為何放射性的影響在身體內有積聚作用?

I suppose you refer to "radiation exposure", not on the inake of radioactive substances into the body.

Scientifically, it is more correct to say that the incurred "radiation risk" has a cumulative effect. That is to say, if you tak two chest x-rays, the risk to your body will be double to that if you only take a single chest x-ray. The risk referred here is the long-term health risk, such as the induction of cancer.

The reason is straight forward. If an amount of radiation causes harm to a certain number of body cells, doubling the amount of radiation would casue double the number of cells to be affected, thus doubling the harm.

This knowledge actually comes from detailed health analysis from Japanese atomic bomb survivors after the Second World War. From the analysis results so far, scientists still believe in the so call "linear non-threshold dose-risk relationship", which in simple terms, is just "doubling the exposure leads to doubling the long term health risk".