Products from overheated fuel

The release behavior of the less volatile fission products from overheated fuels was studied in detail in the Sascha experiments (Albrecht, 1987 a). These results demonstrated that the low-volatility elements (e. g. barium, ruthenium, cerium) were essentially released at the highest temperatures realized in the tests, i. e. from the liquid melt. In Fig. 7.11. some of the measured release rates for low-volatility elements are shown and compared with the assumptions made in the NUREG-0772 report. With the exception of barium (and potentially strontium), the release rates of these elements apparently depend only little on the amount of steam supplied. Barium release rates in a pure steam atmosphere have proved to be lower by a factor of nearly 100 than in an Ar + 5% steam atmosphere or in an Ar -H 5% H2 atmosphere. This behavior was assumed to be due to a reduction of the BaO initially present by still non-oxidized Zircaloy, which converted it into the more volatile elemental form. 

Laboratory investigations like those described exemplarily above (as well as numerous others conducted in many laboratories) have provided valuable information on the basic mechanisms of the release of fission products from overheated nuclear fuel. Insight has been gained into the vaporization behavior of volatile and less volatile fission products, into the chemical forms of the released species, into the impact of environmental conditions (e. g. redox potential), etc. However, these experiments were often carried out under experimental conditions which are not... 

Similar experimental conditions to those in the Sascha experiments, that is a flowing steam-hydrogen atmosphere at atmospheric pressure, were used in the ORNL high temperature tests, the principal aim of which was to determine the impact of specific accident conditions on the behavior of fuel and fission products. In the early ORNL high-temperature experiments (HT series), fuel rod segments fabricated of power-reactor irradiated fuel and encapsulated in Zircaloy cladding were heated for a short time (a few minutes) to temperatures up to 1900 K in atmospheres of various composition. The results obtained in these tests were the main basis for the assessment of fission product release from overheated fuels made in the NUREG-0772 report (US NRC, 1981). 

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]. 

Parker, G. W. etal., Out-of-Pile Studies of Fission Product Release from Overheated Reactor Fuels at ORNL, 1955-1965, ORNL-3 981 (1967)... 

This figure illustrates immediately one of the disadvantages of batch fuel management. The power density, which is proportional to the product of the neutron flux and the fissile material concentration, is just as nonuniform as the neutron flux. If the local power density must be kept below some safe upper limit, to keep from overheating the fuel or cladding, only the fuel at the center of the reactor can be allowed to reach this power density, and fuel at all other points wiU be operating at much lower output. In a typical uniformly fueled and poisoned water-moderated reactor, the ratio of peak to average power density is over 3, so that the reactor puts out only one-third as much heat as it could if the power density were uniform. 

When overheated, hydrocarbons tend to breakdown, leaving carbon residues (coke). This coke builds up on the inside of the heater tubes, slowing the transfer of heat from the tube walls to the product by restricting the flow of product and acting as an insulator. As the control system attempts to maintain the process outlet temperature at the setpoint, the fuel valves will open and the tubes subjected to an increased heat load. With the diminished ability of this heat to be transferred to the process fluid, the temperature of the tubing will increase. 

In order to retain the fission products within the first barrier, it is important to prevent overheating of the reactor fuel. As a first, passive means of ensuring a sufficiently high heat removal from the reactor core in case the main coolant pumps... 

Breaks in the process tubes could cause fuel overheating. However, fuel melting resulting from such a break is highly unlikely. If fuel melting should occur fission products would largely be confined within the graphite stack. 

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