BIogeochemical Gradients and RADionuclide transport. BIGRAD

50多年来，英国的核能发电和武器发展产生了大量的放射性废物。就总量而言，高活性废物中最大的部分（约90%）是中间水平废物（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，英国的ILW应该在地下（200 - 1000米）的“地质处置设施”（GDF）中处置。GDF的安全性取决于减缓从GDF返回到地球表面的放射性。因此，了解控制放射性从GDF向岩石及其外移动的过程是至关重要的。英国的ILW非常多样化，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物质时产生的污泥和有机碎片。英国已经通过将这些放射性废物固定在水泥中来处理它们，并且相当一部分废铁现在已经被固化并等待处置。一旦废物被放置在GDF中，目的是用水泥回填剩余的空间。目前还没有确定英国废物的地点，但预计该地点将在地下水位以下，因此是潮湿的。这意味着，在废物被安置后，随着地下水渗透到废物中，GDF将重新变湿。经过很长一段时间（从数亿年到数百万年），ILW和它的钢容器会降解，水泥会与地下水发生反应，使其呈碱性。这是一种设计特点，因为非常碱性，“生锈”的条件预计会使ILW的大多数放射性成分非常不溶。然而，这种碱性水将与储存库周围的岩石发生反应，形成一个“化学扰动区”（CDZ）。到目前为止，还没有研究检查这种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的放射性核素，即裂变产物锝以及铀、镎和钚，所有这些都将在与核裂变区有关的长期时间尺度上存在。在这个项目中，我们将尝试了解CDZ如何在数千年到数百万年的时间里进化，这样我们就可以预测放射性物质在其中的运动，并帮助评估GDF的安全性。要做到这一点，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。我们将通过实验来了解这些过程，并在此基础上开发计算机模型来预测未来会发生什么。我们将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，以及岩石内的水流如何受到由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，它们与微生物和岩石表面的相互作用，以及微观颗粒携带放射性的可能性；3合成与应用，我们将把所有的实验结果结合在一起，设计、开发和测试我们的计算机模型，以检查CDZ中的放射性核素运输。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的生物地球化学过程；（ii）对CDZ进行预测建模，将所有不同的工作结合在一起。我们的工作将使人们更好地了解污染物在CDZ内流动的控制，提高对地质处置安全性的信心，从而协助英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.