Quantitative Assessment of the Instant Release Fraction (IRF) for Fission Gases and Volatile Elements as a Function of B
- PDF / 91,070 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 15 Downloads / 195 Views
4XDQWLWDWLYH$VVHVVPHQWRIWKH,QVWDQW5HOHDVH)UDFWLRQ ,5) IRU)LVVLRQ*DVHVDQG 9RODWLOH(OHPHQWVDVD)XQFWLRQRI%XUQXSDQG7LPH XQGHU*HRORJLFDO'LVSRVDO&RQGLWLRQV Cécile Ferry 1, Patrick Lovera 1, Christophe Poinssot 1 and Lawrence Johnson 2 1 CEA Saclay, Nuclear Energy Division, Department of Physics and Chemistry, BP.11, F-91191 Gif-sur-Yvette Cedex, France 2 NAGRA, Wettingen, Switzerland $%675$&7 The Instant Release Fraction at container failure time, IRF(t), is here considered as being the sum of (i) the initial labile fraction, corresponding to the sum of gap and grain boundary inventories of certain radionuclides on exit from the reactor, with a further possible contribution from segregation in the rim region and (ii) the time-dependent fraction of radionuclides accumulating at grain boundaries due to α self-irradiation enhanced diffusion through the grains. The initial labile fraction of radionuclides such as 14C, 36Cl, 79Se, 129I, and 135Cs has been estimated based on leaching experiments, post-irradiation fission gas release measurements and studies of solid-state chemistry of spent fuel, along with estimates of fission product segregation in the rim zone. The contribution of the α self-irradiation enhanced diffusion has also been estimated based on a diffusion coefficient decreasing with time proportionally with the volume α−activity of the spent fuel. Its contribution to the IRF is limited for UO2 fuels. The proposed bounding values of the IRF for fuel with a burnup of 55 GWd/tIHM for 14C, 36Cl, 79Se, 129I, and 135 Cs are 11 % at t=0 and close to 15 % at a container failure time of 10,000 y. ,1752'8&7,21 Direct disposal is being studied by many countries as a possible way to manage Spent Nuclear Fuel (SNF) even if it does not in all cases correspond to the current reference scenario (e.g. France). It is in any case of prime importance to obtain a reliable source term for SNF for use in performance assessment calculations. The SNF source term in a water saturated medium is normally described as the combination of two terms [1]: (i) an instantaneous release of radionuclides (RN) at the containment failure time, often referred to as the Instant Release Fraction (IRF(t)). This is a combination of an H[SHULPHQWDOO\YHULILHG rapid preferential release from the gap of some fission products and activation products, and a SHVVLPLVWLFDOO\DVVXPHG rapid release from grain boundaries. In the latter case, there is clear evidence for some accumulation at grain boundaries, but little information on release kinetics, (ii) a slow long-term contribution corresponding to the dissolution of the uranium oxide matrix. Performance assessment calculations illustrate the strong impact of the IRF on doses arising from direct disposal due to the release of weakly sorbing long-lived radionuclides such as 14C, 36 Cl, 79Se, 129I and, to a lesser degree, more strongly sorbing 135Cs. Recent research on the evolution of SNF properties has examined the potential evolution of the pellet before the canister is breached during
Data Loading...
