1.1 Plutonium Isotopes

Table 9.13 lists the isotopes of plutonium important in nuclear technology and some of their important nuclear properties. Plutonium isotopes are produced in reactors by the nuclide chains shown in Fig. 8.5. Typical quantities and isotopic compositions of plutonium in various reactor fuel cycles are listed in Tables 8.4, 8.5, 8.6, and 8.7. In reactors fueled with uranium and pluto­nium, 239Pu is the principal isotopic constituent, but 238Pu contributes the greatest amount of alpha activity. With 235U-thorium fueling, 238Pu is the principal isotopic constituent.

Reaction with 2200 m/s
neutrons

Mass, amu	Half-life	Type+	Effective MeV	Fraction of decays	(n, 7)	Fission	per fission
236.04607	2.85 yr	a SF	5.868	8 X 10‘10		165	2.22
238.049511	86 yr	a SF	5.592	1.7 X 10*9	547	16.5	2.90 2.33
239.052146	24,400 yr	a SF	5.243	4.4 X 10‘12	268.8	742.5	2.871
240.053882	6,580 yr	a SF	5.255	4.7 X 10’8	289.5		2.143
241.056737	13.2 yr	a /3 a SF	0.007	2.3 X 10-5	368	1009	2.927
242.058725	3.79 X 10s yr	4.98	5 X 10‘*	18.5	<0.2	2.15
243.061972	4.98 h	/3	0.239		60	196
244.0641	8 X 107 yr	a SF	4.66	3 X 10‘3	1.7		2.30

Radioactive decay

Cross section, b Neutrons

^SF, spontaneous fission.

radiological toxicity, laboratory work on reactor plutonium must be carried out in airtight glove boxes.

^Pu. The isotope 240 Puis produced by neutron capture in 239 Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons.240 Pu is converted to a fissionable nuclide by neutron capture. Therefore, like 232Th and 238U, it is a fertile material. It undergoes alpha decay, with a half-life of 6580 years, to form 236U, which then decays to 232Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, 240 Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes.

M1Pu. The isotope 241 Pu results from neutron capture in ^Pu. It is fissionable with thermal neutrons and contributes significantly to the energy production in uranium irradiated to high exposure and in recycled plutonium. It undergoes beta decay, with a half-life of 13.2 years, to form 241 Am, which then decays to 237 Np in the 4n + 1 decay series. The decay of 241 Pu results in only low-energy electrons and weak x-rays. Alpha particles are formed in only 2.3 X 10’3 percent of the decays. However, the beta-decay daughter 241 Am emits gamma radiation when decaying, thereby adding to shielding requirements when working with separated reactor-grade plutonium.

^Pu. The isotope 242Pu is formed by neutron capture in 241 Pu. With a half-life of 3.79 X 10s years, it is the longest-lived of all the plutonium isotopes present in any appreciable amount in reactor-produced plutonium. It alpha decays to 238U in the 4n + 2 decay series. Because 242Pu has a small neutron-absorption cross section relative to “’Pu, 240Pu, and 241 Pu, and because its neutron-capture daughter 243 Pu is relatively short lived, 242 Pu of high isotopic purity can be produced by the long irradiation of separated reactor plutonium. After a neutron-exposure fluence of 1.6 X 1022 thermal neutrons/cm2, about 60 g of242 Pu of approximately 99 percent isotopic purity is produced per kilogram of original reactor plutonium [K2]. Because of its long half-life and correspondingly lower radiotoxicity, 242Pu is useful for laboratory chemical research.

243Pu. The isotope 243 Pu, formed by neutron capture in 242 Pu, undergoes beta decay to 243 Am with a half-life of 4.98 h. Because of its short half-life, 243Pu is present only in very small concen­tration during reactor irradiation, and it disappears after irradiated fuel has been stored for a few weeks. The low concentration of 243 Pu results in negligible production of the long-lived 244 Pu in reactors.

244Pu. The isotope 244 Pu is the longest-lived of the plutonium isotopes, with a half-life of 8 X 107 years. It can be produced by neutron absorption in 243 Pu, but because of the short half-life and low concentration of 243 Pu only minute quantities of 244 Pu, of the order of 10’10 percent, are present in reactor-produced plutonium [K2]. Small quantities of244 Pu, as well as 24s Pu and 246 Pu, are present in the residues from nuclear explosions, resulting from the decay of the neutron-rich uranium isotopes 244 U, 245 U, and 246 U formed by multiple neutron capture in the high neutron flux at the initiation of the explosion.

1.2 Plutonium Radioactivity

The radioactive decay properties of the plutonium isotopes that appear in irradiated reactor fuel are listed in Table 9.14. All but 241 Pu and 243Pu are alpha emitters. Because it penetrates matter only weakly, alpha radiation is stopped by the outer layer of dead skin and is not a hazard outside the body. However, plutonium is very effective biologically when deposited in or on living tissue, particularly if by inhalation or by contaminated injuries.241 Pu is a relatively short-lived (13.2-year

Data for 236Pu through 242Pu from Valentine [VI]; data for 243Pu from Keller [К2]. The energy listed for beta decay is the maximum beta energy.

half-life) beta emitter and is of radiological significance because it is the parent of 241 Am, an alpha emitter that accumulates in tissues and constitutes a hazard comparable to that of plutonium [B2].

Personnel working with plutonium must be protected by light shielding. The external radia­tion to be shielded includes gammas from alpha and beta decay, internal conversion x-rays, gammas, and neutrons from spontaneous fission, and neutrons from (a, n) reactions in materials of low atomic number. Neutron yields for various types and forms of plutonium are listed in Table 9.15.

Kilogram quantities of plutonium are fabricated in shielded glove-box facilities [VI]. A

Table 9.15 Neutron yields for plutonium Neutron yield, n/(g Pu*s) Type of plutonium Metal^ Oxide * Low-exposure plutonium § 51 60 High-exposure reactor plutonium 340 538 238 Pu heat source 2,150 13,500

^From spontaneous fission.

* From spontaneous fission and from (a, n) reactions.

§ Plutonium with a relatively low content of 240 Pu, resulting from irradiation of 238 U at low bumup.

Source: A. Valentine, “Capabilities for Control of Plutonium in Processing,” Plutonium Information Meeting, Jan. 1974.

typical box consists of a g in of lead sandwiched between fg in of stainless steel on the interior and •jg in on the exterior. Windows consist of 5 in of lead glass. For neutron shielding 4 in of water, paraffin, or Plexiglas is added. Exhaust ventilation from the glove boxes passes through several layers of high-efficiency particulate filters to remove plutonium aerosols and to provide essentially complete containment of the plutonium being processed.

1.3 Plutonium Electronic Structure

The electronic structures of the ions are simpler than those of the metals. In the case of plutonium, removal of the first two (7s) electrons increases the stability of the 5/level relative to the 6d level, and the electrons become firmly placed in the 5f shell. After depletion of the 7s electrons, the next four electrons are removed from the 5/ shell. A summary of the electronic structures of plutonium (in addition to the Rn core) is given in Table 9.16.

1.4 Plutonium Metal

The phases of plutonium metal and their transition temperatures at atmospheric pressure are shown in Table 9.17. The delta-prime phase exists only in high-purity plutonium; as little as 0.15 w/o (weight percent) impurities results instead in a continuous delta phase [C4]. Because of the high densities of the metallic states of uranium and plutonium, there is considerable incentive to use metal fuel in fast-breeder reactors to obtain high breeding ratio. However, the solid-phase transformations in uranium and plutonium and the susceptibility of these metals to radiation damage have resulted in greater emphasis on nonmetallic forms for high-bumup breeder fuel. The large density changes of plutonium metal, particularly between the alpha and beta plutonium, and the large thermal expansion coefficients, as shown in Table 9.17, can result in serious distor­tion and deformation of the fuel elements when subjected to internal stresses from repeated thermal cycling and from radiation damage. Solutions of molten uranium and plutonium have been considered as a fluid fuel for breeder reactors, but the extensive corrosion of structural materials by this molten metallic fuel is a formidable problem.

Plutonium metal is prepared by calcium reduction of plutonium fluorides or oxides in induction-heated MgO crucibles, under an inert atmosphere of helium or argon. The thermo­dynamics of plutonium reduction are discussed later in this chapter.

Plutonium metal oxidizes readily in the presence of humid air at elevated temperatures. The massive metal is relatively inert to atmospheric oxidation at room temperature, although the presence of water vapor causes unalloyed plutonium metal to disintegrate over long periods even with relatively little oxidation [K2]. The finely divided metal is pyrophoric. Plutonium reacts with halogens at moderate temperatures to form the trihalides. The metal is readily soluble in

Table 9.16 Electronic structures of plutonium ions Plutonium valence Shell 6s 6 P 5/ 6 d Is 0 2 6 6 0 2 2 6 5 1 2 +3 2 6 5 0 0 +4 2 6 4 0 0 + 5 2 6 3 0 0 +6 2 6 2 0 0

Table 9.17 Properties of plutonium metal Phase Temperature ranged °С Crystal structure* Density,* g/cm3 Linear expansion coefficient, § per °С (X 106) a 25-122 Simple monoclinic 19.86 59 p 122-205 Body-centered monoclinic 17.70 30.3 7 205-318 Face-centered orthorhombic 17.14 33.3 8 318-452 Face-centered cubic 15.92 -8.8 8′ 452-476 Body-centered tetragonal 16.00 -63 Є 476-640 Body-centered cubic 16.51 25.6 Liquid *From Rand [R2]. *From Miner and Schonfeld [M5]. ^ Mean value over the indicated temperature range (25 to 122° C for a phase) [C4].

HC1 of all concentrations and dissolves also in 72% HC104, 85% H3PO4, and concentrated tri­chloroacetic acid. Nitric acid shows no visible attack on massive plutonium over a period of several hours. Plutonium reacts slowly with H2 S04 of moderate concentrations, but with concentrated H2 S04 it forms a protective coating that resists further attack.