Tested on two versions of IC2 2.0.5 and 2.0.6.

At 2.0.5 there are no problems with the reactor and the pump, the water comes in and it is enough in full.
On version 2.0.6, the steam drain limit has become -100000% and because of this, the reactor cannot work normally. The videos are attached below.

This version 2.0.6 - https://youtu.be/CuW-H5hC-ME
This version 2.0.5 - https://youtu.be/kVt1FjBwWL0