Poison, from general point of view, is something that's not too friendly and often have opposing or negative effect upon the original situation. Hence, it can be easily deduced here that, neutron poisons are elements or materials that are "poisonous" towards neutron.
However, it must not necessary be bad or negative. In this case, the existence of neutron poisons is critical to ensure that criticality is maintained. In early stage of operation, when the fuel is new and "a lot" or plenty in amout, the number of neutrons generated will occasionally exceed the needed amount and hence will cause the nuclear reactor to be supercritical. This situation, if not monitored or controlled, will cause the reactor to overheat and fuel to melt down, leading to the 2nd Chernobyl accident. That is why control rod exists.
However, relying alone on control rod is not sufficient as control rod in laymen terms is not that capable. Hence, the neutron poisons need to join the "family". Basically, neutron poisons function similarly to control rod by absorbing the excess neutrons or in more technical terms excess positive reactivity in order to make the reactor more stable and balanced.
Generally, these poisons can be divided into 2 types: fixed burnable poison and soluble poison.
Burnable poisons are materials that have high neutron absorption cross section and will eventually have its characteristics declined due to the absorption of neutron. Over core life, the negative reactivity of the burnable poison and the positive reactivity of the fuel will decrease. Hence, it is necessary for both of them to coexist so as to counter for each other especially at the initiation of the nuclear reactor. Fixed burnable poisons are usually in the form of compounds of Boron or Gadolinium that ae shaped into separate lattice pins or plates(similar as to control rods) or added as addictives to the fuel(with more uniform distribution compared to control rods and lessening the disruption to the core power distribution).
Soluble poisons (chemical shim) is a soluble neutron poison that is circulated in the coolant during normal operation. In other words, soluble poison is in liquid form while fixed burnable poison is mostly in solid or powdered form. Two main processes that can be related to soluble poison is boration and dilution. As soluble poison is in liquid form, it is easy to deduce that boration is basically saturation and dilution is basically dilution of the solution. :) Some of the good examples of soluble poisons are boric acid, sodium polyborate and gadolinium nitrate.
That's all from me regarding neutron poisons. Hope the above informations are informative and "easy to digest" for you all. =) Do not hesitate to ask any questions or LIKE the post!
However, it must not necessary be bad or negative. In this case, the existence of neutron poisons is critical to ensure that criticality is maintained. In early stage of operation, when the fuel is new and "a lot" or plenty in amout, the number of neutrons generated will occasionally exceed the needed amount and hence will cause the nuclear reactor to be supercritical. This situation, if not monitored or controlled, will cause the reactor to overheat and fuel to melt down, leading to the 2nd Chernobyl accident. That is why control rod exists.
However, relying alone on control rod is not sufficient as control rod in laymen terms is not that capable. Hence, the neutron poisons need to join the "family". Basically, neutron poisons function similarly to control rod by absorbing the excess neutrons or in more technical terms excess positive reactivity in order to make the reactor more stable and balanced.
Generally, these poisons can be divided into 2 types: fixed burnable poison and soluble poison.
Burnable poisons are materials that have high neutron absorption cross section and will eventually have its characteristics declined due to the absorption of neutron. Over core life, the negative reactivity of the burnable poison and the positive reactivity of the fuel will decrease. Hence, it is necessary for both of them to coexist so as to counter for each other especially at the initiation of the nuclear reactor. Fixed burnable poisons are usually in the form of compounds of Boron or Gadolinium that ae shaped into separate lattice pins or plates(similar as to control rods) or added as addictives to the fuel(with more uniform distribution compared to control rods and lessening the disruption to the core power distribution).
Soluble poisons (chemical shim) is a soluble neutron poison that is circulated in the coolant during normal operation. In other words, soluble poison is in liquid form while fixed burnable poison is mostly in solid or powdered form. Two main processes that can be related to soluble poison is boration and dilution. As soluble poison is in liquid form, it is easy to deduce that boration is basically saturation and dilution is basically dilution of the solution. :) Some of the good examples of soluble poisons are boric acid, sodium polyborate and gadolinium nitrate.
That's all from me regarding neutron poisons. Hope the above informations are informative and "easy to digest" for you all. =) Do not hesitate to ask any questions or LIKE the post!
Good post, I like how you summarize technical topics into layman terms. Keep it up!
ReplyDeletethank you for the compliment.
ReplyDeleteHi,
ReplyDeleteGood and very interesting article. I have a questions here about soluble poison. Do really soluble poisons are used in emergency shutdown systems? and how it is done during SCRAM?
AMREET RAJ SINGH
ME083528
(amreet90@hotmail.com)