Why does uranium become radioactive in a nuclear reactor?

Answer 1

Uranium is already radioactive, it does notbecome radioactive in a reactor. Uranium naturally undergoes alpha decay, emitting alpha particles and transforming to Thorium, another radioactive element. The radioactive decay of these daughter isotopes continues via either alpha or beta decay until a stable isotope of lead is produced.

Perhaps what you meant to ask is "Why does Uranium fission in a nuclear reactor?".

This is because there are low energy neutrons (aka thermal neutrons) in the reactor that can avoid capture by the plentiful Uranium-238 isotope and fission the rare Uranium-235 isotope. Initially when the reactor is being started these thermal neutrons are provided by a device called a neutron source. When the reactor becomes critical (by the operators gradually removing control rods) it sustains a stable fission neutron chain reaction supplying its own neutrons to keep fissioning at a constant rate. The operators then remove the neutron source, as it is no longer needed. The only problem in keeping this chain reaction going is that Uranium-235 fission emits high energy neutrons (aka fast neutrons) which are readily captured by the plentiful Uranium-238 isotope. This problem is solved by a moderator, a substance that rapidly removes energy from the neutrons (slowing them from fast to thermal speeds) before a significant number can be lost in Uranium-238 neutron capture. Typical moderators are: graphite, water, heavy water, hydrocarbons, etc.

Perhaps what you meant to ask is "Why do Uranium fuel rods become more radioactive in a nuclear reactor?".

The fuel rods become more radioactive in the reactor than they were originally because of the highly radioactive fission product isotopes produced by the Uranium-235 fission (as well as radioactive isotopes produced by neutron capture by stable elements in the structural parts of the fuel rods). These isotopes are more dangerous than the original Uranium was, because they undergo beta and gamma decay, emitting beta particles and gamma rays both of which are more penetrating than than alpha particles are. Also the longer the fuel rods remain in the reactor the more of these fission products build up in the fuel rods. The good thing is these fission products have much shorter halflifes than the original Uranium: they decay rapidly to stable non-radioactive elements.

The original Uranium takes billions of years to decay completely to stable lead. The fission products take from hours to centuries to decay completely to stable isotopes. The slowest to decay is gone in about 250 years. Thus, after the decay period the fuel rods are actually less radioactive than they were originally before being put in the reactor. The reactor "burned up" the Uranium, leaving fission product "ashes" which take a few centuries to "cool" completely.

You might ask now "What becomes of the Uranium-238 that captured neutrons during the neutron chain reaction in a nuclear reactor?".

It becomes Plutonium, some of which the reactor "burns" just like the Uranium-235 and some builds up in the fuel rods (like the fission products do). This Plutonium could be reprocessed(along with unused Uranium) to make new fuel rods, but if not, with a halfllife of about 25,000 years it will take about 125,000 years in storage to completely decay back to Uranium via alpha decay, which as I said already takes billions of years to decay to stable lead isotopes.