The detailed measurements of control rod worth and the reactivity coefficients in the asymmetrical control rods arrangement core of "JOYO"; The detailed control rod worth measurement(II)

Omura, Akiko; Yoshida, Akihiro ; Shimakawa, Yoshio; Suzuki, Soju; Kinjo, Katsuya

The "JOYO" modification program, named the MK-III program is now under investigation. The objective of this program is to make "JOYO" more efficient irradiation facility which serve as an irradiation bed for the development of FBR fuels and materials. During the 9th annual inspection, one of six control rods has been relocated from the 3rd row to the 5th row to improve the irradiation capacity, The control rod worth and several reactivity coefficients have been measured before and after the change of the control rod arrangement in detail. The principal results of the measurements are as follows; (1)In the rod drop measurements, the effect of the relative direction of the ex-vessel neutron instrumentation system and neutron source, which affects the accuracy of the measurements, was decreased by operating the measurements based on the experience that has obtained through the last Detailed Control Rod Worth Measurement. (2)The worth of the relocated control rod has decreased to 1/3. (3)The other core neutronics characteristics were not changed by the change of the control rod arrangement. (4)The isothermal temperature reactivity coefficient for the core is -410 %k/k/C, which is almost same at every duty cycle. (5)It was confirmed that the flow rate reactivity coefficient consists of two independent factors, one may causes a reversible reactivity change and the other causes an irreversible reactivity change. Furthermore it is supposed that the threshold flow rate level that occurs an irreversible reactivity change is not the same throughout every duty cycle operation.