The Fermi Chronicles - Part 25: Fission Poisons

There are several kinds of what are called fission poisons in a nuclear reactor. Some are by design. Others a byproduct. Essentially, a fission poison is anything with a very large neutron absorption cross-section. If a materiel absorbs neutrons, those neutrons are then not available for fission and the reaction shuts down. The control rods are made specifically of neutron poisons (boron and hafnium) because their purpose is exactly to absorb and stop fission if need be. Some poison materials are included in the fuel rods because, at least at first, there is an overabundance of fissile material. These types of poisons are called burnable poisons because, as they absorb neutrons, they transform into substances that are not poisons (have a low neutron absorption cross-section) thus unlocking the remaining nuclear fuel as time goes on (clever, eh?). Some reactors use soluble poisons such as borated water used in PWRs (see my post below on the Davis Besse event). Soluble poisons are also called "chemical shims." They are most common as an emergency shutdown feature in most reactor designs. For instance, as a last resort in an emergency situation, the reactor can be flooded with borated water which will both coll and shut down the fission reaction in short order.

Other poisons are produced by the fission reaction itself. These are called fission product poisons. There are many of these, but the two most influential in reactors are Xenon-135 and Samarium-149, as both have a huge neutron absorption cross-section. A typical fission product is Telerrium-135, which becomes Iodine-135 almost immediately (19s half-life) with the emission of a beta particle. I-135 then gives off another beta particle and becomes Xe-135, a neutron poison. Only 5% of Xe-135 is a fission byproduct, while 95% comes from the decay of Iodine-135. The half-life of I-135 is 9.1 hrs, so it hangs around for a long time. Once I-135 absorbs a neutron, it becomes I-136 (and gives of gamma radiation) and is no longer a poison. In this process, it has just been "burned up." There is an equilibrium between I-135 created and burned up at some point, and it is dependent upon reactor power level.

When reactors go through a shutdown, Xe-135 is no longer produced by fission, but is still produced by radioactive decay. The concentration builds up and peaks at its half-life, then it decays into Cs-135. At its peak, however, it is almost impossible to start the reactor up again as there is such an abundance of Xe that it will essentially absorb all available neutrons and the reactor remains below its critical condition. In light of this phenomenon, re-read the module I wrote on the Chernobyl disaster as it is very relevant.

Samarium-149 is another fission fragment decay product that originally started out after fission from Promethium-149 and neodymium-149 (the one?). Unlike Xe-135, however, Sm-149 is stable, so it cannot be decayed out and must be burned up solely by neutron absorption. When a reactor is shut down, Sm-149 is still produced from radioactive decay, but does not itself simply peak and then decay. It peaks and remains. (in the strict technical sense, however, this would be referred to as a maximum, not a peak) Fortunately, Samarium poisoning is minor compared to Xenon poisoning.

I should note that the above poisons only affect thermal reactors. That is, reactors that depend on thermal neutrons for fission. Reactors that depend on fast neutrons and not thermal neutrons, such as FBRs (see my post below on reactor types), are immune from fission poisons.

Previously:
The Fermi Chronicles - Part 24: Reaction k-Factor
The Fermi Chronicles - Part 23: Davis Besse, Ohio, 2002
The Fermi Chronicles - Part 22: Nuclear Events - Chernobyl, 1986
The Fermi Chronicles - Part 21: Nuclear Events - Three Mile Island, 1979
The Fermi Chronicles - Part 20: Nuclear Events - Browns Ferry, Alabama, 1975
The Fermi Chronicles - Part 19: Nuclear Events - Fermi 1, 1966
The Fermi Chronicles - Part 18: Nuclear Events - SL-1 Event, Idaho, 1961
The Fermi Chronicles - Part 17: Nuclear Events - Windscale, UK, 1957
The Fermi Chronicles - Part 16: Nuclear Events - Chalk River, CAN, 1952
The Fermi Chronicles - Part 15: The Nuclear Business Model
The Fermi Chronicles - Part 14: Neutron Moderation
The Fermi Chronicles - Part 13: Nuclear Reactor Types
The Fermi Chronicles - Part 12: Generating Electricity
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post