By the time they will have S-600 in service...
I suspect the S-600 will replace Rocket propulsion with rocket/scramjet propulsion for much greater range and much higher flight speed for interceptions inside the atmosphere... rocket for outside of course.
So the 5th Kirov was cancelled and the CVN was cancelled. There is probably a good chance there is a reactor available that was made for these projects. Since the ship will be having major surgery anyway, maybe they will install a new reactor
There was another ship that used the same hull... it was huge and it was fitted with radar... it was either a spy ship or a space tracking ship... I seem to think it was only scrapped in the last few years... it was too expensive to operate for what it was...
The reality is that they have some very large ship available to them. It is going to be a while before they can afford to build brand new very large ships so for the moment these vessels are an opportunity to keep a long range ship capacity.
They don't have to be super ships with thousands of missiles, because they don't intend to keep them for the next 100 years.
They are not cheap... no big ship will ever be cheap.
Scrap them and you have nothing.
With an upgrade you get a ship able to travel anywhere in the world with decent self defence capacity on its own... remember cruiser sized ships are ships big enough to defend themselves and other ships around them simply because they have air defence missiles and guns in sufficient quantity and type to form a decent IADS on their own. Any other ship operating with them can coordinate their systems and sensors and weapons and make defence even stronger.
This is an opportunity to test new hardware that can be tested in labs but testing in labs is not the same as having it go to sea on a large ship.
Testing new propulsion and sensors and equipment and systems is hugely valuable and will make the new destroyers and new cruisers that they eventually build much better ships because they will be using tried and tested equipment that has been used together.
Imagine not finding out there were arrester gear problems on the K in Syria... imagine a confrontation at sea where it was found... that would have cost lives... which is why you test and why you exercise...
You're right, not Ok-650 but KN-3 reactors, BTW maybe you miss how much electrical energy consumes a combat station like that with all her sensors, hydraulics, electronics of her times, with very low efficiency for today standards, so it's not too wrong to assume that about half of her reactors thermal power was spent for electrical power generation (even less than 30% efficiency conversion).
And all new electrics... fiber optic cables for data transmission, and new electric drives that are rather more effective and efficient... replacing arm launchers and rotary internal launchers with fixed cell launchers... it all makes a difference...
But testing new reactor designs is worth it... even if they only try it on one ship... the purpose being more information and data for designing new cruisers and destroyers. If it works out really well they might consider applying it to the other Kirovs they are upgrading but even a big improvement might not be that important if it is expensive... it might be better to save the money and spend it on more frigates now and fast track the design of the all new destroyers and cruisers.
The point is that if they can test new reactors now on real ships their choices and decisions will have a more sound base in reality and they are less likely to cock the designs up.