PROBLEM TO BE SOLVED: To allow for measuring distribution of conditions within a predetermined range of a control tube using a single set of condition sensing means.SOLUTION: An optical fiber sensors 6 is movably inserted into each of four passages 22 consisting of tubes made of a same material as an outer tube 21 to sense temperature and strain distributions within a longitudinal range of inserted portions simultaneously. Such an arrangement allows a single optical fiber sensor 6 to measure temperature and strain distributions along a predetermined range (range over an axial length) of a simulated fuel rod 4.SELECTED DRAWING: Figure 2
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