Criticality, nuclear systems

The results of the theory of quantum mechanics require that nuclear states have discrete energies. This is in contrast to classical mechanical systems, which can have any of a continuous range of energies. This difference is a critical fact in the appHcations of radioactivity measurements, where the specific energies of radiations are generally used to identify the origin of the radiation. Quantum mechanics also shows that other quantities have only specific discrete values, and the whole understanding of atomic and nuclear systems depends on these discrete quantities. 

To avoid nuclear criticality, each batch of plutonium must be homogeneously dissolved before new batches are added. Total plutonium in the system should be kept below 5 kg so that criticality (nuclear chain reactions) cannot occur. The melter geometry may also limit plutonium content in the system. 

Because, at pH 3.9, E°(Q -QH2) is higher than at pH 4.8 and tc is longer, it is clear that the critical nuclearity will be higher (and the redox potential of the critical cluster wiD also be more positive). This observation confibms previous results obtained with a stronger redox system (E = -0.41 Vnhe) (5) for similar initial conditions, the critical time was much shorter tc 1 ms), implying a smaller critical nuclearity. 

Ryazanov, B.G. (1996) Basic Principles and Nuclear Criticality Safety System in Facilities of Nuclear Industry of MINATOM, Russia, Russian-Chinese workshop on nuclear safety in spent nuclear fuel reprocessing, Beijing Nuclear Design Institute, Beijing, May 24-31. 

Another interesting example dealing with critical nuclear reactor size was proposed by Glasstone and Sesonske (1967), to whom we refer readers for a more detailed description. As in Rice (1993), with certain assumptions and simplifications, the resulting nonlinear system required to calculate the critical size of a nuclear reactor is... 

Emdosed is a comf ation of pikers criticality safety exporiin its, calculations and analyses CHriginaUy published in the Transactions of the American Nuclear Sodety. This com dlation in UCRL-53369 is me of a series of computer-generated reports rdeased from the Nudear Criticality Informatim System (NCC ) at the Lawrence Livermore Natimal Labcnratory (LLNL). 

The compilation was developed as a component of the Nuclear Criticality Information System (NCIS) now under development at the Lawrence Livermore National Laboratory. 

This compilation is one of a series of computer-generated publications released from the Nuclear Criticality Information System (NQS)... 

Brian Koponen/Viktbr E. Hanipel Nuclear Criticality Information System P. O. Box 8d8, Mail Stop L-303 Lawrence Livermore National Laboratory Livermore, CA 4550, . ... 

B.L. Koponen and V.E. Hampel, A Nuclear Criticality Information System, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-86975 Preprint (1981). 

Experimental critical pressure ratio data as a function of length. (Data from Todreas, N.E. and Kaziml, M.S., Nuclear System. I Thermal Hydraulic Fundamentals, Hemisphere, New York, 1990.)... 

Economy dictates that nuclear power plants have high availability. This trade off between safety, reliability and availability has been achieved by making the control and protective systems in,critical reactor systems,triplicated. A two out of three coincidence causes a reactor trip. This scheme permits any one channel to be taken out of service should it develop a fault, or for testing,and still permit the plant to be operated safely. 

All elements of the Personnel Nuclear Accident Dosimeter program except procedures in place at the present time do not require personnel who occasionally enter areas requiring an installed criticality alarm system to wear a CND. The contractor states that compliance may require the purchase of approximately 10,000 additional dosimeters and the procurement of personnel and equipment to service them. 

The nuclear chain reaction can be modeled mathematically by considering the probable fates of a typical fast neutron released in the system. This neutron may make one or more coUisions, which result in scattering or absorption, either in fuel or nonfuel materials. If the neutron is absorbed in fuel and fission occurs, new neutrons are produced. A neutron may also escape from the core in free flight, a process called leakage. The state of the reactor can be defined by the multiplication factor, k, the net number of neutrons produced in one cycle. If k is exactly 1, the reactor is said to be critical if / < 1, it is subcritical if / > 1, it is supercritical. The neutron population and the reactor power depend on the difference between k and 1, ie, bk = k — K closely related quantity is the reactivity, p = bk jk. i the reactivity is negative, the number of neutrons declines with time if p = 0, the number remains constant if p is positive, there is a growth in population. 

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

Our present discussions relate only to the laboratory testing of safety-related secondary systems, as are employed in critical areas such as areas of emergency power supply and reactor power control supply etc. of a nuclear power plant (NPP) according to IEEE 344 and lEC 60980. There are other codes also but IEEE 344 is referred to more commonly. Basically, all such codes are meant for an NPP but they can be applied to other critical applications or installations that are prone to earthquakes. 

It should be clear that a complete FMEA approach is not practical for the evaluation of production facility safety systems. This is because (1) the cost of failure is not as great as for nuclear power plants or rockets, for which this technology has proven useful (2) production facility design projects cannot support the engineering cost and lead time associated with such analysis (3) regulatory bodies are not staffed to be able to critically analyze the output of an FMEA for errors in subjective judgment and most importantly, (4) there are similarities to the design of all production facilities that have allowed industry to develop a modified FME.A approach that can satisfy all these objections. 

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