What is "highly enriched" uranium?

Nuclear Fuel Facts: Uranium

Uranium is a silvery-white metallic chemical element in the periodic table, with atomic number 92.

Energy.gov

Natural Uranium – contains a ²³⁸U concentration of 99.27 percent, ²³⁵U concentration of 0.711 percent and very little ²³⁴U.

Uranium Enrichment - World Nuclear Association

Most of the commercial nuclear power reactors in the world today require uranium 'enriched' in the U-235 isotope for their fuel. The commercial process employed for this enrichment involves gaseous uranium hexafluoride in centrifuges.

World Nuclear Association

Very Highly Enriched Uranium

Directives Program, Office of Management (MA-1.2)

So U-235 is "fissile" while U-238 is not. But U-238 absorbs neutrons and decays into Plutonium-239 which is also fissile.

Plutonium-239 is 29,000 times more radioactive than Uranium-235 and its fission reactions are prone to accidental prompt criticality and generally much more difficult to control.

These are the limitations and realities of designing and operating safe and effective nuclear power reactors as opposed to nuclear weapons.

Rods, pebbles or other piles of low enriched uranium are only allowed to react partially to a slight extent in a power plant, and no complete burn or utilization of the energy theoretically available from the fission of uranium and plutonium is ever achieved because the partially used fuel has to be pulled out and sent to long term storage when too much U-235 has been consumed and too much U-238 has been converted into Pu-239 by absorbing neutrons and emitting beta particles.

Prompt Neutron - an overview | ScienceDirect Topics

ScienceDirect TopicsHambsch, Franz Josef

Delayed Neutrons | Definition & Characteristics | nuclear-power.com

Delayed neutrons are emitted by neutron-rich fission fragments that are called delayed neutron precursors. They are of the highest importance in reactor control.

Nuclear Power

The "moderation" of nuclear fission in a power plant — that is, the ability to produce power without causing a nuclear explosion — depends critically on the availability of delayed neutrons.

The reaction cannot be allowed to go critical with prompt neutrons alone, because that would instantly cause a nuclear explosion. However the prompt and delayed neutrons together much be enough to achieve and maintain criticality for power production.

Almost all the neutrons emitted in the fission of Plutonium-239 are prompt, and it is somewhat more difficult to maintain a controlled reaction without causing a nuclear explosion when too much of this isotope has accumulated as "waste."

A plutonium power plant could easily be built and operated to effectively use waste Plutonium-239 and depleted Uranium-238 as primary fuels by incorporating some neutron-moderating elements such as aluminum and possibly boron.