Replace with uranium to exchange the pressure for heat. or plutonium for pressure AND heat, but adding a auto cool system would be good...
Or use polonium
it's hot enough to make steam when its molten , water from the steam condenser will cool it back down until the reaction starts up again to produce heat and melt it again. alternatively depleted polonium heats itself to 115C which is enough to boil water. High temperature polonium reactors just aren't a thing
How about isolating the POLO from the DEUT, and using the DEUT for heat? The polonium will be cooled, but the DEUT will still produce extra heat.