§ Research · doctoral design study
The Multispectrum CANDU Reactor puts a small helium-cooled fast-neutron core in the middle of a CANDU 6 thermal core. The fast core destroys surplus weapons-grade uranium or plutonium; the thermal core makes power from natural uranium. Neither core is critical on its own.
§1 Problem
Post-Cold-War treaties dismantled thousands of warheads and released large quantities of highly enriched fissile material. Storing it is a proliferation risk; burning it in reactors turns it into power and into isotopes that are far harder to weaponise.
Thermal reactors burn it slowly and breed new plutonium as they go. Fast reactors burn it efficiently but Canada does not have one. The question the thesis asks: can the reactor Canada already knows how to build be given a fast core?
§2 Concept
Select a model. Geometry is to scale from the thesis tables.
Same pitch as the thermal core. Reached subcriticality after 135 days.
Each core is designed subcritical alone. Only the neutrons each sends the other push the combined system to keff = 1. Drain the moderator, void the coolant, or disturb either core and the coupling breaks and the whole reactor shuts down. That is the defining property of a coupled reactor, and it is why the design starts from coupled-reactor theory rather than from a single-core model.
§3 Method
Three stages, each a paper before the thesis.
Two-point coupled-reactor theory was checked numerically on the Japanese Deuterium Critical Assembly, a two-region core whose inner and outer water levels can be varied. MCNP5 and Serpent gave the same multiplication factors and coupling coefficients as the analytical model.
Full-core CANDU 6 (Gentilly-2) models in MCNP6 and Serpent were run through burnup and compared with WIMS-AECL 3.1.2.1, the lattice code the Canadian industry uses. k-effective against burnup agreed; Serpent was the faster of the two.
With the codes trusted, six MSCR models were built in Serpent: fast-core radius, lattice pitch, number of fast channels and fuel (19.9 % U-235 or 13.9 % Pu-239 MOX) were varied and criticality, flux, power, form factor, burnup and actinide inventories were tracked over years of operation.
Eq. Two-point coupled-reactor multiplication factor
k11, k22 are the isolated cores' factors; k12, k21 the coupling coefficients, computed from track-length flux tallies. Thesis Ch. 5, verified on the DCA with MCNP5 and Serpent.
§4 Result
From the thesis abstract and Chapters 7–8. Models without a headline number are marked as such.
| Model | Fast radius | Channels th / fast | Fast pitch | Fast fuel | Outcome |
|---|---|---|---|---|---|
| I | 108.86 cm | 320 / 32 | 28.575 cm | U-235 · 19.9 % | Same pitch as the thermal core. Reached subcriticality after 135 days. |
| II | 108.86 cm | 320 / 148 | 14.575 cm | U-235 · 19.9 % | Destroyed 10.7 % of fissile isotopes over 1000 days without refuelling. |
| III | 137.32 cm | 292 / 52 | 28.575 cm | Pu-239 MOX · 13.9 % | Largest fast core at the thermal pitch. Detailed results in Chapter 8. |
| IV | 137.32 cm | 292 / 240 | 14.287 cm | Pu-239 MOX · 13.9 % | Highest fissile destruction of the plutonium models: 15.3 % over 2600 days. |
| V | 108.86 cm | 320 / 144 | 14.287 cm | Pu-239 MOX · 13.9 % | Mid-size plutonium core. Detailed results in Chapter 8. |
| VI | 80.39 cm | 348 / 76 | 14.287 cm | Pu-239 MOX · 13.9 % | Smallest fast core, highest consumption rate: 420 days to subcriticality. |
The thesis concludes that destruction of fissile isotopes from dismantled weapons can be achieved in an MSCR, with Model II (uranium) and Model IV (plutonium) the strongest performers, and flags that Models II, IV, V and VI carry form factors above the desired safety margin, so power flattening is the next design task.
§5 Sources