Source severe reactor accidents

These experimental studies illustrate the important consequences of air ingression into the reactor vessel when hot fuel residues are present. Ruthenium releases as well as the releases of some other radionuclides are greatly accentuated. The proceedings of the seminar provide also a good digest of much of the world-wide research on severe reactor accident source terms at the time. 

FIGURE 9.7 U.S. NRC protective actions for severe reactor accidents. (Source NRQ 1996)... 

Bowsher, B. R., Jenkins, R. A., Nichols, A. L., Simpson, J. A. H. Silver-indium-cadmium control rod behaviour during a severe reactor accident. Report AEEW-R 1991 (1986) Brockmann, J. E., Tarbell, W. W. Aerosol source term in high pressure melt ejection. Nucl. 

The code Iode has been developed to calculate iodine behavior in a reactor containment and the auxiliary building it is part of the French computer system Escadre which, similar to the US Source Term Code Package, describes the whole sequence of a severe reactor accident. The general philosophy of Iode is to model the main phenomena which may influence the behavior of iodine in the reactor containment IS chemical reactions are modelled, concerning both the water phase and the gas phase (Gauvain et al., 1991). Similar to Impair, radiolysis is not described in detail but is taken into account over its global effect on iodine species. The kinetic data of the reactions were taken from the literature as far as inorganic iodine thermal reactions are concerned other kinetic data were compiled from the elaboration of the Impair 2 code. 

A concern exists that, once a release from a severe reactor accident starts, an evacuation should not be recommended because the evacuees may run into or be overtaken by the plume. However, as mentioned in Section 5.2.3, plume concentrations decrease exponentially with distance from the source. As a result, large reductions in doses to individuals are achieved by evacuation. Conversely, sheltering in most homes can reduce a person s dose by no more than a factor of 2. Also, evacuation precludes the possibility of long term exposure to hot spots. 

R. M. Suminers et a/., MELCOR 1.8.0 A Computer Code for Severe Nudear Reactor Accident Source Term and Risk Assessment Analyses , NUREG/CR-5531, SAND90-0364, Sandia National Laboratories, January 1991. 

Nuclear fusion does not require uranium fuel and does not produce radioactive waste, and has no risk of explosive radiation-releasing accidents, but it takes place at a temperature of several million degrees. Nuclear fusion occurs in the sun, its fuel is hydrogen and, as such, it is an inexhaustible and a clean energy source. The problem with this technology is that, because it operates at several million degrees of temperature, its development is extremely expensive, and it will take at least until 2050 before the first fusion power plant can be built (Tokomak fusion test reactors). It is estimated that it will be 50 times more expensive than a regular power plant, and its safety is unpredictable. In short, the only safe and inexpensive fusion reactor is the Sun ... 

AR193 Radionuclide source terms from severe accidents to nuclear power plants with light water reactors, No. 2, 18 March 1987. 

TA-V installations that could potentially affect or be affected by the HCF include the Annular Core Research Reactor (ACRR), Gamma Irradiation Facility (GIF), Auxiliary Hot Cell Facility (AHCF), Radiation Metrology Laboratory (RML), and the Sandia Pulse Reactor III (SPR III). The GIF provides two cobalt cells for total dose irradiation environments. A new GIF is under construction in the northeast quadrant of TA-V. SPR III provides intense neutron bursts for effects testing of materials and electronics. The RML provides radiation measurement services to Sandia s reactors, isotopic sources, and accelerator facilities. The AHCF provides a capability to handle limited quantities of radioactive material in a shielded cell. These facilities have separate SARs that describe potential accidents. The most severe accidents for all of these facilities involve the release of radiological materials which could necessitate a site evacuation. No physical damage to the HCF could be induced by any of the postulated accidents, nor could any of the HCF accidents physically affect any of the other facilities. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a prestressed concrete containment, the Konvoi pressurized water reactor with a steel primary containment, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. 

Early reactor safety assessments [S-1] hypothesised that severe accidents would entail the prompt release of a significant fraction of a bounding radionuclide (t5q)ically iodine) to the reactor containment. Safety systems were designed, then, for massive, immediate response to this release. Now, it is understood that radionuclide releases will take place by multiple processes over protracted periods and will involve many different radionuclides in different chemical and physical forms. Mitigation methods will have to operate for long periods and may have to change as the sources of radionuclides vary. The inventories of radionuclides available for release from reactor fuel under accident conditions and the processes that lead to releases of these radionuclides are discussed in the next subsections of this report. 

There are, then, several potential sources of radioactive material to the containment atmosphere. The most important of these are thought to be gap release, in-vessel release, ex-vessel release, and late invessel release. A recent assessment of the magnitudes of these releases in terms of fractions of the initial core inventories of important radionuclides has been prepared [S-2]. Results of this assessment are shown in Tables II-2 and II-3. It is important to view the results shown in these tables as somewhat conservative examples of both the timing and the magnitude of radionuclide release. These examples are not prescriptions of accident sources to any particular reactor contaimnent. 

This dociunent describes a representative radionuclide source term to the containments of reactors for use in the traditional design basis accident analysis. The source term is based on the more mechanistic studies of radionuclide behaviour conducted since the accident at Three Mile Island. Although the source is representative of the magnitude and timing of severe accident source terms, it is not intended to represent any specific severe accident sequence nor is it intended to be a boimding source term. 

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. 

This report discusses analyses of the source term consequences of air ingression into the reactor coolant systems during severe accidents. 

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