Imagine you have a toy that goes round and round in a circle. Now, imagine that instead of a toy, there is a huge machine that makes electricity but instead of going round and round, it makes a lot of heat. That machine is called a reactor and it is used in nuclear power plants to generate electricity.
One day, a long time ago, in a place called Three Mile Island in Pennsylvania, something went wrong with the reactor. It was like a toy that started to spin too fast and it got very hot very quickly. The people who were working in the power plant tried their best to stop it, but it was very difficult because they didn't know exactly what was happening.
The heat from the reactor was so strong that it broke some parts inside the machine and it started to release some gases that were not safe for people to breathe. The people in the power plant knew that they needed to get people who lived nearby to leave their homes as soon as possible.
In the end, the people who worked in the power plant were able to stop the reactor from getting any hotter and it stopped releasing the dangerous gases. But, some people got very sick because they breathed in the gases and they needed to go to the hospital.
The accident at Three Mile Island was a very scary event, but it taught us a lot about how to keep people safe when working with the reactors in nuclear power plants. Nowadays, people who work in those places know exactly what to do if something goes wrong and they are ready to act fast to protect everyone who lives nearby.