Investigating the Effect of Chemical Species on Pellet Cladding Interaction in Pressurised Water Reactor Fuel using Hybrid Quantum Mechanics Molecular

在核裂变反应堆中，特别是在压水反应堆（PWR）中，芯块-包壳相互作用（PCI）是核燃料棒中的一种现象，由此燃料芯块的外表面与燃料芯块包壳的内表面接触并相互作用。它发生在功率瞬变、偶然瞬变、燃料操纵以及稳态操作期间。PCI是由燃料-芯块的相对运动产生的，并且在存在挥发性裂变产物（例如碘（I-SCC））的情况下，也可以通过包壳的应力腐蚀开裂（SCC）来显现。在使用无衬里锆合金包壳和UO 2燃料的PWR中，碘应力腐蚀开裂（I-SCC）已被认为是PCI失效的主要原因。然而，要发生I-SCC，必须满足几个要求。裂纹扩展需要一个临界应力。为了引起覆盖在包壳上的氧化膜的开裂，需要临界应变。裂纹尖端需要临界碘浓度，以允许碘连续扩散到裂纹中。最后，为了使系统在I-SCC发生之前达到化学平衡，需要最短的时间。博士项目的目的是研究碘和其他卤素对压水反应堆燃料应力腐蚀开裂过程的影响。攻击物种的确切化学成分尚不清楚。此外，围绕氧在碘存在下的作用以及它是否抑制或增强I-SCC过程存在争议。该项目研究锆的断裂过程以及碘和氧如何影响该过程。现有的研究断裂过程的技术要么使用经验势来描述大型系统中有限类型的原子之间的相互作用，要么使用量子力学技术，该技术允许化学准确性，但仅限于非常小的系统。使用最近开发的混合量子力学分子力学技术，大型系统的调查是可能的，同时通过密度泛函理论（DFT）计算在裂纹尖端保持高的化学精度。为了确定碘对该过程的影响，进行了各种类型的断裂模拟，包括分子静力学、分子动力学和轻推弹性带模拟。

In nuclear fission reactors, especially pressurised water reactors (PWR), pellet-cladding interaction (PCI) is a phenomenon in nuclear fuel rods whereby the outer surface of a fuel pellet is in contact with and interacts with the inner surface of the fuel pin cladding. It occurs during power transients, in incidental transients, fuel manoeuvring as well as during steady state operation. PCI is developed by fuel-pellet relative motion, and can also manifest itself through stress-corrosion cracking (SCC) of the cladding in the presence of volatile fission products such as iodine (I-SCC). In PWRs using unlined Zircaloy cladding and UO2 fuel, iodine stress corrosion cracking (I-SCC) has been recognised to be the main cause of PCI failures. For I-SCC to occur, however, there are several requirements that must be met. A critical stress is needed in order for the crack to propagate. The critical strain is needed in order to induce cracking of the oxide film covering the cladding. A critical iodine concentration at the crack tip is required in order to allow continuous diffusion of iodine into the crack. Finally, a minimum time is required in order for the system to come to chemical equilibrium before I-SCC can occur.The aims of the PhD project are to investigate the effects that iodine and other halogens have on the stress corrosion cracking process in pressurised water reactor fuels. The exact chemistry of the attacking species is not yet known. In addition, there is debate surrounding the role of oxygen in the presence of iodine and whether it inhibits or enhances the I-SCC process. The project examines the fracture process of Zirconium and how iodine and oxygen affect that process. Existing techniques of investigating fracture processes use either empirical potentials for describing the interaction between limited types of atoms in large systems or use quantum mechanics techniques that allow chemical accuracy but are limited to very small systems. Using a recently developed hybrid quantum mechanics molecular mechanics technique, the investigation of large systems is possible while maintaining high chemical accuracy at crack tips via density functional theory (DFT) calculations. Various types of fracture simulations including molecular statics, molecular dynamics and nudged elastic band simulations are performed in order to determine what the effect of iodine is on the process.