PRINCIPLES OF NAVAL ENGINEERING 



thermal neutrons to be absorbed in the fuel. 

 As more neutrons are absorbed in the fuel, 

 more fissions occur, resulting in a higher 

 power level and more heat being generated 

 by the reactor. The additional heat is re- 

 moved by the reactor coolant to the secondary 

 water in the steam generator to compensate for 

 the increased steam demand by the turbine. The 

 temperature of the primary coolant leaving the 

 steam generator increases slightly, lowering 

 the scattering cross section of the moderator, 

 and the reactor settles out at a higher power 

 level. 



The delayed neutron action is a phenomenon 

 that simplifies reactor control considerably. 

 Each fission in a nuclear reactor releases on 

 the average between two and three neutrons 

 which either leak out of the reactor or are 

 absorbed in reactor materials. If the reactor 

 material which absorbs the neutron happens to 

 be the fissionable fuel, and if the neutron is of 

 proper energy level, another fission is likely 

 to result. The majority of the neutrons re- 

 leased in the fission process appear instan- 

 taneously and are termed prompt neutrons ; but 

 other neutrons are born after fission and are 

 termed delayed neutrons . The delayed neutrons 

 appear in a time range of seconds to 3 or more 

 minutes after the fission takes place. The 

 weighted mean lifetime of the delayed neutrons 

 is approximately 12 seconds. About 0.75 per- 

 cent of the neutrons produced in the fission 

 process are delayed neutrons. 



Should a reactor become prompt critical 

 (critical on prompt neutrons), it would be very 

 difficult to control and any delayed neutrons 

 would tend to make it supercritical. The delayed 

 neutrons have the effect of increasing the 

 reactor period sufficiently to permit reactor 

 control. Reactor period is the time required 

 to change the power level by a factor of e_ 

 (the base of the system of natural logarithms). 



A nuclear poison is material in the reactor 

 that has a high absorption cross section for 

 neutrons. Some poisons are classed as burnable 

 poisons and are placed in the reactor for the 

 purpose of extending the core life; other poisons 

 are generated in the fission process and have a 

 tendency to be a hindrance to reactor operation. 



A burnable poison has a relatively high cross 

 section for neutron absorption but is used up in 

 the early part of the core life. By adding a 

 burnable poison to the reactor, more fuel can be 

 loaded into the core, thus extending the life of 

 the core. 



Most of the fission products produced in 

 a reactor have a small absorption cross sec- 

 tion. The most important one that does have a 

 high absorption cross section for neutrons is 

 xenon- 135; this can become a problem near 

 the end of core life. Xenon- 135 is a direct 

 fission product a small percentage of the 

 time but is mostly produced in the decay of 

 iodine-135 as indicated in the following de- 

 cay chain: 



-„I^^^ 6.7hrs ..Xe^^^ 

 53 54 



9. 2 hrs 



55' 



^135 „ „ ,„6 „ 135 



-Cs 2. X 10 yrs cgBa 



Xenon- 135 has a high neutron absorption 

 cross section. In normal operation of the 

 reactor, xenon- 135 absorbs a neutron and is 

 transformed to the stable isotope of xenon- 136, 

 which presents no poison problem to the re- 

 actor. Equilibrium xenon is reached after about 

 40 hours of steady- state operation. At this 

 point the same amount of xenon- 135 is being 

 "burned" by neutron absorption as is being 

 produced by the fission process. 



The second, and perhaps the more serious, 

 effect of xenon poisoning occurs near the end 

 of core life. As indicated by the half- lives 

 shown in the xenon decay chain, xenon-135 is 

 produced at a faster rate than it decays. The 

 buildup of xenon-135 in the reactor reaches a 

 maximum about 11 hours after shutdown. Should 

 a scram occur near the end of core life, the 

 xenon buildup may make it impossible to take 

 the reactor critical until the xenon has de- 

 cayed off. In a situation of this type, the re- 

 actor may have to sit idle for as much as 

 two days before it is capable of overriding 

 the poison buildup. 



THE NAVAL NUCLEAR POWER PLANT 



Since many aspects of the design and 

 operation of naval nuclear propulsion plants 

 involve classified information, the information 

 presented here is necessarily brief and general 

 in nature. 



In a nuclear power plant designed for ship 

 propulsion, weight and space limitations and 

 other factors must be taken into consideration 

 in addition to the factors involved in the design 

 of a shore-based power plant. 



The thermodynamic cycle of the shipboard 

 nuclear propulsion plant is similar to that of 

 the conventional steam turbine propulsion plant. 



626 



