Even if you assumed 50% were inoperable the amount of explosive that would rain on Seoul is insane.
But this is the insanity I don't understand. Why do the Sokors insist on locating their largest city within artillery range of a country it's technically still at war? The government could have easily built, especially during SK's period of rapid growth, a new capital on its southern edge, closer to Japan. If not that, it would have been a rather simple matter, converting some of the other cities into a government center.
Given its central location in the Korean peninsula, Seoul would make an excellent capital for a united Korea, but as it stands, it's a huge security risk and magnet for attack with a mere minutes' advance warning should Kim decide to launch blitzkreig.