The most inner barriers for fission product retention are the fuel matrix and the Zry cladding.

After an avarage burn-up of 30 GWd/t HM the fuel pellets are mostly cracked and brittled.

Given a single free-standing rod without potting, the Zry cladding holds the fuel in place as long as the cladding itself is mechanically stable enough. Some experts assess the time for cladding integrity at 100 years in dry atmosphere, others (much) less. Therefore they fear contamination and reject the dismantling of fuel elements (pulling of rods with seperate disposal of rods and skeleton components).

With the loss of inner geometry, the relocation of fuel debris starts and the risk of criticality enhances.

Unhindered fission product transport and electrochemical reactions accelerate the wall corrosion from inside.

Potting mitigates the effects of INSIDE FEP`s and contributes to long-term retention capability, subcriticality and mechanical stability.

Two different technologies for potting are under discussion:
- dry potting
- wet potting.