Thorium — The Nuclear Physics of Thorium: The Fertile-to-Fissile Cycle
Thorium — The Nuclear Physics of Thorium: The Fertile-to-Fissile Cycle
Why Thorium Cannot Directly Sustain a Chain Reaction
Thorium-232 is fertile but not fissile. The distinction is fundamental:
- Fissile: A nucleus that can be split by a low-energy (thermal) neutron to release energy and additional neutrons, sustaining a chain reaction. The three fissile isotopes are U-235, U-233, and Pu-239.
- Fertile: A nucleus that cannot directly sustain a chain reaction but that, when it absorbs a neutron, transforms into a fissile isotope through radioactive decay.
Thorium-232 (six protons + 142 neutrons = 90 protons total = thorium) when struck by a neutron does not immediately fission. Instead it becomes Th-233 and begins a transmutation chain.
The Breeding Chain: Th-232 to U-233
The thorium-to-uranium-233 breeding chain proceeds as follows:
Step 1***: Th-232 absorbs a neutron → becomes Th-233 (unstable)
- Thorium-232 + neutron → Thorium-233
- This absorption is the key initiating step; Th-232 has a large thermal neutron capture cross section (7.4 barns), making it an efficient neutron absorber
Step 2***: Th-233 undergoes beta decay → becomes Pa-233 (protactinium-233)
- Thorium-233 → Protactinium-233 + beta particle + antineutrino
- Half-life: 22.3 minutes
- This step is very fast; Th-233 essentially immediately becomes Pa-233
Step 3***: Pa-233 undergoes beta decay → becomes U-233 (fissile)
- Protactinium-233 → Uranium-233 + beta particle + antineutrino
- Half-life: 26.967 days (approximately 27 days)
- This is the critical delay in the thorium cycle: approximately one month is required for Pa-233 to decay to U-233
- This delay has significant engineering consequences for reactor design and fuel management
Step 4***: U-233 is fissile — it can sustain a chain reaction
- When U-233 absorbs a thermal neutron, it fissions with high efficiency
- The fission of U-233 releases approximately 200 MeV of energy (comparable to U-235)
- It also releases approximately 2.49 neutrons per fission (the η value) — slightly higher than U-235 in a thermal spectrum
- These neutrons can sustain the chain reaction AND breed more U-233 from thorium
The Breeding Ratio and Why It Matters
A key concept in thorium reactor design is the breeding ratio — the number of new fissile atoms (U-233) created per fissile atom consumed. If the breeding ratio exceeds 1.0, the reactor is a breeder***: it produces more fissile fuel than it consumes. This is the theoretically achievable goal of a Liquid Fluoride Thorium Reactor.
Why is U-233 particularly good for thermal spectrum breeding?
η (eta) value***: This is the number of neutrons released per neutron absorbed by the fissile material. For breeding to be possible in a thermal neutron spectrum, η must be greater than approximately 2.1 (one neutron to sustain the reaction; one to breed new fuel; some margin for parasitic absorption losses).
| Fissile Isotope | η in Thermal Spectrum | Can Breed in Thermal Reactor? |
|---|---|---|
| U-233 | ~2.49 | Yes — the only fissile isotope capable of thermal spectrum breeding |
| U-235 | ~2.07 | No — just barely below the threshold; cannot breed in a practical thermal reactor |
| Pu-239 | ~2.04 | No — insufficient margin in the thermal spectrum |
This is one of the most important facts in nuclear physics for energy purposes: U-233 is the only fissile isotope that can achieve a breeding ratio greater than 1.0 in a thermal neutron spectrum. U-235 and Pu-239 can only breed in fast neutron spectra (requiring fast breeder reactor designs, which are more technically challenging and expensive). The LFTR, using a thermal spectrum, can theoretically breed its own fuel from thorium continuously — a truly self-sustaining energy cycle.
Neutron Cross Sections: Technical Specifications
| Property | Value | Significance |
|---|---|---|
| Th-232 thermal neutron capture cross section | 7.4 barns | High value; Th-232 is an efficient neutron absorber — good for breeding |
| U-233 fission cross section (thermal) | 531 barns | Very high; U-233 is a highly efficient thermal fuel |
| U-233 capture cross section (thermal) | 46 barns | Relatively low compared to fission cross section; good for fuel efficiency |
| U-233 η value (thermal) | ~2.49 | Sufficient for breeding with margin; unique among fissile isotopes in thermal spectrum |
The Protactinium Problem
The 27-day half-life of Pa-233 creates a specific challenge: during reactor operation, Pa-233 is accumulating in the fuel salt. If Pa-233 absorbs a neutron before it decays to U-233, it becomes Pa-234, which decays to U-234 (a non-fissile isotope) — a neutron is wasted and no fuel is bred.
In a high neutron flux environment, neutron absorption by Pa-233 can significantly reduce the breeding ratio. The LFTR design addresses this by continuously removing Pa-233 from the core (where neutron flux is high) to a lower-flux region where it decays safely to U-233, which is then returned to the reactor as fresh fuel. This is one of the key innovations of the LFTR design.
