2013-08-02 Design Meeting Notes

Inner reflector design

  • Update: instrumentation channels should go all the way through (top to bottom).
  • In addition to tie rods in lobe corners, some of the channels will be instrumented with neutron flux measurement.
  • Bottom of buoyant control rod channels must be a blind hole, so that pressure differential on the control rods will induce proper snubbing. Need to calculate required length of blind hole below perforated region of the control rod channel (will affect rod length, therefore stacking problem).
  • Must be careful with stiffness of the channel to cope with pressure differential.
  • Consider having some kind of porous structure at the bottom of the channels to ensure that the rods stop without damaging the structure itself if the channel walls are cracked and the pressure differential is not enough to stop the rod. Need a way to monitor if structures have been damaged.
  • Have another inlet flow chanel in the thick bottom part of the reflector that merges into the control rod channels (more flow area, lower flow velocity). Refined flow areas and pressure differential calculations must be performed. Need to take into account entry losses at the bottom of the inflow channels.
  • Add small channels (1 cm diameter) for flow outlet from the control rod channels. Adjust number and spacing of these channels based on desired flow distribution (based on pebble bed dynamics). Probably 2 columns of small channels per control rod channel.
  • Tip of control rods: load with neutron poison? Amount of graphite? Need to make sure not to insert positive reactivity feedback (based on FHR core neutronics analysis)
  • Plan for the Fall: work on COMSOL coupled stress and fluid analysis (Jae? Alex?)



  • Taper entrance to the combustor (curvature radius should be 1/4 – 1/3 of line radius).
  • Secondary hot duct to vent stack should be smaller (can take higher velocity).
  • Annulus around combustor should be larger (1/4 of total area). Wall should be thin (at pressure equilibrium, takes highest temperature).
  • Diamond structures (insulation) should be thinner to be flexible.


  • Thickness of outer insulation will depend on heat losses and need to have outer wall temperature remain below regulatory limits (wall cannot be >50°C for workers protection). Based on fiberglass insulation thermal conductivity and natural convection heat transfer coefficient.

Core design

  • Block with hot leg can be larger to accommodate size of hot leg (low dose rate).
  • Hot duct liner should extend a bit down into the graphite reflector.
  • Need to analyze thermal stress transient when flow reversal through DHX.
  • Need to have small gap (1 cm?) between fixed and movable metal rings in upper core structure.
  • Flow direction in defueling chute: probably want it to be downards so that upper core structures don’t get heated up, but will make defueling more complicated (pebbles cannot be entrained in flow outside of the core).
  • Shell structure (above cold leg, below hot leg): get elevation transition closer to hot leg.
  • Need to work on detailed design of reactor vessel/fire brick type of insulation/liner/concrete.

White Paper 1 – comments from Syd Ball

  1. There is a big concern in modular HTGRs about adequate mixing of the core outlet coolant that gives this “integral average” temperature referred to in this section.  As the molten salt coolant moves through the core, I’d guess the power and flow distributions would be such that a fair amount of uneven heating of the coolant would occur.  Since the overall core delta-T is small compared to HTGRs, the temperature fluctuations would tend to be much smaller; however, because of the much better heat transfer via the salt, the effects of the fluctuations on the structures it encounters might even be larger.  Might be worth looking at.
  2. Note that the Japanese (HTTR) had a huge amount of trouble with their water cooled cavity cooling system – to the point that they “would do almost anything to avoid using a water-cooled RCCS.”  As the FHR design gets further along, I’d suggest reviewing their tales of woe.
  3. Regulatory foundation:  At some point, it would be useful to review the IAEA work in this area.  There was lots done for modular HTGRs [at least in my experience – & maybe similar stuff for fast reactors, not in my experience base].  Also work done on generic advanced reactors.  See IAEA TECDOCs 1366 & 1570. 1366: Advanced nuclear Plant Options to Cope with External Events. 1570: Proposal for a Teachnology-Neutral Safety Approach for New Reactor Designs.
  4. General discussions about PRA (in 4.1): there was I think a real good point made long ago by Bob Budnitz (NRC) about how PRAs should be thought of for the passively safe reactor designs, where “failure probabilities” need to be assigned to passive systems [e.g., how do you fail a heat transfer coefficient?!].  My favorite VG on this idea is attached.

Syd Ball Figure_PRA for Passive Systems