694 MECHANICAL DESIGN AND PACKAGING 



for substantially complete shielding can be tolerated, but volume and 

 weight are primary factors in the design of an aircraft. Therefore, airborne 

 electronic equipment must exist and operate satisfactorily in a nuclear 

 radiation environment which will vary in intensity depending upon the 

 amount of shielding and type of shielding used and the distance and 

 direction of the equipment relative to the nuclear reactor. 



Various techniques can be used to reduce the intensity of this environ- 

 ment acting on the equipment. Three methods of shielding can be utilized; 

 and the total shield volume can be distributed around specific items or 

 compartments in the aircraft. Partial protection from the effects of nuclear 

 radiation can also be accomplished by locating items such as fuel cells, 

 bomb bays, landing gear, etc. between the nuclear reactor and the compart- 

 ments housing the electronic equipment, thus achieving a blanketing effect. 

 Concentrating the entire required shield volume and weight around the 

 nuclear reactor results in a heavy weight design. Distribution of the shield 

 to specific areas affords maximum protection per unit weight since the 

 required amount of shielding is governed by the distance and direction from 

 the nuclear reactor; further, the nuclear radiation energy, when deflected 

 from its original direction, loses strength. However, the disadvantage of 

 this method is the fact that sections of the aircraft not chosen to be shielded 

 will receive maximum nuclear radiation doses beyond those of the concen- 

 trated shield design. A distributed shield system in combination with the 

 blanketing concept appears to have merit. Since nuclear radiation may be 

 scattered by items external to the power plant and may therefore approach 

 a given location from several directions, the problem of shielding design is 

 further complicated. This fact influences the design of the shield to the 

 extent that, for maximum protection of electronic equipment, the entire 

 compartment must be enclosed. However, for optimum weight distribution 

 of the shielding material, various degrees of shielding thickness can be used, 

 depending on the relative distances and directions of the compartment walls 

 with respect to the nuclear reactor. For example, aircraft compartments 

 located forward of the nuclear reactor will sustain maximum radiation 

 intensity on the aft wall of the compartment. The forward wall will 

 be subjected to the lowest level of radiation intensity, and therefore a 

 relatively thin, even transparent, shield can be installed there. 



13-6 VIBRATION AND SHOCK 



The possibility of mechanical failure under dynamic load presents one of 

 the most serious threats to reliability in the complex electronic components 

 of an airborne radar. In military specifications it is customary to handle 

 the general topic of dynamic loading in terms of the four environments of 

 vibration, shock, acoustic noise, and aircraft acceleration. In particular, 



