During the dissolution of the spent nuclear fuel (SNF) some radionuclides are released to the solution simultaneously from different sources in the fuel. This is of particular importance to some radionuclides that contribute to the Instant Release Fraction (IRF), which govern the initial radiation dose during the dissolution of the SNF. In this work a model that is able to discriminate between the different contributions responsible for the total concentration of a radionuclide in solution was developed. The model permits to establish that uranium and radionuclides that dissolved congruently with the UO₂ matrix came from two sources as a function of time: oxidized phases on the surface of the SNF including fines and the matrix itself. Other radionuclides such as Ru and Rh were released from metallic precipitates with dissolution rates lower than the matrix dissolution rate.

In the case of radionuclides that were expected to contribute to the IRF, this work showed that Cs, Rb and Sr had initial release rates higher than uranium because a fraction of such radionuclides were segregated from the matrix during the irradiation. Actually it was calculated that the fraction of those radionuclides in the grain boundaries in a BWR SNF powder sample from the center part of a pellet (burnup 42 MWd/kgU) was 2.1%, 0.9%, and 0.6% for Cs, Rb, and Sr, respectively. In addition, the model permitted to calculate the duration of their contribution to the IRF, matrix dissolution governed the release of such radionuclides after 137 days, 75 days, and 164 days for Cs, Rb, and Sr, respectively (at these times, the contribution of the release from grain boundaries was lower than the 0.1%).

在废核燃料（SNF）溶解期间，一些放射性核素会同时从燃料中的不同来源释放到溶液中。这对于某些有助于即时释放分数（IRF）的放射性核素特别重要，这些放射性核素控制SNF溶解过程中的初始辐射剂量。在这项工作中，建立了一个能够区分造成溶液中放射性核素总浓度的不同贡献的模型。该模型可以确定与UO ₂完全溶解的铀和放射性核素基质随时间变化有两个来源：SNF表面的氧化相（包括细粉）和基质本身。其他放射性核素（如Ru和Rh）从金属沉淀物中释放出来，其溶解速率低于基质的溶解速率。

对于预期对IRF起作用的放射性核素，这项工作表明Cs，Rb和Sr的初始释放速率高于铀，因为一部分此类放射性核素在辐照过程中与基质分离。实际上，据计算，从颗粒中心（燃耗42 MWd / kgU）开始，BWR SNF粉末样品中晶界中那些放射性核素的C，Rb和Cb含量分别为2.1％，0.9％和0.6％。分别是 此外，该模型允许计算其对IRF的贡献持续时间，基质溶解决定了Cs，Rb和Sr分别在137天，75天和164天后这些放射性核素的释放（此时，晶界释放的贡献低于0.1％。