Inner and Outer Reflector Design (July 18, 2013 meeting notes)

Center reflector design

  • Depth of key holes: currently 4 cm, probably more than we need. We want to make sure that the keys remain inserted into following blocks as the column buckles under DBEs. 1 cm for 25 cm thick blocks is probably enough. Also, instead of sharp breaks, use small fillets. Change the key hole shape accordingly.

Stress concentration:

  • Thickness of the inner reflector blocks is currently 25 cm –> lobes are nearly square. Having this roughly square ratio works best for irradiation induced stresses.
  • Control rod channels: move holes outward so that distances to edges of the lobes are ~ equal. Important for resulting shrinkage of the lobes from neutron irradiation.
  • Coolant injection channels: change shape from circular to key shape to increase distance between channels. This will help to cope with tensile stress from pressure differential between the coolant channels and out of the reflector.
  • For a more uniform neutron irradiation region geometry at surface of reflector, get the center of the curvature radius closer to the surface.
  • Slot size: need to perform pressure loss calculations to determine the right size.
  • Check if the changes in geometry have an impact on neutronics (bigger region of graphite absorber).
  • Small coolant channels between core and control rod/coolant injection channels : have some that go to the control rod insertion channel and some that go to the coolant injection channel. These channels should be ~ 1 cm diameter. Number of rows of channels will be determined based on pressure drop.
  • Injection channels: as we get higher than ~ 70 cm above bottom of the active core region, we don’t want to have too much flow coming through the center reflector à narrow down the size of the coolant injection channels (having a small circular region at the end of the slots will also help to reduce stress concentration at the end of the slots).
  • Add instrumentation channels (8?) at the center of the inner reflector, with keys for alignment.
  • Tie rods to carry tensile stress: possibly locate them at the outer corners of the lobes.

Lower region of the center reflector:

  • Also reduce thickness of the keys to ~ 1 cm. Reduce radial thickness of the keys to ~ 2 cm. All these dimensions will be determined through buckling and FEM analyses. Key structure should not be symmetric (use odd number of keys).

Outer reflector design

  • Block thickness can be double that of the inner reflector blocks (50 cm instead of 25 cm) because of lower neutron dose.
  • Successive layers of blocks are staggered azimuthally so that when gaps open between blocks under thermal expansion (1% of the 6 m circumference, divided by 24 blocks = 2.5 mm) by-pass flow through the cracks is blocked.
  • At the top, transition into metal for neutron shielding, where the DHX wells begin.

Graphite Blocks


  • Maximum size: 1.0 m flat to flat .9 m high
  • " The stacked hexagonal blocks of the PCA’s are 1.15-m flat-toflat, which is comparable to the 1.0-m flat-to-flat, 0.9-m high hot-plenum blocks that have been fabricated for the Japanese High Temperature Engineering Test Reactor [12]."
    "12. T. Iyoku, J. Smita, M. Ishihara, and S. Ueta, “R&D on Core Seismic Design,” Nuclear Engineering and Design, Vol. 233, pp. 225-234, 2004."
  • Outer Reflector:
    • broken up into 24 pieces
    • 8 blocks deep
    • 15 cm inner arc length
    • 30 cm radial length
    • 50 cm axially (depth)
  • Inner Reflector
    • 25 cm axially (depth)
      • smaller than outer reflector blocks because of shrinkage due to neutrons
    • disk pieces
  • Fractional Pieces are acceptable at top or bottom
  • currently keys’ depth is arbitrary (4cm)
  • Outer Reflector
    • axial keying with downcomer
    • circular ribbed keying
    • block
  • Inner Reflector
    • control rods channels are keyed
      • critical part of reactor therefore it is important that they are aligned
      • this should keep the entire center reflector keyed in all directions
    • carbon fiber channels for instrumentation
      • restrain the motion of the blocks
  • Seismic Risk
    • How will keying react to seismic events?
Outer Reflector block offset:
  • 1.5 – 2 cm
Plated Boron rods in the Outer Reflector:
  • plated to avoid risk of poising core with Boron reacted with fluoride