Fission products, containment/release

Relocation of the decay heat source as fission products are released from the fuel and transported to the containment. 

Xe-133 was the main radionuclide released during nuclear incidents such as the Chernobyl and Three Mile Island nuclear accidents. In these accidents, the Xe that was built up as a fission product was released. Because of the unreactive nature of the noble gases, the Xe was hard to contain and hence was released into the environment. 

Figure 3-6 shows the damaged areas of the core as now known from the available information (OECD, 1994). It can be calculated that about 50 per cent of the zirconium present in the TMI-2 core reacted with water to produce hydrogen and that practically all the volatile fission products were released by the core into the primary circuit and hence, through the stuck open relief valve, into the containment building. Forty-five per cent (62 t) of the fuel melted and about 20 t migrated from their original position and collected on the vessel bottom head. 

Fission product containment 1 Fuel clad/sheath 2 Heat transport system pressure boundary 3 Containment envelope P Multiple bamers to release with m-servace inspection mcludmg activi release from fuel, pressure tube leak-bef(xe-break and on-lme detection of leak rate from contamm t... 

From these experiments it can be concluded that fission product iodine released from the reactor core during a loss-of-coolant accident is rapidly converted to Csl (or a similar iodide compound) and only a fraction of significantly less than 10% of the iodine released from the fuel rod segments is transported to the containment as elemental I2 (or HI). This conclusion is supported by the results of the laboratory heating experiments of Groos and Forthmann (1984 a 1984 b) as well as by that of the Isolde tests (Kiihnlein et al., 1993), which showed that more than 90% of the fission product iodine released from the fuel rod segments appeared as nonvolatile iodide. However, in the ISOLDE tests it was also observed that a considerable fraction of the released iodine was deposited onto the surfaces of the stainless steel experimental capsule. 

The PC control EOF is designed to provide a barrier to the uncontrolled release of fission products, contain and condense steam discharged through the safety relief valves and primary cooling system breaks, shield personnel from radiation emitted by the reactor, and provide a protected environment for key equipment important to safety. Entry into this procedure is required at a suppression pool temperature above the limiting condition for operation (LCO), a drywell temperature above LCO, a containment temperature above LCO, a drywell pressure above the high pressure scram set point, a suppression pool water level above maximum level LCO, a suppression pool water level below minimum level LCO, and an SC hydrogen concentration above the alarm set point. 

BP Containment bypass Fission products are released directly from the RCS to the environment via the secondary system or other interfacing system bypass. Containment failure occurs prior to onset of core damage Large release 1... 

Fission products are released through a containment feilure caused by severe accident phenomenon occurring after the onset of core damage but prior to core relocation. 

The prototype containment Is unique In that, because of the flume gates. It Is sensitive to the actual time at which fission products are released from the fuel. Those accidents which Involve the flume gates have been assumed to be associated with a premature release of fission products (l.e., release earlier than would normally have been predicted) to take account of uncertainties in fuel can behaviour due to Irradiation effects or possible gaps between fuel pellets. 

In order to indicate the range of the hazard to the public, the researchers considered three reactor-acddent cases. In the first one, all the fission products were released from the reactor core, but none escaped from the containment building. The second case assumed all of the noble gases (xenon, krypton, and bromide) and iodines plus 1 percent of the strontium were released to the atmosphere. The third case, the major one or worst possible accident considered, assumed that 50 percent of all fission products were discharged to the atmosphere. ... 

It has to protect the control room in the case of a maximum credible accident with fission products being released into the containment vessel so that limited operations should be possible in the control room during the first seven days after the accident. 

Off-Gas Treatment. Before the advent of the shear, the gases released from the spent fuel were mixed with the entire dissolver off-gas flow. Newer shear designs contain the fission gases and provide the opportunity for more efficient treatment. The gaseous fission products krypton and xenon are chemically inert and are released into the off-gas system as soon as the fuel cladding is breached. Efficient recovery of these isotopes requires capture at the point of release, before dilution with large quantities of air. Two processes have been developed, a cryogenic distillation and a Freon absorption. 

Spent Fuel Treatment. Spent fuel assembhes from nuclear power reactors are highly radioactive because they contain fission products. Relatively few options are available for the treatment of spent fuel. The tubes and the fuel matrix provide considerable containment against attack and release of nucHdes. To minimi2e the volume of spent fuel that must be shipped or disposed of, consoHdation of rods in assembhes into compact bundles of fuel rods has been successfully tested. Alternatively, intact assembhes can be encased in metal containers. 

In 1962 the report, TID-14844 was published presenting analysis and assumptions coneeming the behavior of containment (essentially Hazard State 2). The TID report postulated the release of all of the noble gas, 50% of the iodine, and 1% of the radioactive solids to the containment. In addition, TBD-14844 provided assumptions for containment leakage (the TMI-2 containment is intact) and for atmo.spheric transport of the fission products. These results form the basis for Regulatory... 

An accident sequence source term requires calculating temperatures, pressures, and fluid flow rates in the reactor coolant system and the containment to determine the chemical environment to which fission products are exposed to determine the rates of fission product release and deposition and to assess the performance of the containment. All of these features are addressed in the... 

Of these phenomena, the first three in particular, involve thermal hydraulics beginning with the pre-accident conditions. Items 4 through 7 address the meltdown of the core and its influence on (1) hydrogen production, which affects containment loads, (2) fuel temperatures, which affect in-vessel fission product releases, (3) thermal-... 

Computer sensitivity studies show that hole size strongly affects the fraction of fission products released from the containment. The failure location determines mitigation due to release into another building in which condensation and particulate removal occur. The quantity released depends on the time of containment fails relative to reactor vessel failure. If containment integrity is maintained for several hours after core melt, then natural and engineered mechanisms (e.g., deposition, condensation, and filtration) can significantly reduce the quantity and radioactivity of the aerosols released to the atmosphere. 

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