Primary Coolant Routing for Defueling Chute and DHX

Meeting: Nicolas & Raluca

Top of DHX to Hot Leg

Connect top of hot leg to top (inlet) of DHX, above faulted level 1B. (because for FL2, heat removal is done through cavity wall cooling system, and salt level just needs to be sufficient to cover the fuel pebbles).

  • Faulted level 1: due to loss or removal of coolant from the hot and cold legs; at the bottom of the hot and cold leg reactor vessel penetrations
  • Faulted level 2: due to a reactor vessel break and primary coolant filling the free space in the reactor cavity, 0.6m below Faulted Level 1 (per PB-FHR-DESIGN-CALC-004-00A)
  • Faulted level “B”: due to drop in coolant temperature and volume shrinkage, Calculation: 7.2 m^3 [number needs to be updated] salt volume in the RV below Faulted Level 1. Assume salt temperature drops from 900oC to 600oC (same as the assumption in PB-FHR-DESIGN-CALC-004-00A, for dimensional change of reactor vessel in drain volume calculation). This would lead to 8% volume drop, so 0.6 m^3. The level drop of 0.6 m^3 would be 10 cm. (same cross-sectional area assumption as PB-FHR-DESIGN-CALC-004-00A)

Ensure symmetry among the three DHX to hot leg manifolds.

Note: It is important to have cooled downflow and heated upflow in the DHX.

DHX Inlet Manifold

To prevent sudden failure of the DHX when the level faults below the DHX inlet pipe, the top DHX coolant inlets can be designed such that as the level gradually drops the flow rate also gradually drops; (this might be at the expense of some loss in the effective elevation of the natural circulation loop, or might be mediated by internal detailed design of the DHX shell and tubes.) A manifold with most of the pipes at the top of the DHX and gradually fewer and fewer going down, would implement this concept.

Bottom of DHX to Downcomer

Done through penetrations in the core barrel, connecting to the bottom of the DHX shells. no need for a manifold here, just one hole sufices (although mechanical design considerations will need to be discussed here).

Top of the defueling chute to DHX

A penetration in the inner wall of the outer top lid will connect the top of the DHX to the annulus formed by the inner and outer top lids. This annulus will then be connected to the top of the two defueling chutes. The use of the annulus is important for flow redistribution because the DHXs are all located in one quadrant of the core, and the defueling chutes are located at 180oC from each other. From the annulus to the defueling chute, flow can be routed axially, or axially with a radially outwards component, depending on the axial distribution of the coolant ports connecting the defueling chutes to the annulus between the two top lids.

Flow through the defueling chute is downwards during forced circulation and upwards during natural circulation. Allowing upwards flow through the defueling chute in NC is important, to prevent formation of stagnant hot pockets at the very top of the defueling chute.

Fluidic diode functionality

The fluidic diode function needs to be implemented immediately after the dhx inlet manifold. If the check-valve option is chosen, then check valves can be put on the larger manifold lines, and some smaller manifold lines are kept without check valves, to ensure some amount of bypass flow.

We figured out that we can use the DHX bypass flow during forced circulation to provide cooling to the defueling chute. Calculations of the flow resistance in the different branches are needed to verify this, but it might be possible for all of the bypass flow to be routed through the defueling chute. This is achieved by using the two-branch check-valve concept to meet the diode functionality. The check-valve branch would connect the core to the top of the DHX. The small free bypass branch would connect the top of the defueling chute to the top of the DHX.

If it looks good, then we can ask David to sketch some of these (relatively minor) changes in SW, and from there the next steps are (1) pressure drop calculations (Alex can add these to his current set of DP calcs), and (2) COMSOL modeling of the forced and natural circulation cooling modes for the defueling chute.





DHX Outlet Flow Routing

The challenge with DHX flow routing to the cold leg is the we can’t use the inlet toruses at the top because they’re above the coolant fault level. I don’t know if it will be acceptable to break the azimuthal uniformity of the flow in the core during natural circulation decay heat removal, by feeding the DHX outlet directly into the downcomer.

  • Buoyancy forces are significant, and they will help work against the formation of hot spots in the core. (3-D modeling of core porous media flow is needed to check this.)
  • The pebble bed is quite good at mixing flow, so by the time coolant gets to the core outlets, azimuthal temperature uniformity will be reached anyway.
  • I don’t think that we worry about hot regions in the core, except for neutronics, which may be sensitive to funny temperature distributions in the core.
  • Do we worry about temperature gradients to which the graphite walls are exposed?

That being said, there are three options:

  1. We keep all 3 DHXs close to the hot leg, and feed the outlets to a DHX torus that then feeds into the downcomer. This torus needs to be below the elevation of the faulted level. The torus would ensure uniform cold flow distribution around the circumference of the core, thus uniform cooling of the down-comer, and azimuthally uniform temperature distribution at core inlet.
  2. We distribute the 3 DHXs equidistant around the circumference (and maybe change to 4 DHXs, so that we don’t break the 8-symmetry). The DHXs outlet routes directly to the down-comer. The problem is significantly alleviated, but maybe not entirely fixed (2-D modeling of the laminar flow in the down-comer is needed to check this).
  3. We keep all 3 DHXs close to the hot leg, and feed the outlets directly to the downcomer. (2-D modeling of the downcomer and 3-D modeling of the core are needed to check that this will work.)

Outer Reflector and Upper core structures

Coolant flow path design

  • move dhx down, and integrate into lid
  • what is the stand-off, for neutron irradiation? – ask Tommy/Madicken
  • bring outlet plenum down, and run hot duct up through the outer reflector
  • downwards flow through the defueling chute?
  • dead space in the top metalk structure is to be filled with graphite
  • beyond design basis – if the lid looses structure, we want to be able to carry loads through the graphite
  • outlet can be channels, rather than slots, because outer reflector is shielded by graphite pebbles
  • talk to Madicken/Tommy to estimate doses on outer reflector surface, and consider issue of cracking due to differential shrinking

Rationale for the elevation of the skirt

  • at faulted salt level
  • additional salt faulting: 50-60 cm, in the event of vessel faulting, and leaking into the reactor cavity (need to know residual porosity of cavity insulation)
  • need room for the vessel to expand downwards when heating up
  • if the metal structure fails, everywhere, there needs to be structure from the ceramics
  • cavity wall, frozen interface – water boiling, or let the concrete heat up
  • use concrete with modest off-gassing
  • cavity wall: metal composite, tie rods etc, so good conduction path

100 DT:

  • pressure drop vs. materials
  • below 600: changes average temperature on power conversion system
  • above 700: limited by metallics
  • less than 100 DT, high pressure drop, but also higher temp to power conversion system

To Discuss Further:

  1. Stacking problem, elevation for (a) DHX, (b) start point for metallic upper structures
  2. Design of outlet plenum torrous, to ensure uniform azymuthal flow distribution.
  3. Outlet and inlet lines: channels vs. slots
  4. Consider again Option 2 for coolant routing, with hot leg extending down into the central reflector

(notes taken by Raluca, design meeting on 25 Jun 2013)