Voorkomen

Summary of medical aspects

* Plutonium is toxic as a result of its radioactivity: not outside the body, but very much so inside the body. The smoke of burning plutonium contains microscopic plutonium particles, that may settle in the lungs where the "fire" seems to continue. Therefore, they are called: "hot particles", causing microscopic burns in the living tissues. At the edge of them lung cancer may develop. It was calculated that an amount of 27 micrograms of plutonium-dust is sufficient to cause lung cancer (note: one microgram is one thousandth of a milligram). In experimental animals bone cancer has also been found.

* Congenital defects due to plutonium contamination have not been described as yet.

* Plutonium can go off when it is piled up, a 'criticality' disaster. The pile suddenly produces a flash of penetrating radiation causing acute radiation sickness in those who are in the vicinity of it. The smoke coming out of the burning pile may lodge tiny particles containing plutonium deep in the lungs. Fall-out from an explosion of an atomic bomb also contains plutonium dust, and so does the smoke of a nuclear disaster like 'Chernobyl' and 'Windscale' (see page 41).

* Traces of plutonium have been found in the environment where children play, like the sea shores of Cumbria, in the Irish sea and near Cap la Hague on the Channel coast; this fact has caused an outcry by the local population. An increase in the occurrence of leukaemia amongst children has indeed been found, but causality could not be proved.

* Private properties have gone down in value after being contaminated with plutonium dust. Doctors are unable to convince any buyer about the safety of low level radiation. Because of both the facts and the health fear, leaking plutonium may turn out to be an expensive matter.

* Plutonium has a very long half-life. Thus, its toxicity is a potential health hazard for thousands of years. That poses a moral problem; future people may find it but may have forgotten the danger.

* There are no medical applications for plutonium, like those of radium. Plutonium as a power-generator in pacemakers is now obsolete.

Plutonium Production

Plutonium was the second transuranium element of the actinide series to be discovered. The isotope 238Pu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley, California. Plutonium also exists in trace quantities in naturally occurring uranium ores. It is formed in much the same manner as neptunium, by irradiation of natural uranium with the neutrons which are present.

Plutonium has assumed the position of dominant importance among the trasuranium elements because of its successful use as an explosive ingredient in nuclear weapons and the place which it holds as a key material in the development of industrial use of nuclear power. One kilogram is equivalent to about 22 million kilowatt hours of heat energy. The complete detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical explosive. Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world's nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. The various nuclear applications of plutonium are well known. 238Pu has been used in the Apollo lunar missions to power seismic and other equipment on the lunar surface. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals.

The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cm3.

Because of the high rate of emission of alpha particles and the element being specifically absorbed on bone the surface and collected in the liver, plutonium, as well as all of the other transuranium elements except neptunium, are radiological poisons and must be handled with very special equipment and precautions. Plutonium is a very dangerous radiological hazard. Precautions must also be taken to prevent the unintentional formulation of a critical mass. Plutonium in liquid solution is more likely to become critical than solid plutonium. The shape of the mass must also be considered where criticality is concerned.

Uranium and plutonium are composed of several isotopes, some of which are fissile. To produce an explosive device for military purposes requires the percentage of fissile isotopes (U-235 for uranium, Pu-239 for plutonium) present in the material to be of the order of 93%. The levels reached in the nuclear power industry are, however, much lower; less than 5% for uranium and between 50 and 60% for plutonium.

Plutonium containing high quantities of fissile material i.e. Pu-239 in the order of 90-95 %, is known as weapon-grade plutonium. Plutonium containing lower concentrations, in the range of 50-60 % is known as reactor-grade plutonium. The definitions of the various plutonium grades are expressed as a percentage of the isotope Pu-240 which is considered as an impurity for weapons manufacturers.

Plutonium-grade		% of Plutonium- 240
Super-grade		� 3%
Weapon-grade		� 7%
Fuel-grade		between 7% and 18% incl.
Reactor-grade		� 18%

Reactor-grade plutonium is produced in the core of a reactor when uranium-238 is irradiated with neutrons. Unlike weapon grade plutonium (which is relatively pure plutonium-239), reactor grade plutonium is a mixture of plutonium-238, 239, 240, 241 and 242. It is this mixture of isotopes which renders reactor grade plutonium less unsuitable as a weapon-grade material.

The even numbered isotopes (plutonium-238, 240 and 242) fission spontaneously producing high energy neutrons and a lot of heat. In fact, the neutron and gamma dose from this material is significant and the heat generated in this way would melt the high-explosive material needed to compress the critical mass prior to initiation. The neutrons can also initiate a premature chain reaction thus reducing the explosive yield, typically to a few percent of the nominal yield, sometimes called the "fizzle yield". Such physical characteristics make reactor-grade plutonium extremely difficult to manipulate and control and therefore explain its unsuitability as a bomb-making ingredient.

The odd numbered isotope, plutonium-241, is also a highly undesirable isotope as it decays to americium-241 which is an intense emitter of alpha particles, X and gamma rays. Plutonium-241 has a half-life of 13.2 years which means americium-241 accumulates quickly causing serious handling problems.

Weapon-grade plutonium has different characteristics. It contains mainly Pu-239 which has a half-life of 24 000 years and only very small quantities of Pu-241 (unlike reactor-grade plutonium which can contain around 15% Pu-241.) It is thus relatively stable and can be safely handled with a pair of thick gloves

To achieve the high percentages of Pu-239 required for weapon grade plutonium, it must be produced specifically for this purpose. The uranium must spend only several weeks in the reactor core and then be removed. For this to be carried out in a LWR - the prevalent reactor design for electricity generation - the reactor would have to be shut down completely for such an operation; this is easily detectable.

The Isotopic Composition of Reactor and Weapon Grade Plutonium

	Pu-238 (%)	Pu-239 (%)	Pu-240 (%)	Pu-241 (%)	Pu-242 (%)
Reactor-Grade Plutonium (3,7% U-235, 43,000 MWd/t)^1	2	53	24	15	6
Weapon-Grade Plutonium		93	7

1 Source: Plutonium Fuel - OECD Report, 1989.

Plutonium, one of the two fissile elements used to fuel nuclear explosives, is not found in significant quantities in nature. Plutonium can only be made in sufficient quantities in a nuclear reactor. It must be �bred,� or produced, one atomic nucleus at a time by bombarding 238 U with neutrons to produce the isotope 239 U, which beta decays (half-life 23 minutes), emitting an electron to become the (almost equally) radioactive 239 Np (neptunium). The neptunium isotope again beta decays (half-life 56 hours) to 239 Pu, the desired fissile material. The only proven and practical source for the large quantities of neutrons needed to make plutonium at a reasonable speed is a nuclear reactor in which a controlled but self-sustaining 235 U fission chain reaction takes place. The graphite-moderated, air- or gas-cooled reactor using natural uranium as its fuel was first built in 1942. Scale-up of these types of reactors from low power to quite high power is straightforward. ccelerator-based transmutation to produce plutonium is theoretically possible, and experiments to develop its potential have been started, but the feasibility of large-scale production by the process has not been demonstrated.

The �size� of a nuclear reactor is generally indicated by its power output. Reactors to generate electricity are rated in terms of the electrical generating capacity, MW(e), meaning megawatts of electricity. A more important rating with regard to production of nuclear explosive material is MW(t), the thermal power produced by the reactor. As a general rule, the thermal output of a power reactor is three times the electrical capacity. That is, a 1,000 MW(e) reactor produces about 3,000 MW(t), reflecting the inefficiencies in converting heat energy to electricity.

A useful rule of thumb for gauging the proliferation potential of any given reactor is that 1 megawatt-day (thermal energy release, not electricity output) of operation produces 1 gram of plutonium in any reactor using 20-percent or lower enriched uranium; consequently, a 100 MW(t) reactor produces 100 grams of plutonium per day and could produce roughly enough plutonium for one weapon every 2 months. Light-water power reactors make fewer plutonium nuclei per uranium fission than graphite-moderated production reactors.

In addition to production of plutonium, nuclear reactors can also be used to make tritium, 3 H, the heaviest isotope of hydrogen. Tritium is an essential component of boosted fission weapons and multi-stage thermonuclear weapons. The same reactor design features which promote plutonium production are also consistent with efficient tritium production, which adds to the proliferation risk associated with nuclear reactors.

Reactors are generally purpose-built, and reactors built and operated for plutonium production are less efficient for electricity production than standard nuclear electric power plants because of the low burnup restriction for production of weapons grade plutonium. The types nuclear fission reactors which have been found most suitable for producing plutonium are graphite-moderated nuclear reactors using gas or water cooling at atmospheric pressure and with the capability of having fuel elements exchanged while on line. Several distinct classes of reactor exist, each optimized for one purpose, generally using fuel carefully chosen for the job at hand. These classes include the following:

Research reactors. Usually operates at very low power, often only 1�2 MW or less. Frequently uses high-enriched uranium fuel, although most newer models use no more than 20-percent enrichments to make the theft of fuel less attractive. Fertile material ( 238 U for Pu, 6 Li for tritium) can be encapsulated in elements known as �targets� for insertion into the reactor core. The reactor can also employ a fertile blanket of 238 U in which plutonium can be bred. Cooling requirements and shielding requirements are relatively modest. Some research reactors can be refueled while operating, and such reactors are of special concern for plutonium production because they can limit fuel burnup, which enhances the quality of the plutonium compared to that obtained from reactors that require high burnup before shutdown and refueling. Research reactors using nearly 100-percent enriched material produce almost no plutonium in their fuel because the fertile species, 238 U, has been removed. These reactors can, however, be built with a surrounding �blanket� of natural or depleted uranium in which plutonium can be bred efficiently. The Osirak reactor built in Iraq and destroyed by Israeli aircraft was of this type.

Propulsion reactors. Primarily found on submarines and large-surface combatant ships, nuclear reactors have given new operational freedom to the underwater navy and deliver increased time on station combined with high speed for both the submarine service and the surface navy. The United States and Russia have built most of the world�s shipboard reactors. The world�s first nuclear powered cargo ship was the U.S.N.S. Savannah; however, nuclear propulsion power has not been particularly successful in the commercial world. Today, the only operating commercial vessels using nuclear propulsion are Russian icebreakers. To keep the core size small, propulsion reactors generally use highly enriched uranium as fuel. In principle, a propulsion reactor core could be surrounded with a fertile blanket and used to produce plutonium. In practice, this has never been done.

Space reactors and mobile power systems. Nuclear reactors have been used from time to time, usually by the former Soviet Union, to provide on-orbit electrical power to spacecraft. In principle, they will use HEU as fuel to keep the core mass and volume small. Other spacecraft have been powered by the heat released by the radioactive decay of 238 Pu.

Power reactors. These are used to generate electric power. Few use fuel enriched to greater than 5�7% 235 U. Practical power levels range from a few hundred MW(e) (three times that in terms of thermal power output) to 1,000 or 1,500 MW(e)�meaning 3,000�4,000 MW(t). Power reactors designs have included water cooled-graphite moderated (the Soviet RBMK used at Chernobyl), boiling (light) water, pressurized (light) water, heavy water-moderated and cooled, graphite-moderated/helium cooled, and liquid metal-moderated. Most power reactors operate under pressure and cannot be refueled in operation. The RBMK and CANDU reactors are notable exceptions to this rule. The CANDU reactor was developed for the Canadian nuclear power program and is a deuterium oxide (heavy water) moderated reactor which can operate on natural uranium fuel.

Breeder reactors. These reactors generate plutonium at a rate greater (numbers of nuclei per unit time) than they burn their fissile fuel (numbers of nuclei per unit time). Normally, breeders use fast neutrons and irradiate a fissile 238 U blanket. Plutonium produced in the fuel generally has a higher fraction of 240 Pu than that produced in other reactors, but the Pu made in the blanket of uranium surrounding the core is usually of a high quality, containing very little 240 Pu.

Production reactors. These are used to make plutonium (and often tritium) efficiently. Production reactors are frequently graphite-moderated and either air-, CO 2 -, or helium-cooled. The longer a given sample of fuel is irradiated, the greater the build-up of 240 Pu, an isotope which decays by spontaneous fission and which should be minimized in weapon fuel. Consequently, plutonium production reactors usually are designed to be refueled while operating (on-line refueling) so that relatively little 240 Pu is found in the �spent� fuel.

The first nuclear reactor, CP-1, went critical for the first time on 2 December 1942 in a squash court under Stagg Field at the University of Chicago. Construction on CP-1 began less than a month before criticality was achieved; the reactor used lumped uranium metal fuel elements moderated by high-purity graphite. Within 2 years the United States first scaled up reactor technology from this essentially zero-power test bed to the 3.5 MW (thermal) X-10 reactor built at Oak Ridge, Tennessee, and then again to the 250-megawatt production reactors at Hanford. The Hanford reactors supplied the plutonium for the Trinity test and the Nagasaki war drop. Clearly, reactor technology does not stress the capabilities of a reasonably well-industrialized state at the end of the twentieth century.

Some problems did arise with the scale-up to hundreds of megawatts: the graphite lattice changed crystal state, which caused some deformation, and the buildup of a neutron-absorbing xenon isotope poisoned the fission reaction. This latter problem was curable because of the foresight of the duPont engineers, who built the reactor with many additional fuel channels which, when loaded, increased the reactivity enough to offset the neutron absorption by the xenon fission product.

Finally, the problem of spontaneous emission of neutrons by 240 Pu produced in reactor plutonium became apparent as soon as the first samples of Hanford output were supplied to Los Alamos. The high risk of nuclear pre-initiation associated with 240 Pu caused the abandonment of the notion of a gun-assembled plutonium weapon and led directly to the adoption of an implosion design.

Since each fission produces only slightly more than two neutrons, on average, the neutron �economy� must be managed carefully, which requires good instrumentation and an understanding of reactor physics, to have enough neutrons to irradiate useful quantities of U-238. Note, however, that during the Manhattan Project the United States was able to scale an operating 250 watt reactor to a 250 megawatt production reactor. Although the instrumentation of the day was far less sophisticated than that in use today, the scientists working the problem were exceptional. A typical production reactor produces about 0.8 atoms of plutonium for each nucleus of U-235 which fissions.

A typical form of production reactor fuel is natural uranium metal encased in a simple steel or aluminum cladding. Because uranium metal is not as dimensionally stable when irradiated as is uranium oxide used in high burnup fuel, reactors fueled with the uranium metal must be confined to very low burnup operation, which is not economical for electricity production. This operational restriction for uranium metal fuel results in the production of plutonium with only a small admixture of the undesirable isotope, 240 Pu. Thus, it is almost certain that a reactor using metallic fuel is intended to produce weapons grade plutonium, and operation of such a reactor is a strong indicator that proliferation is occurring.

A Heavy Water Reactor would be based on a low-pressure, low-temperature application of nuclear fission technology specifically designed to produce plutonium [or tritium]. The reactor vessel and cooling system configuration (with primary and secondary cooling loops) would be similar to that used in commercial light water reactor nuclear power technology. The HWR would use heavy water as the reactor coolant and moderator. Heavy water, circulated through the core for cooling and moderation, also passes through heat exchangers that are external to the reactor tank. The heat is in turn carried away by the secondary cooling system. The heavy water in the tank surrounding the fuel would represent the bulk moderator.

The cooling system transferring the heat from the reactor to the heat sink can be configured in a wide variety of designs ranging from fresh-water cooling, through evaporative systems, to dry cooling, including some of their combinations. Reactors are either cooled by using saltwater from coastal (estuarine), freshwater cooling pond, lake water or wet cooling towers, depending on the availability of water resources at the particular site.

1 watt = 3.412 BTU per hr

1 MWt = 3,412,000 BTU per hr

1 Ton (heat load) = 12,000 BTU per hr

1 Cooling Tower Ton = 15,000 BTU per hr

BTU = GPM of Water x 500 x Temperature Difference [degrees F]

1 BTU = .293 watt - hour

The heat dissipation system selected, wet or dry, would be dependent on site characteristics. Both wet and dry cooling systems would use water as the heat exchange medium. Wet systems would use water towers and the evaporation process to carry off heat. Dry systems, designed for cold climates, would use water in closed nonevaporative cooling towers to carry off heat to the atmosphere by conduction through radiator-like vanes. In moderate climates, fans would be added to the dry cooling towers to move air over the vanes. There would be some water loss through evaporation in a dry system, but significantly less than with a wet tower. Dry cooling towers would be used for the reactors at all dry sites.

For both once-through and closed-cycle cooling systems, the water intake and discharge structures are of various configurations to accommodate the source water body and to minimize impact to the aquatic ecosystem. The intake structures are generally located along the shoreline of the body of water and are equipped with fish protection devices. The discharge structures are generally of the jet or diffuser outfall type and are designed to promote rapid mixing of the effluent stream with the receiving body of water. Biocides and other chemicals used for corrosion control and for other water treatment purposes are mixed with the condenser cooling water and discharged from the system.

With lake the cooling water from the lake is pumped through a large number of heat exchanger tubes. The cooling water is heated during this process, and is then returned to the lake. The predominant water use at a nuclear reactor is for removing excess heat generated in the reactor. The quantity of water used is a function of several factors, including the capacity rating of the plant and the increase in cooling water temperature from the intake to the discharge. The larger the plant, the greater the quantity of waste heat to be dissipated, and the greater the quantity of cooling water required.

In a once-through cooling system, circulating water for condenser cooling is drawn from an adjacent body of water, such as a lake or river, passed through the condenser tubes, and returned at a higher temperature to the adjacent body of water. The waste heat is dissipated to the atmosphere mainly by evaporation from the water body and, to a much smaller extent, by conduction, convection, and thermal radiation loss.

Recirculating cooling systems consist of either natural draft or mechanical draft cooling towers, cooling ponds, cooling lakes, or cooling canals. Because the predominant cooling mechanism associated with closed-cycle systems is evaporation, most of the water used for cooling is consumed and is not returned to a water source.

In closed-cycle systems, the cooling water is recirculated after the waste heat is removed by dissipation to the atmosphere, usually by circulating the water through large cooling towers constructed for that purpose. Cooling towers are needed when a body of water large enough to provide the cooling, or groundwater is not available in sufficient quantity and there are no other suitable surface water sources available. Closed-cycle cooling towers represents a type of cooling tower that includes both dry cooling towers and hybrid wet/dry cooling towers. Increased cooling tower performance can be achieved by adding surface area or by boosting the flow rate. The former is considerably more expensive than the latter since flow rate can be increased by employing a bigger fan motor allowing increased fan speed.

Wet cooling towers use the same condenser system as in lake cooling, however, the cooling water comes from a large basin at the bottom of the cooling tower. Wet cooling towers use freshwater and achieve 80% of their cooling by evaporation of the cooling water. This evaporation represents a loss of millions of liters of water per year, and dry cooling may be a more attractive option for cooling.

In a natural draft cooling tower the heated cooling water from the condenser is sprayed down the inside of the cooling tower whilst air, under the effects of natural convection flows up through the cooling tower. The air draft evaporates some of the cooling water, lowering the temperature of the remaining cooling water. The draft can also be produced by fans in an induced draft cooling tower. A plume of pure water vapor can often be seen exiting the top of wet cooling towers particularly when the atmosphere has high humidity.

A mechanical draft cooling tower cools circulating water. In the cooling tower, circulating water is directed to the top of the tower and then flows downward through the tower while induced draft fans draw ambient air upward. Heat is transferred to the ambient air primarily through the evaporation of a significant portion of the cooling water. At certain times during the year, a visible plume rises from the tower due to this evaporation cooling process.

A dry cooling tower uses significantly less water. There are two main types of dry cooling technology, the direct system and the indirect system. An air-cooled system operates like a very large automobile radiator. These systems use a flow of air to cool water flowing inside finned tubes. It is essentially a closed loop system where air is passed over large heat exchange surfaces. While air cooling is a reliable and proven technology, it has some technical and economic drawbacks in comparison to a wet mechanical cooling system, which requires the use of significant amounts of water. The principal drawbacks of air cooling are increased noise levels, higher capital costs and larger physical dimensions.

Indirect dry cooling towers use the same condenser system as in lake cooling, however the cooling water is recirculated through banks of finned tubes over which cooling air flows. The air flow is induced in a natural draft cooling tower by convection. The natural draft cooling tower for dry cooling is larger than for an equivalent wet system, since heat transfer rates are much less.

The direct dry cooling mechanical draft cooling tower consists of a concrete structure supporting the mechanical draft fans and exhaust plenum. If fans are used instead of the natural draft tower, a large number of fans would be required to achieve the same heat rejection. This is because the temperature difference between the air and the cooling water is relatively small.

Indirect dry cooling systems tend to have a large capital cost (due to the large cost of the natural draft cooling tower) but low operating cost. In comparison direct dry cooling has a low capital cost but high operating cost (due to large power consumption of the fans). In general, direct dry cooling is favored at sites with low fuel cost (the fan power is less costly), while indirect dry cooling is more suitable at sites with high fuel costs.

The cooling system needs to be located as near as possible to the reactor. It is possible to locate a fan forced cooling system closer to the reactor than a natural draft system. The tube banks may be located directly adjacent to the reactor hall, minimising piping distance, and the fans are located under the tubebanks.

The sources of routine radioactive gaseous emissions to the atmosphere are the air ejector which removes noncondensable gases from the coolant, and gaseous and vapor leakages, which, after monitoring and filtering, are discharged to the atmosphere via the building ventilation systems. The off-gas treatment system collects noncondensable gases and vapors that are exhausted at the condenser via the air ejectors. These off-gases are processed through a series of delay systems and filters to remove airborne radioactive particulates and halogens, thereby minimizing the quantities of the radionuclides that might be released. Building ventilation system exhausts are another source of gaseous radioactive wastes.