Detector environment is largely determined by the type of reactor being instrumented In general, out-of core detectors can be placed in a mildei environment than in-core detectors, especially if out of vessel locations arc — suitable The various environments are described in Chaps 1 5 to 18

The most important single environmental condition is temperature, which can vary from near room level remote from the reactor core to very high in the vicinity of the core of a gas-cooled or sodium-cooled reactor Ionization chamber detectors that can operate in temperatures up to 750°F (400°C) are available and are suitable for most existing reactors Experimental ionization chambers have been operated with varying degrees of success at tempera tures as high as 1400°F (760°C) It is likely that the temperature ratings of commercially available detectors will increase in the future

Gamma background is also an important environmental condition for neutron detectors As reactor power densities are increased in the more sophisticated designs, there will be an increasing need for detectors that can operate in higher gamma backgrounds Improvements in gamma tolerance can be made only with great difficulty The successful use of neutron detectors in high gamma fields depends largely on the ingenuity and skill of the reactor and instrument designers and represents a source of continued difficulty Internal heating may be a problem in gamma fields exceeding 5 X 107 R/hr

High neutron backgrounds in neutron detectors do not normally cause difficulty, since neutrons are the principal instrumentation objective The trend, however, is toward power reactors that have a larger fast neutron fraction in their neutron spectrum Since the sensitivity of neutron detectors, which are primarily thermal neutron detectors, decreases as the energy of the neutrons increases, diffi culties are to be expected The main difficulty is assurance that the detector senses neutrons that truly represent reactor power, 1 e, that vary in a regular way with power variation

In addition, to produce an adequate signal in a fast neutron flux, the detector may require exposure to a high neutron flux This can adversely affect the life and physical characteristics of the detector

Adverse environmental conditions can be avoided by moving the detector For example, start-up detectors that do not have to operate when the reactor is at power can be moved to regions of lower radiation intensity once they are out of range Similarly, other sensors may be moved to a more favorable environment as reactor power is increased The use of detector withdrawal might enable reliable instrumentation where the background is otherwise too severe Safety instruments should not be moved unless they are no longer required Satisfactory operation in the new position must be assured

Excessive neutron flux and gamma gradients may contribute to instrumentation faults In general, it is desirable to operate with a large signal to extend the sensor range In a high flux gradient, a part of a sensor may be operating in excess of its rating This implies operation with a lower input signal than proper use would provide In addition, the most effective gamma compensation is ob tamed in a low gradient gamma field Thus, to the extent that the gamma gradient is related to the neutron gradient, it is likely that compensation may be less effective and there will be a loss of range