Nuclear spent fuel rods

[lighthouse1234]

Are spent fuel rods radioactive and are the spent fuel rods cooling pools discharging radioactive water (tritium) into the environment?

 

[anuttarasammyak]

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.

 

[gmax137]

Yes, there have been instances of fuel pool leakage and consequent contamination. Search using term:

Fuel pool tritium

 

[mfb]

Are spent fuel rods radioactive

Sure.

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.

 

[Astronuc]

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.

are the spent fuel rods cooling pools discharging radioactive water (tritium) into the environment?

In the early days of the industry, before wide 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.

 

[harborsparrow]

"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.

 

[Astronuc]

"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.