Pebble Handling

Our basic concept for fuel handling performs pebble assays and sorting inside the reactor vessel, so that transfers of pebbles to and from the vessel will occur in canisters. We do still need to design to filter and recover graphite dust and broken pebble fragments, but expect to have much less than gas cooled reactors given the lubricity of the salts and the much smaller pebble contact forces due to their near-neutral buoyancy. Currently we have the pebble injection system and defueling chute system well defined, and have identified real estate for the remainder, but no detailed design for the necessary components.

Our current focus, because it determines the height and configuration of the reactor building, is to develop the designs for the control rod drive and DRACS systems, which was the topic of our design meeting today. As the design report notes, we are using water-based thermosyphons as an intermediate heat transfer method between the DRACS loop and an air cooled condenser, for the same reasons that the MSBR design used this approach (we want to use flibe in the DRACS, and the constant 100°C temperature of the thermosyphons makes it easier to design to prevent freezing. Right now we are working to try to design each of the 3 50% capacity DRACS loops to be individual, crane movable modules, so that all of the fabrication can be performed in a factory.

We are finding that the vertical stacking problem is leading us to a taller vessel than we would really like (13 m), but it’s thin walled so the mass is still quite reasonable. We are moving forward using this height since we need to do so to be able to work on other parts of the design, and finding approaches to reduce the height will be deferred to a Mk2 design.

Once the DRACS design is more mature, it will be easier to look further at pebble handling, since the pebble handling and DHX systems need to co-exist in the same areas of the reactor.

(Email discussion by professor Peterson)


2013-08-06 Design Meeting Notes

  • Inventory of primary salt: make a list of sub-systems and estimated volumes
  • DHX: need to generate a RELAP model. For twisted tube heat transfer enhancement, use a multiplier, or perhaps other correlation? Ask Ed, read Chinese paper.

Hot and cold leg routing:

  • Hot leg comes straight through fire brick and cavity; cold leg has a 90° elbow and comes parallel to hot leg.
  • Piping is short enough that no supports are necessary inside the cavity. Pipes will be supported by whatever fitting is used when going through the cavity wall.
  • Pipe slopes for draining: cold leg drains to drain tank. Need isolation valve not to drain whole cold leg. On the hot leg, high point is the hot well with primary pumps.
  • Need to calculate friction losses to know salt elevation above downcomer from swelling.

Reactor cavity:

  • Need to design for heater rods sliding down through holes in the reactor vessel flange (2 heaters per cavity block?)
  • Cover gas: only inside reactor vessel, or also in reactor cavity?
  • Inspection of outside of reactor vessel: have a wide enough gap to be able to insert a probe. Or use heater rod channels? (remove them and insert probes in their place)
  • Rationale to use a skirt for reactor vessel support: requirement to have pipe penetrations not move a lot: their level will not move, and under thermal expansion, the rest of the vessel will change dimensions axially.
  • Need to design the insulating material in the cavity to bear some compressive load without damaging the reactor vessel.


  • Next meeting: level swell; salt volume; full pull length for control rods etc.

Managing Gas Explosion Risk

Design basis events – check with Mike


natural gas explosions
how do we prevent back-flow?
we don’t want gas to go up into the CTAH

New components:
back-flow check valve on hot duct
safety-related valve that isolates CTAH on the air side
question: do the hot and cold air ducts run below grade?

can we have a system to keep gas from flowing if there is no air flowing?

spark, or continuously burning pilot light.

how do we do blow-out?
relief valve on hot and cold ducts

need to add by-pass line to connect hot and cold duct

we need multiple ways to prevent back-flow
do we need active air flow?

air-activated gas valves using air from the air cold leg
how do we route a nat gas leak to a vent stack?

natural gas accidents at combined cycle plants, with the steam generator exploding
explosion-proof wall in-between reactor vessel and CTAH
how about the pressure coming up the salt lines?

alternatives to fuel:
1. depressurize natural gas, then recompress it
2. liquid fuel
3. other?