Critical experiment on half-inserted SUS control-rod measurement of change in power distribution in fuel pin close to control-rod

Takemura, Morio*; not registered; not registered; not registered; not registered

Pressure tube type heavy water reactor has an advantage of load following operation compared with light water reactor. It is planned to prove the possibility of load following operation in the Fugen type demonstration reactor using stainless control-rod (SUS control-rod). Changes in local power distribution and thermal flux distribution due to a small withdrawal of SUS control-rod have been measured for the purpose of confirming the soundness of fuel pin from the viewpoint of the load following operation and fuel design. SUS control-rod (74mm) with the same dimension as those of the demonstration reactor was inserted into the DO moderator of the central region of 0.54wt% plutonium mixed-oxide fuel lattice in 25-cm pitch DCA core. The lower end height of the control-rod inserted was changed from 505mm to 605mm (about 40 change in reactivity). Experimental results for the local power change were compared with calculations obtained from WIMS-D and CITATION codes. The following were concluded from the present study. (1)The maximum power change in outer layer pins of fuel cluster due to a small withdrawal of the control-rod occurs in the nearest fuel pin at the middle position of the axial displacement of the control-rod. (2)Outer layer fuel pin power after 100mm withdrawal of control-rod is (1.120.03) times in maximum as large as that before withdrawal. (3)Local pin power change due to withdrawal of the control-rod occurs mainly in the fuel pins of the nearest and the second nearest fuel clusters to the control-rod. Even in the nearest fuel cluster to the control-rod, power change in the back side fuel pin to the control-rod is very small below 2%. (4)Calculated value of maximum ratio for the outer layer fuel pin power due to 100mm withdrawal of the control-rod overestimate experimental one about 4%.