Chemical Durability of High Burnup Lwr-Spent Fuel in Concentrated Salt Solutions

Abstract

High-burnup (<50 MWd/kgU) spent fuel samples of various sizes were exposed to NaCl solutions under static, anaerobic and reducing conditions. The accumulated corrosion time was about 200 days. Gas phase and leach solutions were analyzed. By dissolving mm sized fragments in large volumes of solution, saturation effects were avoided and upper limits for intrinsic dissolution rates of about 5-20 mg/(m2d) were measured. Surface area normalized reaction rates were significantly lower when using fine grained fuel powder (estimated sample surface area to solution volume ratio S/V ca. 3000 m-1), indicating saturation effects. The maximum concentrations of Pu and Am in the tests are close to reported solubility limited concentrations in pure 5m NaCl solutions in the absence of radiolysis effects. The presence of iron effectively reduces the solution concentration of all measured radionuclides (except Cs).

Coprecipitation of Mono-, Di-, Tri-, Tetra- and Hexavalent Ions with Na-Polyuranates

“…Within the rather large scatter of data, fair agreement between their and our data is obtained, indicating that Th-concentrations are probably not controlled by coprecipitation with U(VI) but that Th is simultaneously precipitating as Th(0H) 4 . However, as precipitated Th(OH) 4 is rather amorphous and only about 1% of the precipitating solids will be Th(OH) 4 , it could not be analysed by solid state techniques as an individual phase in our precipitates.…”

“…The results of the present work give indications both for the coprecipitation of various elements with U(VI) solid phases and for the simultaneous precipitation of various individual phases. Th and Zr concentrations are most likely be controlled by the individual solubility of a phase like Th(0H) 4 . Similar behaviour may be expected for tetravalent actinide such as Np(IV), Pu(IV), etc.…”

“…However, as precipitated Th(OH) 4 is rather amorphous and only about 1% of the precipitating solids will be Th(OH) 4 , it could not be analysed by solid state techniques as an individual phase in our precipitates. The good agreement of the data of Felmy with our data indicates that also the model presented by Felmy [16] may be applicable to our data.…”

“…The activity of an Eu containing endmember a Eu in the solid phase by means of Eq. (1) with ^ Eu, solid = (Kl s /K Q ) Eu = (a ss /a) Eu , aq = (m ss /m) Euaq (4) is given by the europium equilibrium concentration in the aqueous phase (values from ultrafiltrated solutions) in the presence "m|"" (system A) and in the absence of U(VI) "m Eu " (system B) in the same ionic medium. A value of "a Eu = 1.74 · 10~2" is obtained.…”

“…Despite recognition of their principal significance in radionuclide mobility [1], spent fuel related coprecipitation phenomena have been only studied by few research groups [1-3 and the references therein] and a systematic analysis of the retention of radionuclides by coprecipitation is still missing. Recently it has been shown that concentrations of Am in leach solutions of spent fuel in NaCl solutions appear to be about 5 orders of magnitude lower than the equilibrium concentration with respect to pure phases [4]. This discrepancy was tentatively explained by coprecipitation behaviour.…”