Austenitic stainless steel and reactor internal structure

An austenitic stainless steel, comprising:

• a mixed grain structure composed of a columnar crystal having an average crystal grain size of 20 μm or less and an equiaxed crystal having an average crystal grain size of 5.0 μm or less, wherein
• an area proportion of the columnar crystal in the mixed grain structure is 20% or more,
• an average crystal grain size of an entirety of the mixed grain structure is 5.0 μm or less,
• the austenitic stainless steel contains, by weight ratio, Cr:
• 16 to 26%, Ni:
• 3.5 to 26%, Si:
• 0.1 to 1.0%, Mo:
• 0.01 to 7.0%, Mn:
• 0.1 to 2.5%, C:
• 0.01 to 0.08%, O:
• 0.01 to 0.5%, N:
• 0.01 to 0.25%, and further contains Zr:
• 0.1 to 2.8%, and additional elements, and the balance is Fe and inevitable impurities, and
• the additional elements contain one or more of Ti, and Ta.