"Guessing before proving! Need I remind you that is how all important discoveries have been made?" - Henri Poincare
Time for a quick nerdy nuclear post again.
All water-moderated (and water-cooled) nuclear reactors have an issue with a very short-lived radioisotope that is created during reactor operation, and that is the transmutation of Oxygen-16 into Nitrogen-16 in the reactor core.
Neutrons from freshly split atoms have a tremendous amount of energy, which equates to speed. Water is typically used as a "moderator", a fancy term for a material for the neutrons to bounce into, which slows them down. Many materials can moderate neutrons, but materials that contain hydrogen atoms are preferred, because a collision with a low-mass atom transfers more kinetic energy than a collision with a higher mass atom.
Imagine a ping-pong ball fired from a cannon at a bunch of other ping-pong balls, vs being fired at a group of boulders. The ping-pong ball will come to rest more quickly in the ping-pong balls, whereas it will ricochet among the boulders for a good long time before coming to rest. This is the principle of using lighter atoms for neutron moderation. Beryllium and graphite are also excellent moderators, but this is due less to their atomic mass (which is high) than their very high resistance to absorbing a neutron vs hydrogen.
Plain old light water is inexpensive, chemically pretty stable, and can also act as reactor coolant, and so water is usually what's used as a neutron moderator. Water contains hydrogen, but it also contains Oxygen - and fast neutrons also collide with Oxygen atoms in the reactor core.
When a fast neutron hits an Oxygen-16 atom, it sometimes knocks a proton right out of the nucleus, and then neutron takes the proton's place - briefly. The Oxygen-16 has now become Nitrogen-16.
Nitrogen-16 is radioactive, and the creation of a radioactive atom due to neutron capture is called "neutron activation". As radioactive substances go, Nitrogen-16 has one good aspect and one bad aspect. The good aspect is that Nitrogen-16 has a very short half-life, at 7.13 seconds. This means that a minute or two after the reactor is shut down, the radiation from Nitrogen-16 will be gone.
The bad aspect of Nitrogen-16 is that when it decays again to Oxygen-16, it emits a sizzling gamma ray, at 6.13 MeV. During reactor operation, Nitrogen-16 is responsible for the bulk of activity in the cooling system of a water cooled/moderated reactor. Because the coolant is continuously circulated through cooling loops to remove heat from the reactor core, Nitrogen-16 is transported outside of the reactor vessel, and into other places, where it must be shielded against.
In pressurized water reactors, the primary coolant loop is completely contained within a shielded structure, so Nitrogen-16 is not much of an issue.
Below: A pressurized water reactor. The gray border indicates shielding for the primary coolant loop and reactor vessel. The only penetrations are for non-radioactive feed water into the steam generator, and for non-radioactive steam to leave.
On a Boiling Water Reactor, Nitrogen-16 is produced in the core, and is carried out along with the steam into the steam turbine, main condenser, condensate system, feedwater system, and steam drain systems. Therefore all of those things must also be shielded against the N-16 gamma radiation.
Below: A boiling water reactor, with additional required shielding.
So I know what you've been thinking all this time. OK, probably not... How do we shield personnel from Nitrogen-16 who work at a swimming-pool type reactor? These reactors are primarily cooled by natural circulation. Hot water rises out of the core to the surface, and cooler water sinks down the sides of the pool and enters the core to cool it.
Won't the hot coolant rising up off the core of a swimming pool reactor carry highly radioactive Nitrogen-16 straight to the surface, and dose everyone in the area with 6.13 MeV gamma radiation? The answer is yes. Yes it will. So how come you don't see a lot of shielding above swimming pool reactors?
Because there's a fix. It's pretty clever, because it takes advantage of the short half-life of Nitrogen-16. The fix for Nitrogen-16 wafting straight to the surface is a diffuser system. Rather than allowing the hot water to rise directly out of the core to the surface, a pump recirculates water from elsewhere in the tank, and swirls it shortly after it exits the reactor.
Below: A couple of TRIGA swimming pool reactors with arrows indicating the diffuser system nozzles. all to mitigate Nitrogen-16.
This swirling of the Nitrogen-16 laden coolant delays the rise of the warmer coolant to the surface by nearly a minute, allowing most of the Nitrogen-16 to decay underwater. Higher power swimming pool reactors take a suction above the core and sweep that water into a baffled underground tank, which slows transport time and gives the Nitrogen-16 time to decay.
Stuff you never wanted to know, and probably don't care about anyway. LOL
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