Critical mass studies of plutonium solutions
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It may be assumed that the minimum duration of the radiation transient is one millisecond 1 ms. The alarm trip point shall be set low enough to detect the minimum accident of concern. The alarm trip point should be set high enough to minimize the probability of an alarm from sources other than criticality. The spacing of detectors shall be consistent with the selected alarm trip point and with the detection criterion. The location and spacing of detectors should be chosen to minimize the effect of shielding by massive equipment or materials.
Shielding from low-density materials of construction, such as wood framing, thin interior walls, hollow brick tiles, etc. Interlocking with the ventilation system should be provided for shutting off ventilation to prevent release of fission gases outside of the affected area. Consideration should be given that shutting off ventilation does not generate other safety hazards. Initial tests, inspections, and checks of the system shall verify that the fabrication and installation were made in accordance with design plans and specifications.
Following modifications or repairs, or events that call the system performance into question, there shall be tests and inspections adequate to demonstrate system operability. System response to radiation shall be measured periodically to confirm continuing instrument performance.
The test interval should be determined on the basis of experience. In the absence of experience, tests should be performed at least monthly. Records of tests shall be maintained. System designs may incorporate self-checking features to automate portions of this testing. The entire alarm system shall be tested periodically. Each signal generator should be tested at least annually. Field observations shall establish that criticality alarm signals are functional throughout all areas where personnel could be subject to an excessive radiation dose.
All personnel in affected areas shall be notified before testing of the criticality alarm signals. When tests reveal inadequate performance, corrective action shall be taken without unnecessary delay. If portable instrument use is required, the criteria of section 3. Procedures for system testing shall minimize both false alarms and inadvertent initiation of emergency response.
The procedures shall require that the systems be returned to normal operation immediately following tests. The IEC Standard, Radiation Protection Instrumentation — Warning Equipment for Criticality Accidents , holds useful information regarding electrical characteristics and testing procedures for alarm equipment. This document may be used as a guide in these areas. Records of tests and corrective actions for each system shall be maintained.
These records provide information on system operability and help identify sources of failure. The licensee shall develop and implement out-of-service criteria for the nuclear criticality alarm system. If the system is removed from service due to an unforeseen problem, the licensee shall immediately inform CNSC as to the cause of the removal and its expected duration. If an adequate back up alarm system, as described in section 3. Instructions regarding response to criticality alarm signals shall be posted at strategic locations within areas requiring alarm coverage.
Guidance for training of employees and visitors, and for conduct of criticality alarm drills, is provided in section 12, Administrative Practices for Nuclear Criticality Safety. Raschig rings are used inside vessels and tanks containing solutions of fissile material to act as neutron absorbers and prevent a potential criticality accident. Section 4 this section provides guidance for the use of borosilicate-glass Raschig rings as a neutron absorber for criticality control in ring-packed vessels containing solutions of U, Pu, or U.
The chemical and physical environment, properties of the rings and packed vessels, maintenance inspection procedures, and operating guidelines are specified. The purpose of Raschig rings in criticality safety applications is to assure subcriticality for normal and credible abnormal conditions over the operating life of a vessel. General requirements for use of Raschig rings for criticality control are:. Raschig rings shall not be used in applications where credible agitation or movement of the rings can damage the rings sufficiently to compromise their effectiveness as a criticality control.
Applications where the potential for such damage exists include but are not limited to evaporators, portable vessels, pulsed columns, and vessels equipped for sparging . Raschig rings shall not be used in fields of intense ionizing radiation.
Maximum time-averaged radiation dose rates for rings shall be limited to the following :. Light water and other near neutral solutions that do not exceed the free fluoride and phosphate ion concentrations specified in section 4. Subject to these restrictions, acceptable chemical environments include solutions of salts of organic or inorganic acids, hydrocarbons, and solutions of complexing or chelating agents in hydrocarbons.
Results of corrosion tests on borosilicate-glass Raschig rings that support these requirements appear in the literature [22, 23]. Raschig rings shall not be used as a criticality control in basic solutions unless chemical and physical limits for the application have been determined and documented.
If rings are so used, inspection frequencies should be derived from a trending analysis to assure the requirements of this regulatory document are met. Studies of the corrosion of borosilicate-glass in basic environments appear in the literature . The density of glass used for Raschig rings shall not be less than 2. The 10 B isotope content of glass used for Raschig rings shall be no less than 0.
This isotopic content may be determined directly or inferred from:.
Each Raschig ring shall have an average outside diameter no greater than 38 mm 1. These tests shall be performed on the rings to demonstrate their compatibility under normal and credible abnormal conditions of service e. Raschig rings shall be subjected to mechanical tests designed to evaluate glass integrity. These tests shall demonstrate that the rings will remain intact while in service under anticipated normal and credible abnormal conditions. If use is generally static, such that liquid flows easily into and out of a vessel with no dynamic motion between rings, mechanical tests need merely confirm that the glass can withstand the static loading.
If use involves vigorous mixing actions that might cause breakage through movement, mechanical tests must confirm that the rings can also withstand dynamic forces without breaking. Pipes intended for the removal of solution shall be designed and installed to prevent removal of pieces of glass along with the solution e. All regions of the vessel shall be filled with well-settled rings that is, rings that have been gently manipulated during loading such that they are not likely to settle further during use.
Carefully hand-placed rings may have a greater glass volume fraction than even randomly oriented rings; both are allowed. An installation and compaction procedure that minimizes breakage, aids settling, and minimizes voids should be used. The initial loading of rings into a vessel may use unmarked rings that satisfy the requirements of this regulatory document. However, if rings are added to the initial loading to compensate for settling or to replace rings removed for some analysis , the added rings shall be permanently identified to preclude their subsequent use as rings characteristic of that initial loading.
The ring-packed vessel may contain regions free of Raschig rings in apparent contradiction to section 4. These might be formed by pipes embedded in an otherwise well-settled packing of rings. The edge-to-edge spacing between each ring-free region shall be at least mm 12 in. The ring-free region may be produced by a single pipe or a cluster of pipes, provided the cluster of pipes also has an outside diameter less than 64 mm 2.
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These regions may serve any purpose e. The top surface of the Raschig rings within a vessel shall be inspected periodically to detect settling through time and use. This inspection may be visual or non visual e. If a visual method is to be used to inspect the level of the rings, sufficient ports or sight glasses should be provided to allow inspection of the entire upper surface.
See section 4. The glass volume fraction shall be determined each time new rings are installed in the vessel. This shall apply to either a full replacement or a partial addition of rings to compensate for settling. The level of the solution shall not exceed the level of uniformly packed rings. There shall be a method of determining that this condition is met even if the rings should settle through time or use, to preclude the possibility of solution accumulation in a ring-free region.
This protection may be afforded by:. Raschig rings pack differently in different vessels, leading to small variations in the resulting glass volume fraction in the vessel. Table gives the maximum permissible fissile isotope concentration for uranium and plutonium solutions in vessels of unlimited size packed with borosilicate-glass Raschig rings that meet the requirements of this regulatory document [22, 25]. Note: Table shall not be used with mixtures of uranium and plutonium in solution. Low-level contamination of one element with the other is allowed. The definition of low-level contamination shall be justified and documented.
The solutions referred to in the table shall have a hydrogen density not less than 75 g H per litre and not greater than g H per litre. The three columns under the heading "Uranium" are intended to refer to solutions generally characterized by uranium enrichments as follows:. Whenever any combination of isotopes falls into more than one category, the concentration limit of either column may be applied. In all cases, low levels of plutonium contamination are allowed.
Isotope ranges given at the top of each column define the allowed actinide compositions for the concentrations shown. Graphical interpolation between tabulated glass volume fractions is allowed. Raschig rings shall be inspected periodically to determine whether they have settled, whether there have been changes in their physical or chemical properties, and whether solids have accumulated.
A record of the results of inspections of installed rings shall be maintained for each packed vessel. These data shall be used to determine the frequency of inspection through documented analysis. Any change in inspection frequency, and its justification, shall be documented. If settling is detected, rings meeting specifications of this document shall be added to restore full packing.
Those rings shall be permanently identified to preclude subsequent use as samples for maintenance because they are not characteristic of the initial loading. The number of rings added and other appropriate comments shall be recorded and maintained for the life of the set of rings packed into the vessel. Settling trends may be determined by comparison with past results. A record shall be maintained to enable evaluation of, and to set appropriate controls over, the accumulation of fissile solids on the Raschig rings and on the inner surface of the vessel.
The rings in the vessel shall be cleaned or replaced and the vessel walls cleaned if the deposited solids contain more than 50 g of U, U, Pu, or any combination of these isotopes per litre of glass . In-service Raschig rings shall be periodically retested, to determine their current physical properties, by testing of ring samples from representative regions of each vessel. The purpose is to assure that the requirements of this section continue to be met.
Procedures shall be implemented to prevent the inclusion in the sample of Raschig rings that were not part of the initial loading in the vessel see section 4. Control Raschig rings may be used for these tests provided the control Raschig rings remain in the vessel except for test periods not exceeding two weeks per test and a total of four weeks per year.
The vessel may be in continued use while control Raschig rings are removed from it, provided the specifications of sections 4. If any of the tested in service Raschig rings fail to meet any of the ring specifications given in section 4. These appropriate actions may be but are not limited to one or more of the following actions:. Trending analysis of periodic physical and chemical tests may be used to predict the useful lifetime of Raschig rings.
Raschig rings shall be inspected periodically to demonstrate their continued criticality control properties. These required tests shall include ring settling see section 4. The initial interval for inspection of rings shall not exceed:. This initial inspection interval may be set at longer times when justified by a documented and approved analysis. Subsequent intervals between inspections may be based on the analysis of the trends in the data. If records and inspections confirm that no solution has been present in a vessel since the preceding inspection and if the vessel has not been subject to corrosive fumes, then only the settling test see section 4.
If Raschig rings are exposed to solutions in which the free fluoride concentration is greater than 0. Safe and economical operations with fissile materials require knowledge of the subcriticality of configurations that arise in material processing, storage, and transportation. Data from critical experiments have been a principal source of information with which to establish safe practices; however, the need has developed for measurements of limited application that can more expeditiously provide guidance for safe operations with fissile materials in the specific arrangements encountered in industrial environments.
Such measurements are made in some plant process areas and are referred to as in situ nuclear measurements. Personnel protection during in situ experiments depends on the avoidance of a criticality accident. Section 5 this section contains safety criteria and practices for conducting such experiments.
This section is oriented toward measurements of neutron multiplication and thus reflects the preponderance of this experience, but the principles presented in this section may be applied to measurements based on other reactivity indexes, such as the prompt-neutron decay constant. This section provides safety guidance for conducting subcritical neutron-multiplication measurements where the only barrier is the administrative procedures and therefore physical protection of personnel against the consequences of a criticality accident is not provided.
The objectives of in situ measurements are either to confirm an adequate safety margin or to improve an estimate of such a margin. The first objective may constitute a test of the criticality safety of a design that is based on calculations. The second may affect improved operating conditions by reducing the uncertainty of safety margins and providing guidance to new designs.
A written procedure for each new in situ experiment shall be prepared and reviewed in a manner approved by management. Primary responsibility for safety shall be assigned to one individual experienced in the performance of subcritical or critical experiments. Another experienced experimenter shall review the procedure. A record of the status and progress of the experiment shall be maintained with particular emphasis on safety.
Emergency procedures and radiation detection instrumentation appropriate to the experiment shall be provided. The mechanical integrity of equipment to be used in conjunction with the fissile assembly shall be verified prior to the experiment. The proper functioning of all counting circuits, neutron- and gamma-ray-sensing devices, alarms, and other necessary instrumentation shall be verified prior to the experiment.
A source of neutrons shall be present to produce a meaningful indication of neutron multiplication. This source may be inherent in the fissile assembly, i. At least two independent neutron-sensitive counting devices shall monitor the neutron population in the fissile assembly under investigation. A signal continuously indicative of the neutron level shall be audible and may be supplemented by an otherwise apparent signal such as a flashing light. If anyone participating in the experiment expresses doubt of the safety of a particular action or step, the experiment shall be suspended until the doubt is resolved.
The cause of any unexpected behaviour of the assembly and its associated equipment or of any peculiarity in the resulting data shall be resolved before further reactivity additions. A reactivity limit for the fissile assembly shall be defined in the written procedure. This limit may be stated in terms of a maximum value of neutron multiplication or of a fraction of an estimated critical mass, volume, or dimension.
The margin below criticality shall be commensurate with experimental uncertainties; allowance shall be made for effects of neutron reflection brought about by personnel or other movable objects. Plots of reciprocal neutron multiplication as a function of the parameter identifying reactivity change shall be maintained independently by at least two persons using data from two or more neutron-detector channels. These plots shall have a sufficient number of points to permit meaningful extrapolation.
Catalog Record: Critical mass studies of plutonium solutions | HathiTrust Digital Library
The magnitude of reactivity additions shall be guided by extrapolation of the plots of reciprocal neutron multiplication and shall be such that the reactivity limit defined in section 5. Caution is recommended in the interpretation of reciprocal neutron multiplication curves; typical experimental curves are discussed in The Technology of Nuclear Reactor Safety , Vol. Every addition of reactivity shall be authorized by the person assigned primary responsibility for safety in accordance with section 5.
No reactivity addition shall be made until the effects of preceding additions have been evaluated and until the response to be expected from the subsequent addition has been estimated. Alteration in the method of reactivity addition shall not be such as to invalidate the extrapolation of the plot of reciprocal multiplication.
Consideration shall be given to the possibility of inadvertent reactivity additions such as might occur from the instability of slurries, from collapse or formation of voids, from inadvertent transfer of material, or from other conditions. Place of source and detectors shall be such that the neutrons observed are predominantly those produced by the fissile assembly. If a neutron source or detector is to be moved from one location to another, or if attenuating material is to be inserted between the source and detectors, the effect on neutron counting rate of such change shall be measured before further reactivity addition.
Changing the spacing between elements of an array should not be the means of changing reactivity. Data from reciprocal multiplication plots obtained from separate experiments with different spacings may be used to evaluate the effect of element spacing on neutron multiplication.
Section 6 this section provides general storage criteria based on validated calculations, and includes some engineering and administrative practices appropriate to the storage of fissile material [27, 28]. The tabulated mass limits presented in this section are for idealized storage configurations. While these configurations may not be commonly encountered in practice, they do provide bases for establishing safe storage arrays.
Because this section cannot effectively cover all conditions of interest, the use of supplementary information is encouraged [16, 29, 30]. For example, subcriticality of arrays not specified in this section may be confirmed by conducting neutron source multiplication measurements as described in section 5, Safety in Conducting Subcritical Neutron Multiplication Measurements in Situ. Criteria for the range of application of these limits are provided.
All operations with fissile material, including storage, shall be conducted in accordance with section 2, Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors. This section is intended to supplement section 2 by providing storage criteria applicable to many fissile materials. If the limits given in this section are used, an adequate administrative margin of subcriticality shall be applied to ensure compliance with section 2. Methods of storage control and operational practices approved by management shall be described in written procedures.
Persons participating in the transfer and storage of material shall be familiar with these procedures. Limits for storage shall be posted. Additional guidance for administrative practices can be found in section 12, Administrative Practices for Nuclear Criticality Safety.
Limits for the storage of fissile material shall be based on experimental data or on the results of calculations made through the use of validated computational techniques. Storage facilities and structures shall be designed, fabricated, and maintained in accordance with good engineering practices. The design of storage structures should preclude unacceptable arrangements or configurations, thereby reducing reliance on administrative controls. Fissile materials shall be stored in such a way that accidental nuclear criticality resulting from fire or from flood, earthquake, or other natural calamities is not a concern.
Storage areas should contain essentially no combustible materials. Where the presence of significant quantities of combustible materials is unavoidable, as in the storage of combustible scrap, a fire protection system shall be installed. Shelving shall be sturdy and non-combustible. Spacing of storage units may be maintained by the use of birdcage fixtures, covered metal cans, or physical barriers on shelves.
Containers of fissile materials in areas with sprinkler systems shall be designed to prevent accumulation of water. In fissile material storage areas equipped with sprinkler systems, consideration shall be given to the possibility of criticality occurring in an accumulation of runoff water from the sprinkler system. A criticality accident alarm shall be provided in accordance with section 3, Criticality Accident Alarm System. Good housekeeping shall be incorporated as an important part of nuclear criticality safety practices. Tables through list mass limits for array storage of individual units of specified fissile materials.
The information given in the tables may be applied directly to the solution of practical storage problems. If the limits are found to be unnecessarily restrictive for a particular application, they may, at a minimum, serve as lower bounds for comparison with limits derived through the use of other techniques. The limits were derived and subsequently checked through the use of validated computational techniques see section 2, Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors to interpolate within sets of experimental data and to extrapolate from them.
The validated computational techniques employed provide numerical approximations to the solutions of the neutron transport equation for given formulations of neutron cross section data. The basis for the limits is a set of calculational results for individual fissile material units in cubic arrays . These arrays are reflected on all faces by mm of light water. The mass limits in Tables through yielded evaluated array neutron multiplication factors, keff less than 0.
It should be noted that calculations of these arrays through the use of other computational techniques, especially those employing other neutron cross section formulations, may yield different array neutron multiplication factors. The units are spheres of the specified fissile materials, characterized by their main isotopic constituents, centered in cubic cells:.
The fissile material storage limits presented for the oxides are based on void-free mixtures of the dioxides and water at theoretical densities corresponding to the specified ratio of hydrogen to fissile element atoms. These limits may be applied to other oxides, fluorides, chlorides, and nitrates and to other salts that do not exceed the stated ratio of hydrogen to fissile element atoms and that do not exceed the associated fissile element density characteristic of the tabulated mixture.
A number of the tabulated values exceed the critical mass of a water-reflected sphere. Subcriticality of such units shall be provided by appropriate controls, e. Caution in the interpretation of the tabulated values is advised. They are intended to specify capacities of the cells and must be supplemented by good nuclear criticality safety practices.
Other operational considerations may dictate smaller limits. The ratios of hydrogen to fissile material atoms are determined within the fissile region and do not include contiguous hydrogenous materials. Margins inherent in the mass limits specified are sufficient to compensate for incidental moderation such as that resulting from enclosing each unit in a thin plastic bag. The effects of more significant moderation should be evaluated through the use of a validated computational technique.
The mass limits in the tables are also applicable for concrete reflectors up to mm 5 in. Equivalent thicknesses of other masonry materials may be established on the basis of their areal densities . Double batching of certain tabulated unit masses would, in some geometries, result in criticality . Most massive storage units of practical interest, however, would be of much less reactive geometry. If a double-batched cell, reflected by water on all its faces, can be shown to have a value of k eff not exceeding 0. Alternatively, if a double-batched cell can be shown to be subcritical when water reflected on all its faces, double batching in a few 8 or fewer cells in an array satisfying the tabulated requirements will not result in array criticality.
Double batching shall be considered in storage safety analyses and in the establishment of operating procedures. If double batching is credible, it shall be determined that double batching in a single storage cell will not result in array criticality. Administrative controls, limited capacity containers, and storage cell design may be useful for the prevention of double batching.
This reduction is sufficient to include the effect of concrete as a reflector. The mass reduction factors called for in sections 6. If the application of these limits produces an undesirably conservative result, then calculations specific to the system of interest should be performed through the use of a validated computational technique. Consideration should be given to the precision and any bias in the calculational technique used in determining that k eff of 0. Increases in cell size to effect reduction factors may be more desirable than decreases in the mass limits.
Aisles may be provided in the arrays specified in Tables through by removing units from the array or by increasing the total array volume to provide space. The margin of safety is adequate to permit personnel within the resultant storage area. The specified limits allow for thicknesses of steel less than Effects of greater thicknesses of steel or of other materials shall be investigated experimentally or by applying validated computational techniques.
The tabulated limits are not directly applicable to all systems of interest. When the provisions of section 6. Each cell within any array described in Tables through is assigned an index equal to the quotient of and the number of cells in the array . Commingling, in one array, of any of the cells is permitted if the aggregate of the indexes of all the cells within the resultant array does not exceed Interpolation may be made among mass limits, number of cells, and hydrogen content.
Interpolation of U enrichment is permitted. Linear interpolation is not necessarily appropriate. Any tabulated mass limit may be applied to a non-cubic cell equal in volume to that tabulated containing a near-equilateral unit if the largest dimension of the cell does not exceed the smallest by more than a factor of 2.
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The tabulated values may be applied to other than near-equilateral units in non-cubic cells if the unit and cell volumes are maintained and if the ratio of the dimensions that characterize the shape of the unit is approximately equal to the ratio of the corresponding dimensions of the cell. The tabulated mass limits for plutonium containing 5. Section 2, Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors , provides guidance for the prevention of criticality accidents in the handling, storing, processing, and transporting of fissionable materials.
Section 2. Section 7 this section recognizes that, if adequate shielding against radiation and confinement of radioactive materials are provided, the hazards normally attendant with criticality in a facility lacking shielding and confinement are minimized. This section does not apply to those operations requiring entry of personnel inside the shielded process areas wherein fissile and fissionable materials are contained. This section does not include engineering specifications for shield design nor for establishing its adequacy. Nothing in this section shall be interpreted as discouraging additional safety features that can be conveniently incorporated.
This section applies to operations, with U, U, Pu and other fissile and fissionable materials outside of nuclear reactors, in which shielding and confinement are provided for protection of personnel and the public, except the assembly of these materials under controlled conditions, such as in critical experiments. Criteria are provided that may be used for criticality control under these conditions.
This section does not include the details of administrative procedures for control which are considered to be management prerogatives or details regarding the design of processes and equipment or descriptions of instrumentation for process control. The provisions of this section may be applied only in those shielded facilities that meet the following criteria:.
Thus, a storage vault does not qualify unless additions or withdrawals of the fissile material are made by remotely operated devices. The criteria that are presented herein consider only the adequacy of the shielding and confinement for criticality accidents. Additional shielding may be required by the process conditions. Shielding and confinement are considered adequate when the following conditions are satisfied during and subsequent to an accident. However, better shielding and confinement are desirable if practical. Plutonium could also be used to manufacture radiological weapons or as a not particularly deadly poison.
The plutonium isotope Pu is an alpha emitter with a half-life of 87 years. These characteristics make it well suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human lifetime. It is therefore used in RTGs such as those powering the Galileo and Cassini space probes; earlier versions of the same technology powered seismic experiments on the Apollo Moon missions.
It has been largely replaced by lithium-based batteries recharged by induction, but as of there were somewhere between 50 and plutonium-powered pacemakers still implanted and functioning in living patients. Plutonium was discovered in by Dr. Glenn T. Seaborg , Edwin M. McMillan , J. Kennedy , and A. Wahl by deuteron bombardment of uranium in the inch cyclotron of the Berkeley Radiation Laboratory at the University of California, Berkeley , but the discovery was kept secret. It was named after the planet Pluto , having been discovered directly after neptunium which itself was one higher on the periodic table than uranium , by analogy with the ordering of the planets in the solar system.
During the Manhattan Project , large reactors were set up in Hanford, Washington for the production of plutonium, which was used in two of the first atomic bombs the first was tested at Trinity site , the second dropped on Nagasaki , Japan. Large stockpiles of plutonium were built up by both the old Soviet Union and the United States during the Cold War —it was estimated that , kg of plutonium had been accumulated by Since the end of the Cold War, these stockpiles have become a focus of nuclear proliferation concerns. In , the United States Department of Energy took possession of 34 metric tons of excess weapons grade plutonium stockpiles from the United States Department of Defense , and as of early was considering converting several nuclear power plants in the US from enriched uranium fuel to MOX fuel as a means of disposing of these.
During the initial years after the discovery of plutonium, when its biological and physical properties were very poorly understood, a series of human radiation experiments were performed by the U. From the time of April to July , 18 men, women, and children were deliberately injected with solutions containing various concentrations of plutonium by doctors working with the Manhattan Project.
Though the injections were only to occur in what were percieved by the doctors as terminally ill patients at the hospital, in at least one instance this was not the case and the injections, in all cases, were conducted without any kind of informed consent from the subjects of the experiment. The episode is considered today, to be a gross violation of human rights and of the Hippocratic Oath , and is widely regarded as one of the darkest chapters in 20th-century American medical history. While almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores.
These come about by a process of neutron capture by U nuclei, initially forming U; two subsequent beta decays then form Pu with a Np intermediary , which has a half-life of 24, years. This is also the process used to manufacture Pu in nuclear reactors. Some traces of Pu remain from the birth of the solar system from waste of supernovae, because its half-life 80 million yrs is so long. A relatively high concentration of plutonium was discovered at the natural fission reactor in Oklo , Gabon in Since , about 10 tons of plutonium have been released onto Earth through nuclear explosions.
The isotope Pu is the key ingredient to most nuclear weapons. Its manufacture is therefore important to nuclear weapon states. Controlling or preventing the manufacture of refined Pu is also important in preventing nuclear proliferation. Pu is normally manufactured in nuclear reactors. If U is exposed to neutron radiation , the nuclei will occasionally capture a neutron, becoming U This happens more easily with fast neutrons than with slow neutrons , although both can be used.
The U rapidly undergoes beta decay to give Np, which rapidly undergoes a second beta decay, giving Pu Fission activity is relatively rare, so even after significant exposure, the Pu is still mixed with a great deal of U and possibly other isotopes of uranium, oxygen, other components of the original material, and fission products.
The Pu can then be chemically separated from the rest of the material to give high-purity Pu metal. If Pu captures a neutron, it becomes Pu Pu undergoes spontaneous fission at a relatively high rate. As a result, plutonium containing a significant fraction of Pu is not well-suited to use in nuclear weapons; it emits neutron radiation, making handling more difficult, and its presence can lead to a "fizzle" in which a small explosion occurs, destroying the weapon but not causing fission of a significant fraction of the fuel.
The US has constructed a single experimental bomb using only reactor-grade plutonium. Moreover, Pu and Pu cannot be chemically distinguished, so expensive and difficult isotope separation would be necessary to build a nuclear weapon using such a mix. Thus for the purposes of plutonium production, it is necessary to remove the U frequently, before significant amounts of Pu can be converted into Pu A nuclear reactor that is used to produce plutonium must therefore have a means for exposing U to neutron radiation, and for frequently rotating this U A reactor running on unenriched or moderately enriched uranium naturally contains a great deal of U However, most commercial power reactor designs require the entire reactor to shut down, often for weeks, in order to change the fuel elements.
They therefore produce plutonium in a mix of isotopes that is not well-suited to weapon construction. Such a reactor could have machinery added that would permit U slugs to be placed near the core and changed frequently, or it could be shut down frequently, so proliferation is a concern; for this reason, the IAEA inspects licensed reactors frequently.
In fact, the RBMK was built by the Soviet Union during the cold war, so despite their ostensibly peaceful purpose, it is likely that plutonium production was a design criterion. Seaborg chose the letters "Pu" as a joke, which passed without notice into the periodic table. Chemists at the University of Chicago began to study the newly manufactured radioactive element. The George Herbert Jones Laboratory at the university was the site where, for the first time, a trace quantity of this new element was isolated and measured in September This procedure enabled chemists to determine the new element's atomic weight.
Room of the building was named a National Historic Landmark in May Later, large MWt reactors were set up in Hanford, Washington, for the production of plutonium, which was used in the first atomic bomb used at the "Trinity" test in July The "Little Boy" bomb dropped on Hiroshima utilized uranium, not plutonium. Large stockpiles of "weapons-grade" plutonium were built up by both the Soviet Union and the United States during the Cold War.
The U. In , the United States Department of Energy took possession of 34 metric tons of excess weapons-grade plutonium stockpiles from the United States Department of Defense, and as of early was considering converting several nuclear power plants in the US from enriched uranium fuel to MOX fuel as a means of disposing of plutonium stocks. During the initial years after the discovery of plutonium, when its biological and physical properties were very poorly understood, a series of human radiation experiments were performed by the U.
During and after the end of World War II, scientists working on the Manhattan Project and other nuclear weapons research projects conducted studies of the effects of plutonium on laboratory animals and human subjects. In the case of human subjects, this involved injecting solutions containing typically five micrograms of plutonium into hospital patients thought to be either terminally ill, or to have a life expectancy of less than ten years either due to age or chronic disease condition. These eighteen injections were made without the informed consent of those patients and were not done with the belief that the injections would heal their conditions; rather, they were used to develop diagnostic tools for determining the uptake of plutonium in the body for use in developing safety standards for people working with plutonium during the course of developing nuclear weapons.
The episode is now considered to be a serious breach of medical ethics and of the Hippocratic Oath, and has been sharply criticised as failing "both the test of our national values and the test of humanity. While almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores. These come about by a process of neutron capture by U nuclei, initially forming U; two subsequent beta decays then form Pu with a Np intermediary , which has a half-life of 24, years.
This is also the process used to manufacture Pu in nuclear reactors. Some traces of Pu remain from the birth of the solar system from the waste of supernovae, because its half-life of 80 million years is fairly long. A relatively high concentration of plutonium was discovered at the natural nuclear fission reactor in Oklo, Gabon in Since , approximately kg has been released onto Earth through nuclear explosions.
The activation cross section for Pu is barns while the fission cross section is barns for thermal neutrons. The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain bomb grade plutonium. Plutonium is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy.
The other fissile materials are uranium and uranium Plutonium is virtually nonexistent in nature. It is made by bombarding uranium with neutrons in a nuclear reactor. Uranium is present in quantity in most reactor fuel; hence plutonium is continuously made in these reactors. Since plutonium can itself be split by neutrons to release energy, plutonium provides a portion of the energy generation in a nuclear reactor. It is clear to see that with a low flux of neutrons that U will be converted into Pu.
There are small amounts of Pu in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U atom captures a neutron, it is converted to an excited state of U Some of the excited U nuclei undergo fission, but some decay to the ground state of U by emitting gamma radiation. Further neutron capture creates U which has a half-life of 7 days and thus quickly decays to Np Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np by chemical separation of neptunium.
After this chemical separation, Np is again irradiated by reactor neutrons to be converted to Np which decays to Pu with a half-life of 2 days. Plutonium reacts readily with oxygen , forming PuO and PuO 2 , as well as intermediate oxides. Plutonium like other actinides readily forms a dioxide plutonyl core PuO 2. In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties OH - , NO 2 - , NO 3 - , and SO 4 -2 to form charged complexes which can be readily mobile with low affinities to soil. PuO 2 formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO 2 which is resistant to complexation.
It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium. Even at ambient pressure, plutonium occurs in a variety of allotropes. The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. The reasons for the complicated phase diagram are not entirely understood; recent research has focused on constructing accurate computer models of the phase transitions. In weapons applications, plutonium is often alloyed with another metal e.
Interestingly, in fission weapons, the explosive shock waves used to compress a plutonium core will also cause a transition from the usual delta phase plutonium to the denser alpha phase, significantly helping to achieve supercriticality.