Kirk Sorensen’s note: I’d like to introduce my friend and colleague Kirk Dorius to the Energy from Thorium community. Kirk Dorius is a mechanical engineer and intellectual property attorney. I welcome his insights into the technology that underpins today’s solid-fueled uranium reactors.
A typical solid nuclear fuel rod includes a zirconium alloy tube or “cladding” encasing a single column of uranium fuel pellets. The cladding tube is smaller in diameter than your index finger, and is about 14 feet long. The uranium pellets are each about the size of the tip or your pinky finger, with the energy equivalent of 17000 cubic feet of natural gas, 1780 pounds of coal or 3.5 barrels of oil. The pellets are stacked in the tube with allowance for pellet expansion during fission and heating of the uranium. Once the uranium pellets are loaded into the cladding tube, zirconium end caps are welded in place to form a complete loaded fuel “rod.”
The fuel rods are then arranged in “bundles” or “fuel rod assemblies”, e.g., 14×14 or 17×17 arrays, which are then inserted into the core with a number of control rods being retractable from the bundle to initiate fission and insertable into the bundle to stop fission. Many rod bundles are oriented vertically in the reactor core with a substantial flow of water passing upward through the bundles to convey the fission reaction heat to a steam turbine for generation of electricity.
The zirconium cladding serves to hermetically isolate the uranium pellets and accumulated fission byproducts from exposure to the water flow in the core or cooling tank or to the atmosphere. The thin-walled cladding is transparent to radiation but is naturally affected by the high heat stresses and heat loading in the core. The rods are preemptively retired after a finite core cycle of several years to maintain cladding integrity even though only a very small fraction of the uranium is “spent.” This finite core cycle is also limited by accumulation of fission byproducts, particularly nuetron absorbers, inside the fuel rod.
A retired or spent nuclear fuel (“SNF”) rod is placed in a water cooling tank for an initial cool-down period during which the more highly radioactive (shorter half-life) isotopes rapidly decay. During this period, the rapid decay still generates substantial decay radiation and heat, albeit only a small fraction of the fission radiation and heat that is generated during reactor operation. After this initial cool-down period, the slower decay of the remaining longer-half-life isotopes generates a moderate amount of decay radiation and heat, which is readily absorbed by a concrete “dry cask” during long-term storage.
A typical nuclear plant can have hundreds of active fuel rod bundles in each core, thousands of SNF rods in short-term cool-down tanks and fuel from tens of thousands of SNF rods in long-term dry cask storage. The cooling tanks at the compromised Fukushima Daiichi nuclear plant collectively house around 11,000 SNF rods with a portion of those housed in the cooling tanks above reactors 1-4.
Water in the cool-down tanks acts as a neutron moderator, radiation shield and coolant, so long as the water level around the rods in the tank is maintained. If the SNF rods are left exposed and uncooled long enough, rapid oxidation (often called “burning”) and extreme heat stress can eventually compromise the cladding, expose the uranium, generate hydrogen, and release fission byproducts. Unmoderated and uncooled SNF rods can produce sufficient radiation and heat that even brief close proximity worker exposure is unacceptable. Should the cooling tank levels drop too low for too long, it could be challenging to restore the cooling tank water levels from a safe distance.
Hopefully, the cooling tank water levels at the Fukushima Daiichi nuclear plant will be restored and the situation stabilized soon.