BIogeochemical Gradients and RADionuclide transport. BIGRAD

在50多年的核能发电和武器开发中，英国产生了大量的放射性废物。就总量而言，高放射性废物中最大的部分（> 90%）是中级废物（ILW）。ILW不产生热量，但含有长寿命的放射性同位素，因此不能在地球表面附近处理。政府最近决定，联合王国的ILW应在地下（200 - 1000米）的“地质处置设施”（GDF）中处置。GDF的安全性取决于减缓放射性从GDF返回到地球表面。因此，了解控制放射性从GDF到岩石和岩石以外的运动的过程至关重要。联合王国的ILW种类繁多，包括废弃的核燃料、用于盛装燃料的金属容器以及处理这些放射性材料时产生的污泥和有机碎片。联合王国已将其中许多放射性废物固定在水泥中加以处理，现在已将大部分的ILW用水泥固定，等待处置。一旦废物被放置在GDF中，意图是用水泥回填剩余空间。尚未确定联合王国废物的场址，但预计该场址将在地下水位以下，因此是潮湿的。这意味着，在废物被放置后，GDF将随着地下水通过废物的渗透而重新润湿。经过很长一段时间（从数百年到数百万年），ILW及其钢容器将降解，水泥将与地下水发生反应，使其变得非常碱性。这是一个设计特征，因为非常碱性的“生锈”条件预计会使ILW的大多数放射性成分非常不溶。然而，这种碱性水会与储存库周围的岩石发生反应，形成一个“化学干扰区”（CDZ）。到目前为止，还没有研究检查这个CDZ的化学，物理和生物发展，以及它如何影响GDF放射性污染物的流动性。我们选择研究四种长寿命放射性核素，即裂变产物锝以及铀、镎和钚，所有这些都将在与CDZ有关的长时间内存在。在这个项目中，我们将试图了解CDZ在数千年到数百万年的时间里将如何演变，因此我们可以预测放射性通过它的运动，并帮助评估GDF的安全性。为此，我们需要研究CDZ发展过程中发生的化学、物理和生物变化，以及这些不同因素相互作用的方式。我们将使用实验来理解这些过程，并在此基础上开发计算机模型来预测未来会发生什么。我们的工作计划分为三个部分：1地圈演化，我们将研究岩石和矿物的相互作用，以及岩石中的水流如何受到GDF水引起的化学和微生物变化的影响; 2放射性核素的形式，反应和运输，我们将研究放射性核素的化学形式和溶解度，它们与微生物的相互作用，以及与岩石表面，和微观粒子携带放射性的潜力; 3合成和应用，我们将把所有的实验结果放在一起，设计，开发和测试我们的计算机模型，以检查放射性核素在CDZ中的传输。为了确保我们有效地将项目的不同部分联系起来，我们确定了两个“交叉主题”（CCT）-（i）CDZ中的地球化学过程;以及（ii）CDZ的预测建模，这将把所有不同的工作联系在一起。我们的工作将使人们更好地了解对整个CDZ污染物流动性的控制，提高对地质处置安全性的信心，从而协助联合王国完成处置放射性废物的关键任务。

Over 50+ years of nuclear power generation and weapons development, the UK has created large quantities of radioactive wastes. In terms of total volume, the largest fraction (> 90 %) of the higher activity waste is Intermediate Level Waste (ILW). ILW does not produce heat but contains long-lived radioisotopes, and so cannot be disposed of near the Earth's surface. The Government has recently decided that the UK's ILW should be disposed of underground (200 - 1000 m) in a 'Geological Disposal Facility' (GDF). The safety of a GDF depends on slowing the return of radioactivity from the GDF to Earth surface. Understanding the processes which control the movement of radioactivity out of the GDF and to the rock and beyond is therefore critical. The UK's ILW is very diverse and includes discarded nuclear fuel, the metal containers used to hold fuel, as well as sludges and organic debris produced when processing these radioactive materials. The UK has treated many of these radioactive wastes by immobilising them in cement and a substantial fraction of ILW has now been cemented and awaits disposal. Once the wastes have been placed in the GDF, the intention is to backfill the remaining space with cement. No site has been identified for UK wastes as yet, but it is expected that the site will be under the water table and therefore be wet. This means that, after the waste is emplaced, the GDF will rewet as groundwater percolates through the wastes. Over a long time (from hundreds to millions of years) the ILW and its steel containers will degrade, and the cement will react with the groundwater to make it very alkaline. This is a design feature, as very alkaline, 'rusty' conditions are expected to make most radioactive components of the ILW very insoluble. However, this alkaline water will react with the rock around the repository to form a 'chemically disturbed zone' (CDZ). Up until now, no studies have examined the chemical, physical and biological development of this CDZ and how this affects the mobility of radioactive contaminants from the GDF. We have chosen to study four long-lived radionuclides, the fission product technetium as well as uranium, neptunium and plutonium all of which will be present over the long timescales relevant to the CDZ. In this project, we will try and understand how the CDZ will evolve over thousands to millions of years, so we can predict the movement of radioactivity through it, and help assess the safety of the GDF. To do this, we need to study the chemical, physical and biological changes which occur as the CDZ develops, and the way in which these different factors interact with each other. We will use experiments to understand these processes and, based on these, we will develop computer models to predict what will happen in the future. We have divided our work programme into three parts: 1 Geosphere Evolution, where we will examine rock and mineral interactions, and how water flow within the rock is affected by chemical and microbiological changes caused by the water from the GDF; 2 Radionuclide Form, Reaction and Transport, where we will examine the chemical form and solubility of radionuclides, their interactions with microorganisms, and with rock surfaces, and the potential for microscopic particles to carry radioactivity; 3 Synthesis and Application, where we will bring all the experimental results together and design, develop and test our computer model to examine radionuclide transport in the CDZ. To ensure we link the different parts of the project effectively, we have identified two 'cross cutting themes' (CCTs) - (i) biogeochemical processes in the CDZ; and (ii) predictive modelling of the CDZ, which will tie all the different pieces of work together. Our work will provide improved understanding of the controls on contaminant mobility across the CDZ, improve confidence in the safety of geological disposal, and hence assist the UK in the crucial task of disposing of radioactive wastes.