- DHX sits at bottom of metal lid
- Arrangement on plate for modularity
- Large radius elbows for DRACS loop to allow for flexible inspection instruments to be inserted
- DHX to TCHX centerlines will be ~6m, which is a distortion compared to scaling from CIET (would be ~8.5m)
- Each DHX should remove 2% of nominal power. 2/3 failure logic.
- Level difference between two hot leg penetrations in hot well creates a seal loop. 0.5m of head can be accommodated if isolation valve fails.
- Look at calculation for hot well height required above penetrations for thermal expansion (600°C to maximum accident condition temperature) + level swell from pump operation.
Reactor cavity cover – refueling deck hatches
- Need to have a center circular hatch to pull out center reflector
- Hatch for reactor vessel
- Hatches for 3 DRACS (see picture)
- Seal for DRACS hatch could be integrated in frame design of DRACS
- Because of high number of penetrations in cover, may need to have it be steel
- Missile shield above has fewer penetrations and could be concrete
- Design objective: 2.0 m of head from cold leg to hot leg (level swell in cold leg standpipe)
- May need full 2.0 m available inside reactor vessel to accommodate full level swell (therefore need to increase vessel height to 13 m), although probably lower because the level swell in the control rod insertion channels doesn’t take head losses from cold leg and downcomer
- If reactor vessel’s height increases by 1 m, control rod drive must be even longer (top must remain uncovered) (see picture)
- Stress analysis for inner reflector: use COMSOL as an easy first step (+ allows for coupling with TH analysis).
- Trick the model into taking shrinkage and swelling due to neutron irradiation into account.
- Find references on implementation of dose data.
- Find ratio between peak stress for optimized design and simplified design.
- Outer reflector blocks slide in radially (cold core): use tie rods to lift up blocks and key into upper core structure.
Upper core structures
- Dose limit on upper core structures should be ~ the same as that on core barrel.
- Make sure metallic components in upper core structure don’t start to bulge in under force from buoyant fuel and graphite structures underneath.
Cold leg flowpath
- Deliberate fraction of by-pass flow from the cold leg to small annular slot in the upper core region to keep it ~ at same temperature as downcomer.
- This small by-pass will feed into the hot leg through a nozzle that shouldn’t see hot salt à use same design as combustor in turbine. The high temperature gradient along the hot leg pipe should be designed to be located a few 10s cm away from the core barrel, away from inside of the vessel and the joint with the cold leg by-pass.
- Other by-pass to pebble injection lines (opposite of hot leg). Pebble injection system is part of the upper core internals.
- Shielding of core barrel from recirculated fuel is not too big of an issue: gammas don’t affect integrity of materials, and it is a sub-critical multiplication region, so the only neutron irradiation comes from inside the core, meaning the fuel in the injection lines doesn’t change anything.
- Both key rings for outer reflector blocks and core barrel need to be shielded from neutrons from the core though. Investigate where to optimally locate boron absorbers (currently between rings and core barrel; move inwards to protect rings as well?).
Core barrel design
- Avoid stress concentration under creep regime.
- Design to accommodate bends, changes in diameter, etc.
- Keep access for inspection along core barrel.
- Hot leg and cold leg should be at same elevation to have same influence on LOCA level. Have one above and one under the inner flange by having the flange make a 45° angle with core barrel.
- Downcomer flow: want to remain ~ 2 m/s, 0.6 m3/s à need 0.3 m2 flow area. With a 3.5 m diameter vessel (12 m circumference), the gap should be ~ 2.5 cm. If using a 2 cm flange, this means only a 5 mm gap.
- Assume possibility that all joints may self-weld, especially tapering; investigate gasket materials in the design.
Pebble defueling mechanism
- Material flowpath schematic for defueling mechanism: separate graphite vs. fuel pebbles, detect broken pebbles.
- Design of canister system if used vs. manipulate pebbles individually through pneumatic systems.
- See IAEA paper on pebble fuel: rationale for letting the pebbles cool down for ~ 4 (?) days betwen several passes is to let some of the high gamma emitters decay before analysis for burnup (Cs accounting).
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:
- 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.
- 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).
- 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.)