On the behaviour of spent fuel under simulated early canister-failure conditions

• This paper reports on the results of a long term experiment on spent fuel leaching, canister corrosion and radionuclide immobilization under repository conditions corresponding to early canister failure. A γ-dose of 850 mGy per hour, and Ar-flushed synthetic groundwater were used. During the first 287 days, the fractions of inventory in aqueous phase per day (f/d) increased nearly constantly for each radionuclide (except for 100Mo), with higher fractions for fission products than for actinides. After this initial stage, iron, cast iron and copper coupons (of̃30 mm2 size) were inserted into the leaching vessel: the concentrations of 238U, 237Np and 99Tc decreased by 80%, 97% and 88% to relatively stable levels (500ppb, 0.2ppb and 0.6ppb respectively). 239Pu concentrations slightly increased to the level of PuO2 solubility (0.5ppb). The leaching process for Cs, Mo and 90Sr was not influenced by the added metals. The pH and Eh values and corrosion potential were measured. The polarization resistance (Rp) was obtained from fitting of electrochemical impedance spectra, and then corrected by the surface area of the sample. The corrosion current density Icorr was calculated according to the Stern-Gary relationship, and using the same Tafel constants (26 mV)for all samples, i.e., Icorr = 0.026/Rp. The momentary corrosion rates of iron, cast iron and copper (Cu/Cu+) calculated based on the Faraday's law are in reasonable agreement with the values estimated from observation of thickness of corrosion layers using SEM microscope. The copper coupon appeared much more stable against corrosion than the other coupons. The corrosion layer and radionuclides deposited on corrosion products on iron coupons were analyzed by SEM-EDS and SIMS. Si was found to be coordinated with all iron corrosion products; however, redox sensitive elements U and Pu were mainly coordinated with the inner layer of iron corrosion products even though some 1-2 μm sized uranium-silica rich particles were found on the outer side of the iron corrosion layer. The findings of this work contribute to an improved understanding of the behaviour of spent fuel under near field repository conditions.

Subject headings

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Energiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Energy Engineering (hsv//eng)

Keyword

Canister-failure conditions

Polarization resistance

Tafel constants

Actinides

Computer simulation

Corrosion

Failure analysis

Groundwater

Leaching

Radioisotopes

Spent fuels

Publication and Content Type

ref (subject category)

kon (subject category)