Your math is ballpark correct but you're using globalist units :) The large nuclear units evaporate about 8 million lbm/hr of water at around 1000 psia. Enthalpy of evaporation at that pressure is 650.4 Btu/lbm, so the unit is using 5,203,200,000 Btu/hr to evaporate that water. Now coal has varying heating values depending on the grade. Hard anthracite is around 13,700 Btu/lbm. Bituminous is around 14,300 (higher because it has more volatiles - flammable gases). Lignite (which the Krauts love to burn) is only 7200 due to all the water in it. Let's be generous and use the highest value - bituminous - which will result in the smallest weight of coal to burn. This comes out to 363,900 lbm/hr or 101 lbm/sec. This big difference is due to your assumption of 4.1 MJ/lb. It's off by a factor of three as far as good U.S. coal is concerned (4.1 MJ/lb --> 3856 Btu/lb). But to the main point, now determine the volume of coal for 18 months of operation. Won't fit in containment :)