Nuclear power plants provide about 17 percent of the world's electricity.Some countries depend more on nuclear power for electricity than others. In France, for instance, about 75 percent of the electricity is generated from nuclear power. There are more than 400 nuclear power plants around the world.
　　To build a nuclear reactor (核反應(yīng)堆),what you need is some mildly2 enriched uranium(鈾). Uranium is a fairly common element on Earth,incorporated3 into the planet during the planet's formation(形成). U-235 is one of the few materials that can undergo4 induced fission(誘發(fā)性核裂變). Raising and lowering the control rods(桿,棒條) allow operators to control the rate of the nuclear reaction.When an operator (操作員) wants the uranium core to produce more heat,the rods are raised out of the uranium bundle(捆,束). To create less heat, the rods are lowered into the uranium bundle.The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel. The uranium bundleactsasanextremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a steam turbine(汽輪機), which spins5 a generator(發(fā)電機) to produce power.
　　Well-constructed nuclear power plants have an important advantage when it comes to electrical power generation——they are extremely clean. Compared with a coalfired power plant, nuclear power plants are a dream come true from an environmental standpoint (立場,觀點). Unfortunately, there are significant problems with nuclear power plants. Mining and purifying(提純) uranium has not, historically, been a very clean process. Improperly func- tioning nuclear power plants can create big problems. The Chernobyl6 disaster is a good example.Spent fuel from nuclear power plants is toxic (有毒的) for centuries, and, as yet, there is no safe, permanent(永久的) storage facility for it.