Nuclear Control Rod Drive Mechanism Segmentation for Scram Precision
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing control rod drive mechanisms (CRDMs) in nuclear reactors face challenges such as complex linkage issues, precision problems during normal operation, potential damage during scram events, and reliability concerns in high-pressure and high-temperature environments, especially in internal designs where the CRDM is placed within the pressure vessel.
Innovation Solution
A control rod mechanism featuring a hollow lead screw driven by a motor, with a latch assembly that disconnects from the connecting rod during a scram condition, allowing the control rod to move independently while the lead screw remains operatively coupled to the motor, and a support system for multiple CRDM units at different heights within the reactor vessel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a separable roller-nut assembly is used to engage the lead screw, then the control rod can be quickly inserted during scram by releasing the magnetic force, but the complex linkage adversely impacts gray rod insertion precision during normal operation
Solution Approach 1:
The system is divided into two independent but coordinated mechanisms: a ball nut mechanism for precise gray rod control and a separate roller-nut scram mechanism. The roller-nut assembly can be quickly released during scram without affecting the precision of the ball nut mechanism during normal operation, thus resolving the contradiction between scram speed and insertion precision.
Solution Approach 2:
A clutch mechanism acts as an intermediary between the ball nut mechanism and the control rod. During normal operation, the clutch engages the ball nut for precise control. During scram, the clutch disengages and allows the roller-nut assembly to directly drive the control rod for rapid insertion, eliminating the precision issues associated with the separable roller-nut linkage.
2Ease of operation
If the roller-nut is reattached to the lead screw after scram, then normal operation can resume, but it may not be immediately apparent when contact is reestablished, thus introducing a positional offset
Solution Approach 1:
A position detection system provides continuous feedback on the control rod position and the engagement status of the roller-nut assembly. When the roller-nut reattaches to the lead screw after scram, the feedback system detects the change in engagement state and signals the control system, allowing for immediate correction of any positional offset and ensuring accurate position measurement is maintained.
3Speed
If the lead screw is scrambled during emergency, then the control rod can be quickly inserted, but the threading or structural integrity of the lead screw may suffer irrecoverable damage
Solution Approach 1:
The drive mechanism is segmented into a ball nut mechanism for normal operation and a roller-nut mechanism for scram. During scram, only the roller-nut assembly engages with the lead screw, while the ball nut mechanism remains disengaged. This segmentation allows the lead screw to be driven during scram without the complex linkage of the ball nut mechanism, reducing the risk of structural damage while maintaining scram speed.
Solution Approach 2:
The system is designed with the understanding that scram events may cause damage to the roller-nut assembly or lead screw threading. The ball nut mechanism and its associated precision components are kept separate and protected from scram forces. After scram, the damaged roller-nut assembly can be replaced without affecting the integrity of the ball nut mechanism, thus cushioning against irrecoverable damage to the overall system.
4Device complexity
If the CRDM is placed internally within the pressure vessel, then the number of feedthroughs is reduced, but the complex electro-mechanical CRDM faces difficult structural challenges in the high pressure and high temperature environment
Solution Approach 1:
The CRDM is segmented into functionally independent modules: the ball nut mechanism for precise control, the roller-nut mechanism for scram, and the position detection system. Each module can be optimized for its specific function and replaced independently if damaged by the high pressure/temperature environment, thus improving overall reliability while maintaining the internal configuration that reduces feedthrough complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances precision control, reduces the risk of damage during scram events, and improves reliability by allowing the control rods to move independently, ensuring safe and efficient operation even in emergency conditions like a loss of coolant accident.
Implementation Method 1
a motor operatively coupled with the hollow lead screw to drive the hollow lead screw linearly toward or away from the reactor core
Implementation Method 2
the roller-nut assembly is clamped onto the lead screw by an affirmative magnetic force acting against biasing springs
Implementation Method 3
a release mechanism configured to cause the latches of the latch assembly to open responsive to a scram condition
Data Source
AI summary
A control rod drive mechanism (CRDM) comprises a lead screw, a motor threadedly coupled with the lead screw to linearly drive the lead screw in an insertion direction or an opposite withdrawal direction, a latch assembly secured with the lead screw and configured to (i) latch to a connecting rod and to (ii) unlatch from the connecting rod, the connecting rod being free to move in the insertion direction when unlatched, and a release mechanism configured to selectively unlatch the latch assembly from the connecting rod.