Integral Molten Salt Reactor Core Replacement
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Solution Overview
Problem
Molten salt nuclear reactors face challenges with graphite moderator lifetime, reactor vessel longevity, and primary heat exchanger maintenance due to neutron exposure, leading to increased costs and complexity, particularly in achieving desired 50-60 year operational lifetimes without graphite replacement and ensuring safe storage of radioactive materials.
Innovation Solution
The integration of a graphite moderator and reactor vessel into a single replaceable unit with a higher power density, along with a buffer material for decay heat absorption, allows for simplified maintenance and extended operational periods, enabling the IMSR to function as both a reactor and storage unit, and utilizing a buffer material for phase transition latent heat absorption to manage decay heat.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If graphite moderator is used in MSR to achieve softer neutron spectrum and improved reactor control, then reactor control and fissile inventory are improved, but graphite lifetime is limited due to neutron irradiation causing shrinkage and expansion
Solution Approach 1:
The graphite moderator is divided into multiple replaceable elements or modules. When one element reaches its lifetime limit due to neutron irradiation, it can be removed and replaced with a fresh element, while other elements continue to operate. This segmentation allows continuous reactor operation without shutting down for complete graphite replacement.
Solution Approach 2:
The graphite moderator elements are designed to be nested within the reactor vessel structure, allowing easy insertion and removal. The replaceable elements are positioned within the core region where they can be accessed through the vessel top or side, enabling maintenance without complete reactor disassembly.
2Duration of action of stationary object
If reactor is designed to be large and low power density to extend graphite lifetime, then graphite can remain in vessel for design life, but reactor size and material requirements increase
Solution Approach 1:
Instead of building a large reactor to extend graphite lifetime, the system uses multiple smaller graphite elements that can be independently replaced. This allows maintaining a compact reactor size while achieving extended operational life through modular replacement of irradiated elements.
Solution Approach 2:
The system changes the operational parameter from extending individual graphite element lifetime to extending overall reactor lifetime through periodic replacement of elements. This allows higher power density operation while maintaining extended service life through a replacement schedule based on accumulated neutron fluence.
3Power
If periodic graphite replacement is planned to maintain high power density, then power density is maintained, but reactor must be shut down and opened every few years increasing complexity
Solution Approach 1:
The graphite moderator is segmented into multiple replaceable elements. This segmentation allows replacement of only the irradiated elements while leaving other elements in place, enabling maintenance during shorter shutdown periods and reducing the complexity of complete reactor opening and reassembly.
Solution Approach 2:
Fresh graphite elements are prepared and positioned in advance before shutdown. During the brief shutdown period, pre-positioned elements are quickly installed, minimizing the duration and complexity of the replacement procedure. The replacement process is streamlined through pre-planned access paths and positioning mechanisms.
4Ease of repair
If reactor vessel and primary heat exchangers are integrated with graphite core as sealed unit, then maintenance is simplified and costs reduced, but unit must be replaced as whole after design lifetime
Solution Approach 1:
The reactor system is segmented into a replaceable core unit containing graphite elements and associated components, and a permanent vessel structure. The core unit can be replaced as a module, avoiding the need to replace the entire vessel and heat exchanger system, thus reducing replacement time and resource loss.
Solution Approach 2:
The integrated core unit is designed as a finite-life component that is replaced after a predetermined number of fuel cycles or when graphite elements reach irradiation limits. The permanent vessel and heat exchanger systems are recovered and reused for subsequent core units, minimizing waste and reducing overall replacement time.
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 approach extends the operational life of molten salt reactors by simplifying maintenance, reducing costs, and ensuring safe storage of radioactive materials, while effectively managing decay heat through phase transition and convective heat transfer, thereby enhancing the economic viability and safety of nuclear power plants.
Implementation Method 1
The absorption of decay heat is effected by the buffer material phase transition latent heat, the phase transition being that of solid phase to liquid phase.
Implementation Method 2
The absorption is also effected by convective heat transfer when the buffer material is in the liquid state. The convective heat transfer occurs between the reactor vessel and a heat sink in thermal contact with the buffer material.
Data Source
AI summary
The present relates to the integration of the primary functional elements of graphite moderator and reactor vessel and/or primary heat exchangers and/or control rods into an integral molten salt nuclear reactor (IMSR). Once the design life of the IMSR is reached, for example, in the range of 3 to 10 years, it is disconnected, removed and replaced as a unit. The spent IMSR functions as the medium or long term storage of the radioactive graphite and/or heat exchangers and/or control rods and/or fuel salt contained in the vessel of the IMSR. The present also relates to a nuclear reactor that has a buffer salt surrounding the nuclear vessel. During normal operation of the nuclear reactor, the nuclear reactor operates at a temperature that is lower than the melting point of the buffer salt and the buffer salt acts as a thermal insulator. Upon loss of external cooling, the temperature of the nuclear reactor increases and melts the buffer salt, which can then transfer heat from the nuclear core to a cooled containment vessel.