FHR External Design Review Meeting

Identified gaps

1. Alloy selection

2. Tritium => robust oxide permeation barrier – diffusion bonding of anodized alloys with structural metals

3. Detailed design & development of validated models for DRACS

4. Normal shut-down cooling system.

5. Material inputs

6. Power conversation system maintenance (including impact of air ducts).

7. Concept of maintenance and removal of equipment (using separate hot machine shop)

8. Thermal stresses management in upper reactor internals (100 deg C DT)

9. NACC mods-ducts from single side.

10. Can we eliminate requirement for off-sire power for safety? Station black-out.

11. Construction cost/schedule model

12. Need for pilot-scale plant first?

13. DRACS gaps – # modules?

14. Where to put thermocouples in the primary system

15. Approach to trace heating (circulate gas?) – in TCHX

16. Complete and document DRACS design calculations.

17. Study reactor coping times, and instrumentation needed.

e.g.: how much does the DRACS system salt heat up if it takes the energy during the first 24 hours after SCRAM – i.e. no heat sink). Coping times (a) time to metallic components overheating; (b) time to freezing and plugging up

18. Instrumentation summary. e.g. how to we measure the power extracted through TCHX? We have a gap on the mass flowmeter, and we have a gap on the bulk temperature sensor.

19. RELAP validation data: CIET, also ORNL salt loop.

20. Assess the thermal hydraulics behavior of the pebble bed. Pebble bed heat transfer (separate effects tests). Build oil loop to scale the salt pebble bed at ORNL?

21. Add an ISFSI inside PA

22. Volume of salt in pebble handling system

23. How to insulate the pebble handling system

24. Canister cooling system

25. Argon activation – what is the solubility in flibe?


Other discussion notes

Question: to what temperature do you need to raise the pebbles to get the tritium out of them?

Does Ar cover gas activate under neutron irradiation? If not, the alternative is Helium.

  • Raluca TO DO: Find Argon solubility in flibe

core pressure drop: must keep head below 2 meters

can we do an inductively heated oil-cooled packed bed?

can we get a fairly uniform power deposition in the bed? – talk to Grady about how to do uniform power density in an inductively-heated bed.

IR absorption for flibe hasn’t been measured in the 600 deg C range

we need a PIRT for the pebble bed core


sensitivity study for the friction factor coefficients in the core; is it more important than getting heat transfer coefficients? is anisotropy important?


Pebble Bed Modeling, in preparation for design review on May 7-8


  • Thermal radiation
  • Salt as a participating media
  • Optimize flow distribution in the core
  • Friction coefficients
  • Benchmark problems?
  • PIRT: reference design (set of designs to be considered: plate fuel – anaisotropic porous medium with correct modeling for dispersion across channels; ordered packing pebbles; random packing pebbles), and well designed transients (reactivity insertion, ATWS)
  • develop a preliminary PRA?




  • will change inlet conditions so that m(z) can be specified
  • will change outlet conditions, to add a porous zone with no friction loss; test that model converges, and that model computes zero pressure drop on this “ghost” zone; test that we can specify a z-dependent porosity.


  • will set up optimization problem, and do a couple of test runs


  • will do core flow optimization

12-19-2013 Design Meeting

Flow from the central reflector, as currently designed, is potentially a source of lots of bypass flow.
You could change the geometry of the slots to increase the flow resistance.
The main problem with this type of bypass flow will mean that some of the flow coming out of the core region will be above 700 degrees.
if the delta T is 20%, that’s a problem, because the core outlet will have to e mixed.
thermal power is generally flow rate * delta T

There are two different atws events.
one is LOHS : the pumps though, are still running. so, we have the capability to tune the amount of bypass flow going through the DHX. The flow pattern in this case is that the flow going through the core is 98% of the flow. 2% of the flow may be going through the DHX heat sink. However, we may not be removing heat efficiently in the DHX because it’s designed to avoid being a parasitic heat sink. A good question is what kindof parasitic heat load we get out with the DHX? Another question is whether we want those pools to be constantly be sitting there at 100C? A lower temperature may be better to avoid the concrete degradation. Having a liner with active cooling for the water pools is good. That feature would also be a good example for best practice because you can add a leak checker.

the other LOFC is less of a problem because of natural circulation.

Mike : For completeness, there’s another ATWS scenario that includes just one pump shutting down. This is an imbalanced situation in which you may see in the pumps or other features as well.
We may need to think about the single component failure accidents as a design basis concern. For this reason, the situation in which one pump shuts down is no longer a BDBE, but, rather, a DBA.

Per: We have the question about when to interrupt the supply of energy to something. So, since there are many diverse ways to interrupt power to something, good failures are failures that occur during power interruption and fail to a safe state. Still, there is sometimes the situation in which, mechanically, the component won’t fail successfully to a safe state. Also, there may be something that keeps power interruption from happening successfully, so we want to try to make sure that the safety systems are effectively physically secured because all you have to do is to interrupt malevolent actors before thety can break into containment to access equipment that they could destroy to disable expected power interruption and other safety situations.

Nico : Using Tommy’s decay heat curve, the pools can be 30 cubic meters large. This is based on 24 hours of use, using two tanks and full evaporation. This is without any re-condensing.
Per: We also have to think about the way the freezing problem is handled. You may be able to do this by activating some sells and allowing air flow only into some cells under cold ambient conditions. In this case, then, you just cool a subset of the condenser surface.

Mike: could we use our very nice heat sink to help the condenser behavior ? Or, use our very nice heat source to help the condenser behavior in this freezing situation? or
Per: Maybe we could just actively cool the secondary liner system between the pools and the concrete. Additionally, the cooling water provided to the motors may be provided by some other heat management system in the building. (fan coolers, HVAC system… ) We also should have redundant ways to cool the reactor cavity, as an investment protection system.
Raluca: Do we have a plan for such a redundant manner of concrete cooling.
Per: We could have cooling tubes that are fed by more than one cooling system. You could have three tubes that alternate going up each of the flat surfaces that alternate between three trains of service water supplied by different manifold pipes. The more difficult thing is how do we provide cooling to the thermal shield.
Raluca: That’s a good project for the 170 class!

Raluca: So, for the design of the DRACS, in addition to a safe failed state, should we also design for no-power instrumentation.
Per: First, what is a safe failed state for the DRACS? It’s likely to be a fully operational state. That means that in the longer term, we need to have energy to support the dracs and possibly need heat to avoid freezing in the dracs and heat exchangers. It may be possible to design the dracs in such a way that they fail elegantly when the salt starts to freeze in them. The dracs will certainly stop heat removal from the core.

Raluca: If you froze the downcomer, then you could potentially have freezing in the core.
Per : Temperature measurements will be important. The question is, can we measure flow?
Raluca:  What about pressure drop?
Per: The delta Ps might not be an easy way to determine flow.
Raluca: It might be that the difference in elevation between the surfaces, plus the pressure drop would give the flow.
Per : That’s worth checking

Tommy: Whatever happened to us using dowtherm for freezing experiments?
Per: It may be difficult to argue that dowtherm would behave like salt.

Does sinap have a reports repository in english?

We’ll want to focus in comsol on the biggest losses.
the ratio of DHX pressure drops to the core in the Annular AHTR was no more than two.
This points to having a strong basis for characterizing the DHX pressure drops, etc.
We’re likely to see the same stratification in natural circulation.
We should see reynolds drop for lower power levels as well.
Once we do this with better analysis, then there will be a reasonable basis with which to do some parametric analyses and sensitivity studies to see where we can make changes and gain efficiency.
Comparison between the DHX and TCHX will be important too.
Do we want to use twisted tubes in the TCHX?

Per: Discusses the plan for filling ciet.
Question: how do we pump this container if we want to empty it? (there is a little pump that can be used to prime it before siphoning.)
Also, how do we purge gas out of it?
Raluca: you can do it through ciet.

2013-12-04 Design Meeting Notes

Upper core internals

  • With the 3 DHXs in the space between the inner and outer lids, there is significant room for pebble handling containers.
  • Refueling deck could be thinner (1 m rather than 2 m; higher at the bottom, same at the top) and add steel or lead locally where additional shielding is needed.


  • Fill tank: needs to be big enough to hold whole DRACS salt inventory. Locate it inward compared to TCHX (needs to fit in footprint of DRACS hatch).
  • Hot and cold leg should not penetrate the RV wall if we want to mount the DRACS modules on frames. Extend upward (above cap).
  • Hot and cold leg elbows should be bends.
  • Space out the TCHXs 120° rather than 90° (Aligned with DHXs).
  • Frame should be key shaped structure going through cavity cap. Reactor will have to be defueled when pulling out the DRACS (breach in containment).
  • Condenser can be on the outside, at the base of the chimney structure, at grade level.
  • Water tanks for makeup water: look at ESBWR design. Bottom can be aligned with bottom of TCHX. Shape doesn’t matter. Locate inside key shape, inside containment, against wall.s
  • Partition the water tanks with independent valve systems so that if a line breaks, they don’t drain completely.

Reactor cavity

  • May want to change the shape of the reactor cavity wall to key shape (not flat walls everywhere but rather circular walls where not interfering with hot/cold leg of primary system.
  • Having flat outer walls of the cavity would help for manufacturing (cf AP1000). Could be done by having 12 flat sections (spaced by 30°).
  • Outer reactor building wall below grade could be thinner (~ 50 cm). Look at IAEA report for water proofing.

2013-11-19 Design Meeting Notes

Fuel Volumes :

We need to make sure that the cad model and mcnp are in agreement about the volume of the core. The flutes on the inner graphite reflector.

The angle of repose of the fueling chute also affects this volume, but the cad model takes this into account by providing a fueling chute height that characteristically represents the angle of repose as a flat inlet instead.

Calculating the relative volumes of blanket and fuel pebbles needs to be addressed as well in this volume calculation. The width of the chute divider will matter a bit, but we can use the area fractions quoted in Mikes thesis for this.

Set the average number of passes to 8. The real average burnup should be 180MWd/kg. The average amount of time that the pebbles spend in the core is about 1.4 year, so fuel recirculation is about .18Hz. Per estimates then that the pebbles spend about 102.7 hours residence time in the defueling chute (The defueling is 1.03 m^3). This means that there is enough of the decay time before the assay such that they can be assayed and immediately returned. You’ll need some slight pebble storage time after defueling for “surge”, but not a lot.

For reliability, we probably want two assays. — Mike

Core Description in Report : 

Jim Rushton suggested a new section that is a description of the core in addition to a description of the pebbles. A figure that shows a cad model of the core, with the MCNP model of the core side by side. Tommy and David will do this soon.

Building Considerations :

DRACS and defueling/assay apparati should have placeholder spaces in the cad model so that we can be certain there is an appropriate amount of available space in alotted the building.

David reports that the crane plan previously discussed was reasonable, according to crane experts he talked to at ans.

For the building size concerns, we should make full list of the components that will take up space in the reactor building. Tommy notes that we should look into figuring out what’s on the analogous list for the NGNP. The PBMR technical description document will also be helpful in this effort. This should include controls and power components as well. Mike, David, and Nicholas will meet about this list offline.

The footprint may be relatively large for a reactor producing this small amount of electricity. Because it is light, however, the basemat can be thinner than conventional reactors.

Crane ideas :

A couple of additional ideas from the IAEA report on construction :

Lift Crane :

Instead of using a big heavy lift crane, a “lift tower” could be used. They are less expensive and may be reasonable for the assembly of all of the things inside the reactor cavity during construction.

  • A main feature of these is that you don’t need a big counterweight.
  • The heat recovery steam generator could be assembled separately, but for everything else, a lift crane may be possible for lifting objects into the reactor building and along that lengthwise axis.
  • Consideration from Lakshana : How do you rotate things that you’re lifting with a crane?
  • The reactor vessel definitely still has to be dropped in with a “sky crane”
  • We might want to put the natural gas portion outside a fence, in the open air. If that’s the case, we could potentially do so carefully in order to be able to run a rail for the lift crane above the dint(?) valve (so, it should be below grade)

Design Meeting

Fuel zoning:

  • make bottom partition adjustable
  • are wall effects important for relative volumes of pebbles from difference zones?
  • 1.4 years fuel lifetime – 0.12 Hz pebble recirculation rate needed
  • 102.7 hours of cooling time in the defueling chute – based on 1.03 m^3 (well under 10% of total core volume)

Plant layout, and vessel dimensions:

  • Can we design for using lift towers instead of cranes for installation of moules
  • Design the diffuser after the turbine to be removable, as well as the small venting line
  • Need to consider laydown space for the turbine, to be able to open them up
  • Consider puting both air ducts on the same side of the turbine

Security and plant layout

  • common access tunnel – cooling water line and electrical cables
  • no building enclosures for intake vent and gas turbine
  • security fence will be in front of the CTAH, and you’d like to be able to see people approaching the fence.

Salt inventory

  • need new estimates for out of core salt inventory, based on CAD models. Current

Next important steps

  • get the DRACS modules in the CAD
  • other key systems that need to fit into the CAD?
  • ex-core pebble handling?
  • need to go through NGNP list of items in the reactor building
  • need to go through PBMR technical description document

BDBE Source Term Calculation

  • CTAH tube break – hypothetical release
  • Cesium inventyory in the primary coolant
  • Fuel reliability requirements

2013-10-25 Design Meeting Notes

Design report

  • Include section on rationale for important decisions that were made in the past (coolant, NACC, etc.).

CAD model

  • CAD model is broken right now (some pipes were not built to be moved).
  • Need rupture disk on low pressure end of air duct.
  • CTAHs could be open-pressurized.

Startup/Shutdown cooling

  • When in shutdown cooling mode, air at outlet of compressor is at ~400°C, by-passes heat exchanger, goes through 2 stages of expansion, therefore ends up at significantly colder temperatures.
  • May want to by-pass some flow from compressor outlet through CTAHs to heat air up and mix with cold air.
  • For startup, need to integrate with neutronics model. 2 startup conditions:
    • From fresh fuel
    • From equilibrium fuel
    • Because the 2 CTAH loops are independent, need to design pump control strategy (2 pumps) to keep salt temperatures coming from 2 CTAHs into cold leg at same temperature (in order to avoid large delta T across reactor vessel). Other adjustable inputs to achieve this are control rod positions.
    • Need to validate some control strategies with CIET (distortion from having only one pump and one heat exchanger).
    • Also need to assess risk of freezing from cold air exchanging heat with salt.

Plant arrangement (see pictures)

  • Switch to cylindrical building, ~ 24 m diameter, ~ 35 m high
  • Need some kind of holes over CTAH vessels
  • Minimum distance between inner wall and center of any element that will need to be pulled out is determined by crane capacity (investigate and select crane) (see picture)
  • Space between refueling deck and roof of building must be tall enough to accommodate reactor vessel
  • Use AP1000 and NGNP (pre-conceptual design) for overall dimensions

2013-10-25 Crane 2013-10-25 Elevation view 2013-10-25 Plant arrangement

Ballpark sizing of water pools for TCHX

  • 1% total power for 72 hours (1% is a good average over 72 hours)
  • (2.4 MWt * 72 hrs * 3600 s/hr) / (2.4 m^3/kg * 1000 kg/m^3) = 260 m^3
  • 4 m high vessels (3 pools), meaning 8 m^2 area total: too big
    • Hence need for recondensation, using 3 air stacks, spaced 120°, in boxes against outer wall of the building


  • Look at ESBWR emergency core cooling condenser
  • Horizontal tubes for water, square lattice
  • Variables:
    • Tube diameters
    • Pitch to diameter for lattice