First Application of Large Reactivity Measurement through Rod Drop Based on Three-Dimensional Space-Time Dynamics

TL;DR

This paper introduces a novel three-dimensional space-time dynamic approach to improve reactivity measurement accuracy in nuclear reactors using rod drop experiments, achieving significant reactivity detection over 2000 pcm.

Contribution

It proposes a modified rod drop technique that constrains neutron flux shape functions based on 3D dynamics, reducing perturbations and improving measurement accuracy.

Findings

Measured reactivity over 2000 pcm using the new method

Reduced reactivity perturbation by constraining flux shape functions

Validated the approach with Monte Carlo and transient analysis codes

Abstract

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.