Production of Minor Actinides in Thermal Nuclear Reactors and Nonproliferation

67 Pages·2003·0.69 MB·English

Production of Minor Actinides in
Thermal Nuclear Reactors and
Nonproliferation
(Report 1 under contract DE-AC01-02NN40287 between the Analytical Center for Non-
proliferation and the U.S. Department of Energy under the auspices of the Nuclear
Cities Initiative)
2
Abstract
This report summarizes the results of the first stage of investigations to evaluate the
urgency of the problem and examine possible ways to enhance security of a special group of
radioactive materials, known as minor actinides. This work is being performed under an
agreement between the Analytical Center for Nonproliferation and the U.S. Department of
Energy.
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Contents
Introduction................................................................................................................................5
1. Generation of Minor Actinides in Uranium-Uranium Nuclear Fuel.....................................7
2. General Physical and Radiation Properties of Minor Actinides............................................7
2.1. Neptunium......................................................................................................................8
2.2. Americium....................................................................................................................10
2.2.1. Am-241..................................................................................................................10
2.2.2. Am-243..................................................................................................................10
2.2.3. Am-242m...............................................................................................................11
2.3. Curium..........................................................................................................................12
2.3.1. Сm-242..................................................................................................................12
2.3.2. Cm-244..................................................................................................................13
2.3.3. Cm-245..................................................................................................................13
3. Characteristics of Gamma and Neutron Emission by Materials Containing Minor
Actinides............................................................................................................................14
3.1. Gamma Emission by Np-237 Dioxide..........................................................................15
3.2. Neutron Emission by NpO ..........................................................................................16
2
3.3. Gamma Emission by AmO ..........................................................................................16
2
3.4. Neutron Emission by AmO .........................................................................................17
2
3.5. Gamma Emission by CmO ..........................................................................................17
2
3.6. Neutron Emission by CmO .........................................................................................19
2
4. Estimation of Production of Minor Actinide in Thermal Nuclear Reactors........................19
4.1. Problem Definition.......................................................................................................19
4.2. PWR Reactors...............................................................................................................23
4.3. Other Types of Nuclear Reactors..................................................................................29
4.3.1. BWR Reactors.......................................................................................................29
4.3.2. LWGR Reactors.....................................................................................................30
4.3.3. AGR Reactors........................................................................................................31
4.3.4. PHWR Reactors.....................................................................................................31
4.3.5. GCR Reactors........................................................................................................32
5. Estimated Production of Minor Actinides in Nuclear Power Reactors...............................33
5.1. General Approaches to the Problem.............................................................................33
5.2. Isotope Production Estimation Procedure.....................................................................34
5.3. Total Estimated Production of Minor Actinides in Nuclear Reactors..........................36
5.4. Sensitivity of Estimated Isotope Production to the Burnup of SNF.............................38
6. Worldwide Production of Minor Actinides in Nuclear Power Reactors.............................40
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7. Radioactive Effects of Minor Actinides..............................................................................52
7.1. General Approach.........................................................................................................52
7.2. Dose Coefficients and Specific Toxic Doses................................................................53
7.3. Plutonium Equivalents for Minor Actinides.................................................................54
7.4. Chemical Equivalents of Minor Actinides....................................................................55
7.5. Some Sensitivity Issues................................................................................................56
7.6. Estimated Plutonium and Chemical Equivalents of Minor Actinides for Heavy-
Water Reactors.....................................................................................................................58
8. Critical Mass Characteristics of Minor Actinides................................................................60
8.1. General Problem Definition..........................................................................................60
8.2. Critical Mass Characteristics of Minor Actinides Produced in PWR Reactors............61
8.3. Critical Mass Characteristics of Minor Actinides Produced in PHWR Reactors.........62
9. Spent Nuclear Fuel as a Security Barrier to Prevent the Use of Minor Actinides...............63
9.1. Dilution in Spent Nuclear Fuel.....................................................................................64
9.2. Radiation Security Barrier............................................................................................64
Conclusion...............................................................................................................................66
References................................................................................................................................67
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Introduction
This report summarizes the results of the first stage of investigations to evaluate the
urgency of the problem and examine possible approaches to enhance security of a special
group of radioactive materials, known as minor actinides. Minor actinides include long-lived
and relatively long-lived isotopes of neptunium (Np-237), americium (Am-241 and Am-243)
and curium (Cm-242, Cm-244, and Cm-245) produced in nuclear reactors. Security problems
associated with these materials are considered in two aspects:
• critical mass parameters associated with potential applications of such materials
to the development of nuclear explosive devices;
• radioactive properties associated with potential applications of such materials to
the creation of threats to human beings and the environment.
An essential fact about minor actinides is that they are generated in spent nuclear fuel
(SNF) of nuclear reactors along with reactor-grade plutonium, the amount of which is much
greater. That is why it is important to evaluate the relative role of the minor actinides
compared with reactor-grade plutonium as applied to the two security problems mentioned
above. An important task pertinent to the second problem is to analyze the relative role of the
minor actinides compared with hazardous chemical substances.
The problem of minor actinides occurred mainly due to the extensive spreading and
buildup of nuclear power based on light-water and heavy-water, as well as gas-graphite and
water-graphite reactors, and due to the proliferation of radiochemical SNF reprocessing
facilities. The variety of reactor designs, nuclear fuels and SNF burnup rates yields a wide
range of ratios between absolute buildup rates of different minor actinides, and as compared
to the buildup rate and composition of reactor-grade plutonium. In order to determine these
ratios, a special numerical model for was developed, which allows simulating isotope kinetics
as applied to different reactor designs. This model made it possible to obtain absolute and
relative values of minor actinide buildup rates.
The amount of minor actinides produced in different countries was estimated using a
properly adapted IAEA database characterizing the status of the world nuclear power as of
late 2001 according to the total burnup, electric power and time of operation of individual
nuclear reactors. The analysis carried out for the purposes of this study involved data on 530
active and shutdown reactors located in 32 countries of the world. Based on the results of this
analysis and evaluation of minor actinide buildup rates in different types of nuclear reactors,
for each of the 32 countries, assessments of the amount of minor actinides and reactor-grade
plutonium produced in their nuclear reactors were made.
The relative role of the minor actinides was analyzed using the term ‘plutonium
equivalent’ as applied to the comparison with reactor-grade plutonium, and ‘chemical
equivalent’ as applied to the comparison with chemical substances. The relevance of different
minor actinides in the context of the two security aspects of interest was determined based on
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the analysis of their production in two major types of light-water and heavy-water nuclear
reactors. At that, some minor actinides proved to be rather relevant, whereas the influence of
other minor actinides on various security aspects turned out to be by far less significant.
This study also included systematization of the major radiation properties of the minor
actinides. Based on these data, the levels of their penetrating radiation were shown to be
comparatively weak to provide considerable radiation security barriers while handling
separated minor actinides. From this standpoint, it might be much more secure to keep them
in SNF, where the predominant background radiation is produced by γ-active fission
products. Note that, in this case, security can be enhanced by strong dilution of the minor
actinides (and reactor-grade plutonium) in a body of a low-level material.
The major outcome of the first stage of this effort is that the minor actinides have been
shown to be relevant to the security problem associated with the proliferation of nuclear and
radioactive materials, and that specific buildup features and characteristics of these materials
have been determined.
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1. Generation of Minor Actinides in Uranium-Uranium Nuclear Fuel
In a standard uranium-uranium (235U , 238U ) fuel used in nuclear reactors, the
x 1-x
isotope of neptunium, Np-237, originates from U-235, and the isotopes of americium, Am-
241 and Am-243, and curium, Cm-242, Cm-244 and Cm-245, from U-238 as a result of the
following series of nuclear reactions:

n,γ n,γ β- n,γ β-
235U 236U 237U 237Np 238Np 238Pu
6.75 d 2.12 d
n,f
n,γ β- β- n,γ n,γ
238U 239U 239Np 239Pu 240Pu 241Pu
23.5 min 2.36 d
n,f
n,γ β-
241Am 242Am 242Cm 245Сm
β- EC n,γ
14.4 yr
n,γ n,γ β- n,γ β-
241Pu 242Pu 243Pu 243Am 244Am 244Сm
4.96 hr 10.1 hr
n,f

At that, the parent isotope of uranium, U-235, and the daughter isotopes of plutonium,
Pu-239 and Pu-241, fission making the major contribution to the energy release and
maintenance of the chain reaction.
2. General Physical and Radiation Properties of Minor Actinides
This section specifies basic physical and radiation properties of the isotopes of
neptunium, americium and curium under consideration [1].
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Table 2.1 – Basic physical and radiation properties of the isotopes of neptunium, americium
and curium
Density, Heat capacity, Melting heat,
Melting point, оC
g/cm3 J/g⋅K kJ/g
Neptunium metal 20.25 637 0.124 0.22
Americium metal 13.65 1,180 0.107 0.06
Curium metal 13.51 1,340 – –
Neptunium oxide NpO 11.1 2,560 0.246 0.235
2

2.1. Neptunium
Isotope Np-237 with the half-life Т = 2.14⋅106 years belongs to long-lived isotopes
1/2
of neptunium produced in considerable amounts in thermal reactors. Its radioactive decay to
the nearest long-lived daughter nucleus can be schematically shown as follows [1]:

Np-237 (T = 2.14⋅106 years; α) → Pa-233 (T = 27 days; β) →
1/2 1/2
→ U-233 (T = 1.59⋅105 years; α)
1/2

The amount of isotope Рa-233 produced in decay of Np-237 makes:

τ(Pa-233)
m(Pa-233)=m(Np-237) 1−exp(−t/τ(Pa-233))≈
 
τ(Np-237)

τ(Pa-233)
≈m(Np-237)
τ(Np-237)
t(cid:0)τ(Pa-233)

whereτ≡T /ln2 is the isotope lifetime.
1/2
It is obvious that at t >> τ the daughter and the parent isotope have the same activity
and

m(Np-237) m(Pa-233)
C(Np-237)= =C(Pa-233)= .
τ(Np-237) τ(Pa-233)

Since the lifetime of Pa-233 is short compared to the characteristic time (several years
or decades) used in this study for the neptunium-containing material, we have to assume that
isotopes Np-237 and Pa-233 are in radiation equilibrium in this material and have the same
activity.
In contrast to this, for U-233, t << τ, and its amount produced as a result of Np-237
decay makes:
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t t(cid:0)τ(Pa-233)
m(U-233)=m(Np-237)
τ(Np-237)
t(cid:0)τ(U-233)

and, consequently,

t
C(U-233)=C(Np-237) <<C(Np-237).
τ(U-233)

Therefore, the contribution of U-233 and its decay products to the total activity of the
Np-237-containing material will be neglected.
Thus, the radioactivity of the neptunium-containing material are determined by two
radionuclides: Np-237 itself and Ра-233. Table 2.2. characterizes these radionuclides in terms
of radioactivity [3, 4].
Table 2.2 – Radioactivity of Np-237 and Ра-233
Type of
Isotope C Q E E E W
0 decay α β γ
Np-237 0.7 α 4.96 4.77 0 33
2.1⋅10-2
Pa-233 0.7 β 0.57 0 0.064 200

where C is the specific activity of the material per 1 kg of Np-237 (Ci/kg);
0
Q is the decay energy (MeV);
E is the energy of α-particles (MeV);
α
E is the average energy of β-particles (MeV);
β
E is the total energy of γ-rays (keV);
γ
W is the heat release (W/kg).
Tables 2.3 and 2.4 present basic lines and intensities I of γ-ray emission from the
decay of Np-237 and Pa-233 (percent per 1 decay) [2, 4].
Table 2.3 – Basic lines and intensities of γ-ray emission from the decay of Np-237
Е (keV) 29.37 57.1 86.5 143.25 151.4 195 237.9
γ
I 15 0.39 12.4 0.43 0.23 0.184 0.063
Table 2.4 – Basic lines and intensities of γ-ray emission from the decay of Pa-233
Е (keV) 75.35 86.8 300.3 312.2 340.8 375.45 398.6 415.8
γ
I 1.39 1.97 6.6 38.6 4.47 0.68 1.39 1.75
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2.2. Americium
Long-lived isotopes of americium produced in considerable amounts in thermal
reactors are Am-241 and Am-243. The amount of isotope Аm-242m is much smaller, but its
presence in americium separated from SNF can have a great impact on the material’s neutron
radiation properties.
2.2.1. Am-241
The radioactive decay of Am-241 to the nearest long-lived daughter nucleus can be
schematically shown as follows:

Am-241 (T = 4.32⋅102 years; α) → Np-237 (T = 2.14⋅106 years; α)
1/2 1/2

As long as this scheme contains no intermediate short-lived nuclei, the radioactivity of
this process is determined by the decay of Am-241 only, whose properties are described in
Table 2.5 [3, 4].
Table 2.5 – Radioactivity of Am-241
Type of
Isotope C Q E E W
0 decay α γ
Am-241 3.44⋅103 α 5.64 5.48 29 1.11⋅102

Table 2.6 gives basic lines and intensities of γ-ray emission accompanying the decay
of Am-241 [2, 4].
Table 2.6 – Basic lines and intensities of γ-ray emission from the decay of Am-241
E (keV) 26.3 33.2 59.5 99 103 125.3
γ
I 2.4 0.126 35.9 2⋅10-2 2⋅10-2 4.1⋅10-3
E (keV) 208 335.4 376.6 619 662.4 722
γ
I 7.9⋅10-4 5⋅10-4 1.4⋅10-4 6⋅10-5 3.6⋅10-4 2⋅10-4

2.2.2. Am-243
The decay of Am-243 to the nearest long-lived daughter nucleus takes the following
form:

Am-243 (T = 7.38⋅103 years; α) → Np-239 (T = 2.35 days; β-) →
1/2 1/2
→ Pu-239(T = 2.42⋅104 years; α)
1/2

Thus, the radioactivity of the material containing Аm-243 is determined by the
radioactivity of this isotope and Np-239 being in equilibrium with it [3, 4].