Reactivity Control Device for Nuclear Fuel Storage
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Solution Overview
Problem
High capacity dry storage canisters for nuclear fuel lack sufficient neutron absorption capability to meet the NRC's sub-criticality criterion, necessitating reliance on boron credit or burn-up credit, which is uncertain and jurisdiction-dependent.
Innovation Solution
A reactivity control device comprising a top tube sheet with vertically elongated neutron absorber rods and a floating guide plate, which is inserted into guide tubes of the fuel assembly to enhance neutron absorption and control reactivity without relying on boron or burn-up credit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high capacity dry storage canisters use tightly packed fuel assemblies to maximize storage density, then the number of fuel assemblies accommodated is increased, but the neutron absorption capability becomes insufficient to meet sub-criticality criteria
Solution Approach 1:
The neutron absorption function is segmented from the canister structure itself and distributed through individual absorber rods inserted into each fuel assembly's guide tubes. This allows each fuel assembly to have dedicated neutron absorption capability while maintaining tight packing density in the canister.
Solution Approach 2:
Neutron absorber rods serve as intermediary elements inserted between the canister structure and the fuel assemblies. These rods provide the necessary neutron absorption function without requiring the canister itself to have complex absorbing structures, enabling both high density and sub-criticality assurance.
2Reliability
If traditional control rod assemblies are used to ensure sub-criticality, then neutron absorption capability is sufficient, but the device complexity and cost increase
Solution Approach 1:
The complex control rod assembly mechanism is extracted and replaced with simple, passive neutron absorber rods that can be manually inserted into guide tubes. This eliminates the need for complex drive mechanisms, positioning systems, and control electronics while maintaining sufficient neutron absorption capability.
Solution Approach 2:
The invention uses simple, inexpensive neutron absorber rods that can be easily replaced if needed, rather than expensive, complex control rod assemblies. The rods are straightforward cylindrical elements filled with neutron-absorbing material, dramatically reducing system cost and complexity.
3Device complexity
If reliance is placed on boron credit or burn-up credit to achieve sub-criticality, then additional neutron absorption structures are avoided, but the uncertainty and regulatory jurisdiction dependence increase
Solution Approach 1:
Neutron absorber rods are pre-inserted into the fuel assemblies during storage preparation, providing immediate and certain neutron absorption capability. This eliminates the need to rely on future burn-up accumulation or soluble boron additions, ensuring sub-criticality is achieved from the outset regardless of regulatory jurisdiction.
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
The device effectively decreases the reactivity of nuclear fuel assemblies, allowing for safe storage and transport without boron or burn-up credit, providing a cost-effective alternative to traditional control rod assemblies.
Implementation Method 1
an array comprising a plurality of vertically elongated neutron absorber rods fixedly attached to the top tube sheet
Data Source
AI summary
A reactivity control device for storing nuclear fuel includes a top tube sheet, plurality of neutron absorber rods fixedly attached to the sheet, and a floating guide plate slideably mounted on the absorber rods for upward and downward movement between a lower position and an upper position. The reactivity control device is insertable into a spent fuel assembly in either wet storage in a spent fuel pool or dry storage in a canister. The absorber rods are inserted into empty guide tubes when the fuel assembly is removed from the nuclear fuel core normally occupied by control rods when in the reactor vessel. During installation, the device is lowered to insert the absorber rods into the guide tubes. The guide plate first engages the fuel assembly as the rods continue to slide through the plate until the tube sheet then engages the plate signaling the device is fully installed.