It's already in need of replacement, apparently it was assessed some years ago and found to be weakening.
This is lifted from the wiki article at
http://en.wikipedia.org/wiki/Chernobyl_disaster
"A New Safe Confinement was to have been built by the end of 2005; however, it has suffered ongoing delays and is currently expected to be completed in 2012. The structure will be built adjacent to the existing shelter and then will be slid into place on rails. It is to be a metal arch 105 meters (344.4 feet) high and spanning 257 meters (842.9 feet), to cover both unit 4 and the hastily built 1986 structure."