Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions

Three mile island nuclear power plant

America’s worst accident at a civilian nuclear power plant occurred on March 28, 1979, at Three Mile Island. Operators were trying to repair a problem with piping in the water circulation system, and they accidentally blocked the flow of water. This stopped the removal of heat from the reactor, and began a series of events that would change the way America thought about nuclear power.

Within ten seconds after the flow of water was blocked, the reactor vessel became hotter and the pressure rose. Safety control rods were automatically thrust down into the reactor, causing the chain reaction to stop completely. Pressure, however, continued to build. A relief valve opened but failed to close. As a result, too much water escaped, and the reactor began to melt down.

[1] Large quantities of radioactivity began to leak from the reactor. [2] Most of the radioactivity was contained, and it is likely that no one received a harmful amount of radiation. [3] “Three Mile Island” became the reason that many Americans are afraid of nuclear power to this day. [4]

which word refers to the part of a nuclear reactor in which the nuclear reaction takes place?