Self-Powered Neutron Detector

Four other types of radiation detectors are the self-powered neutron detector, wide range fission chamber, flux wire, and photographic film. 

In very large reactor plants, the need exists to monitor neutron flux in various portions of the core on a continuous basis. This allows for quick detection of instability in any section of the core. This need brought about the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation. Figure 29 illustrates a simplified drawing of a self-powered neutron detector. 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

One disadvantage of the self-powered neutron detector is that the emitter material decays with a characteristic half-life. In the case of rhodium and vanadium, which are two of the most useful materials, the half-lives are 1 minute and 3.8 minutes, respectively. This means that the detector cannot respond immediately to a change in neutron flux, but takes as long as 3.8 minutes to reach 63% of steady-state value. This disadvantage is overcome by using cobalt or cadmium emitters which emit their electrons within 10 14 seconds after neutron capture. Self-powered neutron detectors which use cobalt or cadmium are called prompt self-powered neutron detectors. 

A description of how self-powered neutron detectors, wide range fission chambers, flux wires, and photographic film detect radiation is summarized below. 

Self-powered neutron detectors (SPNDs) use a material such as cadmium that has a high cross section for low-energy neutrons and produces copious gammas or... 

This chapter discusses in detail all the neutron detection methods mentioned above, as well as the Bragg crystal spectrometer, the time-of-flight method, compensated ion chambers, and self-powered neutron detectors (SPND). Other specialized neutron detectors, such as fission track recorders and thermoluminescent dosimeters, are described in Chap. 16. 

The self-powered neutron detectors are divided into those with delayed response and those with prompt response. The characteristics of these types of self-powered detectors are presented in Secs. 14.10.1 and 14.10.2. 

The funds required will depend on the local capability to fabricate equipment and the equipment already available (e g., self powered neutron detectors, micro-ammeters, activation foils, and a computer). If this equipment is already available, it should be possible to construct an in-core facility locally for under US 5000. 

Neutron detectors (particularly fission chambers and self-powered neutron detectors) ... 

Alarm typewriter indication showed self-powered neutron detectors responding to high temperature down to 4 level of the core. 90% of the core exit thermocouples >700T. 

The main reason to monitor neutron flux in a reactor is that it is proportional to the power density, and this is the variable which we are concerned about. There are mainly five types of neutron detectors, BF3 proportional counters. Boron ( B) lined detectors, fission chambers, He proportional counters, and self powered neutron detectors Two other instruments are widely used to monitor the reactor power, calorimeters, used to monitor power density, and N-16 detectors, used to monitor integral reactor power... 

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