Plutonium isotope

At about the same time, the artificial isotope plutonium-239 [15117-48-3] was discovered and was recognized as also being fissionable. This led to the conjecture that a controlled chain reaction might be achieved and that neutrons could be used to produce enough plutonium for a weapon. 

The isotope plutonium-238 [13981 -16-3] Pu, is of technical importance because of the high heat that accompanies its radioactive decay. This isotope has been and is being used as fuel in small terrestrial and space nuclear-powered sources (3,4). Tu-based radioisotope thermal generator systems dehvered 7 W/kg and cost 120,000/W in 1991 (3). For some time, %Pu was considered to be the most promising power source for the radioisotope-powered artificial heart and for cardiovascular pacemakers. Usage of plutonium was discontinued, however, after it was determined that adequate elimination of penetrating radiation was uncertain (5) (see PROSTHETIC AND BIOMEDICAL devices). 

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Americium does not exist in nature. All of its isotopes are man-made and radioactive. Americium-241 is produced by bombarding plutonium-239 with high-energy neutrons, resulting in the isotope plutonium-240 that again is bombarded with neutrons and results in the formation of plutonium-241, which in turn finally decays into americium-241 by the process of beta decay. Both americium-241 and americium-243 are produced within nuclear reactors. The reaction is as follows Pu + (neutron and X gamma rays) —> " Pu + (neutron and X gamma rays) —> Pu—> Am + beta minus ([ -) followed by " Am—> jNp-237 + Hej (helium nuclei). 

Plutonium is the most important transuranium element. Its two isotopes Pu-238 and Pu-239 have the widest applications among all plutonium isotopes. Plutonium-239 is the fuel for nuclear weapons. The detonation power of 1 kg of plutonium-239 is about 20,000 tons of chemical explosive. The critical mass for its fission is only a few pounds for a solid block depending on the shape of the mass and its proximity to neutron absorbing or reflecting substances. This critical mass is much lower for plutonium in aqueous solution. Also, it is used in nuclear power reactors to generate electricity. The energy output of 1 kg of plutonium is about 22 million kilowatt hours. Plutonium-238 has been used to generate power to run seismic and other lunar surface equipment. It also is used in radionuclide batteries for pacemakers and in various thermoelectric devices. 

Scientists later discovered a third isotope, plutonium-239, also could undergo nuclear fission. Plutonium-239 does not occur in nature but can be made synthetically in nuclear reactors and particle accelerators. 

The most stable isotopes of plutonium are plutonium-242 and plutonium-244. The half lives of these two isotopes are 373,300 years and 80,800,000 years respectively. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Consider the isotope plutonium-242, with its half life of 373,300 years. In 373,300 years (one half life), only half of a sample prepared today would still be plutonium-242. The rest would have broken down into a new isotope. 

We are now in position to derive equations that will give the degree uf bumup nuclear fuel can experience before it ceases to be critical. First, we must determine how the concentration of each nuclide that affects the neutron balance changes with time. We consider fuel that at time zero contains N s atoms of U per cubic centimeter, atoms of U, and no other uranium isotopes, plutonium, or fission products. This fuel is then exposed to a thermal-neutron flux 0(0, which may be a function of time. The variation in concentration of each nuclide in this fuel with time is obtained as follows. 

OOIS Ci Pu is released annually from a reprocessing plant. What will be the corresponding release of Pu and Pu for typical isotopic plutonium composition of LWR fuel ... 

Studies in laboratory animals have also shown the influence of metals on the toxicokinetics of plutonium. Pretreatment of rats with a subcutaneous injection of cadmium or copper followed by an intravenous injection of plutonium-239 or plutonium-238 resulted in changes in the distribution patterns of plutonium, but not in total retention of either isotope. Plutonium retention of both isotopes, following pretreatment with either metal, was increased in the spleen and the kidneys, as compared to animals treated with plutonium only (Volf 1980). Copper pretreatment appeared to increase the retention of plutonium in the liver, while cadmium pretreatment appeared to decrease plutonium retention in the liver. These differences in retention of plutonium in the liver may reflect different properties of the respective metal- binding proteins or different mechanisms of action (Volf -1980). 

Plutonium exists in trace quantities in naturally occurring uranium ores (Weast 1980). Plutonium is produced by the bombardment of uranium with neutrons. The most important isotope, plutonium-239, is produced in large quantities from natural uranium in nuclear reactors (Weast 1980). Plutonium- 240, -241, and -242 are produced from successive absorption of neutrons by the plutonium-239 atoms. The successive absorption of two neutrons rather than one by uranium leads to the production of plutonium-238. Plutonium-237 is usually produced by the helium ion bombardment of uranium-235. 

The isotope, plutonium-238, is not useful for nuclear weapons. However it generates significam heat through its decay process, which make it useful as a power source. Using a thermocouple, a device that converts heat into electric power, satellites rely on plutonium as a power source. Tiny amounts also provide power to heart pacemakers. 

In this manner the nonfissionable uranium-238 is transmuted into the fissionable isotope plutonium-239 (Figure 23.13). 

The other commonly used fissionable isotope, plutonium-239 (Pu-239), is very rare in nature. But there s a way to make Pu-239 from U-238 in a specif fission reactor called a breeder reactor. Uranium-238 is first bombarded with a neutron to produce U-239, which decays to Pu-239. The process is shown in Figure 5-4. 

Although many heavy nuclei can be made to undergo fission, only the fission of naturally occurring uranium-235 and of the artificial isotope plutonium-239 have any practical importance. 

Uranium-238 is the most abundant isotope of uranium, constituting approximately 99.2830 percent of natural uranium. It is not fissionable, but it is a fertile material, forming the fissile isotope plutonium-239 as a result of neutron bombardment. 

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