燃料丸端面製造缺陷是2000年中期前後，造成沸水式反應器(Boiling Water Reactor, BWR)燃料破損的重要機制之一，稱為非典型燃料丸與護套作用(non-classical Pellet Cladding Interaction, PCI)，而除了BWR燃料，壓水式反應器(Pressurized Water Reactor, PWR)電廠也發生少量類似的破損機制。燃料丸不規則端面除了造成較高之應力集中，破損案例肇因分析顯示，氫化鋯也扮演著關鍵角色。此外日本對高燃耗燃料行為的相關研究中，也發現在進行功率躍升實驗時，氫化鋯於護套外側集中，形成初始裂縫，並往內部成長，而成為另一種不預期的破損機制。護套中的氫原子隨溫度梯度往低溫區域擴散，並隨著環境變化析出成為氫化物，由於與鋯合金母材機械效應的差異，往往成為核燃料破損的起始點或是加劇的主因。
本研究探討燃料丸外緣因製造缺陷造成燃料棒與護套發生局部相對低溫現象，進而造成氫於護套部分區域之擴散與氫化物集中效應。研究過程先依據可預期之燃料丸製造缺陷，分析局部溫度差異，參考已建立之溫度梯度模型與氫濃度分佈模型取得氫化物局部差異性，接著導入有限元素法以及破壞力學理論建立分析模式。並分析由高燃耗燃料所造成之LENS型態氫化物析出，建立護套由外往內成長之裂縫模式將與先前由內往外成長的裂縫模式將有所不同，並在裂縫形成後評估其穩定性建立初步之裂化機制。
燃料丸缺陷與分析氫化鋯生成與外側裂縫研究成果對近年來所發現之燃料丸製造缺陷以及裂縫由護套外側往內部成長之破損機制將有所貢獻，可以協助提升核燃料之運轉績效。

The pellet fabrication defect “Missing Pellet Surface (MPS)” became an important failure mechanism of BWR fuel in the past decade, and the unexpected high stress induced on cladding inner surface was identified as “non-classical PCI”. Similar failures were found for few PWR rods at the same time. The failure root cause analysis of some leaking rods indicated that hydrides may play an important role of the outside-in cracking. The study of high burnup fuel behavior performed by Japanese also revealed the cladding outside-in crack during power ramp test and may become another issue of high burnup fuel integrity. The cladding temperature gradient will induce the hydrogen re-distribution, and the precipitated hydride could initiate cladding crack and rod failure.
Our study is to investigate the cladding local temperature distribution with the MPS, then to identify the local cold spot area with higher hydrogen content and possible more hydride. The approach of pellet defect is based on the fabrication experience. A finite element model of cladding with non-homogeneous hydride distribution was applied in the analyses of the formation of outside-in crack and its stability. We analysis the high burnup fuel caused by the LENS types of hydride precipitation, the establishment of cladding the outside-in crack pattern will vary with the previous inside-out crack pattern. After the crack formation, We assess its stability and establish preliminary cracking mechanism..
The established model is helpful to understand the hydride effect of Pellet-Cladding Mechanical Interaction (PCMI), either for the fabrication concern or high burnup fuel reliability.

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