  Reactivity measurement is an essential part of a zero-power physics test,
which is critical to reactor design and development. The rod drop experimental
technique is used to measure the control rod worth in a zero-power physics
test. The conventional rod drop experimental technique is limited by the
spatial effect and the difference between the calculated static reactivity and
measured dynamic reactivity; thus, the method must be improved. In this study,
a modified rod drop experimental technique that constrains the detector neutron
flux shape function based on three-dimensional space-time dynamics to reduce
the reactivity perturbation and a new method for calculating the detector
neutron flux shape function are proposed. Correction factors were determined
using Monte Carlo N-Particle transport code and transient analysis code for a
pressurized water reactor at the Ulsan National Institute of Science and
Technology and Xi'an Jiaotong University, and a large reactivity of over 2000
pcm was measured using the modified technique. This research evaluated the
modified technique accuracy, studied the influence of the correction factors on
the modification, and investigated the effect of constraining the shape
function on the reactivity perturbation reduction caused by the difference
between the calculated neutron flux and true value, using the new method to
calculate the shape function of the detector neutron flux and avoiding the
neutron detector response function (weighting factor) calculation.
