It is pointed out that the group of derivde nuclides generated by the interaction of 238 U and reactor neutron flux actually forms a new type of unstable nuclide series

i.e.the fifth unstable nuclide series.The role of fission products played in the series is similar to the stable Pb isotope（or 209 Bi）

the last generation daughter nuclide in the radioactine decay series. So that fission products should be regarded as the generation of derived nuclide of the fifth unstabale nuclide series.Since in radioactive decay as well as its whole family of radioactive daughter nuclide of the fifth unstabale nuclide series.Sincein radioactive decay series the total quantity of atoms lost due to radioactive decay of the parent nuclide as well as its whole family of radioactive daughter nuclides transforms entirely to stable nuclide Pb

this means tuansformatiom ratio equals to unity .Thus the peculiar property of transformation ratio approaching unity in the fifth unstable nuclide seriex is understandable.As a full set of the fifth unstable nuclide series is very difficult to lbtain

idea on the quasi-fifth unstable nuclide series

which consists of only the upper generations of the derivde nuclides which are in equilibrium with their parent

is introduced.