(350b) Stress and Diffusion in Stored Pu Zppr Fuel from Alpha Generation

ZPPR (Zero Power Physics Reactor ) is a research reactor that has been used to investigate breeder reactor fuel designs.  The reactor is currently in shut down status. Its plutonium containing metal fuel elements are enclosed in stainless steel cladding with gas space filled with helium-argon gas and welded air tight.  The fuel elements which are 2 inches by 0.2 inches up to 8 inches long were manufactured in 1968.  Some of these fuel elements have failed releasing contamination raising concern about the general state of the remaining fuel elements.  Inspection of the large number of fuel elements could lead to contamination release so analytical studies have been conducted to estimate the probability of failed fuel elements.

This paper 1) calculates the initial gas loading in a fuel element and its internal pressure after it has been brought into the atmosphere at ZPPR, 2) shows that the amount of helium generated by decay of Pu‑239 to U‑235 over 46 years since manufacture is significantly greater than this initial loading, 3) determines the amount of fuel swelling if the helium stays fixed in the fuel plate and 4) the amount of Helium which diffuses out of the fuel plate into the fuel plenum assuming the helium does not remain fixed in the fuel plate but can diffuse to the plenum and possibly through the cladding. Since the literature is not clear as to which possibility occurs, as with Schroedinger’s cat, both possibilities are analyzed. The paper concludes that 1) if the gas generated is fixed in the fuel, then the swelling it can cause would not cause any fuel failure and 2) if the helium does diffuse out of the fuel (in accordance diffusivities estimated from the literature), then it is likely that fuel element bulging will occur.