Carbon transformation in nuclear reactors In a nuclear reactor, does stable carbon 12C in graphite rods turn into unstable 14C? If so, is it because of induced radioactivity? 
 A: 14C is obtained from 13C present in the graphite , by one neutron capture .
In the gas-cooled reactor, it is also possible to obtain 14C from 14N and 17O included in carbon dioyde gas, also by one neutron capture.
A: The reaction that produces $^{14}C$ depends on the reactor type. Graphite reactors have thermal spectrum (take RBMK as an example), that is neutrons are being slowed down to escape the resonance absorption region. In this case you can breed $^{14}C$ by two reactions $^{13}C(n,\gamma)^{14}C$ and $^{17}O(n,\alpha)^{14}C$. Both of these reactions are neutron induced, where the latter is a real problem, since it happens in the coolant of commercial power reactors, not only in graphite ones. Power-plants usually vent $^{14}C$ from the coolant during the night, to avoid it being captured by photosynthesizing plants. In general, $^{14}C$ is a low range radiation type emitter($\beta$).
The real problem with Carbon is the energy deposited in the graphite if the core temperature is low. This energy might be released spontaneously and can cause rapid temperature rise. An event like that caused fire at one of the British graphite reactors. This was not an issue for RBMK, due to the higher core temperature.  
