Nuclear criticality prevention

Nuclear criticality, preventing, 17 547 Nuclear cycle, water treatment issues in, 23 235-236... 

Experimental criticality data are needed for soluble neutron absorbers (poisons) to accurately determine their effectiveness in the criticality prevention and control of solutions bearing fissile material. These data are needed to validate, criticality control calculational techniques that are used in the analysis of poismwd systems. This paper summarizes the results of a series of experiments performed at the Battelle-Pacific Northwest Critical Mass Laboratory, on water-flooded uranium rod lattices which vere poisoned with soluble neutron absorbers. The poisons utilized, B, Cd, and Gd, were chosen due to their large neutron-absorption cross sections. The exqxerlments were oriented toward providing data on the effectiveness of these materials as soluble absorbers for use in nuclear criticality prevention. Data on integral critical exq >eriments are provided against which calculational techniques can be checked. 

The experiments were oriented toward providing data on the effectiveness of Gd as a soluble absorber for use in nuclear criticality prevention. Data on integral critical experiments are presented against which calculational techniques can be checked. 

Another unique consideration is the prevention of nuclear criticality within the cells. In the dissolver and first cycle, criticality is prevented by the presence of the uranium-238, which absorbs neutrons. Later in the process, the plutonium is separated from the uranium. Criticality is prevented by proper design of the vessels and piping. This includes the cell floor and sumps, where materials would collect in case of leakage from the equipment. To prevent criticality, the vessels are limited in either diameter or thickness. Vessels and piping are placed in arrangements designed to avoid a critical array. 

Select the parameters for preventing accidental nuclear criticality (establishing subcriticality) for all criticality-related operations at each step of the process. 

The achievement of proper operating conditions, prevention of incidents, or mitigation of incident consequences, resulting in protection of workers, the public, and the environment from nndne radiation hazards. This covers nuclear power plants as well as all other nuclear facilities, the transportation of nuclear materials, and the use and storage of nnclear materials for medical, power, industry, and military uses. In addition, there are safety issues involved in products created with radioactive materials. The Office of Nuclear and Facility Safety establishes and maintains the Department of Energy (DOE) reqnirements for nuclear criticality safety. The DOE s detailed requirements for criticality safety are contained in Section 4.3 of the DOE Order 420.1, Facility Safety. Criticality safety requirements are based on the documented safety analysis leqnired by 10 CFR 830, Subpart B. 

The primary objective of nuclear criticality safety is the avoidance of even a single nuclear critic ty accident. The major prize we are working for in preventing such events is the lives of those directly involved. 

Nuclear criticality safety is charged with prevention of criticality in all operations with rissile materials outside of reactors. As with other safety-related activities, the balance of risks, costs, and benefits is an important guideline. 

This paper presents a sununaiy of evaluations made by the Babcock Wilcox Company (B W) in support of its customers. General Public Utilities. (GPU), to assess the potential for reciiticality of the damaged Three Mile Island Unit 2 Reactor (TMI-2) core and to provide a basis for recommendations to prevent die occurrence, of such an event. The TMI-2 incident occurred on March 28, 1979 on March 31, B W formed a Criticality Analysis Task Force composed of penoniiel from the Company s reactor physics unit and their nuclear criticality safety unit. The Task Force at maximum strength utiliaed ten individuals and completed all pertinent evaluations in two weeks advisory activity continued for several months. 

It is obvious from the above that cooperative relationship is necessary for the development of standards and guides to define methods for good nuclear criticality safety practices and the establishment of acceptable methods for their implementation. The NRC participates at all levels of the standards development process. Members of the NRC staff have participated, as individuals and as members of Subcommittee ANS-8, in the development of standards for determining the potential for nuclear criticality of fissionable material outside reactors, for the prevention of accidental criticality, and for coping with accidents should they occur. The NRC is represented on National Standards Committee... 

The American National Standard for Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors is a basic standard that contains general guidance for the prevention of criticality accidents. The guidance is in the form of required and recommended administrative... 

Such nuclear reactions are controllable by keeping the sample size small. Most of the neutrons escape from the sample instead of causing further reactions. The smallest mass of sample that can cause a sustained nuclear reaction, called a chain reaction, is called the critical mass. Another way to control the nuclear reaction is to insert control rods into the nuclear fuel. The rods absorb some of the neutrons and prevent a runaway reaction. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

The proteasome is a large multisubunit proteolytic complex that participates in the degradation of proteins critical for cell cycle regulation. The proteasome inhibitors can overcome drug resistance. This effect is thought to be mediated through prevention of activation of NF-kB (nuclear transcription factor-icB) by proteasome inhibition. NF-kB is considered as one of the key factors involved in apoptotic pathways... 

Some of the subgroups which are of special interest are the Office of Scientific Personnel the Office of Documentation the Nuclear Data Project which publishes Nuclear Data Tables, Nuclear Reaction Graphs, and Nuclear Theory Cards Office of Critical Tables, which publishes the Directory of Continuing Numerical Data Projects and the Prevention of Deterioration Center which publishes Prevention of Deterioration Ahstracts, Environmental Effects on Materials and Equipment Abstracts, and PDC Newsletter. The Cardiovascular Literature Project which recently left NAS-NRC is now operating as the Washington office of the Institute for Advancement of Medical Communications. 

Benedict, M Pigford, T.H. Levi, H.W. Prevention of criticality in processing plants. In Nuclear Chemical Engineering, 2nd Ed. McGraw-Hill New York, 1981 547-455. 

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