Backup Control Rod Positioning for Fast Reactor Void Reactivity
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Solution Overview
Problem
Fast breeder reactors face challenges in managing void reactivity, which can lead to core damage due to positive void reactivity coefficients, especially when the primary cooling system fails, and existing methods to reduce structural material volume in sodium plenums have limited effectiveness.
Innovation Solution
The implementation of a sodium plenum structure with the tip of the primary control rod inserted into the core fuel region and the backup control rod positioned above the upper end of the core fuel region, reducing void reactivity by increasing neutron leakage and absorption, thereby enhancing safety during transient events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the primary cooling system fails and scram fails simultaneously, then coolant temperature rises leading to core damage, but implementing passive countermeasures like SASS increases system complexity
Solution Approach 1:
The patent implements a self-actuated reactor shutdown system (SASS) that uses magnetic force to hold control rods in an elevated position during normal operation. When coolant temperature rises indicating cooling system failure, the magnetic force is lost and control rods automatically fall to shut down the reactor. This beforehand cushioning mechanism prevents core damage by preparing the shutdown capability in advance without requiring active intervention during emergencies.
Solution Approach 2:
The SASS is designed to be self-actuating rather than requiring external power or control systems. The control rods are held by magnetic force during normal operation and automatically drop when the magnetic field fails due to coolant temperature rise. This self-service mechanism eliminates the need for complex external shutdown systems while maintaining high reliability.
2Reliability
If structural material volume in sodium plenum is reduced to decrease void reactivity, then void reactivity coefficient improves, but effectiveness is limited
Solution Approach 1:
The patent segments the control rod system into two independent systems: primary control rods with tips inserted into the core fuel region for normal power adjustment, and backup control rods with tips positioned above the core fuel region for void reactivity control. This segmentation allows each system to perform its specific function optimally without compromising the other.
Solution Approach 2:
The backup control rods act as an intermediary mechanism specifically for controlling void reactivity. By positioning their tips above the core fuel region and using them in conjunction with the primary control rods, the system can adjust neutron leakage and absorption characteristics to control void reactivity without significantly impacting normal power operation.
3Reliability
If backup control rod tip is positioned in sodium plenum region, then void reactivity is reduced through increased neutron leakage, but neutron absorption capability may be compromised
Solution Approach 1:
The patent applies local quality by positioning the backup control rod tips specifically in the sodium plenum region above the core fuel region. This localized positioning allows the backup control rods to influence neutron leakage from the core into the plenum, thereby controlling void reactivity. The primary control rods remain inserted into the core for their neutron absorption function, while the backup rods provide localized control of void reactivity through their strategic positioning.
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 configuration effectively reduces void reactivity, improving the reliability and safety of fast reactors by increasing neutron leakage and reducing the required plutonium enrichment, thus mitigating risks during unprotected loss-of-flow and over-power conditions.
Implementation Method 1
The void reactivity is reduced by arranging a tip of the backup control rod above an upper end of the core fuel region for operation, thereby increasing neutron leakage from the core fuel region to the sodium plenum region
Implementation Method 2
Among neutrons generated by a fission reaction that occurs in the fuel assembly loaded in the core fuel region, the neutrons that leak from the core fuel region are absorbed by the depleted uranium (U-238) in each fuel rod of the blanket fuel assembly loaded in the blanket fuel region
Implementation Method 3
The primary control rod is used for changing a reactivity associated with burnup and adjusting the power density distribution
Implementation Method 4
when a temperature of a coolant outlet rises, a magnetic force for holding the control rods is lost and the control rods automatically falls to shut down the nuclear reactor
Implementation Method 5
when a temperature of a coolant outlet rises, a magnetic force for holding the control rods is lost and the control rods automatically falls to shut down the nuclear reactor
Data Source
AI summary
A fast reactor core includes a sodium plenum installed above the fuel. The sodium plenum is capable of reducing a void reactivity. During operation, a tip of a primary control rod is inserted in a core fuel region, and a tip of a backup control rod is arranged near an upper end of the sodium plenum.