NuMI Decay Pipe

1
NuMI Decay Pipe
2 m diameter 677 m long 3/8 inch 0.95cm
thick Welded stiffener rings to meet vacuum
code Water cooled by 12 plastic-coated
copper pipes embedded in concrete
2
Decay Volume

• Vacuum is the default (1 torr adequate)
• helium fill is the backup if a significant leak
  develops
• Entrance window 1.5 mm thick stainless steel, 1
  meter diameter welded to thicker
  material that extends to 2 meter diameter.
• Designed to meet the requirements of the ASME
  Pressure Vessel Code
• No shutter
• Alignment tolerance is pair of circles, 97.155 cm
  and 100.965 cm radii
• Alignment is achieved by utilization of a GPS
  network on the surface, transferred underground
  by sight-risers, geo-theodolite, etc.

3
Radiation Shielding Design

• Decay pipe
• surrounded by poured concrete shielding
• (Tunnel Boring Machine was 6.55 m in diameter)
• Space for a 112 cm width walkway on one side is
  provided.
• Pipe and tunnel centers are offset as needed to
  shield groundwater.
• (The limit on concentration of radio-nuclides in
  groundwater at the tunnel walls depends on the
  amount of water inflow)
• Walkway
• residual gamma activity 2 mSv/hr (30 day
  activation, 1 day cool-down)
• Hadron Absorber
• Design goal is residual gamma activity lt 1 mSv/hr
  on the walkway side

4
Sample Decay Pipe / Shielding Cross Section
Target hall to absorber secondary access
Decay pipe
concrete radiation shielding, density 2.1 g/cm3
Relative centers vary along length
5
Energy Deposition, Cooling

• Decay region power deposition
• 63 kW in 1 cm thick steel decay pipe
• 52 kW in shielding concrete
• Peak deposition in the steel is 360 W/m ( 50 m
  from target hall)
• Drops to 20 W/m (at 610 m)
• Heat is removed by twelve copper water-cooling
  pipes, limiting decay pipe temperature to 50
  deg C
• Absorber core
• 8 modules of aluminum with dual water-cooling
  paths
• 30.5 x 129.5 x 129.5 cm3 each
• 8 kW peak power in one module (normal beam
  conditions)
• followed by 10 plates of steel, each 23.2 cm
  thick.
• Total power in Absorber region 60 kW
• (nearly entire 400 kW beam power in accident
  condition)

6
Absorber / CavernCross Section
Steel blocks Absorber core Concrete
blocks
beam west
Steel block dimensions are 132 x 132 x 66 cm3
Egress path
7
Absorber Core
Steel slabs Aluminum slabs
Two water lines from each Al. block run
back through holes in other blocks.
1.3 m