(a) Temperature Control Temperature control of in strumentation systems is usually not given sufficient atten tion For example, although the overall average temperature of all components may not be excessive, many “hot spots” can develop through improper attention to cooling require ments Even though such hot spots may not cause immediate failure, the) eventually show up in system failures and poor mean time before failure (MTBF), with resulting high maintenance costs Although temperature considerations are basically a design function, no designer’s product can function effectively if operated in an environ ment m which it was never intended to operate For this reason those in charge of the installation must make certain that all equipment is operated within the designed environ mental limits, whether thermal, vibrational, radiation, or any other

Since most instrumentation equipment is located m enclosed cabinets, proper ventilation must be provided to avoid convection cooling that may allow heat from a lower chassis in a cabinet to pile up in the top chassis, thereby effectively “baking” every component in the equipment Under these conditions some units that functioned well in a

55°C test oven have been known to fail when operating in a “room-temperature” relay rack For this reason each chassis placed in a cabinet must be checked for power consump­tion before installation and provisions made for the total cooling load demanded per cabinet The type of equipment in the cabinet must also be considered, for example, a log amplifier requires closer temperature control than a rela

If temperature might be a problem (either high temper­ature for indoor installation or low temperature in winter at outdoor locations), provisions for correcting the problem should be made before the equipment is installed, not after Since the instruments in control rooms are usually a large source of heat, air may be drawn from the room through vents placed in the bottom of each cabinet and then drawn out of the top of the cabinet into the building’s central air-conditioning return. Additional cooling capacity ma be required in the air conditioning system at the time of plant construction to handle the load of the control room If proper size ducts are used from the cabinets to the air return, the airflow through the equipment and in the room will he silent, low speed, and unobtrusive If the air input to the room is filtered, the control room will stay clean as well as cool, providing a more pleasant and lower maintenance environment Each cabinet mav also be equipped with a thermometer so that the cabinet temperature can be monitored

During set up and testing of cabinet-mounted equip ment, temperature-sensitive materials, such as tempilac, may be placed in areas of high-component density and low airflow or on components that require critical temperature control to be effective If after 8 hr of operation the temperature sensitive materials indicate proper operating temperatures, the airflow should be turned off or blocked, and the effect of the ensuring temperature rise on the equipment operation should be noted If any significant changes occur, an alarm annunciator should be installed to signal and warn of loss of sy stem airflow

Other than the electronic package located m the control room, the most critical area regarding temperature regula tion is around the reactor itself Energy dissipated in the reactor shielding material produces heat which raises the temperature of any detectors or other sensors in close proximity to the core Those responsible for installations should be certain that the ambient temperatures of each sensor location do not exceed those recommended by the manufacturer and that any connecting cable is rated for the environment in which it must perform.

(b) Vibration Control. Every attempt should be made to mount instrumentation in vibration-free areas When it is necessary to plate instruments in high-у ibration positions, it is most important that neither components, portions of the case, nor wires of any kind resonate at any of the vibrational frequencies involved since metal fatigue will most certainly cause ultimate failure

The easiest cure for vibrational problems is to shock mount the equipment and fasten securely all wiring harnesses by using anti wicking tools on connector solder points Anti-wicking tools prevent solder from flowing within stranded wire to a point beyond which the insula­tion has been stripped from the wire Other approaches mav also be required, including silicone rubber encapsula tion of wires and connectors, special internal vibration dampening of equipment, etc For further information on vibration control, see Defense Department Wire Specifica­tion MIL-W-5088C 01 MIL W-9160D

(c) Selection of Insulation for Radiation Environment.

Wiring insulation exposed to environmental extremes of radiation should be carefully selected It would be desirable to select wire that could withstand radiation exposures for the life of the plant (about 40 у ears) Extensive irradiation research programs have been conducted and numerous tables have been compiled on radiation damage to wire conductors and insulating materials

This chaptei contains several tables on radiation effects on materials These tables are typical and may or may not agree with specific results obtained by other research organizations

There is no substitute for experience gained in oper ating nuclear plants It has been found that out-of-core detector wiring and some m-core yviring may be good for only 24 months or less Replacement of this wiring at refueling time is considered standard operating procedure

Tabic 10 1 shoyvs the radiation stability of plastic insulating materials Klein and Mannal concluded that only inorganic insulation materials could function yyithin the reactor primary shield since radimon dose rates up to 101 2 rads/hr are often experienced [21] I he same type of insulation will be required in the containment vessel of a fast breeder reactor, where levels are expected to reach 10s rads/hr Outside the primary shield but inside the containment vessel of a thermal reactor, the dose rates may range from 0 5 to 160 rads/hr, and temperatures up to 70°C may be expected

On the basis of the foregoing assumptions and a 40-y ear plant lifetime, wiring inside the containment yessel may be expected to absorb 5 X 107 rads under normal conditions, a power excursion or other nuclear accident may add another 4X 106 rads Auxiliary structures, eg, residual heat remoyal compartments, outside the containment yessel mav be expected to receive dose rates 1/100 that of objects yyithin the containment yessel, but, in the event of an accident, these areas must be able to yvithstand much higher ley els

The temporary effects of radiation on elastomer based cables include thermoluminescence, decrease in electrical resistance, and gas generation I ong term effects include cither embrittlement or softening of the insulation Present theories tend to support the view that the cumulatnc

radiation damage to a substance in or near a nuclear reactor depends on the rotal energt absorbed b the material and is not a function of the type of radiation Accordingh, neutron damage to cables can be determined by referring to the tables for gamma-radiation damage and adjusting the total dose to account for the neutron energy Tables 10 2 to 10 6 show degradation as a function of absorbed dose caused by gamma radiation on various parameters of a cable and for various types of cable insulations Ггшп these findings, Blodgett and I isher presented the data shown as Table 10 7. The table lists and rates matenals that may be used successfully in various nuclear environments

10- 3.4 Installation Symbols

The designers of nuclear power plants use different symbols on drawings for the installation of instrumentation and electrical systems Although there are some relevant electrical codes and standards, there still appears to be lack of uniformity throughout the industry

Table 10 8 lists symbols typical of those currently used in the nuclear industry.