Delayed neutrons in nuclear fission chain reaction 1
Critical state Temporal flow Loss by leakage Loss by Absorption If the number of neutrons (the number of fission reactions) is practically constant over time, it is called the "critical state". 2
Period of one cycle Temporal flow The period of one cycle Loss by leakage Loss by Absorption Light water reactor (LWR):~10-5 sec Very short 3
Neutron multiplication factor, : Critical : Sub-critical : Super-critical 4
Another, but consistent definition of : Critical : Sub-critical : Super-critical 5
Reactivity, : Critical : Sub-critical : Super-critical 6
Reactor power control Power increase: Critical Super-critical Critical Power decrease: Critical Sub-critical Critical 7
Reactor power control Reactor Power Reactivity 0 (= critical) 8
Two types of neutrons born from nuclear fissions Prompt Neutrons Released just after fission reactions Averaged number: 2.4 (U-235) Prompt neutrons Fission products Fission 9
Two types of neutrons born from nuclear fissions Delayed Neutrons Released from radioactive decays of some unstable fission products with time delay Relative abundance of delayed neutrons to total number of fission neutrons ( ) is only 0.64% (U-235). Prompt neutron Delayed neutron Fission product Fission Delayed neutron precursor Radioactive decay 10
Power control only with prompt neutrons Period of one cycle: 2 10-5 [sec] (LWR) Total number of cycles during 1 Sec. : 50,000 Neutron multiplication per cycle: Neutron multiplication after 1 sec. : Neutron multiplication After 1 second After 10 seconds 1.0001 148 5.2 10 21 1.00001 1.65 148 0.9999 0.007 1.9 10-22 Uncontrollable! 11
Two types of neutrons born from nuclear fissions The relative abundance of the total number of generated neutrons are delayed neutrons (i.e., delayed neutron precursors). neutrons After one cycle precursors neutrons neutrons and precursors If is less than unity, the number of neutrons should decrease immediately. 12
Two types of neutrons born from nuclear fissions Order of seconds If 1 is less than unity, the number of neutrons in this fission family will become zero immediately, but some precursors are generated. These precursors will generate neutrons in future and they produce new fission family. 13
Roles of delayed neutrons in reactor dynamics Suppose a situation that delayed neutrons are needed to attain the critical condition. A period of effective cycle becomes longer. (~0.08 sec) Very important! 14
Power control with delayed neutrons In the case where delayed neutrons are needed to attain the critical condition: Period of effective cycle:0.08 sec. Total number of cycles during 1 sec.:12.5 Neutron multiplication After 1 second After 10 seconds 1.001 1.01 1.13 1.003 1.04 1.45 Controllable! The existence of only 0.6% yield of delayed neutrons enables us to use nuclear power in a controlled way. 15
Comments on the stationary critical state If a nuclear reactor is in a stationary critical state, the number of delayed neutron precursors should be much larger than that of neutrons. Even if we introduce so many neutrons instantaneously to a critical reactor, reactor power should immediately decrease because the number of delayed neutron precursors is not enough large to be in a stationary condition. 16
It s time to answer the question 1. 17