Monte Carlo Simulation Research on the Spontaneous Fission Yield of 240Pu

104 Pages·2015·2.16 MB·English

Monte Carlo Simulation Research on the
240
Spontaneous Fission Yield of Pu

A dissertation submitted to the
Graduate School
of the University of Cincinnati
in partial fulfillment of the
requirements for the degree of

Doctor of Philosophy

in the Department of
Nuclear & Radiological
Engineering
of the College of Engineering
and Applied Sciences
by

Tianyou Xie

University of Cincinnati
Nov 2015

Committee Chair: Henry B. Spitz, Ph.D.

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Abstract

This research describes results of mathematical simulations to predict the
240
spontaneous fission yield of Pu thereby guiding selection of a suitable
fission product for use as a radiochronometer for weapons grade plutonium.
The PUREX process is effective in separating plutonium and uranium in
irradiated nuclear fuel from fission products, which may make it possible to
240
use one or more of the spontaneous fission products from Pu as a
radiochronometer. However, the fission product inventory from the
240
spontaneous fission of Pu reported in the literature is quite variable.
The non-uniform induced fission probability at multiple neutron energies
makes the induced fission yield too complex to be listed as a single value.

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The neutron flux distribution from a source containing Pu is a
combination of both spontaneous and induced fissions. Fission products
generated in this source are due to a combination of spontaneous and
induced fission processes, each with a unique neutron energy distribution
and fission product yield. The physical conditions of the source (i.e., size,
shape, etc.) will have a significant influence on the neutron flux distribution
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240
and the induced fission probability associated with Pu. Thus, it is likely
that there is a relationship between the combined (spontaneous and induced)
fission yield and the physical conditions of the source. Exploring this
relationship, a table of values will be generated to predict the combined
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fission product yield for any source geometry containing Pu. Minimizing
variations in the source geometry should stabilize the combined fission
product yield and provide a means to determine the spontaneous fission
240
probability for Pu. This research has identified that the combined fission
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product yields from Zr and Xe exhibit less sensitivity to the physical
source parameters than other fission products making them good candidates
as radiochronometers for age dating weapons-grade plutonium.

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Contents
Abstract ............................................................................................................................... 2
Chapter 1 Introduction ........................................................................................................ 7
1.1 Introduction ............................................................................................................... 7
1.1.1 Weapons-grade Plutonium ................................................................................. 8
1.1.2 Spontaneous Fission........................................................................................... 9
1.1.3 Chronometer .................................................................................................... 11
1.1.4 Monte Carlo Method & MCNPX .................................................................... 13
1.1.5 Current Study ................................................................................................... 15
1.1.6 Challenge of Chronometer Study..................................................................... 20
1.2 Objectives and Specific Aims ................................................................................. 22
Chapter 2 Neutron Energy Effect on Fission .................................................................... 24
2.1 Background ............................................................................................................. 24
2.1.1 Fission Neutron Energy ................................................................................... 30
2.1.2 Simulations Using Fission Product Photopeaks .............................................. 31
2.1.3 Independent Fission Probability Study ............................................................ 32
2.2 Method .................................................................................................................... 34
2.2.1 Single Neutron Energy Model ......................................................................... 34
2.2.2 Varied Energy Selection .................................................................................. 35
2.3 Result & Discussion ................................................................................................ 36
2.4 Conclusions ............................................................................................................. 39
Chapter 3 Micro Position Model....................................................................................... 41
3.1 Background ............................................................................................................. 41
3.2 Method .................................................................................................................... 44
3.2.1 Uniform Thickness Model ............................................................................... 47
3.2.2 Uniform Volume Model .................................................................................. 51
3.2.3 Neutron Energy Distribution............................................................................ 52
3.3 Result & Discussion ................................................................................................ 53
3.3.1 Neutron Energy Distribution............................................................................ 53
3.3.2 Equal Thickness Model.................................................................................... 55
3.3.3 Equal Volume Model ....................................................................................... 58
3.3.4 Fission Yield Combination .............................................................................. 61
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3.4 Conclusion .............................................................................................................. 62
Chapter 4 Macroscopic Parameter Model......................................................................... 64
4.1 Background ............................................................................................................. 64
4.1.1 Plutonium Fission Yield Curve ........................................................................ 64
4.2 Method .................................................................................................................... 65
4.2.1 Simulation Parameters Selection ..................................................................... 68
4.2.2 Macro Parameter Model .................................................................................. 69
4.3 Result & Discussion ................................................................................................ 73
4.4 Conclusion .............................................................................................................. 83
Chapter 5 Conclusion ........................................................................................................ 85
5.1 Research Accomplishments .................................................................................... 85
5.2 Future Work ............................................................................................................ 86
References ......................................................................................................................... 88
Appendix A ....................................................................................................................... 90
Appendix B ....................................................................................................................... 98
Appendix C ..................................................................................................................... 102

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Chapter 1
Introduction
1.1 Introduction

Plutonium is a radioactive actinide metal whose isotope, plutonium-239, is
1
one of the three primary fissile isotopes (uranium-233 and uranium-235 are
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the other two), Plutonium-241 is also highly fissile. A fissile isotope has a
nucleus that may fission when struck by a thermal neutron.

Twenty radioactive isotopes of plutonium have been characterized. The
longest-lived are plutonium-244, with a half-life of 80.8 million years,
plutonium-242, with a half-life of 373,300 years, and plutonium-239, with a
half-life of 24,110 years. All of the remaining radioactive isotopes have
half-lives that are less than 7,000 years. This element also has eight
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metastable states, all with have half-lives less than one second.

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1.1.1 Weapons-grade Plutonium

The primary component of nuclear weapons is uranium or plutonium metal,
233 235 239
enriched in a fissile isotope ( U, U or Pu). These isotopes are defined
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as Special Nuclear Material (SNM). The development of nuclear weapons
has relied predominantly on the production of high-enriched uranium (HEU)
or weapons-grade plutonium for fission primaries. However, spontaneous
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fission neutrons from Pu may cause a premature detonation of the device
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unless the weapon design accommodates this neutron source.

The commercial nuclear power industry is a significant contemporary source
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of SNM. Plutonium is created by irradiation of U in a nuclear reactor,
under tailored conditions of neutron bombardment and post irradiation decay,
followed by radiochemical reprocessing. A major difference in used
commercial nuclear fuel and weapons-grade plutonium is the concentration
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of Pu. Plutonium is considered as “weapons grade” if it contains at least
239 240 5
93% Pu and <7% Pu. Reactor grade plutonium contains more than 8%
240
Pu. Plutonium is also used for civilian nuclear power production in
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mixed-oxide (MOX) fuel, which nominally contains 60% Pu and 25%
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Pu.
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Weapons grade plutonium is generated in nuclear fuel that has been
irradiated for no more than 7 months. The economics of commercial nuclear
power plants dictates that fuel be retained in a reactor core for about 4 years
to achieve optimum performance and cost effectiveness. The typical 1-GW
e
nuclear reactor uses about 1 ton of fissile material per year which generates
about 200 kg of plutonium. The annual global production of plutonium in
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commercial nuclear power reactors is about 70 t.

1.1.2 Spontaneous Fission

In nuclear physics and chemistry, nuclear fission is either a nuclear reaction
or a radioactive decay process in which the nucleus of an atom splits into
smaller parts (lighter nuclei). The fission process also produces fast
neutrons and photons (as gamma rays) with the release of a large amount of
energy.

Fission is a form of nuclear transmutation because the resulting fragments
are not the same element as the original atom. The two nuclei produced are
most often of comparable but slightly different sizes, typically with a mass
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