Initial phase of Pu-238 production in Idaho National Laboratory

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Highlights

  • Pu-238 is a strong alpha emitter with the half-life of 87.7 years, used as a thermoelectric generator in deep space probes.

  • Pu-238 is produced from Np-237 in a two-step process: via neutron capture to the Np-238 then decays to Pu-238 by beta.

  • Sensors were inserted into the ATRC low power critical reactor to test the Pu-238 production and check the neutron flux.

  • Production of Pu-238 was confirmed at the level expected from modelling via detecting intermediate Np-238 gamma peaks.

  • Careful pre- and post-irradiation Np-237 radiation monitoring including sensors matrix effects was implemented.

Abstract

The initial phase of Plutonium-238 (Pu-238) production for radioisotope thermal generation is described here in detail. Two dosimeters with/without cadmium sleeve containing Neptunium-237 (Np-237) and other neutron sensors were inserted into the low power Advanced Test Reactor Critical (ATRC) for Pu-238 production testing. The gamma-ray energy measurements from Np-238, the short-lived intermediate product, confirmed that sizable amount of Pu-238 can be produced in the full power Advanced Test Reactor (ATR). The Pu-238 production determined from the irradiation experiment was in accord with the modelling predictions. Detailed studies of the Au/Cu sensors for thermal and epithermal flux analysis and their consistency for sensors in different locations, bare and in Cd-sleeve, provided confidence in the Pu-238 production data.

Section snippets

PFS program history and objectives

Pu-238 is an alpha emitter with a half-life of 87.7 years with a sufficient decay heat to power a deep space satellite or a space probe as a radioisotope thermoelectric generator (RTG). NASA missions: Mars’ Curiosity rover, the New Horizons spacecraft flyby of Pluto, and the most recent Perseverance launched to Mars, July 2020 were powered with a Pu-238 RTG. The capability to produce Pu-238 was lost in the late 1980s after the target irradiation reactors at the Savannah River Site were shut

ATRC operating conditions

The ATRC facility is a testing facility adjacent to the ATR, which runs at low power with nearly identical neutron profile to the ATR, see Table 1. Typically, nuclear material experiments are tested and mimicked in the ATRC before they are irradiated in the ATR to determine targets reactivity and safety by analysis of neutron flux and gamma background induced by the irradiation.

PFS-1 experimental design

Original dosimeters were shipped to the INL from the Pacific Northwest Laboratories (PNNL) in 1992, with detailed records of their weights, activities and measured doses, see Table 2, (U-235-U238Dosimeter R, 1992). They were unopened until preliminary measurements in 2017. The irradiation assembly in the ATRC consisted of four NpO2 dosimeters, in two separate dosimetry capsules, 1-X and 1-Y. It also included a number of other dosimetry wires, some were found in the INL, but others were ordered

Source production – details

Np-237 is a long-lived (>106 years) alpha radioisotope decaying to Protactinium (Pa-233) which decays to Uranium (U-233) in equilibrium (see Fig. 4). Almost all of the Np-237 feedstock was generated by reactors and refinement from past weapon activities (Preliminary Design Review, 2016). It decays in the combination of alpha, beta, and gamma emanations. The Np-237 most prominent gamma ray, 143.249(20) keV, has a branching ratio 0.443(8) % compared to over 38.5(4) % branching ratio for Pa-233

Summary

Careful pre- and post-irradiation measurements of NpO2 dosimeters and other sensors with and without Cd sleeve for thermal and epithermal neutron production in the ATRC reactor were performed by use of gamma spectroscopy to validate Pu-238 production in the INL.

The pre-irradiation average activity was determined from Pa-233 isotope's most intense gamma peak at 311 keV at 0.70(4)+-5% μCi/mg which was identical in the range of error with the nominal specific activity at 0.704 μCi/mg.

The

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The project was funded by DOE Office of Science, DE-AC07-05ID14517 contract for PFS ATR. It was released for publication thanks to the support of A. Zillmer and K. Lively from INL Nuclear Power and Isotopes Production Dept. We are grateful for the expertise provided by Dr. D. Nigg with experiment's plan design and N. Manwaring, for ATRC plan execution. We appreciate support of the RML laboratory, especially Dr. M. Reichenberger providing information about RML detectors/software, and technical

References (16)

  • Atlas of Nuclear Resonances ENDF-VIII.0:...
  • ATR handbook user manual
  • M.D. Gaithersburg
  • T. Katoh et al.

    Meas. Of thermal neutron capture CS and res. Integral of the reaction 127I(n,γ)128I

    J. Nucl. Sci. Technol.

    (1999)
  • W. Killian et al.

    PCGAP Users Guide and Algorithm Desc

    (2000)
  • RML Data for Flux Run 17-4 in TP-04-17

    (2017)
  • W. Lindsey et al.

    Production of plutonium-238 with minimum plutonium-236 concentration

    J. of Nuclear Technology

    (1972)
  • CSM-10584 monitor procurement for INL

    Rev.

    (2017)
There are more references available in the full text version of this article.

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