Fission product release mechanism, design

The reference fuel design, quality, performance models, and methods discussed in Section 4.2.5.2.2.3 were used to calculate the fuel particle failure and the gaseous and metallic fission product releases as a function of time. The key attributes for fuel quality are summarized in Table 4.2-4 and the design is in Table 4.2-16. The following fuel particle failure mechanisms were considered in the analysis ... 

III-l. The main source of radiation in a nuclear power plant under accident conditions for which precautionary design measures are adopted consists of radioactive fission products. These are released either from the fuel elements or from the various systems and equipment in which they are normally retained. Examples of accidents in which there may be a release of fission products from the fuel elements are loss of coolant accidents and reactivity accidents in which the fuel cladding may fail due to overpressurization or overheating of the cladding material. Another example of an accident in which fission products may be released from the fuel rods is a accident in handling spent fuel, which may result in a mechanical failure of the fuel cladding from the impact of a fuel element that is dropped. The most volatile radionuclides usually dominate the accident source term (the release to or from the reactor containment). Recommendations and guidance on the assessment of accidents are presented in Section 4 of Ref. [III-l]. 

Over the more than 40 years since the first nuclear fission reactor was constructed numerous designs of reactor have been evolved by variation of the basic parameters such as fuel type, moderator, and coolant. One possible classification is by intended use, e.g., research, plutonium production, electricity generation, or propulsion units for submarines or surface ships. In this chapter we will concentrate on power reactors, both on account of their practical importance and because of the complexity in engineering design introduced by the need to convert the energy released by nuclear fission into a mechanical or electrical output. Many of the characteristics of the various reactor types have been touched on in earlier chapters, but the objective in the present chapter is to provide a systematic summary of the main classifications of reactor prior to the more detailed descriptions to be given in the following chapters. 

The primary gas envelope can also be considered a barrier against radionuclide release. However, for the short-lived fission gases, the dominant removal mechanism is radioactive decay. For the condensable fission products, the dominant removal mechanism is deposition or plate-out on the various helium wetted surfaces in the primary circuit. The primary pressure boundary, consisting of conventional steel pressure vessels, is designed to ASME Section HI. Through-wall cracks are considered unlikely. The chemically inert helium coolant minimizes corrosion and eliminates the complications associated with internal cladding, and only materials for which extensive data exist are to be used in the construction of the vessels. 

This subsection should provide relevant information on the systems for the removal and control of fission products. In addition, the following specific information should be presented to demonstrate the performance capability of these systems considerations of the coolant pH and chemical conditioning in aU necessary conditions of system operation effects on filters of postulated design basis loads due to fission products and the effects on filter operability of design basis release mechanisms for fission products. 

The report had to include a description of the reactor and the site, a detailed plan of operation, a schedule of chemical processing and disposal of reactor fission products, the methods of disposal of radioactive effluents, and a description of the safety mechanisms of the reactor. More specific information was required on potential hazards, incorporating the data used in the safety-evaluation procedure described by Teller. The designer had to list all the known potentially hazardous features and include the experimental information, calculations, and assumptions used in evaluating those hazards. The report required information on steps taken to minimize the risks and an estimate, if a failure should occur, on the extent of any release of radioactive material and the damage to be expected. ... 

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