Spent Nuclear Fuel Questions

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Questions About Spent Nuclear Fuel and Spent Fuel Pools
Hi, I have some questions about nuclear fuel, I hope I could get some direction here.

Firstly, based on research I've done it appears that spent fuel rods come out of the reactor around 5000 F, is this true?
Secondly, if the previous statement is true how does the water in the fuel pool not get instantly turned to steam or am I missing something big here?
Finally, why is nuclear fuel discarded and cooled instead of being used as an alternate heat source? ie. being used to boil water since it still has a lot of heat coming off the fuel. Once again is this possible or am I missing something big here?

Fyi, I'm a sophomore in high school and I've never posted on here, I have no background in Nuclear, just a newfound passion, sorry for any mistakes I make, any feedback would be very helpful and welcome.
 
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  • #2
anuttarasammyak
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I do not know its temperature but I have heard engineers at reprocessing plant saying the wet surface of spent fuel dry immediately after pulled out of the water.
Pool water is cooled outside the pool and circulated to keep the water temperature constant.
Economy would decide whether generated heat would be used or not. Heat generation degrades by time. Safety measures preventing radioactive nuclei in spent fuel to leak to heat users and to pollute environment might be costly.
I am not a nuclear engineer. I hope this is certified/corrected by engineers onsite.
 
  • #3
Astronuc
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Summary:: Questions About Spent Nuclear Fuel and Spent Fuel Pools

Firstly, based on research I've done it appears that spent fuel rods come out of the reactor around 5000 F, is this true?
The temperature of 5000°F would be close to the melting point of UO2, and it is approximately 5070°F or ~2800°C. With burnup, i.e., with cumulative fission, the melting point decreases. However, we never allow the fuel to melt under normal operating conditions. Under normal operation, the maximum temperature (a the centerline of the ceramic pellets) in the fuel rod should be typically less than 2550°F (1400°C) and the surface temperature between 370 to 450°C, depending on the power level or heat flux.

Once a reactor shuts down, the fission process stops, so that part of the heat generation stops. However, there are a plethora of fission products that continue to undergo beta decay, and they produce 'decay heat' which decreases rapidly at first, but gradually decreases as the short-lived (short half life) radionuclides decay to more stable nuclides. The decay heat is removed in order to keep the spent fuel cool, usually for several years until the assemblies are placed in dry storage. The water in the spent fuel pool is kept cool so that the water does not boil. The spent fuel pool water should have a surface temperature well below boiling.

Decay heat diminishes over time.
At the moment of reactor shutdown the decay heat is about 6.5% 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%.
https://www.ne.anl.gov/pdfs/nuclear/spent_fuel_nutt.pdf (Nutt is no longer at ANL)

An example of decay heat - https://www.osti.gov/pages/servlets/purl/1435337

A typical spent fuel pool is about 40 feet (12 meters) deep and can be 40 or more feet (12 meters) in each horizontal dimension. The pool walls are constructed of reinforced concrete typically having a thickness between 4 and 8 feet (1.2 to 2.4 meters).
https://www.nap.edu/read/11263/chapter/23

The water provides shielding from the beta and gamma radiation, as well as cooling. There is some contamination on the fuel from metal corrosion products that form oxides on the fuel surface while it is resident in the core. For that reason, the pool water is filtered. In the spent fuel racks, adjacent to the fuel, the water does get hot, and one will see thermal effects in the water at the top of the rack, especially for freshly discharged fuel. Older fuel produces much less heat.

The water from the spent fuel pool is at relatively low temperature (less than ambient boiling temperature) and it would not be economical to use that heat for a heat source. That plant doesn't need it, and it would be costly to send off-site. Some NPPs do use process heat for district heating, but that is typically not the case where NPPs are sited many 10s of miles from a population center.

Summary:: Questions About Spent Nuclear Fuel and Spent Fuel Pools

Fyi, I'm a sophomore in high school and I've never posted on here, I have no background in Nuclear, just a newfound passion, sorry for any mistakes I make, any feedback would be very helpful and welcome.
One's passion and curiosity are welcome at PF. One asks some good questions.

When spent fuel assemblies are placed in dry storage, the heat load is balanced by mixing older vintage or low burnup fuel with newer (younger vintage) or high burnup fuel. Modern fuel is designed to achieve greater discharge burnup than much older fuel of the 1960s through 1980s.
 
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  • #4
slapp
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Thank you so much, I appreciate the answers.

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? And finally, since the rate of decay in nuclear fuel is pretty fast which would make it pretty bad for heat generation, setting aside that the fuel might irradiate nearby objects?
 
  • #5
gmax137
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I'm not sure where you are getting these fuel temperatures (1400F) but that is way way higher than reality. Here's a link that describes the fuel centerline temperature as 275F for fuel in the spent fuel pool, with a loss of pool cooling (so the pool water is boiling at 212F). The heat production in the spent fuel by the time it is placed into the spent fuel pool is very low. The maximum fuel temperatures are therefore also very low, nowhere near the temperatures seen during operation in the core.

https://www.nrc.gov/docs/ML1122/ML11223A323.pdf
 
  • #6
artis
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@slapp as others have said those temps are nowhere near the actual temps of the spent fuel , otherwise that would be fuel meltdown, meltdown of nuclear fuel is typically understood as the degradation of the fuel cladding which then releases the fuel elements themselves in various forms starting from fine powder/dust to aerosols and gasses if temps get higher.
You can google nuclear fuel , it is typically in metallic oxide form for simple reasons like higher temperature endurance.
https://en.wikipedia.org/wiki/Nuclear_fuel
This metallic oxide is prepared in small pellets , close to half the size of hand held AA battery size for a simple comparison, such pellets are then packed into hollow long rods. Rods are made from metals that have low neutron absorption (for obvious reasons , because chain reaction needs neutrons and one wants to increase neutron economy). These rods are hermetically sealed at the factory where they are produced.
The idea is that ideally they should stay sealed their entire life and even after while taken out and put inside spent fuel pond. If they get cracked or open then problems begin as radioactive materials can and do leak.

The reason spent fuel is not used for power production is because almost all commercial NPP's use water/steam cycle for production of power. This cycle is a Carnot cycle after the physicist Sadi Carnot
https://en.wikipedia.org/wiki/Carnot_cycle

The basic idea is this , in order to get any meaningful efficiency out of the water/steam cycle one needs to have a high temperature and temperature difference between the hot and cold side. Spent fuel doesn't produce high temperature in pool in the first place and if allowed to boil very little steam would be produced, this steam would be low temperature as compared to the one coming from the reactor and so the additional complexity doesn't pay back for the little gain.
Reactor output steam temperature is around 300 degrees Celsius, spent fuel pools are under atmospheric pressure so boiling would happen at 100 degrees C so this steam would be of no more than 100 C.


Just to add to @Astronuc quote the spent fuel pools are made from thick concrete , as is pretty much everything within a power reactor building, but the inside is lined with stainless steel. You don't want bare concrete in touch with water as water seeps into concrete and possible contaminants in water would too making the concrete a hazard. Stainless steel is easy to clean and decontaminate from surface pollutants.
 
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Astronuc
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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.
 
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  • #8
anuttarasammyak
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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?
Temperature have meanings only with mutual arrangements in mind of us. You know just tiny heat generation in the system may cause very high temperature of the system when energy does not go out of the system.
 
  • #9
Rive
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based on research I've done it appears that spent fuel rods come out of the reactor around 5000 F
I wonder where could you find such information. In what context?

Only during a meltdown would be such temperatures any realistic, but then it's already not a fuel rod. Was it some Fukushima or Chernobyl related source?
 
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  • #10
artis
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I wonder where could you find such information. In what context?
Well I think it might have been in the context of Chernobyl reactor number 4. There the fuel indeed came out at 5000F , probably higher, the only problem is, it came out not exactly the way the designers and operators had planned... also a bit too fast and all at once (speaking about shortening refueling time...) Yes, my attempt at mediocre joke. :biggrin:
 
  • #11
Rive
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May be a joke, but has the necessary amount of truth in it o0)
 

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