QUOTE(Sebastian Balos @ Thu 29 Oct 2009 0608)

We don't know anything about their nuclear "fuel" level, or perhaps Typhoons were near their end of their service life, since Deltas were used mainly from waters that are closer to USSR/Russia. Refurbishment of older subs may be a tricky business, pretty expensive and long lasting, so they decided to go for a new subs.
I see these new subs as the more economic replacement for other subs.
The problem with the Typhoon is that the R-39 missiles exceeded their service life years ago, the last ones were disposed of by 2004, while attempts to produce an improved variant met with failure. IIRC the USSR breakup separated many of the contractors and organizations involved in R-39 production, but don't quote me on this point. Regardless of the exact details this has left the Typhoon without missiles making them unable to perform their mission and three units of the class have been scrapped. Still one of them, Dmitriy Donskoy, has been refurbished and refitted to carry the new Bulava missile, while Arkhangelsk and Severstal are being kept in caretaker status and might follow in due time.
While the Typhoon are relatively quiet something better than them (and the Delta) incorporating the latest advancements was badly needed in order to mantain a survivable deterrent.