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Fast Neutron Hodoscope


Mission statement

Provide key measurements and monitoring of fuel movement.


System description

The hodoscope is a unique special-radiation detector designed to measure the neutrons emitted from fission taking place in fuel located in test items. It provides a highly sensitive measure of when and where initial fuel system failures occur during transient nuclear events.

During an experiment, if the transient is severe enough to damage the fuel, it can melt and relocate in the test item. If the fuel relocates within the test item, the fission rate in the original area decreases, and fission begins at new areas. In areas with less fuel, the neutron emission rate is decreased, and areas newly occupied with fuel start to produce neutrons.

Principle of operation:

1. A row of fuel elements in the TREAT core is left empty between the test vehicle and the hodoscope system

2. A steel collimator with hundreds of small slits is placed external to the core, each viewing a small area of the test vehicle

3. Fast neutron detectors are placed at the outside surface of the collimator

4. Fission is induced in the fuel sample by the reactor during the transient

5. Fission-neutrons from the test specimen leak from the core, through the slits, and are measured in the detectors

During a test, data is recorded from >720 detectors every millisecond. The maximum instantaneous data rate is expected up to 400,000 events per millisecond.



Provides basic data for predicting the safety margin of fuel designs and the severity of potential accidents.

Serves as a proving ground for fuel concepts designed to reduce or preclude the hazards associated with potential accidents.




  • Outer dimensions: 361 mm × 1,337.9 mm × 1,822.4 mm
  • Slot layout: 10 columns × 36 rows (360 slots)
  • Pixels at core center: 33.05 mm (height) × 9.35 m (width)
  • Field of view: 1,242.0 mm (height) × 68.6 mm (width)


  • Proton-recoil scintillators (PRS; ZnS in Lucite, "Horynak" buttons)
  • Proton-recoil proportional counters (PRPC; methane gas)

Data acquisition

  • Fully digital control system for PRS high-voltage; high-speed signal digitization and on-board processing