What started the 3 mile accident?

Answer 1

This initially started because the feedwater pumps in the secondary circuit stopped (for an unknown reason). This automatically shutdown the turbine and reactor, but due to the then lack of a heat sink, the reactor primary circuit started to pressurise, and the relief valve (PORV) opened (as designed). But the valve failed to reseat and this was not noticed by the operators. This led to continued loss of reactor coolant and eventually to partial melting of the reactor fuel. The complete sequence in more detail can be read in Wikipedia article 'Three Mile Island accident', which I recommend. It is not technically difficult to understand.

Answer 2

The Three Mile Island accident in 1979 was caused by a series of mechanical and human errors, compounded by design flaws in the reactor. It began with a malfunction in the plant's cooling system that led to a partial meltdown of the reactor core. The incident raised concerns about nuclear safety and led to increased regulations and safety measures in the nuclear industry.