Actinide Abundance in the Thorium Fuel Cycle

Plutonium Abundance

A widespread number of publications make claims that indicate that the thorium fuel cycle does not produce any plutonium. A list of some of these is given below with quotes:

“The absence of ²³⁸U in the process also means that no plutonium is bred in the reactor.”

[Dean 2006]

“For incineration of WPu [weapons grade Pu] or civilian Pu in ‘once-through’ cycle, (Th, Pu)O₂ fuel is more attractive, as compared to (U, Pu)O₂, since plutonium is not bred in the former and …”

[IAEA 2005]

“It [their reactor design] is transportable and does not produce plutonium or other transuranic elements in significant quantity.”

[Thorenco LLC 2009]

“In the absence of ²³⁸U, the cycle produces virtually no plutonium or other transuranic elements

“[title] (a) Producing no plutonium [body text] This is true of the pure thorium cycle …”

[Wilson and Ainsworth 2002]

“The fuel cycle can also be proliferation resistant, stopping a reactor from producing nuclear weapons-usable plutonium.”

[Beedie 2007]

The rigor of these statements has been tested plutonium [Coates and Parks In Press 2010]. While under specified circumstances it is true that the thorium fuel cycle does not produce plutonium in significant quantities, the statements do not accurately convey the mechanics of actinide generation and incineration in a fast thorium reactor. For a fast thorium reactor operated over an extended duration all of the actinides tend towards equilibrium abundances, regardless of the initial fuelling conditions. Fast thorium reactors can therefore act as generators or incinerators of plutonium, subject to the initial fuel mix and duration of burn up.

Dean, T., 2006, “New Age Nuclear” COSMOS Magazine, 8. www.cosmosmagazine.com/node/348/ [Accessed 23th Feb 2010]

IAEA, 2005, “Thorium fuel cycle — Potential benefits and challenges”, International Atomic Energy Agency, IAEA-TECDOC-1450

Thorenco LLC, 2009, “Thorium: The Preferred Nuclear Fuel of the Future” www.thorenco.com/index.html [Accessed 23th Feb 2010]

Wilson, P.D. and Ainsworth, K.F., 2002, “Potential Advantages and Drawbacks of the Thorium Fuel Cycle in Relation to Current Practice: a BNFL View.” In Thorium Fuel Utilization: Options and Trends, International Atomic Energy Agency, IAEA-TECDOC-1319, 305-315

Beedie, M., 2007 “Thorium: Cleaner Nuclear Power?” power-technology.com website. www.power-technology.com/features/feature1141/ [Accessed 9th March 2010]

Coates, D.J. and Parks, G.T., 2010, “Actinide evolution and equilibrium in fast thorium reactors” In press at Annals of Nuclear Energy

Actinide Evolution and Equilibrium in the Thermal and Fast Fuel Cycles

Evolution of the relative abundance of plutonium in a 28.5 tonne fast reactor for two different initial fuel mixes. The spectrum is taken from [Coates and Parks In Press 2010].

In the thermal energy spectrum certain nuclides among the actinides either do not fission, or have very small fission cross sections. The transmutation of these nuclides therefore tends only to occur as a result of neutron interactions, typically (n,γ). In cases where the transmutation products of these actinides have a long half-life, the net effect is a continual build up of these nuclides and their (n,γ) products over any reasonable timescale that the reactor might be operated. The reactor actinide mix is therefore in a continual state of evolution and does not reach a steady state.

In a fast reactor all actinides are fissionable. Due to this the actinide abundance in fast reactors can reach steady state equilibrium on a timescale comparable with reactor lifetimes plutonium [Coates and Parks In Press 2010]. The equilibrium is a balance between the rate of population of a nuclide (through neutron capture and the decay of parent nuclei) and the depopulation of that nuclide (through neutron capture, decay of the nucleus and also from induced fission). The equilibrium abundance can largely be controlled by the reactor neutron flux profile. For an example case of a lumped reactor with a homogeneous flux profile containing 28.5 tonnes of fuel, it has been shown that this equilibrium is more or less reached after > 70 years of operation (subject to the initial fuel mix). This assumes that fission fragments are periodically removed from the reactor and that it is topped every 5 years with additional thorium, see the figure to the right. In the figure the resulting abundance of plutonium isotopes are shown for a pure and a 15% plutonium enriched thorium fuel. In each case the equilibrium plutonium abundance is identical. The fast thorium reactor can therefore be either a breeder of burner of plutonium, as is also the case for all other actinides.

It is important to recognise that adding ²³⁸U into an equilibrated fast reactor initially results in a significant growth in the plutonium content before the fuel mix returns to equilibrium. If extracted after ~ 5 years, the plutonium will be between 76% and 82% ²³⁹Pu enriched.

Coates, D.J. and Parks, G.T., 2010, “Actinide evolution and equilibrium in fast thorium reactors” In press at Annals of Nuclear Energy

The Role of Protactinium

Neutron capture cross-section data for ²³³Pa according to the NNDC database [NNDC 2010].

Protactinium has a significant role in the thorium fuel cycle, it is necessarily populated as ²³²Th breeds fissile ²³³U. The most abundant isotope is ²³³Pa, which is primarily populated via the reaction channel:

²³²Th(n,γ)²³³Th $\rightarrow$ β^-($T_{1/2} = 22$ minutes) $\rightarrow$ ²³³Pa ($T_{1/2} = 27$ days).

Through β^--decay, ²³³Pa directly populates fissile ²³³U, however a characteristic of ²³³Pa is its large neutron capture cross-section. The cross-section data is given in the spectrum in the figure to the right. In cases where the reaction, ²³³Pa(n,γ)²³⁴Pa, occurs ²³⁴U is populated shortly following, due to the half-life of ²³⁴Pa being only $T_{1/2} = 6.7$ hours. If ²³⁴U does not fission itself (in the fast spectrum this is possible, but at thermal and epithermal neutron energies neutrons are nearly always captured) then ²³⁵U is populated. Energy is released from the fission of ²³⁵U, however the decay requires a total of 4 neutrons. The half-life of ²³³Pa ($T_{1/2} = 27$ days) may be compared to ²³⁹Np ($T_{1/2} = 2.4$ days), which is equivalently a β^--decaying mediator, but for ²³⁸U(n,γ)²³⁹U and fissile ²³⁹Pu. The affect of ²³³Pa in thorium fuel compared to ²³⁹Np having a short half-life in uranium fuel is that the time delay between populating the fission precursor nucleus and the release of fission energy is longer for the thorium fuel cycle. The build up of reactivity is therefore longer when switching on the reactor and concordantly the cool down period following a shutdown is also extended.

NNDC, Updated October 2009, “σigma Evaluated Nuclear Data Files (ENDF) Retrieval and Plotting” Version 3.1, National Nuclear Data Center, USA. http://www.nndc.bnl.gov/sigma/ [Accessed 9th March 2010]