Experimental Determination of the Bare Sphere Critical Mass of Neptunium-237. [View all]
The paper I'll discuss in this post is this one: Criticality of a 237Np Sphere (Rene Sanchez et al., Nuclear Science and Engineering, Nuclear Science and Engineering, 158:1, 1-14 (2008)).
Neptunium is the only actinide element that is easy to obtain in an isotopically pure form simply by chemically isolating it. This is because all of the isotopes except Np-237, which has a half-life of 2,144,000 years, that are known and which form readily in thermal spectrum nuclear reactors - which represent almost all of the world's commercial nuclear reactors - are short lived. The half-life of Np-238, the parent of plutonium-238 is 2.117 days, and the half-life of Np-239, the parent of plutonium-239 is 2.356 days. Thus even in a continuous on line isolation system from a critical nuclear fluid of the types now under discussion, chiefly molten salt type reactors, any isolated neptunium would decay, with a few weeks time to essentially pure Np-237.
Neptunium is routinely formed in the operation of commercial nuclear reactors. In thermal reactors, neptunium has a high neutron capture cross section and its fission is rare. Chiefly it is transmuted into plutonium-238, the accumulation of which has the happy result, in high enough concentrations (albeit not necessarily routinely formed concentrations), to make reactor grade plutonium that is essentially unusable in nuclear weapons. (As a practical matter, it is much easier to make nuclear weapons from natural uranium by separating the U-235 than it is to make it from reactor grade plutonium, and since it is impossible for humanity to consume all of the natural uranium on the planet, it will never be possible to make nuclear war impossible.)
In a fast neutron nuclear spectrum, neptunium can form a critical mass, and thus can be utilized as a nuclear fuel (or in theory, a nuclear weapon).
I personally favor fast spectrum nuclear reactors, since they represent the potential to ban all energy related mining, dangerous natural gas wells, fracked and "normal," dangerous petroleum wells, fracked and "normal," all the world's coal mines, and in fact, all of the world's uranium mines for many centuries to come, utilizing the uranium already mined and the thorium already dumped by the lanthanide industry.
The so called "minor actinides," generally including neptunium, americium, curium and sometimes berkelium and californium, all have useful properties; there has been a lot of discussion in the scientific literature of using neptunium and americium as constituents of nuclear fuels, to eliminate the often discussed, but entirely unnecessary waste dumps for the components of used nuclear fuel.
From the introduction of the paper:
Neptunium-237 is a byproduct of power production in nuclear reactors. It is primarily produced by successive neutron captures in 235U or through the n, 2n reaction in 238U. These nuclear reactions lead to the production of 237U, which decays by beta emission into 237Np (Equation 1):
It is estimated that a typical 1000-MW electric reactor produces on the order of 12 to 13 kg/yr of neptunium.2 Some of this neptunium in irradiated fuel elements has been separated and is presently stored in containers in a liquid form. This method of storage is quite adequate because the fission cross section for 237Np at thermal energies is quite low, and any moderation of the neutron population by diluting the configurations with water would increase the critical mass to infinity. However, for long-term storage, the neptunium liquid solutions must be converted into oxides and metals because these forms are less movable and less likely to leak out of containers.
As noted in Ref. 3, metals and oxides made out of neptunium have finite critical masses, but there is a great uncertainty about these values because of the lack of experimental criticality data. Knowing precisely the critical mass of neptunium not only will help to validate mass storage limits and optimize storage configurations for safe disposition of these materials but will also save thousands of dollars in transportation and disposition costs.
The experimental results presented in this paper establish the critical masses of neptunium surrounded with highly enriched uranium (HEU) and reflected by various reflectors. The primary purpose of these experiments is to provide criticality data that will be used to validate models in support of decommissioning activities at the Savannah River plant and establish welldefined subcritical-mass limits that can be used in the transportation of these materials to other U.S. Department of Energy facilities. Finally, a critical experiment using an a-phase plutonium sphere surrounded with similar HEU shells and using the same setup used for the neptunium experiments was performed to validate plutonium and uranium cross-section data.
A brief excerpt of the materials utilized in these experiments:
…The analysis showed that the sphere was 98.8 wt% neptunium, 0.035 wt% uranium, and 0.0355 wt% plutonium. There were also traces of americium in the sphere. Table I shows the elements found in the chemical analysis of the sprue. Approximately 1% of the mass of the sphere was missing because the sprue sample did not dissolve completely.
To reduce the gamma-radiation exposure to workers, which comes mostly from the 310-keV gamma ray from the first daughter of 237Np, 233Pa, the neptunium sphere was clad with a 0.261-cm-thick layer of tungsten and two 0.191-cm-thick layers of nickel. The gamma radiation at contact with the bare sphere was reduced from 2 R/h to 300mR/h for the shielded sphere. Table II shows the dimensions, weights, and calculated densities of the neptunium sphere and different cladding materials. The total weight of the sphere, including cladding materials, was 8026.9 g. Figure 2 illustrates how the neptunium sphere was encapsulated. Except for the tungsten layer, both of the nickel-clad materials were electronbeam welded. In addition, a leak test was conducted for the nickel-clad layers to ensure that the neptunium metal and possibly some neptunium oxide produced in the event of a leak were contained within these materials and not released into the room or the environment.
Table 1:
This is a highly technical paper, and it is probably not of any value here to excerpt all that much of it. Nevertheless, there is a great deal of public mysticism about nuclear technology, mysticism that is killing the world, since nuclear energy is the only technology that might work to ameliorate, stop, or even reverse climate change. There is so much mysticism and misinformation that completely scientifically illiterate morons like say, Harvey Wasserman, can find people ignorant enough to believe he is, in fact, an "expert" on nuclear issues. (He's not. He is an abysmally ignorant fool, whose ignorance is killing people right now.)
With this in mind, I thought it might be useful to show some diagrams and photographs of the work that was performed here and that is found in the original paper:
A student of nuclear history will recognize that these experiments are very much like the experiments with the "demon core" that killed the nuclear weapons scientists Harry Daghlian and Louis Slotin in separate experiments in 1946. The remote equipment here is obviously designed to prevent that sort of accident from recurring.
The authors explored a number of different systems and reflectors, including both polyethylene and steel. In the process of conducting these studies, they refined some nuclear data on uranium isotopes, a valuable outcome.
From their conclusion:
Currently the main use for Np-237 is as a precursor for Pu-238 for use in deep space missions. Production of this important isotope has resumed at Oak Ridge National Laboratory, albeit on a small scale.
If we are interested in saving the world - there isn't much evidence that we are - neptunium can play a larger role in doing so, and thus this historical work is of considerable value.
A related minor actinide, which is also a potential source of Pu-238, although this plutonium will always be contaminated with Pu-242 owing to the branching ratio of the intermediate Curium-242, is americium-241.
It was estimated, in 2007, that the world inventory of these valuable elements was, as of 2005, was about 70 tons of Np-237, and 110 tons of Americium. It is desirable, critical actually (excuse the pun) that these materials be put to use.
I wish you a pleasant Sunday.
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