Nuclear criticality safety assessment

The three steps of the criticality safety evaluation—contingency analysis, limit determination, and control specification—are presented in a document generally referred to as a Nuclear Criticality Safety Evaluation (NCSE) (although some sites separate out the first step into a separate document referred to as a Nuclear Criticality Safety Assessment (NCSA)). Within a given organization or processing site, the structure and format of NCSA/NCSEs are usually strictly proscribed for consistency of development and ease of use. 

VII. 14. The SAR should provide sufficient information or references to demonstrate that the computer code, nuclear cross-section data and technique used to complete the criticality safety assessment are adequate. The computer codes used in the safety assessment should be identified and described in the SAR, or adequate references should be included. Verification that the software is performing as expected is important. The SAR should identify or reference all hardware and software (titles, versions, etc.) used in the calculations as weU as pertinent version control information. Correct installation and operation of the computer code and associated data (e.g. cross-sections) should be demonstrated by performing and reporting the results of the sample problems or general validation problems provided with the software package. CapabiUties and limitations of the software that are pertinent to the calculational models should be discussed, with particular attention to discussing limitations that may affect the calculations. 

VII.38] BOWDEN, R.L., THORNE, P.R., STRATFORD, P.I., The methodology adopted by British Nuclear Fuels pic in claiming credit for reactor fuel bumup in criticality safety assessments , ibid., pp. lb.3-10. 

Critical Experiments and Analysis. The nuclear safety parameters of hydrogenous systems containing mixtures of uranium and plutonium will be based largely on calculations that are keyed to relatively few critical experiments. We must continue to emphasize the devel-ment of reliable computational techniques and evaluated differential cross-section data, and to select critical experiments that will provide for maximal benefits. Credence in computed data must be established by thorough reviews of possible errors and independent assessments by responsible authorities in nuclear criticality safety. 

Criticality Safety Assessment work necessitates nuclear, chemical, and engineering studies. In all three fields it is often necessary to use calculation techniques to derive parameters needed in the assessment... 

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. 

Applicability of the model to the experiment and feasibility of three-phase calculations in such rather complex configurations has therefore been shown, both qualitatively and quantitatively. Numerical modelling of hydro-mechanical three phase coupling can, as in the presented example, yield useful and probably critical support for the design and the safety assessment in the context of e.g. nuclear waste disposal. 

Criticality. The process plant is designed on the principle of safe geometry under all conditions (e.g., the use of HARP tanks). Continuous monitoring is provided by an approved criticality detection and alarm system. Where additional operational control is required to maintain safe conditions, this is specified in the appropriate nuclear safety assessments and Criticality Clearance Certificate. The latter specifies limits and conditions that need to be complied with during operations for example, in respect of limited tap density, moisture content, isotopic inventory and mass. It identifies any systems or instrumentation that demonstrate that compliance is maintained during operation. In addition key points of the plant are monitored by neutron monitors to give early warning of the unanticipated build-up of solid plutonium which could lead to the development of unsafe conditions. 

Calculational methods used to determine the neutron multiplication should be validated, preferably against applicable measured data (see Appendix VII). For irradiated nuclear fuel this vahdation should include comparison with measured radionuclide data. The results of this validation should be included in determining the uncertainties and biases normally associated with the calculated neutron multiplication. Fission product cross-sections can be important in criticality safety analyses for irradiated nuclear fuel. Fission product cross-section measurements and evaluations over broad energy ranges have not been emphasized to the extent that actinide cross-sections have. Therefore, the adequacy of fission product cross-sections used in the assessment should be considered and justified by the safety analyst. 

The criticality safety specialist in Industry must be competent to establish practical limitations on the designs of equipment and the actions of men and machines that process fissionable materials so the probability of accidental criticality is infinitely small. Presumably, graduation from an accredited college with a degree in physics or nuclear engineering evidences a certain capacity for nuclear work. Does that education provide, the skill and knowledge required by the. criticdlity safety specialist If not, what is the process by which he becomes competent By what criteria may one assess competence in this hi ly specialized field., of nuclear endeavor ... 

Criticality safety analysis is a discipline aimed at the prevention or termination of inadvertent nuclear chain reactions in nonreactor environments. Although the physics and mathematics are the same as for reactor analysis, the basic approach is different because the goal is different. Because the analysis goal of criticality safety exclusively is to establish a conservative assessment of the criticality state of a given proposed operation, and the systems being analyzed have so much variety, the methodology takes a completely different form than reactor analysis methodology. 

This approach is expensive. It is attractive for particularly complex products which cannot be tested in real service conditions, and for those where the consequences of malfunction justify the costs of testing. It applies particularly to safety-critical products in the nuclear industry which could be subjected to abnormally harsh conditions, either for the environmental exposure or the end assessment or both. 

DOE Order 5480.19, Conduct of Operations (COO) (DOE 1990), requires that the line organization responsible for operations at nuclear facilities have an Organization and Administration document that clearly describes job responsibilities, resources, and support infrastructure. The COO order also requires written procedures for critical operations, configuration control for the facility safety equipment, and formal processes for inspection of equipment, operations, and procedure improvement. The order further specifies that records of operations and maintenance be made and maintained and that managers assess operations and identify problems and correct them in a timely manner. 

In the performance-based design and operation of modem engineered systems, the accurate assessment of reliability is of paramount importance, particularly for civd, nuclear, aerospace and chemical systems and plants which are safety-critical and must be designed and operated within a risk-informed approach (Pata-lano et ah, 2008). 

Accurate prediction of criticality is essential not only for the quality design of plutonium recycle cores tmt also for the safety of handling fdutonium assemblies. To assess the ffllL capability in this regard, multigroup cross sections based on the latest nuclear data file, ENDF/B-IV, were used in conjunction with the lattice analysis code, HAMMER, and the two-dimensional (2-D) mnltigroup discrete ordinate code, TW0TRAN, to analyze a series of mixed-oxide-fneled critical experiments. These critical experiments which covered a wide range of water-to-fiiel ratios and Pu compositions represent a set of excellent benchmarks for validation of both the analytical methods and nuclear data for criticality prediction of mixed-oxide fuel assemblies typical of pin tonium recycle cores in LWRs. 

The safety and the environmental effects from a repository for spent nuclear fuel should be evaluated for very long time periods, typically hundreds of thousands of years. For time periods until the next glaciation, or similar expected drastic changes of the biosphere, the individual dose received by members of a hypothetical critical group could be calculated and assessed with some confidence. For longer time periods similar predicted doserates are used as "safety indicators" together with... 

A-1. The inclusion of pre-existing software (PSW) components into the apphcation software may not only be beneficial for productivity but may also increase the safety of a software system if introduced in a proper way. The benefit stems from the fact that PSW components often have been used in maty applications, and their operating experience, when assessable and representative, can be taken into account. Reusable software components may have been developed to suitably high standards in other industries for use in safety critical apphcations and, therefore, may be reusable in the nuclear indusby. Licensees may wish to make use of such software given that the appropriate assessment has been undertakea... 

Health and Safety Executive, Advisory Committee on the Safety of Nuclear Installations (ACSNI), ACSNI Study Group on Human Factors, 2nd Report - Human Assessment A Critical Overview, HSE Books, Sudbury (1991)... 

Carvalho,P.V.R.,Santos,I.L., Gomes, J.O. BorgesM.R.S. 2008. Micro incident analysis framework to assess safety and resilience in the operation of safe critical systems a case study in a nuclear power plant. Journal of Loss Prevention in the Process Industries 21 277-286. 

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