A Review of Plutonium Destructiveness, Complexity, and Hazards ⁱ

Plutonium will be with us for a long time, and not only because it has a radioactive half-life of 24,000 years and therefore is dangerous for more than 200,000 years. Plutonium will be with us because nuclear weapon states are deeply devoted to having it as a military presence, the global nuclear power establishment is deeply devoted to pushing it as the fuel of the future, and the personal and political opinions of scientists often carry more weight than their scientific opinions.

A passage from the most recent issue of Los Alamos Science, No. 26-which is must reading for plutonium foes and friends alike-illustrates this reality:

This emphasis on the military use of plutonium suggests that without the military applications, support for “peaceful uses” of plutonium 239 would be meager. Plutonium may be a nuclear weapons physicists’ dream (see sidebar), but the dreams of physicists do not always come true, as is evident in the case of the now defunct Superconducting Super Collider project of the 1980's.

So while the pro-plutonium inertia is powerful, it is not omnipotent and the future of this element and other special nuclear weapons materials is not set in stone. As the debate continues to unfurl, it is important for people to know that this most secret of elements is the most complex metal in the periodic table; and its presence in deployed nuclear weapons threatens life as we know it.

Plutonium in Nuclear Explosives

Plutonium-239 is a fissile material well-known for its use as the primary trigger in most nuclear explosives (Figure 1-1). All grades of plutonium (see Table 2-1) are considered useable in nuclear explosives, but weapon-grade plutonium--which contains more than 92% plutonium-239--is preferred for nuclear weapon arsenals because lower amounts of plutonium-239 found in fuel and reactor grade pose a much higher risk of “pre-initiation” of the trigger due to corresponding higher amounts of plutonium-240. Use of lower grades also makes fabrication of the plutonium trigger, or pit, more difficult.ⁱⁱⁱ Because of its use in weapons of mass destruction, plutonium accounting is conducted to the level of grams, and large security forces are necessary to guard it.

However, the use of fuel or reactor grade plutonium is considered an easier path for a nonweapons state or a terrorist group because: easiest way to make a nuclear weapon is with reactor-grade plutonium because:

there is much more of it in the world, approximately 1300 metric tonnes in irradiated nuclear fuel, and another xx MT separated and awaiting use as reactor fuel.

it does not require the use of a “neutron generator.” As the Department of Defense puts it, “ a nuclear device used for terrorism need not be constructed to survive a complex stockpile-to-target sequence, need not have a predictable and reliable yield, and need not be efficient in its use of nuclear material.”^iv

Figure 1-1. A simplified illustration of how a precise detonation of chemical high explosives surrounding a subcritical mass of fissile materials generates enough force to initiate, or trigger, the nuclear detonation. Source: Los Alamos Science. Number 23. 1993. Page 55.

Plutonium Chemical Complexity

If anything contributes to plutonium’s demise as a military tool it will be its inherent chemical instability. The future of the plutonium triggers in the U.S. nuclear weapons stockpile is the focus of intense debate both internally and externally to the weapons labs and in the Pentagon. In particular, the lack of understanding of how plutonium ages is driving calls for renewed large-scale pit production. Lawrence Livermore National Laboratory spins it this way, “predicting kinetics is crucial to avoiding surprise requirements for large-scale refurbishment and remanufacture of weapons components.”^v

Plutonium is cited by the nuclear weapons labs as the most complex metal in the periodic table and continues to baffle people who best understand it (see sidebar). U.S. and Russian weapons scientists do not even agree on the “phase diagram” for the easily machinable delta-phase plutonium that dominates nuclear weapons stockpiles.^vi Its traits are commonly described as unstable, unpredictable, anomalous, and dramatically variable in the open literature. The litany of difficulties includes:

an inherent instability marked by adverse reactivity as a metal or an oxide powder with common items like air, water, and oils, which also “makes it difficult to keep track of plutonium inventories.”^vii

corrosion from hydrides and oxides from the outside-in and from radioactive decay from the inside-out;

runaway corrosion reactions;

an ability to cling “tenaciously” to anything and everything;^viii resulting in buildups of plutonium in ductwork, piping, and ventilation systems;

ultra-sensitivity to temperature and pressure changes, with marked increases in density with phase changes (Figure 1-3);

an “anomalously low melting point;”

pyrophoricity: spontaneous ignition at certain temperatures and certain particle sizes.

Figure 1-2. This diagram is commonly used to illustrate plutonium complexity, showing the contrasts between the dramatic and abrupt six phase changes of plutonium as it is heated compared to the stability of iron. Some of the key traits of the different phrases include:
· Alpha-phrase plutonium is brittle and difficult to machine, like cast iron.
· Small amounts of aluminum alloyed with delta-phase plutonium stabilize the plutonium and produces a metal as machinable as aluminum. However, because aluminum emits neutrons upon absorbing alpha particles from the decay of plutonium, it raises the risk of pre-initiation, or early criticality, of the plutonium trigger.
· Gallium alloyed with delta-phase plutonium retains the benefit of a product nearly machinable as aluminum and far less prone to plutonium oxidation without raising the risk of pre-initiation, and therefore the plutonium-gallium alloy is the most common in plutonium pits.
To make plutonium fuel, DOE intends to destabilize plutonium by removing gallium during purification.

Plutonium Hazards

The combination of radioactivity and chemical instability makes plutonium in the workplace an inherently unsafe enterprise even after it is produced and separated.

Add to this the need for precise accounting to the gram level and large protective forces to guard vaults and other storage areas, and the costs of dealing with plutonium become exorbitant.

Primary among the numerous aspects of the plutonium radiation hazard is the fact that it takes 24,400 years for it to lose one half of its radioactivity, meaning that it will remain dangerous for hundreds of thousands of years and react adversely when exposed to common environments.

Alpha Radiation and Decay

Plutonium-239 emits high levels of alpha radiation (Figure 1-3). Although alpha radiation can be stopped with paper, it causes damage in many ways and from several phenomenon.

Figure 1-3. The first part of the plutonium-239 decay chain. Plutonium decays to Uranium 238 by emitting an alpha particle, in this case a helium nucleus. The energy from this process drives several reactions that are poorly understood.
Source: Los Alamos Science. Number 26. 2000.

1. Damage to the plutonium over time. The recoil energy from the decay generates 85 kilo-electron-volts of kinetic energy in the uranium nucleus, of which 60 keV remains when the nucleus collides within the matrix and displaces plutonium atoms in the metal.^ix Over the course of decades, this action can damage plutonium enough to keep weapons designers leery of the “reliability” of the plutonium triggers.

The helium nucleus has far more energy when released, 5 million-electron-volts, but this is said to lose all but 0.1 percent of its energy through collisions with electrons before capturing a few electrons and “settling in” as a helium atom.^x Over the course of decades, helium atoms accumulate to the point of creating bubbles, another grave concern of weapons designers. Helium buildup also poses a health and safety risk. For example, in 1963 a plutonium pit tube broke during a weapon disassembly process at Pantex and contaminated workers and the facility with plutonium contaminated helium gas.

2. Damage to other metals over time. Plutonium decay basically damages everything in its path, and this impact is most measurable on elements that experience “void swelling” from radiation, meaning they swell in size over time.^xi The effects of this over the course of decades is poorly understood because plutonium has never been allowed to age for decades, but some implications are obvious:

Beryllium, which is used as a neutron tamper within pits and as cladding on many plutonium pits (see Part III) serving to protect the plutonium from oxidizing, experiences “gas-driven” swelling;.

Aluminum, which is used in cladding on some pits, suffers from void swelling.

Iron, Chromium, and Nickel, the key ingredients in stainless steel used for plutonium storage cans, experiences void swelling;

Zirconium, used to clad nuclear fuel, experiences void swelling.

3. Damage to live tissues. If the uranium nuclei from decay damages metal as dense as plutonium, the impacts on living tissue are quite obvious. Plutonium is said to be “harmless” if ingested as a metal, but this is an obvious fallacy since even plutonium metal has a layer of plutonium oxide present at all times,^xii oxides are always present to some degree on metals, and the chemical reactions with common materials that worry metallurgists and weapons designers are certainly a concern inside the human body.

Plutonium is most hazardous in a powder form. Much debate has occurred over how much plutonium oxide can cause lung cancer within a few decades, with estimates ranging from a few micrograms to 30-60 micrograms to 2 milligrams. There seems to be little debate over how much will kill a person:

Ingestion of 500 milligrams, or one half of a gram, is considered the acute lethal dose;

Inhalation of 20 milligrams is considered the acute lethal dose;^xiii

A good scale for reference is a typical Sweet N’ Low packet which contains one million micrograms of sugar substitute.

4. Radiolysis of common materials. Alpha particles react with materials such as air and water to cause “radiolysis” of common materials (Figure 1-4). Plutonium metal oxidizes readily in air and plutonium oxide generates gases that can rupture storage containers. Plutonium is most hazardous in a powder form.

Figure 1-4. Simplified illustration showing various reactions brought about by alpha decay.
Source: Los Alamos Science. Number 26.

The literature is filled with reports about ruptured containers and massive oxidation of entire metal pieces. For example, in 1983 Los Alamos reported the formation of a black powdered suboxide in “casting skulls” left over from plutonium pit fabrication, and when containers of skulls were opened, the plutonium suboxide would ignite “almost explosively.”^xiv

To avoid these undesirable reactions, DOE finally established a long-term storage standard for plutonium in 1994, but has had trouble meeting that standard (see Part II, Section B.) Called the 3013 standard, it requires that plutonium metals and oxides be stored in two sealed metal containers free of organic materials. Reaching this standard requires heating of oxides to temperatures greater than 900 degrees Celsius.

A few near-term implications of this chemical fact include:

1. Nitric acid processing, which DOE plans to use to purify plutonium oxide as the first step towards making plutonium MOX, greatly increases the likelihood of explosions, spills, and criticality events. The plutonium pit disassembly and conversion facility is planned as the main source of plutonium oxide for a plutonium fuel (MOX) factory. Early plans for the PDCF require the plutonium oxide product to meet the long term plutonium storage (3013) standard.^xv

2. The dangers of nitric acid plutonium processing are aggravated if the plutonium oxide was produced or treated at temperatures greater than 600 degrees Celsius. Oxides heated to temperatures between 600 and 1000 C “require somewhat more stringent procedures” when dissolving in acids, and plutonium oxide powder heated to temperatures over 1000 Celsius “require extreme measures.”^xvi Since the long-term storage standard requires plutonium to be heated at temperatures well above 600 degrees C,^xvii it is incompatible with the needs of plutonium fuel production.

Aging Plutonium and Americium-241

Plutonium-241, which is present in all grades of plutonium, decays into the more radioactive and dangerous americium-241, an intense gamma ray emitter that is 100 times more toxic than plutonium 239. Weapons plutonium was routinely purified to eliminate americium, which of course produced stockpiles of americium. If plutonium decay is allowed to run its course, radiation levels in U.S. plutonium will peak in the next 38 to 60 years (Figure 1-4).

Figure 1-4. As plutonium-241 decays to Americium-241, weapon grade plutonium becomes more hazardous and radioactive. Americium levels peak after 70 years. Source: Peterson, 1993. RFP-4910.