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There are seven long lived fission products:

Technetium-99 halflife 211,000 years
Tin-126 halflife 230,000 years
Selenium-79 halflife 295,000 years.
93zirconium halflife 1,530,000 years,
Caesium-135 halflife 2,300,000 years
Palladium-107 halflife 6,500,000 years)
Iodine-129 halflife 15,700,000 years

There are also a number of radioactive isotopes created by neutron abortion, most notably several isotopes of plutonium including the nuclear weapons fuel PU239.

Neutrons are most readily attenuated by hydrogenous materials (stuff with a lot of hydrogen) like water or plastic. That is because a neutrons atomic mass is nearly the same as the hydrogen nucleus (proton) and hydrogen nuclei have a relatively high cross section for scattering. Once the neutron reaches thermal equilibrium, it can be readily absorbed by something with a high cross section for absorption. Usually boron because it is cheap and plentiful.