Nuclear weapons interfere with carbon dating
Uranium and plutonium are composed of several isotopes, some of which are fissile.
Some nuclear weapons are typically designed so that a pulse of neutrons will start the nuclear chain reaction at the optimum moment for maximum yield; background neutrons from plutonium-240 can set off the reaction prematurely, and with reactor-grade plutonium the probability of such "pre-initiation" is large.
The even numbered isotopes (plutonium-238, 240 and 242) fission spontaneously producing high energy neutrons and a lot of heat.
Dealing with the second problem with reactor-grade plutonium, the heat generated by plutonium-238 and plutonium-240, requires careful management of the heat in the device.
The Pu-240 has a high spontaneous rate of fission, and the amount of Pu-240 in weapons-grade plutonium generally does not exceed 6 percent, with the remaining 93 percent Pu-239.
Higher concentrations of Pu-240 can result in pre-detonation of the weapon, significantly reducing yield and reliability.