Neptunium-235 Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by: •
Electron capture: the decay energy is 0.125 MeV and the decay product is
uranium-235. •
Alpha emission: the
decay energy is 5.2 MeV and the decay product is
protactinium-231.
Neptunium-236 Neptunium-236 has 143 neutrons and a half-life of 153,000 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. Neptunium-236 is a
fissile material; it has an estimated
critical mass of , though precise experimental data is not available (as sufficient material for criticality is not). is produced in small quantities via the (n,2n) and (γ,n) capture reactions of , however, it is nearly impossible to separate in any significant quantities from its parent . 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 or reactors. Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from or the
isomer . The most favorable reactions to accumulate were shown to be
proton and
deuteron irradiation of
uranium-238. 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 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, also forms in
spent nuclear fuel as the decay product of
americium-241. 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 production When exposed to neutron bombardment can capture a neutron, undergo beta decay, and become , 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 stockpiles The long
half-life (88 years) of 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. is therefore a key radionuclide for the production of , which is essential for deep space probes requiring a reliable and long-lasting source of energy without maintenance. Stockpiles of 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
Savannah River Site(operating in
South Carolina from 1950 to 1988) the development of
atomic weapons, are now almost exhausted. The extraction and purification of sufficient new quantities of from
irradiated nuclear fuels is therefore necessary for the resumption of 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 239U 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 239Np dominating "the spectrum for several days". == References ==