XIV

Neptunium-235※

Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by:

• Alpha emission: the decay energy is 5.2 MeV and the decay product is protactinium-231.
• Electron capture: the decay energy is 0.125 MeV and the decay product is uranium-235

This isotope of neptunium has a weight of 235.044 063 3 u.

Neptunium-236※

Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods:

• Electron capture: the decay energy is 0.93 MeV and the decay product is uranium-236. This usually decays (with a half-life of 23 million years) to thorium-232.
• Beta emission: the decay energy is 0.48 MeV and the decay product is plutonium-236. This usually decays (half-life 2.8 years) to uranium-232, which usually decays (half-life 69 years) to thorium-228, which decays in a few years to lead-208.
• Alpha emission: the decay energy is 5.007 MeV and the decay product is protactinium-232. This decays with a half-life of 1.3 days to uranium-232.

This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material; it has an estimated critical mass of 6.79 kg (15.0 lb), though precise experimental data is not available.

Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of
Np
, however, it is nearly impossible to separate in any significant quantities from its parent
Np
. It is for this reason that despite its low critical mass and high neutron cross section, it has not been researched extensively as a nuclear fuel in weapons/reactors. Nevertheless,
Np
has been considered for use in mass spectrometry and as a radioactive tracer, because it decays predominantly by beta emission with a long half-life. Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from
Np
or the isomer
Np
. The most favorable reactions to accumulate
Np
were shown to be proton and deuteron irradiation of uranium-238.

Neptunium-237※

Np
decays via the neptunium series, which terminates with thallium-205, which is stable, unlike most other actinides, which decay to stable isotopes of lead.

In 2002,
Np
was shown to be capable of sustaining chain reaction with fast neutrons, as in a nuclear weapon, with a critical mass of around 60 kg. However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for light water nuclear power plants (as opposed to fast reactor or accelerator-driven systems, for example).

Inventory in spent nuclear fuel※

Np
is the only neptunium isotope produced in significant quantity in the nuclear fuel cycle, both by successive neutron capture by uranium-235 (which fissions most but not all of the time) and uranium-236, or (n,2n) reactions where a fast neutron occasionally knocks a neutron loose from uranium-238 or isotopes of plutonium. Over the long term,
Np
also forms in spent nuclear fuel as the decay product of americium-241.

Np
is considered to be one of the most mobile radionuclides at the site of the Yucca Mountain nuclear waste repository (Nevada) where oxidizing conditions prevail in the unsaturated zone of the volcanic tuff above the water table.

Raw material for
Pu production※

When exposed to neutron bombardment
Np
can capture a neutron, undergo beta decay. And become
Pu
, this product being useful as a thermal energy source in a radioisotope thermoelectric generator (RTG or RITEG) for the production of electricity and heat. The first type of thermoelectric generator SNAP (Systems for Nuclear Auxiliary Power) was developed and used by NASA in the 1960's and during the Apollo missions to power the instruments left on the Moon surface by the astronauts. Thermoelectric generators were also embarked on board of deep space probes such as for the Pioneer 10 and 11 missions, the Voyager program, the Cassini–Huygens mission, and New Horizons. They also deliver electrical and thermal power to the Mars Science Laboratory (Curiosity rover) and Mars 2020 mission (Perseverance rover) both exploring the cold surface of Mars. Curiosity and Perseverance rovers are both equipped with the last version of multi-mission RTG, a more efficient and standardized system dubbed MMRTG.

These applications are economically practical where photovoltaic power sources are weak or inconsistent due to probes being too far from the sun or rovers facing climate events that may obstruct sunlight for long periods (like Martian dust storms). Space probes and rovers also make use of the heat output of the generator to keep their instruments and internals warm.

Shortage of
Np stockpiles※

The long half-life (T_½ ~ 88 years) of
Pu
and the absence of γ-radiation that could interfere with the operation of on-board electronic components. Or irradiate people, makes it the radionuclide of choice for electric thermogenerators.

Np
is therefore a key radionuclide for the production of
Pu
, which is essential for deep space probes requiring reliable and long-lasting source of energy without maintenance.

Stockpiles of
Pu
built up in the United States since the Manhattan Project, thanks to the Hanford nuclear complex (operating in Washington State from 1943 to 1977) and the development of atomic weapons, are now almost exhausted. The extraction and purification of sufficient new quantities of
Np
from irradiated nuclear fuels is therefore necessary for the resumption of
Pu
production in order to replenish the stocks needed for space exploration by robotic probes.

Neptunium-239※

Neptunium-239 has 146 neutrons and a half-life of 2.356 days. It is produced via β decay of the short-lived uranium-239, and undergoes another β decay to plutonium-239. This is the primary route for making plutonium, as U can be made by neutron capture in uranium-238.

Uranium-237 and neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout from a nuclear detonation, with Np dominating "the spectrum for several days."

References※

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.