What could propel a 500,000-ton battleship to 42 knots

What power plant could drive a hypothetical 500,000-ton battleship at 42 knots?

A nuclear plant is the most plausible option; an oil plant would be enormous and a gas-turbine solution would require many turbines.

The proposed 500,000-ton Japanese battleship is an impossible construction, even by modern standards. Taking its 42-knot requirement as a propulsion thought experiment, nuclear power is the most plausible answer. Oil-fired machinery would require an enormous plant and fuel burden. A gas-turbine solution might provide enough power density, but only with a very large number of turbines. The spectacular speed claim quickly becomes a question of machinery mass and power-to-weight ratio.