Lapas attēli
PDF
ePub
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

8

Fig. 1. Power Reactor Fuel Cycle. Uranium ore is mined, milled, and refined, and the resulting U,O, is converted to UF for enrichment to approximately 3% 235 U. Reactor fuel, fabricated from virgin, enriched uranium, or mixtures of uranium and plutonium, provides power in the reactor. Spent-fuel elements are stored at reactor sites or at specially designed away-from-reactor storage facilities. In a "once-through" fuel cycle, the spent fuel is stored permanently, and the remaining 235U and the plutonium formed as a by-product of power generation are not used. In a complete fuel cycle, the uranium and plutonium are recovered from the spent fuel and recycled to provide raw materials for new, "mixed-oxide" fuel elements. Safeguards efforts are concentrated on preventing diversion of separated plutonium between the fuel reprocessing and fuel fabrication steps.

tinued development of nuclear power and endorsement of plutonium-based nuclear energy systems, including commercial development of the fast breeder reactor in appropriate countries, in order to avoid a projected shortage of uranium. fuel by the end of this century. The INFCE report urged that technical safeguards against proliferation be applied in "a consistent and predictable" (reasonably standardized) way that would not discourage the peaceful development of nuclear energy by creating doubts and uncertainties about the future availability of fuel supplies.

Despite numerous problems and difficulties. nuclear energy is rapidly becoming a major energy source in an

increasing number of countries. It
currently supplies over 10% of all elec-
tric power generated in the United States
and 20% or more of total electric power
in some industrialized countries, such as
Belgium. Sweden, and Switzerland.
Some of the more advanced developing
countries, such as India and South
Korea. have significant and growing
nuclear power programs, while many
other developing countries are actively
seeking to acquire this new source of
energy. Recent projections indicate that
nuclear power plants will supply nearly
one-quarter of the world's electrical
energy by approximately the year 2000.
As IAEA Director General Sigvard
Eklund noted at a recent LASL collo-

quium, the driving force behind the worldwide growth of nuclear energy is not difficult to understand when viewed against the background of economic. political, and supply-assurance problems associated with the world's shrinking supply of hydrocarbon fuels. With the growing demands for fossil fuel, the cost of oil, for example, has risen by a factor of 5 or 6 in nearly as many years.

Hand in hand with the promise of nuclear energy come some challenging. and recently much publicized, problems and concerns. The accident at Three Mile Island near Harrisburg. Pennsylvania. in 1979, focused worldwide attention on the problems of nuclear reactor safety. TMI also has had

[graphic]

Fig. 2. Structure of the Safeguards System. The functional relationships among the elements of a safeguards system and the normally required management and process control elements of a nuclear fuel cycle facility are indicated by the arrows. Process and item operations are contained within a physical protection barrier (dark outline box) that is part of the physical protection and materials control components of the safeguards system. Materials control is provided by monitoring both the process line and the item operations; the item operations also are controlled. Materials measurements and accounting data are derived from measurements of nuclear materials in process operations. Coordination of each of the components of the safeguards system provides facility safeguards status information to both management and process control coordination.

implications for nuclear energy generally, bringing increased attention to the problems of nuclear waste, weapons proliferation, and nuclear material safeguards. Full realization of nuclear power's great potential for meeting world energy needs will clearly depend on how effectively such problems are addressed, including how effectively nuclear safeguards can be implemented on both the national and the international levels.

During their lifetime, nuclear reactor fuel materials undergo a variety of physical and chemical processes in various plants and facilities collectively known as the nuclear fuel cycle (Fig. 1). To maintain strict accountability and control of sensitive fissionable materials throughout the nuclear fuel cycle, we

must be able to take a rapid and ac-
curate inventory of these materials in
each facility at any given time. This re-
quirement is especially important if a
diversion, theft, extortion, or blackmail
threat should occur. We must be able to
ascertain quantitatively what, where, and
how much material is present in any
facility at any time. Even more to the
point, we must be able to ascertain how
much material may be missing from a
facility at any time.

Unique Role of Measurement and Accounting Systems

Effective safeguards (Fig. 2) depend on a combination of three basic components: (1) physical protection, (2) materials measurement and accounting,

and (3) materials control, including process monitoring. Each component is necessary for a fully effective overall safeguards and security system, but only the materials measurement and accounting component can determine the amount and location of material in a plant at any given time. This capability for determining nuclear material inventories with adequate sensitivity and timeliness provides an overall quantitative check on the combined effectiveness of all other safeguards and security measures at a facility.

Under DOE sponsorship, LASL has developed and demonstrated new automated chemical analysis and NDA instruments that can measure the various forms of nuclear materials rapidly and accurately and thereby

provide the high degree of incisiveness required of modern materials measurement and accounting systems. In the application of analytical chemistry. methods for safeguards and accountability, it is extremely important to obtain analysis samples that are truly representative of the material being measured. Reliable inventory confirmation further requires precise and accurate analyses of the amounts and isotopic compositions of fissionable materials. (uranium, plutonium, and thorium) in widely diverse physical and chemical forms, including pure products, reactor fuels having complex chemical compositions, and numerous types of scrap. Multiphase scrap and materials containing highly refractory components are particularly difficult to dissolve and analyze, while characteristically heterogeneous solid-waste materials in general are simply not amenable to meaningful assay by conventional sampling and chemical analysis techniques.

Major objectives of the LASL analytical chemistry safeguards program are (1) development of fast, effective dissolution techniques and analytical methods for uranium, plutonium, and thorium determinations; (2) design and construction of automated analyzers for these determinations; (3) evaluation of mass spectrometric measurements of uranium and plutonium isotopic distribution; (4) preparation of wellcharacterized plutonium standard reference materials for distribution by the National Bureau of Standards and for use in DOE safeguards standards intercomparison programs; (5) preparation of plutonium and uranium reference. materials for calibration of NDA instrumentation used in the dynamic materials accountability (DYMAC) system at the LASL plutonium processing facility; and (6) participation in an interlaboratory program devoted to measurement of plutonium isotope halflives.

An example of newly developed automated chemical analysis instrumentation is LASL's automated controlledpotential plutonium analyzer, which determines low-milligram amounts of plutonium with high (0.1%) precision at an average rate of one sample per 30 minutes. The combination of high measurement precision and a specially developed high tolerance for impurity elements makes this relatively low cost analyzer directly applicable to the analysis of a wide variety of nuclear materials.

Because representative sampling of some types of scrap and particularly of heterogeneous solid waste is a particularly plaguing problem, it is not surprising that in the early days of the LASL safeguards program one of the first CMB-identified requirements was for NDA instruments to measure scrap and waste materials. The inherently rapid NDA methods also offered the capability for measuring essentially every individual contained unit of feed or product material. For example, in the assay of reactor fuels, NDA techniques made it possible to measure the total fissionable material loading of each individual reactor fuel rod and to certify, on a routine production basis, the pelletto-pellet uniformity of uranium fuel loading. Such certification of uniform loading is an important quality control factor in avoiding "hot spots" in the fissioning fuel, and thereby also an important factor in reactor safety. Other "spin-off" benefits of modern nondestructive and destructive measurement techniques developed for safeguards include better in-plant process control, quality assurance, operational safety, and more efficient management of recycle and waste materials.

Major goals for acceptable performance of NDA instruments were set forth in the period from 1965 to 1970, concurrent with steadily increasing pressures to rigorously quantify and

reduce uncertainties in measured nuclear material inventories. Characteristic measurement times for individual items were usually under 10 minutes and desired accuracies for the various material categories were 0.2-3.0% for well-characterized, uniform feed and product materials: 2-10% for recoverable scrap materials; and 5-30% for poorly characterized nuclear waste.

Fissionable nuclide characteristics exploited for "passive" assay are the gamma-ray, neutron, and alpha-heat emissions accompanying the natural radioactive decay of the nuclides. Supplementing passive NDA techniques. "active" assay methods use external neutron sources to induce fissions in a sample; the fissions are then measured by counting fission neutrons or gamma rays. Gamma-ray and x-ray densitometry also provides rapid, accurate determination of the concentrations of uranium, plutonium, and thorium in typical solutions and solids.* The principal neutron and photon measurement techniques and instruments currently in use or being developed for measuring fuel-cycle materials are summarized in Tables I and II. Calorimetry, a technique based on the measurement of radioactive decay heat of contained materials, also has been implemented widely for measurement of plutonium.

Advanced Materials Accountancy and Control

In conventional safeguards practice. the accountability of nuclear materials within a facility and the detection of unauthorized removals have relied almost exclusively on discrete-item counting (as opposed to the more difficult task of measuring bulk process materials) and on material-balance accounting following periodic shutdown, cleanout, and

*See "Nondestructive Assay for Nuclear Safeguards. in this issue.

[graphic]

physical inventory. The classical materials balance usually is drawn around the entire facility or a major portion of the process. It is formed by adding all measured receipts to the initial measured inventory and subtracting from this sum all measured removals and the final measured inventory. During routine production, material control is vested largely in administrative and process controls, augmented by secure storage for discrete items and sealed containers.

Although periodic shutdown-cleanout operations will always be important in determining the amount of bulk nuclear material holdup in process equipment, pipes, pumps, traps, and filters, the use

of this procedure alone has inherent limitations in sensitivity and timeliness. Sensitivity is limited by measurement uncertainties that might obscure the diversion of relatively large quantities of SNM in a large throughput plant. Timeliness is limited by the practical difficulties, the expense, and hence the infrequency of process shutdown, cleanout, and physical inventory; thus a loss of material could remain undiscovered until the next physical inventory.

Recently developed NDA technology, state-of-the-art conventional (destructive) measurement methods, and special inplant sensors, combined with computer and data-base management technology,

provide the necessary technical basis for much more effective methods of safeguarding nuclear facilities. For example, the greater sensitivity and timeliness requirements on SNM control now being imposed by DOE and NRC can be achieved by subdividing a nuclear facility into discrete accounting envelopes, called unit processes, and drawing individual material balances around them. A unit process is chosen on the basis of process logic, the time material resides within the unit process, and the ability to perform quantitative measurements and draw a material balance. Thus, by subdividing a facility into unit processes and measuring all material flows across unit process boundaries, the

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][merged small][merged small][merged small]
« iepriekšējāTurpināt »