In a series of startup physics tests, measuring the control rod worth is a critical means to determine whether the actual worth of the control rods matches the design values. This test ensures that the reactivity of the reactor can be precisely controlled through the control rods, thereby ensuring the safe operation of the reactor. Traditional methods for this measurement include the boron dilution method and the dynamic rod worth measurement method. Although these methods are now widely used in nuclear power plants (NPP), there is still potential for further improvement in both safety and economic performance. Subcritical control rod worth measurement does not require equipment transformation and on-site operations, which makes it easier to implement whilst control rod worth measurement under deep subcritical conditions is totally a different technique with respect to traditional control rod worth measurements, which changes the reactor condition from the Low Power Physics Test (LPPT) window after reaching criticality to the Criticality Approach Test (CAT) window before reaching criticality. The test conditions vary from near-critical condition to deep subcritical condition, making the core much safer.

This study aims to minimize the risk of core re-criticality during control rod withdrawal by measuring control rod worth within existing test windows without occupying the critical path.

Subcritical control rod worth measurement was implemented within the window of startup physics tests, saving outage time and improving the economic efficiency of the power plant, hence no additional work was required for subcritical control rod worth measurement test, except for collecting relevant data while measuring the worth of control rods during the test. Two functional modules, namely the spatial correction factor calculation module and the data processing and display module were developed for subcritical control rod calibrating system. Under deep subcritical conditions, the traditional point reactor model was no longer applicable due to the significant impact on neutron flux distribution caused by the external neutron sources, therefore, a different approach was adopted to calculate the effects of external neutron sources on neutron flux distribution under deep subcriticality by carrying out a spatial correction of count rate to ensure a linear relationship between the corrected count rate and the subcriticality. Finally, two verification tests of subcritical control rod worth measurement were conducted on the AP1000 reactors at Sanmen Nuclear Power Plant (NPP), the count rate of the source range detector and reactor condition data were collected, and processed for quality improvement and spatial correction of count rate.

Verification results show the linearity of the corrected count rate achieved is above 0.999, and the maximum subcriticality provided is approximately 0.1, indicating that the experimental conditions are in a deep subcritical state. All sets of control rod worth values meet the acceptance criteria requirement of a relative deviation of 10% or an absolute deviation of 75 pcm.

The results met the acceptance criteria of control rod worth, providing preliminary verification of the accuracy and reliability of calculation system under deep subcritical conditions.