Constraints by Experimental Data for Modeling of Radionuclide Release from Spent Fuel
- PDF / 634,955 Bytes
- 10 Pages / 420.48 x 639 pts Page_size
- 29 Downloads / 181 Views
CONSTRAINTS BY EXPERIMENTAL DATA FOR MODELING OF RADIONUCLIDE RELEASE FROM SPENT FUEL
and J. BRUNO ýERND GRAMBOW , L.O. WERME , R.S. FORSYTH Berlin, FR*SKB, Stockholm, Sweden; *Hahn-Meitner-Institut Studsvik, Nykbping, Sweden; Royal Institute of Technology, Sweden
Stockholm,
ABSTRACT Comparison of spent fuel corrosion data from nuclear waste management rojects in Canada, Sweden and the USA strongly suggests that the release of OSr to the leachant can be used as a measure of the degradation (oxidation/dissolution) of the fuel matrix. A surprisingly quantitative similarity in the 9 0 Sr release data for fuel of various types (BWR, PWR, Candu), linear power ratings and burnups leached under oxic conditions was observed in the comparison. After 1000 days of leachant contact, static or sequential, the fractional release rates for 9 0 Sr (and for cesium nuclides) were of the order of 10- 7 /d. The rate of spent fuel degradation (alteration) under oxic conditions can be considered to be controlled either by the growth rates of secondary alteration products, by oxygen diffusion through a product layer, by the rate of formation of radiolytic oxidants or by solubility-controlled dissolution of the matrix. These processes are discussed. Methods for determining upper limits for long-term 9Sr release, and hence fuel degradation, have been derived from the experimental data and consideration of radiolytic oxidant production. INTRODUCTION The evaluation of spent fuel as a principal barrier for radionuclide release from a repository has to rely to a high degree on laboratory studies and on the extrapolation of laboratory data to long periods of time. Such extrapolations are only justified if the release mechanisms are understood and quantitatively modelled. The interpretation of release data is difficult because the fuel is a heterogenous and complex material. In order to find constraints for the contributions to fission product release from the fuel/clad gap, from segregated phases at grain boundaries and from the U02 matrix, we have compared a large body of data from various fuel types from research projects in Canada, USA and Sweden. The contribution of the initial release from the fuel/clad gap and from cracks can be identified by an analysis of the time dependence of the total release, and usually this initial fraction is leached out in the initial 2 weeks of water contact. Release due to selective attack of segregated material at the grain boundaries may occur over long periods of time (such as in laboratory corrosion tests) and may, therefore be difficult to distinguish from release by matrix dissolution, since this also may be non-congruent. Identification and determination of the inventories of possible grain boundary segregation as a function of burnup, linear heat rating etc. will be necessary in order to adequately model their contributions to the total release rate [1]. With respect to the fuel matrix contribution, we propose here to consider the release of 9 0 Sr as a monitor of matrix dissolution, and on this bas
Data Loading...
