1. 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

2. 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?

3. 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

4. The Role of Source Neutrons1
The Role of Source Neutrons

5. 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

6. 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., Accelerator-Driven Systems For Nuclear Waste Transmutation. Annual Review of Nuclear and Particle Science, Volume 48,

7. 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

8. 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

9. Sub-critical 15% Pu Enriched Reactor Operation
K = (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

10. 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

11. 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 =
100% Thorium
Fast Spectrum

12. ADS Shut-down Considerations2
ADS Shut-down Considerations

13. 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

14. 3
The 233Pa Effect

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

16. 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

17. Equilibrium Shift on Flux Change4
Equilibrium Shift on Flux Change

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

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

20. 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

21. 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

22. Post Shut-down Criticality Excursion5
Post Shut-down Criticality Excursion

23. 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

24. 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

25. Thorium Reactor – Post Shutdown Criticality Increase

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

27. Thorium Reactor – Post Shutdown Time to Criticality

28. Discussion

29. 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

30. 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)

31. 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 > 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)

32. The End