Why isn't neptunium used in nuclear reactors in nuclear power plants?

Why isn't neptunium used in nuclear reactors in nuclear power plants? Uranium is, and plutonium is. But neptunium isn't and it is in the middle of them. Is it like it is too hard to make it do fission? Can someone please kindly explain it to me?

• Fission is not an issue. It's hard to make neptunium, period. – Ivan Neretin Mar 20 '18 at 11:37
• Actually, neptunium (Np-237) is regularly used in nuclear reactors; however, not as nuclear fuel but as so-called fission dosimeter in irradiation capsules. (I have just received a delivery of six such capsules that will later be installed in a pressurized water reactor.) – Loong Mar 20 '18 at 11:47

There are several reasons that neptunium may not be used.

The first is the abundance of neptunium. From Wikipedia, it states the best source is from spent fuel rods. Basically, someone would need to handle very radioactive material to obtain a small amount of neptunium.

Next would be the likelihood of fission when struck with a neutron. If it is the same or less than uranium, I would go with uranium. It would have to be substantially more than uranium to offset the cost of refinement.

Also, there is the energy output per gram. I don't think it would be much greater than uranium, probably less.

The isotopes of neptunium that have long half-lives are $\ce{{}^{236}Np}$ and $\ce{{}^{237}Np}$. These are not produced in great abundance from $\ce{{}^{235}U}$ or $\ce{{}^{238}U}$.

To make plutonium from uranium, the uranium must absorb neutrons without fission. This is much more likely to happen then to make $\ce{{}^{236}Np}$ or $\ce{{}^{237}Np}$

• Technically, neptunium is an intermediate when uranium takes up neutrons. Starting with U-238, we go to U-239, which decays by electron beta emission to Np-239 and then Pu-239. The plutonium isotope, with a longer half-life than the neptunium intermediate, is primarily what we see. – Oscar Lanzi Apr 25 '18 at 22:54

Production rate is low for reactors using low-enrichment uranium. You get about one atom of 237Np per thousand atoms of 239Pu, so its easier to get the Pu out and use it rather than running and running your production reactor to get the 237Np.

Neptunium is not a good fuel, firstly there is the activation problem

Neptunium-237 can adsorb neutrons and either undergo fission or it can capture neutrons to form neptunium-238. This will beta decay to form plutonium-238 which is a horrible isotope of plutonium to work with in bulk. Plutonium-238 has a half life slightly shorter than 100 years, it has a very high specific activity and it does lots of horrible things that plutonium-239 does not do. A small trace of plutonium-238 will greatly increase the alpha activity of the plutonium produced by the reactor. This will make reprocessing and the spent fuel handling harder.

Then there is the problem of production.

The normal route to neptunium-237 is that a uranium-235 nucleous adsorbs a neutron, it forms an excited state of uranium-236. Normally we hope that the excited state of uranium-236 undergoes nuclear fission. But sometimes it manages to relax into its ground state.

The uranium-236 can adsorb another neutron to form uranium-237 which then beta decays to neptunium-237. An alternative route is to subject uranium-238 to fast neutrons such as fusion neutrons, this causes the uranium-238 to undergo the n.2n reaction. These very fast neutrons will not appear commonly in a normal nuclear reactor.