抄録
Core-tank of UTR-B is separated in six sections. Each section has a length of 83.3mm in east-west direction, and holds a fuel element which has a length of 76.2mm in the same east-west direction.
It may be expected that the reactivity of this reactor would be changed, if each fuel element is shifted towards the center line of the core-tank or in the opposite direction.
We examined this possible reactivity change due to the slight shift of the fuel elements, and obtained the following results; if each fuel element is shifted toward the center line of the core-tank, the reactivity of the reactor increases and the critical mass decreases. If each fuel element is shifted outward in the opposite direction, the reactivity of the reactor decreases and the critical mass increases.
This effect is smallest in near the center line of the core-tank, and the farther the position is off the center line, the larger the effect becomes. But in near the side of the core-tank, this effect comes small again.
This may be affected by the thermal neutron flux distribution and the importance function. We have measured the thermal neutron flux distribution in the core-tank, and calculated the importance function.
The curve of the statistical weight vs. position of the core-tank had shown to have a same tendency of the experimental results.
