Is radiation poisoning always fatal?

No.

It depends on the exposure and the amount in a dose; large doses and huge exposure is lethal and is always fatal, however, smaller amounts are not entirely fatal.

For instance, there is an average radiation exposure for humans such as x-rays. It's typically a light dose and, of course, isn't fatal for the human as x-rays are fairly common and standard in the medical world.

Hope this helps!