Nuclear Instrumentation Systems

In a car you need to know how fast you are driving so you do not exceed the speed limit. In a nuclear power plant, the operator must know how much neutron power is being produced so that he or she does not exceed the regulatory power limit. Neutron Power level (or power level, for short) can actually span about 11 orders of magnitude (11 powers of 10) from the lowest power level to full power. No one type of detector can be used. In many plants, 3 levels, and methods, of detection are used. These levels are called:

The neutron detectors, used by the operator, may be installed inside or outside the reactor, depending on the type of reactor. If detectors normally used by the operator are outside the reactor, these detectors must be periodically recalibrated with detectors that are either installed in the reactor core or that pass through the reactor core.

This figure illustrates the span of the various detector types. The detector ranges overlap to ensure accuracy of the indication. This system also provides signals to the Reactor Protection System that can initiate automatic shutdown of the reactor should control go awry. The upper and lower bounds of each range are shown, together with the units (like speed can be miles per hour or km/hour).

Power Range (percent, %)   10-3 %
PR - IR Overlap      
Intermediate Range (amps)   amps  
SR - IR Overlap      
Source Range (counts/sec, cps) cps    

The neutron detectors are typically cylindrical with a central wire. The wire and the outside of the detector have voltage differences between 10 and 2500 volts dc. The detector often has a gas that will interact with the neutron to produce a charged particle. The charged particles then are drawn to the appropriate central wire or outside of the detector. These events produce either a single pulse or a current depending on the type of detector.

Gases used in neutron detectors usually include boron or uranium. When a neutron reacts with boron, it produces 2 charged particles - an alpha particle (helium nucleus 2 He 4 with a +2 charge). When a neutron reacts with the uranium it fissions creating 2 fission products that are both highly positively charged.

Copyright 1996-2004.  Joseph Gonyeau, P.E.. The Virtual Nuclear Tourist. All rights reserved. Revised: March 15, 2001.