Are spent nuclear fuel rods radioactive?

In summary, spent fuel rods from LWRs (both PWR/VVER and BWR) contain radioactive materials such as unused fuel, fission products, and transmuted fuel and non-fuel components. The fuel rods also produce tritium through ternary fission, but the Zr alloy cladding retains most of it. There may be some leakage of tritium into the cooling water and spent fuel pool, but there are systems in place to clean and contain the water. Older reactors with different cladding materials may have had issues with tritium contamination, but most of it has decayed over time. Different types of reactors may have their own unique issues with spent fuel.
  • #1
lighthouse1234
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Are spent fuel rods radioactive and are the spent fuel rods cooling pools discharging radioactive water (tritium) into the environment?
 
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  • #2
AFAIK most of the tritium is retained in fuel cell and fuel rods. Some amount of unexpected leakage from fuel surface to pool water takes place actually.
 
  • #3
Yes, there have been instances of fuel pool leakage and consequent contamination. Search using term:

Fuel pool tritium
 
  • #4
lighthouse1234 said:
Are spent fuel rods radioactive
Sure.
lighthouse1234 said:
and are the spent fuel rods cooling pools discharging radioactive water (tritium) into the environment?
There is nearly no tritium in the spent fuel pools, and the water is not getting into the environment unless there is an accident. There is no chain reaction happening in the spent fuel pool so the neutron flux is small. Traces of tritium can come from the fuel rods.
 
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  • #5
lighthouse1234 said:
Are spent fuel rods radioactive
Yes. Spent or used fuel contains the unused fuel, the fission products (2 atoms per fission), and the transmuted fuel and non-fuel components. In the early years of commercial nuclear energy, the fuel was discharge with burnups (GWD/tU) of about 25-33 GWd/tU, or roughly 2.5-3.3% FIMA (fissioned initial metal (U) atoms). In modern times, discharge burnups are at least 2x greater, with batch burnups being something like 50-60 GWd/tU (or 5 to 6% FIMA). Since there are regulatory/statutory limits on peak rod burnup, or peak pellet burnup, the actual discharge burnup is slightly less, although technically, the fuel could continue to higher burnups. Within a batch of fuel, there is a relatively broad range of burnup due to the axial and radial gradients in the core/fuel, with the most severe gradients being on the periphery of the core (typically one row in from the outermost assemblies and the corner assemblies in a PWR). The BWR situation is more complicated because control elements, which are inserted part-time in the core during operation for reactivity control, cause local radial and axial gradients in the fuel closest to the control elements (control blades).

On spent fuel, corrosion products may accumulate, typically oxides of metals like Fe, Cr, Ni, Zn, and trace impurities, and in a neutron field, they will become activated (i.e., radioactive). While there is an ongoing 'reactor water cleanup' (RWCU) system, in which cooling water is filtered (and demineralized), some of the corrosion products will find their way into the spent fuel pool. However, the spent fuel pool also has a cooling water cleanup and heat removal system.

Tritium is produced in fuel rods through ternary fission. Since the fuel rod cladding is composed of a Zr alloy (approximately 0.975 to 0.985) a substantial portion of the tritium is held inside the fuel rod. Some tritium is produced in PWR cooling water (soluble boron and lithium buffer), and cooling water is contained with the RWCU system. In BWRs, there is no soluble boron in the cooling water, but tritium is produced in the boron in the control rods, and some tritium will leak (permeate) out the stainless steel tubes containing the tritium. BWRs also have a RWCU system with filter/demineralizers to clearn the water.

lighthouse1234 said:
are the spent fuel rods cooling pools discharging radioactive water (tritium) into the environment?
In the early days of the industry, before wide use of Zr alloys (Zircaloy-2 (in BWRs) and Zircaloy-4 (in PWRs)), cladding was often made of 304, 347 and 348 stainless steel. Tritium from fission would leak out of the fuel rods into the coolant, and into the spent fuel pool following discharge. Some older plants have had issues with tritium in ground water onsite. Since tritium decays over time (half life ~12.3 years), a lot of the tritium from 50-60 years ago has decayed, and otherwise at very low levels, generally less than regulatory limits.
 
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  • #7
harborsparrow said:
"Spent fuel rods" from exactly what kind of reactor? See https://en.citizendium.org/wiki/Nuclear_power_reconsidered for some of the options. A discussion without knowing exactly what reactor is kind of, well, makes me wonder.
I provided the context of LWR (both PWR/VVER and BWR), since that is the greatest volume of spent fuel in the US and Eu, but that could apply to CANDU. AGR and Magnox fuel is similar, but different, and of course there are fast reactor fuels and research reactor fuels.

New fuel designs using different materials will have certain unique issues.
 
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  • #8
Astronuc said:
I provided the context of LWR (both PWR/VVER and BWR), since that is the greatest volume of spent fuel in the US and Eu, but that could apply to CANDU. AGR and Magnox fuel is similar, but different, and of course there are fast reactor fuels and research reactor fuels.

New fuel designs using different materials will have certain unique issues.

The only significant difference regarding Tritium and CANDUs is during operation. The spent fuel from a CANDU is not different in any drastic way. It starts as natural Uranium so has less fission products than, for example, BWR spent fuel. The amount of Tritium in a spent fuel pool is quite low for nearly any commercial type of reactor.

During operation a CANDU produces significant Tritium. The "lore" is that it produces more Tritium per kg of Uranium than the reactors that the US military runs to top up their weapons. This is because a CANDU puts about 50 tons of heavy water in close contact with the fuel. So the D's are in large quantity in the highest neutron flux, so catch some and become T's.

CANDUs must do something to remove Tritium from the coolant during operation, or they have trouble meeting exposure limits (for workers) and release limits. Ontario Power Generation has a facility to extract Tritium from heavy water. They then store it as metal hydrides. Canadian policy is we can't sell it to any country with a weapons program, so most of it is just sitting and decaying in storage.
 
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  • #9
Grelbr42 said:
They then store it as metal hydrides. Canadian policy is we can't sell it to any country with a weapons program, so most of it is just sitting and decaying in storage.
Thanks, I did not know this.
 
  • #10
Grelbr42 said:
Canadian policy is we can't sell it to any country with a weapons program, so most of it is just sitting and decaying in storage.
The decay product 3He is quite valuable.
 
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  • #11
Astronuc said:
The decay product 3He is quite valuable.
You are correct. And there is apparently new needs for this isotope due to such things as fusion research, just for one. And, if I recall correctly, something about detectors for certain chemicals. (He deliberately avoids sensitive search terms. Having various nuclear things attached to my name is enough.)

However, I am unable to report what happens to the decay product. You have scratched my interest so I am going to ask around.
 
  • #12
Grelbr42 said:
However, I am unable to report what happens to the decay product. You have scratched my interest so I am going to ask around.
3He has interesting physical properties. It diffuses faster than 4He, and it is very hard to contain, unless in a hermetically sealed container.

https://www.mpg.de/18784452/news-from-mpi
https://www.nature.com/articles/s41586-022-04761-7

https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=3312&context=etd

It is a strong thermal neutron absorber.
https://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=14963&mf=3&mt=103&nsub=10
 
  • #13
Grelbr42 said:
However, I am unable to report what happens to the decay product.
3He is the decay product (of tritium). It's used for refrigeration, neutron detectors, medical imaging and more.
 
  • #14
mfb said:
3He is the decay product (of tritium).
I think the question was not about the general use of 3He, but whether it's actually collected at the deposit site.
 
  • #15
To paraphrase Madison, if men were angels we could use spent fuel rods to power our cars.
 
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  • #16
bob012345 said:
To paraphrase Madison, if men were angels we could use spent fuel rods to power our cars.
When I lived in New England, I wanted to mix the pellets with asphalt and pave my driveway. No more shoveling!
 
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  • #17
And the runoff into your garden would produce rare and interesting foliage.
 
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  • #18
Vanadium 50 said:
And the runoff into your garden would produce rare and interesting foliage.
Perhaps like in The Day of the Triffids.
 
  • #19
And some lead BVDs
 
  • #20
bob012345 said:
To paraphrase Madison, if men were angels we could use spent fuel rods to power our cars.
Probably not.

Immediately after the reactor shuts down, the decay heat is some few percent of the power of the reactor. In the range of 3 percent or so, depending on the specific design. Within a few minutes this has fallen to less than 1 percent. After a week it is down to less than 0.2 percent. It continues to decay after that, with the shorter lived isotopes gone, and the longer decaying at lower power.

A (very roughly) 50 kg fuel bundle from a CANDU is in the range of 500 kW during operation is down to 10 kW inside of a week after reactor shutdown. And it continues to fall as the isotopes decay.

If you wanted to run a steam engine and get 100 horse-power out, and putting in an arbitrary 30% efficiency, you would need round-about 1100 kg of fuel alone. That's over half the weight of a Tesla model S.
Plus the boiler and pistons and such. Plus some shielding to protect you from your fuel. And a Tesla S has either 695 hp or 1000 hp, depending on the specific configuration.

So, probably not going to be using spent fuel to power cars.
 
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  • #21
Grelbr42 said:
Probably not.

Immediately after the reactor shuts down, the decay heat is some few percent of the power of the reactor. In the range of 3 percent or so, depending on the specific design. Within a few minutes this has fallen to less than 1 percent. After a week it is down to less than 0.2 percent. It continues to decay after that, with the shorter lived isotopes gone, and the longer decaying at lower power.

A (very roughly) 50 kg fuel bundle from a CANDU is in the range of 500 kW during operation is down to 10 kW inside of a week after reactor shutdown. And it continues to fall as the isotopes decay.

If you wanted to run a steam engine and get 100 horse-power out, and putting in an arbitrary 30% efficiency, you would need round-about 1100 kg of fuel alone. That's over half the weight of a Tesla model S.
Plus the boiler and pistons and such. Plus some shielding to protect you from your fuel. And a Tesla S has either 695 hp or 1000 hp, depending on the specific configuration.

So, probably not going to be using spent fuel to power cars.
Well, it was meant as humor but afterwards I thought cars require too much power but houses don't so from now on I'll downgrade my joke to say houses.
 
  • #22
Even taking disasters like Chernobyl into account nuclear power is still a viable alternative. Given the exigencies of the situation, namely the inability of renewable energy sources to produce a reliable power supply, there is no doubt that nuclear fission reactors offer a welcome alternative. It all depends on how efficiently the nuclear power plants are run and how well they are organized. The fuel rods used in nuclear reactors have a useful life of about 18 months, after which the have to be stored in pools in which the water is kept cool by means of circulating pumps. The rods stay in the pool for around 10 years until they become cool enough to handle but remain highly radio-active for the next 10,000 years. If the pools containing the depleted rods are not properly monitored, the water will begin to boil and evaporate from the heat generated by the spent rods. When this happens the spent fuel rods will start to disintegrate releasing highly toxic radio-active flakes and
materials into the air.

The problem is that not much of the fuel by weight is used in a nuclear reactor., therefore if 100,000 tons of ore are mined and after processing about 27.6 metric tons is available for nuclear fuel rods, then after 18 months of use there are still 27.6 metric tons of depleted fuel rods to take into consideration and dispose off. There are many fuel rods, which are stacked together to make an assemble which might hold from 140 to 200 individual fuel rods, if a single fuel rod assembly weighs about a 1000 Kg then of 27.6 metric tonnes would be the equivalent of about 27 fuel assemblies i.e., enough to power a nuclear reactor producing 1GW annually for 18 months. Out of the 27.6 metric tonnes of fuel used in spent fuel rods, 90% by volume is low level waste, 7% intermediate level waste and 3% highly toxic waste. However, this waste should not be taken lightly, if not stored and cared for properly spent fuel rods, pose a serious environment and health hazard.
A plus point is that it is possible to recycle spent nuclear fuel rods after undergoing vary processes over a period of about twenty years
 

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