D J Coates, G T Parks Department of Engineering, University of Cambridge, UK Safety Considerations for the Design of Thorium Fueled ADS Reactors ThorEA Workshop Department of Engineering, University of Cambridge 19th September 2011

Motivation for the Research ADS reactors provide additional reassurance against criticality excursions through sub-critical operation Commercial Driver: Small sub-critical margin (hence small accelerator) Safety Driver: Large sub-critical margin (hence large accelerator) What are the quantifiable safety implications of the size of the sub-critical margin adopted How close is too close and why?

Contents: 1 The Role of Source Neutrons 2 Reactor Shut-down Considerations 3 The 233Pa Effect 4 Equilibrium Shift on Flux Change 5 The Post Shut-down Criticality Excursion

The Role of Source Neutrons 1 The Role of Source Neutrons

Accelerator Boost to the Neutron Balance (Thermal) As burn-up progresses the reactivity of the fuel decreases Additional neutrons can be added to the system with an accelerator

Source Neutrons to Enable Operation “The neutron balance can be regulated such as the reactor would be below criticality if the additional neutrons by the accelerator were not provided” Wikipedia, Subcritical Reactor Principle, 2010 “ Uses a particle accelerator instead of the fuel itself as a neutron source…..causing the chain reaction to go critical or self sustaining” New Energy and Fuel Website, 2010 “ A full fuel load does not provide sufficient numbers of neutrons to sustain the fission chain reaction, instead an accelerator provides the additional neutrons” “The addition of an accelerator to enable subcritical operation supplements the neutron economy further” Bowman, C, D., 1998. Accelerator-Driven Systems For Nuclear Waste Transmutation. Annual Review of Nuclear and Particle Science, Volume 48, 505-56

Additional Neutrons to Achieve Criticality ? Spallation source neutrons to make-up the shortfall The source neutrons are non-multiplying and do not supplement the fission neutrons to enable criticality to be achieved Contribution from fission neutrons

The Source Neutrons Do Not Directly Affect Criticality The source neutrons have no direct effect on the reactor criticality and the reactor will remain sub-critical regardless of the number of neutrons added Note: A power excursion is possible by adding too many neutrons Criticality can only be achieved through a change in the reactor properties: Fuel composition Materials Geometry

Sub-critical 15% Pu Enriched Reactor Operation K = 0.9847 (Fast Reactor) Time Interval Source neutrons Neutrons at start Neutrons absorbed System losses Neutrons generated Neutrons remaining t1 10 5.536 0.885 6.322 9.901 t2 19.901 11.017 1.762 12.583 19.705 t3 29.705 16.444 2.629 18.781 29.413 ti 1016.9 562.91 90.01 642.92 1006.9 ti+1 The neutron population continues to increase until the number of neutrons absorbed and lost is equal to the source plus the neutrons generated The source neutrons are non-multiplying, the neutron loss/production ratio within the reactor remains unaffected by their presence

The ADS Advantages are Underplayed “ An ADS drives nuclear reactions that will stop if the proton beam from the accelerator stops” Nuclearinfo.net This is also true for critical reactors through the insertion of control rods The ADS advantage here is limited to : The faster quasi-electronic switching of the accelerator trip The non-intrusive nature of the shut-down A more significant advantage comes from the sub-critical operating margin which provides further reassurance against a criticality excursion

Operation is Possible for any keff and Accelerator Combination keff and accelerator size do not determine operational viability - this is largely a commercial judgement The accelerator power can be adjusted to produce the desired reactor power output ADS reactors are typically judged against current (critical reactor) efficiencies Keff = 0.0638 100% Thorium Fast Spectrum

ADS Shut-down Considerations 2 ADS Shut-down Considerations

Critical and ADS Shut-down Critical – Control Rod Insertion Has an inherent reduction in the reactivity of the system as a direct consequence of the action Intrusive – requires a clear path ADS- Accelerator Trip No associated inherent reduction in the reactivity of the system Non intrusive The system must be sub-critical for this to work The ADS trip requires the reactor to be sub-critical and remain sub-critical to be effective

3 The 233Pa Effect

Thorium Reactor – Growth in Nuclide Mass The 233U population is largely generated through 233Pa decay and lags the 233Pa population on start-up

Drop in reactivity due to 233Pa decay time on start -up Thorium Reactor – Drop in reactivity due to 233Pa decay time on start -up EA-MC Code Plot Produced by Carlo Rubbia

Equilibrium Shift on Flux Change 4 Equilibrium Shift on Flux Change

Thorium Reactor – Steady-State Equilibrium Populations Adjustments in the neutron flux produce movements in the actinide equilibrium positions

Thorium Reactor – Post Flux Reduction Population Shift As the actinide equilibrium positions shift - the mass of 233Pa evolves into much slower decaying 233U

Thorium Reactor – Post Flux Reduction Criticality Increase The 233U has a larger fission cross-section and produces an increase in reactivity The change in reactivity is time and not burn-up dependent

The Criticality Increase is Self-limiting In a shut-down situation (without control rod intervention) the reactivity increase will be accompanied by an increase in neutron flux This shift in the flux will push the equilibrium populations back towards the starting populations and thus will tend to reduce the reactivity of the system

Post Shut-down Criticality Excursion 5 Post Shut-down Criticality Excursion

ADS operating modes to compensate for reactivity variations: Use rods to continually flatten the reactivity variations and maintain fixed keff Use fixed rods to set maximum keff and use the accelerator to compensate for reactivity movements Note: The bare reactor is critical and requires rods to achieve sub-critical operation 15% Pu Enriched Thorium Fuel Fast Spectrum Bare Reactor

The Fission Reaction Dies Out When The Accelerator Stops ? “ An ADS drives nuclear reactions that will stop if the proton beam from the accelerator stops” Nuclearinfo.net “If the particle beam is switched off, it is impossible for the fuel to enter a chain reaction and cause a meltdown. Instead, the rate of fission will immediately begin to slow and the fuel will eventually cool down and die out” ” COSMOS magazine

Thorium Reactor – Post Shutdown Criticality Increase

Thorium Reactor – Post Shutdown Power Increase The analyses are discontinued at the point at which the reactor reaches criticality

Thorium Reactor – Post Shutdown Time to Criticality

Discussion

The Fission Reaction Dies Out When The Accelerator Stops ? ADS Thorium Reactor The Fission Reaction Dies Out When The Accelerator Stops ? Yes - But only if: The reactor is sub-critical at the moment of tripping and the margin of sub-criticality is sufficiently large to accommodate the subsequent increase in reactivity or the accelerator trip is followed by a control rod insertion Note: accurate measurement of the sub-critical margin is required

Does an ADS Thorium Fueled Reactor with a Small Sub-critical Margin Provide Enhanced Safety ? Yes In sub-critical conditions the accelerator can enable a rapid preliminary shutdown. (But must be followed up by a control rod shut-down)

Conclusions Following a reduction in power thorium reactors undergo an increase in fuel reactivity due to the decay of 233Pa. The size of the reactivity increase is a function of the relative difference in neutron flux and nuclide populations at the time of the event For fast reactors at keff > 0.990 control rods will be necessary to prevent a post shutdown criticality excursion from the 233Pa decay The effect is not an issue for critical thorium reactors (which use rods)

The End