Defueling and Salt Inventory

  • If we de-fuel the core without adding coolant, the level of the coolant and the pebble bed will drop below the defueling chute, making it impossible to fully defuel the core.
  • So we’ll need to use the coolant from the CTAH to make up for the coolant level drop in the core, with defueling.
  • Sampe problem for initial fueling.
  • Unless we use some form of stand-in pebbles, but they can’t be graphite, because we create an overmoderated core when we stansition between fuel and stand-in pebbles.

ATWS Modeling

Inputs for TH from neutronics:

  1. power dristribution
  2. energy deposition in the graphite (neutron-gamma heating)
  3. energy deposition in the coolant (1% – generally neglected)
  4. averaging method for the coolant and kernel temperatures (right now: volumetric averaging weighed on power density)
  5. averaging method for graphite temperature (flux weighing)

does BU swelling/shringking change temp reactivity of the graphite?

  • BU-dependence of thermal expansion coefficient
  • Do we see less number of pebbles in the bed because of the swelling of the central reflector? – ask Mike

What’s the contribution of voiding vs. scattering in the graphite reactivity coefficient? – Madicken to check this

To Do: Add “ID key” for input decks – Madicken and Katy to do this

Temperature Reactivity Coefficients as of Right now:

  • All coolant (core + channels): -0.64 pcm /K
  • Coolant in the inner reflector channels: +0.36 pcm/K (likely on the order of +0.6 pcm/K, because the error bars are on this order)
  • Collant in IR + OR channels: +0.61 pcm/K
  • Coolant in the OR channels: ~0
  • Graphite IR: +1.45
  • Graphite OR: +0.67
  • Both graphite blocks: +1.89
  • Graphite pebbles: [Madicken to update]
  • Graphite in the fuel pebbles: [Madicken to update]
  • Graphite in reflector pebbles + fuel pebbles: [Madicken to update]
  • All graphite in the core (pebbles+blocks: [Madicken to update]

Uncertainty in reactivity coefficients:

  • Madicken to do a one-page design calculation on uncertainty calculation of thermal expansion coefficients; Katy/Jeff to review this; Raluca to approve it

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.


Core power level

What might limit the system is the 2.5m meter active core envelop peterson wants to keep.

If you increase power density more neutrons are lost to Xe because it equilibrate to a higher concentration at higher power densities and the fuel temperature reduces reactivity because the negative temperature feedback and that reactivity penalty eats into your burnup.

Also, the higher flux intensity will damage the inner graphite reflector at a higher rate so you’ll hit your radiation damage limit faster.

So you can go to 300MW, but you have to sacrifice something in terms of power density or burnup or shutdown frequency.


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.