Central Graphite Reflector

  1. anisotropic and isotropic
  • for now we’re assuming that isotropic is best, because it’s easier to manufacture thin slices
  • anisotropic has less crack propagation
  1. suppliers: SGL carbon in Germany, Graphitek US, Tiotin in Japan – see NGNP white paper on this
  2. great resources: Tim Burchill, at ORNL
  3. What is the criterial on peak stress? – ask Tim – can we releave stress by accomodating for geometry changes, and does that allow us to go to higher dose rates?
  4. Creep in graphite is also important; some of the stresses are released through creep
  5. Dave Holcomb should talk to Tim Burchill to figure out these questions

Initial simple analysis:

  1. whole reflector – cylindrical simple model – look at axial stress distribution, then consider how the slicing releaves these stresses
  2. single lobe – get dose and temperature gradients to figure out the hoop stresses on the lobe

Reactivity Control

  • are blades and control rods independent control mechanisms?
  • can we use ATWS as our independent S/D mechanism?
  • need to understand what NRC requires in terms of independence. check against GDC. look at what GE does in BWRs (poison injection)
  • the buoyant rods aren’t enought at cold conditions to S/D with 0.95, but 0.90 might be good enough
  • what is the lifetime of the S/D rods? the tips burn out, because they’re used as control rods.
  • need to overcome Xe –
  • after Mark 1 report is done – need to do neutronics benchmark exercises with another group, to verify our work

TH modeling and Tritium Management

Need a system-level TH code

  • Heat transfer coefficients in the core
  • Loss coefficients in the bottom of the central reflector


  • graphite pebbles – what kind of residence time and type of graphite to use for the graphite pebbles?
  • tritium diffusion through oxidized kanthal
  • outline future steps in the report – important for advisory commitee
  • do we need a gas recovery system for tritium clean-up from the cover gas?
  • is tritium absorption and diffusion in the graphite sensitive to deutron dose (irradiation damage) of the graphite?

Fuel recirculation – External design review

Full core unload: graphite pebbles are refueled first and replaced by fuel pebbles to allow for more fuel cooling time; and need to have sufficient control rod worth to keep the core subcritical. In this situation can the S/D blades be used to keep the core subcritical?

One canister every 2-3 weeks; Is criticality safety an issue? How about if it’s immersed in water?

How much time is required for an assey on a pebble? 10s – reported in the IAEA report.

Foreign debree identification and removal. What happens to the pieces of fuel that are heavier than the coolant?

Defueling chute functional requirement:

  • entrain heavy pebble fragments and remove them our of the core.
  • where do they drop of out the flow, and how do we take them out?
  • eg: likely to drop off in the bottom of the core, or in the hot well

Vacuum pump device that goes to the bottom of the vessel, and cleans it out.


Coolant flowpaths – External Design Review Meeting

  • minor bypass paths – show with think dotted lines
  • total bypass flow budget: 10%
  • consider replacing the fluidic diode with a check valve with a melting disk
  • check valve: ball sealing with drag only, not a spring

Central reflector: show pebbles as a scale reference

Can pebbles be trapped in the slots of the central reflector?

Need to specify pumping power in the report.

Can we predict the total bypass flow through gap openings due to differential thermal expansion? (Regis)

  • Is there a critical dimension? If the space is below XX mm, then is the flow resistance so high, that it becomes negligible?
  • PBMR was convinced that GA was off on the by-pass flow on a factor of 3 or 4