Understanding the Effects of ERWs and Salted Devices

Introduction

In most atomic weapons, neutrons generated during the fission process are intended to trigger further fission events, thus increasing the total energy yield (blast, thermal and prompt radiation) of the device before it disassembles. In thermonuclear bombs, neutrons can be used to trigger fission events in the non-fissile uranium- 238 tamper as well.

However, two nuclear weapon types exist where released neutrons are not intended to accelerate the fission chain reaction. One is an enhanced radiation weapon, also known as an ERW or neutron bomb, where fusion neutrons are not confined so as to decrease the ratio of blast and thermal energy released to prompt radiation and also increase the neutron component of prompt
radiation. The figures in this article illustrate how the partition of energies released differs between a fission device and an ERW. The range to lethal effect from prompt radiation from an ERW is equivalent to the range to lethal radiation effect of a fission weapon with 10 times the overall yield. A second device is a “salted” weapon, where non-radioactive materials such as cobalt-59 are
placed into the weapon so that the neutron flux creates radioactive activation products. The intent is to create radioactive contamination for a longer time over a larger area compared to the activation products and fallout from a non-salted weapon.

History

Samuel Cohen is generally credited as the inventor of the neutron bomb in 1958. [2] The United States was apparently preparing to deploy it to Europe during the Carter administration, but did not do so owing to media and political pressure. USSR President Leonid Brezhnev informed a group of U.S. senators at the Kremlin that they had tested a neutron bomb “years ago,” but did
not begin production. [3]

France tested its first such weapon in 1980 [4], and informally told Allied officials in 1982 that it intended to go ahead with production, though it is not believed that France did so. [5] China successfully tested an ERW on Sept. 29, 1988, reportedly as a hedge against the Soviets, but did not deploy it. [6] There is speculation that Israel produced and tested a neutron bomb in 1979, but
considerable doubt exists that the detected explosion was a nuclear device. In a speech in Parliament on Nov. 24, 2012 Lord John William Gilbert, the former defence minister for the UK, threatened militants with an enhanced radiation reduced blast warhead, or neutron bomb, to “create cordons sanitaire” along the Pakistan and Afghanistan border; [7] however, the UK has never admitted it has produced or possesses such a device. Gilbert may have conflated neutron bombs with salted weapons.

The initial gamma radiation from activation and fission products from a fission or fission- fusion-fission device is very intense. The half-lives of some of these isotopes are generally within hours or days, [8] and the dose rate falls off rapidly. Other fission products have much longer half-lives, some of several years. However, several isotopes can be irradiated by the neutrons emitted during a fission-fusion weapon detonation that have intermediate half-lives and emit high-energy gammas or beta particles. A salted weapon therefore furnishes an intermediate peak of activity that would render an area unsafe for habitation or human presence for several months or years.

Figure 1. Energy Partitioning of Standard Fission/Fusion Weapon [1]

Radiation Biophysics

The biological effects of neutron irradiation differ from those of gamma irradiation. A key concept in comparing the behavior of different radiations upon biological tissues is relative biological effectiveness. The relative biological effectiveness of neutrons, or RBEn, is defined as the ratio of: RBEn = (Dose from reference radiation to produce a given biological effect)/(Dose from neutron radiation to produce the same effect) The reference radiation used is generally either 250 kVp x-rays or cobalt-60 gamma rays. RBEn is a function of several variables including:

• Neutron energy
• Type of tissue irradiated
• Tissue thickness and depth in tissue
• Dose rate
• Specific biological effect (e.g., incapacitation versus lethality versus carcinogenesis)

The average energy of neutrons emitted by fission reactions is around 2 MeV; [11] neutrons emitted by a deuterium-tritium fusion reaction have energies of approximately 14.1 MeV. Atmospheric attenuation, while more rapid for neutrons than gamma photons, is slower for higher energy neutrons. Accordingly the neutron/gamma ratio decreases more slowly over distance from ERW energy neutrons. RBEn is at a maximum around 1 MeV, so although higher energy neutrons, while penetrating further through the atmosphere, have relatively less biological effectiveness.

Type of tissue irradiated is important. Materials with high Z (atomic mass), such as lead, best attenuate gamma radiation, while neutron radiation is best attenuated by hydrogen- rich compounds (e.g., paraffin and, in human tissue, fat). Tissues with high lipid concentrations, such as the brain, fat and muscle, and tissues with high water concentration, such as the gastrointestinal epithelium, will therefore be more sensitive to the effects of neutron irradiation. RBEn is higher in the gastrointestinal tract than in skin, cartilage and hematopoietic tissue (in that order). [12, 13] As with any form of radiation, the dose rate decreases with depth in tissue; however, this is more so with neutron than gamma irradiation and the RBEn decreases with depth as it traverses mammalian tissue. [14]

In assessing the medical effects of neutron radiation, the fact that RBEn varies with the endpoint under consideration is very important. It was thought that the RBEn for radiation carcinogenesis was as high as 20-50 or even more [15]; more recent work however has noted, in the Japanese atomic bomb survivors that the neutron RBE is dependent upon the accompanying gamma RBE, particularly at low neutron doses. [16] The Radiation Effects Research Foundation has used a constant RBEn of 10, although this may be larger at low doses. Very high supralethal radiation doses such as 50 Gy, which are capable of causing incapacitation in primates, have an RBEn of less than one. [17] Animal studies using doses around the LD50 (median lethal dose) have yielded RBEn values from 1 to 4, with values decreasing inversely with size. For simplicity the RBEn for human lethality used by the RERF and most Department of Defense and North Atlantic Treaty Organization publications has been assumed to be 1.

Studies of neutron versus photon effects in tissues have shown differences in gene expression related to DNA damage, cell cycle delays, oxidative stress degeneration, regeneration, apoptosis and transcription. [18] Double strand breaks and non-DSBclustered DNA lesions are the hallmarks of high-linear energy transfer radiations. Neutrons decrease DNA flexibility more than photons, and alter DNA conformation as well. As a result, laboratory studies show that mixed field (neutron and gamma) irradiation increases the mortality, decreases survival time, decreases the latency period in acute radiation sickness or syndrome and delays the time to healing of wound and/or burn injuries in animals with combined radiation and traumatic injury. [19-22] Consequently,
the clinical presentations, medical countermeasures employed and outcomes of irradiation with a high neutron component will differ.

Medical Response and Treatment

After any nuclear detonation first responders will go to the areas likely to contain the greatest numbers of casualties capable of survival if given prompt medical care. [23] Their length of stay will be guided by external radiation exposure levels rather than the weapon type employed. Either an ERW or salted weapon will increase the external irradiation from activation products around
(or below) the detonation point (ground zero) and the amount of time medics will have to treat and evacuate survivable patients will be shorter. In addition, while only 15 percent of casualties in Japan had radiation injuries alone (most had combined injury from blast, thermal and prompt radiation), this percentage will likely be higher with salted or ERW weapons, owing to the reduction in blast and thermal effects relative to prompt radiation.

Casualties that spend any length of time in a fallout field are at risk for inhaling radioactive materials, as well as receiving external irradiation. For a salted weapon, it is important to know the isotopes comprising the fallout, as the medical countermeasures used for removing incorporated radioactive materials will vary depending upon the chemical nature of the isotopes.

The presenting symptoms of the prodromal phase of acute radiation syndrome are nausea, vomiting, diarrhea, fatigue, weakness and anorexia. This is followed by a latent phase where symptoms improve, but then the manifest illness phase, when the casualty worsens, sets in and lasts until death or recovery. In mixed field irradiation the latent phase is shorter, and there is more
pronounced hypothermia. [25] These symptoms will likely be more pronounced in casualties receiving a large neutron dose.

Figure 2. Energy Partitioning of Enhanced Radiation Weapon [1]

Very shortly after radiation exposure the lymphocyte count decreases rapidly. Except at the highest doses, there is an initial neutrophil spike, followed by a decline in numbers. Platelets and erythrocytes also decrease soon after the white cell counts drop. Deaths occurring around the second or third week post-exposure are generally due to infection and hemorrhage aggravated by the depleted blood elements.

However, it was noted that mice at the Nevada Test Site, shielded from gamma irradiation by lead hemispheres, died from prominent gastrointestinal signs and symptoms such as vomiting, bloody diarrhea and loss of appetite with relative sparing of the bone marrow. Deaths were earlier, around 4 to 10 days. This observation was verified in laboratory experiments in pigs, [26] and the conclusion drawn was that neutron irradiation aggravated gastrointestinal tract injury with relative sparing of the blood elements. ERW casualties will likely require greater attention to sepsis (which, in neutron-irradiated animals, was noted to be predominantly Gram-negative whereas cobalt-60 irradiated animals had predominantly Gram-positive bacteria [19,22]) and a relative decrease in blood transfusion and transplant requirements. [26]

The hematopoietic syndrome, or hematopoietic subsyndrome of ARS, is a clinical diagnosis assigned to casualties with one or more cytopenias following acute radiation exposure. The World Health Organization convened a panel of experts in 2009 to develop a harmonized approach to the medical management of acute radiation exposure, including HS. [28] A strong recommendation
was made for administering granulocyte or granulocyte macrophage colony-stimulating factor in HS, and only a weak recommendation for stem cell transplants, after cytokine treatment has failed. One study ascertained that G-CSF is effective not only after photon irradiation, but also after mixed field irradiation, in accelerating hematopoietic recovery and improving survival. However, a thrombopoietin analogue effective in stimulating recovery of platelets after gamma irradiation did not improve survival after mixed-field induced injury. [18]

Table 1. Significant activation products. Data selected and abbreviated from [8,9]. The asterisk (sodium-22*) indicates that if the neutron energy is above 11.5 MeV, sodium-22 can be an activationproduct by a (n, 2n) reaction. Such a threshold would imply fusion neutron energies, which an ERW could produce. Also, the type and amounts of activation products in an urban environment would depend heavily upon the type of cement and aggregate used in the concrete.

A basic principle of ARS treatment is prevention and treatment of sepsis with environmental control, selective gut decontamination and use of antibiotic therapy. Because different organisms may induce sepsis in casualties receiving a prominent neutron dose, as from an ERW, it will be necessary to identify the causative organism and start appropriate treatment; other than that, management is not changed.

Control of hemorrhage and restoration of hematopoietic elements with cytokine stimulation, treatment with immunomodulators and transfusions are also similar, with the exception regarding platelet stimulation noted above. Restoration and maintenance of fluid and electrolyte balance, and treatment of shock, will not vary. Supportive treatment in general has been shown to improve survival by a dose-modifying factor (the ratio of median survival doses in dogs given standard therapy versus control animals) of 1.30 in gamma-irradiated animals, but only 1.21 in dogs given neutron radiation. [20] Overall, it appears that for irradiated casualties with the same clinical picture and treatment countermeasures, the prognosis for ERW casualties is less favorable. [31]

Radioprotectors, such as sulfhydryl compounds, protect the animal or human by scavenging damage-causing free radicals or by donating hydrogen atoms to facilitate DNA repair. Thousands of these compounds have been synthesized and have had dose reduction factors, defined as the ratio of the dose of radiation required to cause lethality in the presence of the drug divided by the dose required in its absence, of two or more. [29] Although effective in animals and cancer patients undergoing radiation therapy, and carried but not used by Soviet soldiers and U.S. astronauts, their side effects (nausea, vomiting) and toxicity have so far precluded their routine use by persons expecting to be exposed to radiation. As a rule, they are somewhat less effective against fission neutron than gamma only radiation however. [30]

The long-term risks of carcinogenesis and cataractogenesis are increased in casualties receiving neutron irradiation from an ERW. Inhabitation of an area contaminated by a salted weapon could increase these risks as well, but this is due solely to the environmental behavior and amount of radioactive material deposited, not to different biological effects.

Conclusion

Though the technical sophistication required to construct an ERW or a salted device will most likely preclude their actual employment (in favor of a “standard” fission or thermonuclear weapon), their different effects should at least be understood in principle by emergency responders and other medical care personnel to facilitate responder safety and optimal patient treatment.

Table 2. Significant fission products. Source: Data selected and abbreviated from [8]. The CDC and NCI have developed a list of 19 isotopes from atmospheric nuclear weapon testing posing the greatest health hazards [10]; these are the top four in terms of health hazards in the author’s estimation.

For first responders there will be no significant changes in protective equipment, rescue procedures or on-scene emergency treatment. Stay times in contaminated areas will be determined by external exposure dosimetry regardless of weapon type. If the on-scene dosimetry readings are disproportionally higher than expected from the blast damage, the possible detonation of an ERW
or salted weapon should be considered.

Specific differences in medical planning and casualty care that can be caused by these weapons include:

• Elevated radioactivity at hypocenter, increasing external radiation risks to both responders and casualties (for both ERW and salted weapons)

• Increased incidence of radiation only injuries with respect to combined injuries (both)

• Shorter latent phase of ARS with ERW

• Wound and burn healing is slower in neutron-irradiated combined injury

• Increased incidence of nausea, vomiting, and other GI signs and symptoms with respect to dose from neutron irradiation

• Sepsis may be caused by different organisms in neutron versus gamma only irradiated casualties (and in combined injury versus radiation injury alone— not discussed earlier)

• Transfusion requirements may be decreased in ERW patients

• Doses of radiation high enough to cause incapacitation might be seen from an ERW

• Neutron irradiation increases risk of late carcinogenesis and cataractogenesis

Although the use of such weapons has been threatened, the likelihood of their use is small. The nations that have developed neutron bombs have, to all external appearances, dismantled or mothballed their arsenals (though not forgotten the technology of development).

American scientist Leo Szilard originally proposed salted weapons in 1950, but the U.S. has never tested such a weapon above ground. The British tested a 1 kt bomb incorporating a small amount of cobalt as a radiochemical traced at their testing site in Maralinga Range, Australia, but the experiment was a failure. In 1971 the Soviet Union conducted an underground “taiga” nuclear salvo using three bombs that created enough cobalt-60 to generate half of the gamma dose measurable at the test site 40 years later. No atmospheric testing of such a device has been performed. Nevertheless, while medical emergency response planners should be focused primarily on dealing with the effects of a standard fission nuclear weapon or improvised nuclear device, prudence requires appropriate consideration of the possibility that a neutron bomb or salted nuclear weapon could someday be used.

Table 3. Decorporation therapy recommendations. Source: Data selected and abbreviated from [23]. Tantalum-182 treatment is not discussed in this reference, presumably because its chemical toxicity is low.

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