR and BWR nuclear reactors ?

Answer 1

Both reactors use steam to spin the turbines which generate electricity. the difference of the two is that in a Boiling Water Reactor (BWR) the water is in direct contact with the enriched uranium fuel in one contained vessel making steam. In a Pressurized Water Reactor (PWR) the heated water goes into 3 tubes which are in contact with another container of water, or the Steam Generator, there the heat transfer is made turning the water, that is not contained in the reactor vessel, into steam which is not only uncontaminated but also what spins the turbines. Chernobyl was a BWR type reactor.

Answer 2

PWR are pressurized water reactors. The primary coolant system is pressurized (usually to a very high pressure). This means that a) the PCS is a closed system (you can't have high pressure with out it) and b) because of the high pressure, the water remains a liquid even when heated to a high temperature. They have several advantages including safety because the primary coolant water is always contained.
BWR are boiling water reactors. The water in the reactor is allowed to boil, i.e. turn from a liquid to a gas. This means they are always at atmospheric pressure. BWR's may or may not be closed systems. Typically, they are not closed systems.

Chernobyl was not a closed system. Once the roof was removed (blown off), the reactor was exposed to open air. Allowing not only radiation waves out but also radioactive particles, which are the primary danger in SPREADING radiation.

Answer 3

In a PWR - pressurized water reactor- system you have a primary coolant system that circulates through the reactor core. The water absorbs the heat released by fusion of the nuclear fuel, and then passes into a heat exchanger. In this heat exchanger the hot primary water is used to boil the water on the secondary side of the heat exchanger system. This secondary steam is then used to drive steam turbines for producing electricity.

In a BWR or Boiling Water Reactor, there is only one system. The water that goes through the reactor core is heated to boiling temperature and the steam is used directly to turn the steam turbine for producing electricity.

In general a pressurized water reactor is cleaner, in that the contaminated primary coolant is contained in a closes system. Whereas in a boiling water reactor system, the contaminated steam travels throughout the entire steam system. This is a much larger system and prone to more minor leaks.