Fissile Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce non-fissile Pu-240 and worse, then Pu-241 which is a fairly strong neutron emitter.

" Fissile " is distinct from " fissionable.

Currently under discussion are a Fissile Material Cut-off Treaty ( FMCT ), a pact to prevent an arms race in outer space ( PAROS ), nuclear disarmament, and negative security assurances ( NSA ).

Fissile materials can be used to fuel thermal reactors, with a neutron moderator ; fast-neutron reactors, with no moderators ; and nuclear explosives.

In the arms control context, particularly in proposals for a Fissile Material Cutoff Treaty, the term " fissile " is often used to describe materials that can be used in the fission primary of a nuclear weapon.

The Fissile Material Cutoff Treaty ( FMCT ) is a proposed international treaty to prohibit the further production of fissile material for nuclear weapons or other explosive devices.

Image: Russet Lake, Fissile Mountain. jpg

* Avoiding Nuclear Anarchy: Containing the Threat of Loose Russian Nuclear Weapons and Fissile Material.

Under 1 % of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel.

U-235 is fissionable by thermal ( i. e. slow-moving ) neutrons.

There was some hope that the enrichment process would be improved and fissionable isotopes of U-235 could, at some future date, be extracted from the depleted uranium.

This is differentiated further into " source material ", consisting of natural and depleted uranium, and " special fissionable material ", consisting of enriched uranium ( U-235 ), uranium-233, and plutonium-239.

Among the heavy actinide elements, however, those isotopes that have an odd number of neutrons ( such as U-235 with 143 neutrons ) bind an extra neutron with an additional 1 to 2 MeV of energy over an isotope of the same element with an even number of neutrons ( such as U-238 with 146 neutrons ).

Near the outer edge of the cylinder heavier gas molecules containing U-238 collect, while molecules containing U-235 concentrate at the center and are then fed to another cascade stage.

The main by-product of enrichment is depleted uranium ( DU ), principally the U-238 isotope, with a U-235 content of ~ 0. 3 %.

However, modern reactors are only moderately enriched with U-235 relative to U-238, so the U-238 continues to serve as a denaturation agent for any U-235 produced by plutonium decay.

Naturally occurring uranium is not fissile because it contains 99. 3 % of U-238 and only 0. 7 % of U-235.

( Natural uranium is about 99. 27 % U-238, 0. 72 % U-235 — the fissile isotope, and 0. 0055 % U-234 ).

Natural uranium metal contains about 0. 71 % U-235, 99. 28 % U-238, and about 0. 0054 % U-234.

K-25 was a gaseous diffusion plant designed to separate U-235 from U-238 ; the S-50 liquid thermal diffusion plant was located nearby.

When 3 % enriched LEU fuel is used, the spent fuel typically consists of roughly 1 % U-235, 95 % U-238, 1 % plutonium and 3 % fission products.

Uranium in nature consists primarily of two isotopes, U-238 and U-235.

Naturally occurring uranium consists of approximately 99. 28 % U-238 and 0. 71 % U-235.

The three naturally-occurring actinide alpha decay chains given below — thorium, uranium / radium ( from U-238 ), and actinium ( from U-235 )— each ends with its own specific lead isotope ( Pb-208, Pb-206, and Pb-207 respectively ).

Wu worked on the Manhattan Project ( she helped to develop the process for separating uranium metal into the U-235 and U-238 isotopes by gaseous diffusion ).

She helped to develop the process for separating uranium metal into the U-235 and U-238 isotopes by gaseous diffusion.

No isotopes of uranium have a half-life of 25, 000 years though, naturally occurring forms have half-lives of 4. 46 billion years ( U-238 ), 700 million years ( U-235 ), and 245, 000 years ( U-234 ).

The rotation creates a strong centrifugal force that draws more of the heavier gas molecules ( containing the U-238 ) toward the wall of the cylinder, while the lighter gas molecules ( containing the U-235 ) tend to collect closer to the center.

U-234 is converted to U-235 more easily and therefore at a greater rate than U-238 is to Pu-239 ( via neptunium-239 ) because U-238 has a much smaller neutron-capture cross-section of just 2. 7 barns.

Depleted uranium also contains less U-235, but in spite of its half-life that is much shorter than the one of U-238, the concentration of U-235 in natural uranium is low enough ( about 0. 7 %) so that the U-235 depletion does not result in a significant reduction in radioactivity.

IUPAC prefers that isotope symbols be written in superscript notation when practical, for example < sup > 12 </ sup > C and < sup > 235 </ sup > U. However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.

" Examples of fissile isotopes are U-235 and plutonium-239.

In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element.

* they have a high power density in a small volume and run either on low-enriched uranium ( as do some French and Chinese submarines ) or on highly enriched uranium (> 20 % U-235, current U. S. submarines use fuel enriched to at least 93 %, compared to between 21 – 45 % in current Russian models, although Russian nuclear-powered icebreaker reactors are enriched up to 90 %),

It is likely that the fissile material of an old bomb which is due for refitting will contain decay products of the plutonium isotopes used in it, these are likely to include U-236 from Pu-240 impurities, plus some U-235 from decay of the Pu-239 ; due to the relatively long half-life of these Pu isotopes, these wastes from radioactive decay of bomb core material would be very small, and in any case, far less dangerous ( even in terms of simple radioactivity ) than the Pu-239 itself.

The two fissile materials used in nuclear weapons are: U-235, also known as highly enriched uranium ( HEU ), oralloy ( Oy ) meaning Oak Ridge Alloy, or 25 ( the last digits of the atomic number, which is 92 for uranium, and the atomic weight, here 235, respectively ); and Pu-239, also known as plutonium, or 49 ( from 94 and 239 ).

Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.

A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater.

For instance, the curve of yield against mass for Pu-239 has a more shallow valley than that observed for U-235 when the neutrons are thermal neutrons.

Since all Pakistani bomb designs are implosion-type weapons, they will typically use between 15 – 25 kg of U-235 for their cores.

Using U-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus.

Using U-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus.

This reduces the chance that the neutrons are captured by U-235 atoms and initiate fission, thus reducing the reactor's power output and placing an inherent upper limit on the temperature of the fuel.

Low power pool-type reactors such as the SLOWPOKE and TRIGA have been licensed for unattended operation in research environments because as the temperature of the low-enriched ( 19. 75 % U-235 ) uranium alloy hydride fuel rises, the molecular bound hydrogen in the fuel cause the heat to be transferred to the fission neutrons as they are ejected.

The " special nuclear materials " are also plutonium-239, uranium-233, and enriched uranium ( U-235 ).

They are labeled " End-cap and Neutron Focus Lens " and " Reflector Wrap "; the former channels neutrons to the U-235 / Pu-239 Spark Plug while the latter refers to an X-ray reflector ; typically a cylinder made out of an X-ray opaque material such as uranium with the primary and secondary at either end.