Core Fission Reactor Concept a Giant Expensive Water Boiler

 You probably have boiled water in a metal pot, pan, kettle, metal cup, well it turns out that if you zoom out & look at a basic or simple view of a nuclear fission reactor, it has a lot in common with a tube boiler, except the tubes in the nuclear reactor are not filled with burning propane, liquified natural gas, white gas, butane, gasoline, diesel, coal dust, or other combustable substance like wax, heat oil, heavy bunker oil, wood dust, peanut oil, biofuel, alternative fuel, in a nuclear reactor the tubes are filled with enriched uranium dioxide with about -20% Uranium 235 fissile content & mostly cheaper more abundant Uranium 238 which is fertile. 

Thorium is also fertile. Fertile elements can but hit by neutrons to become fissile or able to be used as nuclear fuels in a reactor. In the reactor core many fuel rods filled with pellets of ceramic sintered pellets of enriched uranium dioxide, for bundles, these fuel rods are laser welded closed, made of an iridium alloy of incredibly value as metal alloy, needed for its neutron resistance. Bundles of these iridium alloy laser welded closed tubes filled with ceramic pellets of uranium are assembled into a core stack of many bundles arrayed rectilinear. The core contains either many dozens or hundreds of these bundles of nuclear fuel rods. 

What happens when enriched uranium pellets are in close proximity their neutron emissions strike other nearby uranium. When the neutron of sufficient energy hits uranium, it causes fission by breaking the uranium into daughter products or smaller atoms. Thorium & uranium 238 can be applied around the core as a blanket that absorbs stray neutrons, thereby converting them into fissile isotopes like plutonium, that can then participate in the chain reaction of high neutron flux density in the core, which cause all the tubes to heat up & boil water. When the nuclear chain reaction maintained with enough neutron density of high energy neutrons the entire core becomes hotter than boiling water, & the metal tubes of the fuel rods & all the related surface area of those tubes acts like a giant tube boiler, in a boiling water reactor or pressurized building water reactor.