On the debate of Liquid Metal Reactor , I have found few more reason from other forum,
Thought It would be good to share

Thanks to Shankar From BRF:

extreme reactivity of sodium with water is always a clear and present danger that includes moisture in air that you breath out within the close confines of a sub .In a land based reactor it is protected by heavy argon gas more commonly called as cover gas which layers the liquid metal reactor and prevents the atmospheric moisture from getting in and reacting with hot liquid sodium. Prob lem is even the cover gas can get contaminated by nuclear reactio products like Xenon which are radioactive but have a short half life of about 7 days or so. In a land based reactor these gases can be held in cryogenic liquid form or adsorbed form till the radioactivity level is down to acceptable . Similar treatment of radio active off gases inside a submarine is diffcult and a further penalty on space and payload .

The russian alfa class used a different type of liquid metal coolant namely lead -bismuth which had to be kept at 125 celcius plus no matter what .The reactors had to be kept running alwys since providing power and steam from outside when in harbour to keep the liquid metal coolant operational was too trouble some .In fact one of the reactors was a lost case since the fuel rods got stuck in fozen liquid metal coolant .

A total of 8 alfa class liquid metal cooled fast reactor powered submarines were built and assigned to northern fleet out of which 7 are confirmed de commssioned and the remaining one may be still operational but not confirmed . Refueeling even by part is not possible with liquid metal cooled reactors and thats why they are one lift time fuelling type with a rated reactor life of 70 years .

K-27 a november class boat was modified to try out the newly developed lquid metal cooled reactors for alpha class . During sea trials reactor power was totally lost causing radioactive gases to leak into operating area .9 crews were killed . The submarine K-27 was removed from service .

All this with first generation liquid metal cooled reactor using lead and bismuth now we use in our fast breeder reactor liquid sodium which has much higher operating temperature and many times more reactive .