Passive Reactivity Control via Molten Fuel Thermal Expansion

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Solution Overview

Problem

Fast spectrum nuclear fission reactors face challenges in achieving stable operation and efficient thermal generation due to the need for precise control of nuclear reactivity, which is not adequately addressed by existing technologies.

Innovation Solution

The introduction of a passive reactivity control nuclear fuel device that utilizes thermal expansion of molten nuclear fuel under high neutron flux to provide negative power feedback, thereby stabilizing the reactor operation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If active reactivity control systems are used, then reactor stability can be maintained, but device complexity and operational difficulty increase

Engineering Contradiction:

Improvereactor stabilityVSAvoidcontrol system complexity

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

The fuel device performs reactivity control automatically through its own thermal expansion, without requiring external control systems. The molten fuel's temperature-dependent density change naturally regulates neutron flux, making the system self-regulating and eliminating complex active control mechanisms.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The patent replaces mechanical/electronic control systems with a passive physical mechanism - thermal expansion of molten fuel. This substitution eliminates the need for sensors, actuators, and control algorithms, reducing device complexity while maintaining stability.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Device complexity

If passive reactivity control is implemented, then device complexity is reduced, but reactivity control precision may be insufficient

Engineering Contradiction:

Improvecontrol system complexityVSAvoidreactivity control precision

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The invention exploits the temperature-density parameter change of molten fuel to achieve reactivity control. As temperature increases, fuel density decreases, naturally reducing neutron flux. This parameter change provides sufficient precision without complex control systems.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The system implements a natural feedback loop where temperature increases lead to fuel expansion and reduced density, which in turn reduces neutron flux and power generation. This self-feedback mechanism provides precise control automatically.

Inventive Principle:

Principle #23Feedback

3Speed

If molten fuel is used for reactivity control, then fast-acting control is achieved, but thermal management challenges increase

Engineering Contradiction:

Improvecontrol response speedVSAvoidthermal management difficulty

Core Design Contradiction:

SpeedVSTemperature

Solution Approach 1:

The patent converts the harmful effect of heat into a beneficial control mechanism. Thermal expansion of molten fuel, which would normally be considered a thermal management problem, is utilized as the basis for reactivity control. The heat generated by fission directly drives the fuel's density change, providing fast-acting control.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The system operates with molten fuel, utilizing the liquid phase and its thermal expansion properties. The phase state of the fuel (molten) enables rapid response to temperature changes, achieving fast-acting control while managing thermal effects through the inherent properties of the molten state.

Inventive Principle:

Principle #36Phase transitions

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 solution effectively introduces a fast-acting, passive reactivity control mechanism that enhances reactor stability and safety by providing a negative power feedback loop, which helps in maintaining the reactor in a subcritical state during high neutron flux conditions.

Implementation Method 1

thermal expansion of a liquid/molten nuclear fuel under high neutron flux

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS12272466B2Passive reactivity control in a nuclear fission reactor

Publication Date: 2025.04.08 TERRAPOWER LLC

AI summary

A nuclear reactor includes a passive reactivity control nuclear fuel device located in a nuclear reactor core. The passive reactivity control nuclear fuel device includes a multiple-walled fuel chamber having an outer wall chamber and an inner wall chamber contained within the outer wall chamber. The inner wall chamber is positioned within the outer wall chamber to hold nuclear fuel in a molten fuel state within a high neutron importance region. The inner wall chamber allows at least a portion of the nuclear fuel to move in a molten fuel state to a lower neutron importance region while the molten nuclear fuel remains within the inner wall chamber as the temperature of the nuclear fuel satisfies a negative reactivity feedback expansion temperature condition. A duct contains the multiple-walled fuel chamber and flows a heat conducting fluid through the duct and in thermal communication with the outer wall chamber.