slapp said:
Your response on the operating temperature of UO2 clears up a lot and makes a lot of sense. If I'm understanding it correctly the hottest the UO2 would be is 1400F (under normal, safe conditions) and the fuel that is stored in the fuel pool wouldn't be 5000F?
More or less correct, but I mentioned 1400°C, rather than 1400°F (~ 760°C). The 1400°C (~2550°F) is somewhat of a guide. A fuel designer would like to minimize the peak temperature, and the core designer designs a core to minimize the power peaking (energy generation per unit mass/volume) in order to minimize the peak temperature. Power peaking will shift in the fuel axially and radially as fuel is consumed (and enrichment is depleted). We set limits on the energy generation of the fuel such that we constrain the maximum operating temperature of the fuel, which also relates to release of fission gases (Xe and Kr isotopes) and rod internal pressure. We also prefer to limit the migration of chemically aggressive fission product species, e.g., Te and I, which would attack the cladding surrounding the fuel pellets.
At the end of a cycle, the reactor shuts downs, so the energy generation from fission stops (mostly), while decay heat continues, but which decreases with time as fission products decay. Short-half life radionuclides decay in seconds to minutes to hours, so the fuel is already cooling as the reactor cools. There is a residual heat removal system attached to the primary system that allows heat to be removed before any fuel is removed from the core.
Usually, the oldest fuel is removed from the core to the spent fuel pool, but some plants will do a full-core offload, which means all fuel is removed and the youngest fuel is temporarily stored in the spent fuel pool. The youngest fuel (having operated one or two cycles) is subsequently returned to the core with fresh (unirradiated) fuel for the next cycle of operation.
Note the part "At the moment of reactor shutdown the decay heat is about 6.5% [or more like 7%] of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it falls to 0.2%." Fuel assemblies located at the edge of the core operate 30% or less of core average power, while interior assemblies operate at or above core average power. In PWRs, some assemblies, usually those one row in from the core periphery operate with a substantial power gradient, whereby the innermost row of fuel rods operate as much as 30 to 40% above core average power (peaking factor ~1.3 to 1.4) while the outermost row of fuel rods operates about 30 to 40% below core average power (peaking factors ~0.7 to 0.6). Peaking factor is just a way to describe the local power level with respect to some reference level, such as core average power.
BWRs are more complicated because they employ control rods in the core, and so many assemblies will see strong power gradients when operating adjacent to a control rod. Such fuel assemblies experience strong axial power gradients in the fuel next to the top of the control rod, as well as radial (lateral) gradients across the fuel assembly. During BWR operation, the total amount of control rod volume decreases during the cycle, in other words, the length of control rod inserted decreases and the number of control rods used decreases. However, it's complicated since different groups of control rods are used periodically at different times during the cycle, and some are inserted further in (more deeply) while others are inserted less (more shallow). Deep and shallow control rods are then interchanged to balance the enrichment depletion (and thus energy generation) in the core. Toward the end of cycle in a BWR, the reactor reaches a condition known as All Rods Out (ARO), in which all control rods are withdrawn, and power distribution can be controlled with core flow (flow control), in which the water level is moved up or down depending on flow rate (more water means more moderation in the lower part of the fuel assembly). At this point, the reactor power is coasting down (gradually decreasing) sometimes down to 60% of rated power.
