Physics & Reactivity

Nuclear physics: fast spectrum

Traditional light-water reactors (LWRs) use water to moderate (slow) neutrons. ReCore explores a sodium-cooled fast spectrum: neutrons remain at high energy.

Why “fast” matters

Fast neutrons can more readily fission many heavy actinides (plutonium, americium, curium, …).

• In a thermal reactor: many of these nuclides build up as long-lived waste because slow neutrons often absorb without fission.
• In a fast reactor: they can become fuel if recovered and returned to the core — the physical basis of actinide multi-recycle (a fuel-cycle goal, not automatic).

Fertile vs fissile: uranium utilization

Natural uranium is mostly U-238 (~99%) with little U-235. Thermal reactors mainly burn U-235 (and some bred Pu). Much of the fertile U-238 ends up as depleted uranium or remains unused in once-through spent fuel.

Transmutation as fuel creation

In a fast spectrum, neutrons can convert fertile U-238 into fissile Pu-239. Over multi-recycle operation, that path can extract far more energy per tonne of natural uranium than once-through LWR use — often summarised as roughly two orders of magnitude under idealized closed-cycle assumptions. That is a fleet fuel-cycle potential, not a single-core guarantee, and it still depends on recycle losses, breeding design, and industrial pyro capacity.

The sodium temperature margin

Coolant physics matter as much as fuel physics.

Thermal inertia and pressure

• Water: boils at 100 °C at 1 bar; LWR cores keep water liquid with ~150 bar class pressure. A large break can flash coolant.
• Sodium: boiling point ~881 °C at atmospheric pressure. The concept hot leg is ~600 °C, giving about 281 K margin to boil if pressure and inventory are maintained.
• Implication: large subcooling to boiling is a favourable safety margin, not a proof that decay heat is removed in every accident sequence without engineered systems. Natural circulation is a design intent.

Thermal vs fast spectrum

Thermal reactors

Most operating reactors are thermal.

• Moderator: water (or historically graphite, etc.) slows neutrons to fission scarce U-235 efficiently.
• Risk character: moderator and void behaviour must be carefully managed; some historical designs (e.g. RBMK) had unfavourable void coefficients.

Fast reactors (ReCore approach)

Fast reactors operate without a moderator so neutrons stay energetic.

• Fuel flexibility: many heavy nuclides can be fissioned if the spectrum and fuel form allow.
• Inherent feedbacks: metallic fuel expansion and related effects provide strong negative temperature feedback that assists safety under many transients. That does not make an uncontrolled excursion “physically impossible” in all conceivable configurations, nor replace engineered shutdown and decay-heat removal. Licensing still requires analysis of design-basis and beyond-design-basis sequences.

Void coefficient

The void coefficient describes how reactivity changes if coolant density falls or voids form.

• Positive (unsafe trend): power tends to rise as coolant is lost (historical RBMK concern).
• Negative (favourable): power tends to fall as coolant density drops.
• Concept intent: core geometry and materials chosen so sodium density loss reduces reactivity. Exact coefficients must be calculated for the final core design — not asserted by slogan.

Negative temperature coefficient

As metallic fuel heats, expansion increases mean free paths and reduces reactivity — a classic temperature feedback. Combined with other effects (structure, coolant), this pushes the system toward lower power when hot. Engineered protection systems remain essential.

Delayed neutrons in a plutonium core

A small fraction of neutrons appear late, after beta decay of certain fission products. That delay is what a control system actually uses. For U-235 the delayed fraction is about 0.65%. For Pu-239 it is about 0.21% — roughly a third. A core started on civilian plutonium therefore sits closer to prompt critical than a uranium thermal core at the same excess reactivity. The number for a real core (β_eff) also depends on spectrum and on which isotopes are present. It has not been calculated for this screening geometry.

The practical consequences are ordinary fast-reactor practice, not a new hazard class:

• control and shutdown have to be fast, and redundant;
• Doppler broadening and axial expansion of metal fuel act in milliseconds, without a signal;
• a Curie-point latch, studied on some SFR programmes as a self-actuated shutdown, is a candidate for that speed. It is not installed in this concept, and it does not replace the engineered shutdown systems.

Even-mass plutonium (Pu-240, Pu-242) and the minor actinides, which absorb in a thermal spectrum and often survive a MOX pass in a light-water reactor, have meaningful fission cross sections once the neutrons stay fast. That is why a fast core can keep using plutonium after a thermal reactor is finished with it. It is a cross-section fact. It is not a measured burnup for this plant.