Seen here is a $1.75 Pulse of the Oregon State TRIGA Reactor (OSTR)

The video has been slowed to about 1/6th real time. The pulse in this video had a peak power of nearly 900 MW, and lasted about 4 milliseconds.

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For those curious, nuclear reactors need three things to effectively operate: fuel, a moderator, and a coolant. Fuel and coolant are pretty self explanatory. The moderator however, is a substance that slows neutrons from high energy to low energy by collisions. Imagine neutrons as billiard balls hitting other billiard balls and eventually going from relativistic speeds down to about 2200 m/s (still very fast). We call these slow neutrons “thermal” neutrons.

OSTR and other reactors like it use a Uranium-Zirconium-Hydride (UZrH) fuel with an erbium burnable poison to extend fuel life. The uranium and zirconium are homogeneously mixed in metal form. The hydrogen is interlaced in the zirconium matrix and acts as a moderator inside the fuel. This is very different from a power reactor that uses ceramic fuel with water acting as both coolant and moderator. The UZrH fuel allows OSTR to have lots of excess reactivity to startup and shutdown as frequently as needed, and it provides an intrinsic safety feature: a massive negative temperature feedback. This means as fuel temperature increases, reactivity sharply declines, so much so that the reactor can shut itself down faster than the control rods can drop into the core.

Conventional nuclear reactors are controlled by control rods made of a neutron absorbing poison. To increase reactor power, rods are slowly removed to induce supercriticality (more neutrons out than in). Once the power level is reached, rods are inserted to some level to keep the reactor critical (critical meaning the number of neutrons put in to the system is equal to the net number of neutrons generated).

What this video demonstrates is a prompt supercriticality, or a pulse. Prompt neutrons are generated instantaneously from fission, and are uncontrollable. Reactors are controlled with delayed neutrons that are generated from the decay of neutron rich nuclei and decay on the order of seconds, allowing effective control of the reactor. For OSTR 99.9925% of the neutrons are prompt, the other 0.0075% are delayed. This is just enough to allow effective control.

A pulse is induced by bringing the reactor critical at 100 W using 3 of the 4 control rods available. The fourth control rod, called the transient rod, is pneumatically driven and can be ejected to whatever height wanted. This rapid insertion allows the reactor to go supercritical on prompt neutrons alone, allowing the power excursion to evolve on the order of microseconds. In just 4 milliseconds, the reactor went from 100 W to 1000 MW and back down below 1 MW, all from nuclear physics. The control rods drop in about half a second later.

The blue glow seen is called Cherenkov Radiation, and is an artifact of beta particles (electrons originating from nuclear decay) moving faster than the speed of light in the medium (greater than 225 million meters per second for water). As the electrons slow down, they emit photons in the ultraviolet and visible light spectrums, most of which is blue light, as shown.

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