As an external defect, cracks cannot be neglected, and crack detection is crucial for the safety monitoring of nuclear containments. Because of the low continuity and tiny size of cracks, traditional crack-detection methods sometimes fail to detect cracks in complex backgrounds. This paper proposes a crack-detection method featuring a template that can adaptively determine the crack width. The search starts from the local minimum point of the gray level, and the crack width is determined by comparing the gray difference between the seed region and its neighborhoods on a pixel basis. Then, the block-iterative algorithm is used to intersect with the results of the template method followed by the morphological method for further noise removal. Compared with the traditional methods, the developed method can detect tiny cracks with a higher recognition ratio. The applicability of this method was verified by experiments from a nuclear containment in southern China.