So I take it the other half of the building doesn't pose a danger? Or are they deconstructing that later and then sealing things off? Or building another arch to confine the other half?

The primary goal of the NSC is to prevent the reactor complex from leaking radioactive material into the environment and the secondary goal is to allow a future partial demolition of the old structure.

To add to what @skinnydaddy1 has said, if you look at a satellite image you can see that only the part of the building covered by the new arch, housing reactor number 4, was destroyed in the 1986 explosion. There isn't any need to cover the intact reactor number 3 which was actually still used for another 14 years after the loss of reactor number 4:

https://goo.gl/maps/GxvACfnvtpM2

As the whole building was reinforced, shielded concrete, only the obliterated half needed containment. In fact, this is the permanent fix, as most of the damaged area was already contained, but not particularly long term.

A short 3 min recap of the Chernobyl story is here.


The Chernobyl Disaster: How It Happened