Fission products from the fuel

In the following description of the reactions occurring during this stage of a severe core damage accident, three different topics will be discussed the release of fission products from the fuel, the release of constituents of the core structural and control rod materials (although these two sources develop almost simultaneously in the reactor pressure vessel so that the volatilized substances can be assumed to enter the gas flow as a mixture) and, finally, volatilization of substances during the molten core - concrete interaction phase. The current state of the art will be discussed with special emphasis on the important chemical phenomena no attempts will be made to establish numerical values of source terms from the results of these experimental and theoretical efforts. 

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An important parameter in the evaluation of the safety of a reactor system is the release of fission products from the fuel. The fuel in the high-temperature gas-cooled reactor (HTGR) consists of spherical particles (U, ThC2) that are coated with a material presenting a diffusion... 

With systems where a rapid increase in heat output can lead to a surge in coolant pressure, the whole of the reactor system can he surrounded hy a sealed containment system. There can therefore he several barriers to prevent or restrict the escape of radioactive fission products from the fuel to the surroundings ... 

The reactor is designed against these accidents The analysis of these accidents is done with conservatism The analysis results show excellent safe response of the reactor to accidental events Within the fi-amework of DBA no accident would lead to relevant release of fission products from the fuel elements... 

The MSRE secondary coolant was kept extremely clean during the 4-year operation of the reactor, and no in-line cleaning was required. It is likely that the AHTR will not require a complex clean-up system for the coolant, but that more extensive cleanup may be needed on a period determined by the release of fission products from the fuel. 

Release of fission products from the fuel to the environment is prevented by the fuel matrix, the fuel cladding, the reactor pool and the reactor building (confinement). Cracking of the concrete of a confinement building, due to environmental conditions over a long period of time, may reduce its ability to confine a radioactive release. 

There are six barriers in the transmission path of radioactive fission products from the fuel to the environment. 

Due to its universal applicability and excellent safety features, the modular HTR power plant is suitable for erection on any site, but particularly on sites near other industrial plants or in densely populated areas. TTie principal safety feature of the HTR-Module is based on the fact that, even in the case of failure of all active cooling systems and complete loss of coolant, the fuel element temperatures remain within limits at which there is virtually no release of radioactive fission products from the fuel elements, due to ... 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

The minimum design objective for the ECCS is to limit the release of fission products from the fuel. While specific acceptance criteria may differ from country to country, typical requirements in this regard are listed below. 

The main advantage of a two fluid system is that the processing-out of fission products from the fuel salt is simplified by the absence of fhorium. The prime method is known as vacuum distillation (ORNL 3791, 1966) and was developed in 1964. After removal of all UF4, the carrier salt would be evaporated off af low pressure and high temperature (1000°C) and recycled, leaving most fission products behind in the still bottoms. 

As can be seen from the schematic diagram in Fig. 1.11., four independent, passive barriers prevent the release of fission products from the fuel to the environment in the case of accident ... 

As was discussed above, the release of fission products from the fuel rods which failed during the burst, refill and reflood phases of the loss-of-coolant design basis accident is limited to their gap inventories due to the comparatively low maximum... 

It should be noted that the melted uranium will be resolidified as a consequence of heat removal by the graphite cooling system vlthln a short time, perhaps as short as five minutes. This will terminate melting, tend to stabilize the system, and minimize release of fission products from the fuel. 

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]. 

The probability of releasing significant quantities of fission products from the fuel elements is believed to be extremely low. However, it is not possible to reduce this probability to zero. Thus, it is prudent to provide for limiting the potential releaee of fission products to the environs in the event they escape from the fuel elements in significant quantities. 

Core cooling shall be provided in the event of a loss of coolant accident so as to minimize fuel damage and limit the escape of fission products from the fuel. The coohng provided shall ensure that ... 

If such power excursions are part of the design for normal operation, the equipment and installations for both monitoring and protection should be able to withstand high power levels. Power excursions give rise not only to intense direct radiation fields, but also to fuel degradation and the release of fission products from the fuel if it is not designed to cope with such excursions. 

In addition to destroying the UO2 matrix, fuel liquefaction accelerates the release of fission products from the fuel. However, minor alloying components or impurities can have large effects on such releases. For instance, tin, which is a 1% component of zircaloy, may act as a getter for tellurium resulting in significant holdup or retention of this fission product. Both fuel liquefaction and retention of tellurium in the presence of tin illustrate that chemical reactions are crucial to the understanding of severe accidents. 

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