Release of fission products

Release of fission products to the environment Source Term... 

The behaviour of irradiated uranium has been studied mainly with respect to the release of fission products during oxidation at high temperatures The fission products most readily released to the gas phase are krypton, xenon, iodine, tellurium and ruthenium. The release can approach 80-100%. For ruthenium it is dependent upon the environment and only significant in the presence of oxygen to form volatile oxides of ruthenium. 

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... 

To evaluate fission product release in a reactor, it is necessary to supply the appropriate particle geometry, diffusion coefficients, and distribution coefficients. This is a formidable task. To approach this problem, postirradiation fission product release has been studied as a function of temperature. The results of these studies are complex and require considerable interpretation. The SLIDER code without a source term has proved to be of considerable value in this interpretation. Parametric studies have been made of the integrated release of fission products, initially wholly in the fueled region, as a function of the diffusion coefficients and the distribution coefficients. These studies have led to observations of critical features in describing integrated fission product releases. From experimental values associated with these critical features, it is possible to evaluate at least partially diffusion coefficients and distribution coefficients. These experimental values may then be put back into SLIDER with appropriate birth and decay rates to evaluate inreactor particle fission product releases. Figure 11 is a representation of SLIDER simulation of a simplified postirradiation fission product release experiment. Calculations have been made with the following pertinent input data ... 

In considering the operational safety and accident analyses of sodium-cooled fast reactors, similar information on the release of fission products from sodium is needed. Although the extent of vaporization can often be calculated from thermodynamic considerations (3, 4), appropriate transport models are required to describe the rate phenomena. In this chapter the results of an analytical and experimental investigation of cesium transport from sodium into flowing inert gases are presented. The limiting case of maximum release is also considered. 

Table 2.3. Release of fission products in weapons tests and accidents...

It is convenient to consider reactor accidents alongside weapon explosions so that the release of fission products can be compared, but the mode of dispersion is quite different. The configuration and thermal capacity of power reactors are such that bomb-like explosions are not possible. In the Chernobyl accident, nuclear overheating, a steam explosion and steam/zirconium reactions all contributed to the disruption of the reactor (U.S.S.R. State Committee, 1986), but the longdistance environmental effects were due to the subsequent releases of fission products from the damaged reactor. 

Can failures occur from time to time. The release of fission products from them depends on the temperature and type of fuel. If the fuel is uranium metal, as in the Windscale and Magnox reactors, and the can fails, the uranium will oxidise in air or C02. In laboratory experiments, the mass median aerodynamic equivalent diameter (MMAD) of the particles produced by oxidation of uranium increased from about 40 ptm when the temperature of oxidation was 600°C to 500 jum at 1000°C (Megaw et al., 1961). At high temperature, a coherent sintered oxide layer formed on the uranium and this hindered the formation of particles. 

In Advanced Gas Cooled (AGR), Pressurised Water (PWR) and Boiling Water (BWR) reactors, and in the Russian RMBK, the fuel is U02. Experiments in the UK and USA, reviewed by Farmer Beattie (1976), showed less than 1% release of fission product iodine and caesium from punctured U02 fuel cans at about 1000°C in air or steam, rising to 10-50% release at 1800°C. At 2800°C, the U02 melted and there was nearly complete release of volatile nuclides (I, Te, Cs, Ru) but only small release of refractory alkaline earth and rare earth nuclides. 

Various estimates of the release of fission products from Windscale... 

The estimates of release of fission products from Windscale, which are inevitably subject to error, are not essential to the assessment of the radiological effects, which are based on local measurements of activity in milk and other foodstuffs, and in the thyroids of members of the public. 

The nuclear fuel pellets are generally filled in thin-walled cladding tubes to hinder leaching by coolant in the reactor core and to prevent release of fission products into the coolant circuit. In light-water reactors, for example, zirconium alloy (zirkaloy) cladding is used. 

Gamer RJ, Morley F. 1967. Agricultural implications of a release of fission products from a criticality incident. Health Phys 13 465-475. 

Assumes fission-product tritium diffusing through fuel cladding oi escaping through pin-hole cladding failtires is equivalent to release of fission-product tritium from 0.5% of the fuel. Calculated as average over irradiation cycle. 

PCI may lead to cladding failure and subsequent release of fission products into the reactor coolant. 

B. The Release of Fission Products from Damaged Fuel... 

In any reactor, as must be obvious, fuel is prevented from rising to damaging temperatures, which could cause release of fission products in many cases, by maintaining a sufficient balance between the rate of nuclear heat generation and the rate of heat removal by the coolant system. Clearly,... 

Accidents 4 and 5 both involve release of fission products from fuel failures induced by off-design transients. It is of interest that the applicant... 

The release of fission products under accident conditions will clearly... 

Large-scale experimental releases of fission product activity are clearly ruled out because of the implications on the safety of the public, described in Section V,F, as are also the smaller scale releases referred to earlier in Section V. Therefore, for verification of our conclusions we have to rely on limited experience from those few accidents that have occurred and that have released fission products (see Section I,D), but much more on our store of knowledge of all the factors involved, i.e., types of reactor accidents (Section II) through fission product release (Section III) and dispersion of a release in the atmosphere (Section IV), to analysis of the radiation and radiobiological hazards and risks to exposed members of the population. In view of these several steps involved in the estimation of hazard, it is reassuring that the many different authors who have written on the topie reach conclusions which are generally similar and differ only in limited areas. 

There are multiple barriers to the release of fission products from an HTGR core the fuel kernel, the particle coatings, the fuel rod matrix, and the fuel element graphite. The effectiveness of the individual barriers to fission product release may depend upon a number of factors Including the chemistry and half-llfes of the various fission products, temperature, and Irradiation effects. These barriers are described briefly below. 

A good overview on the retention capability of modem fuel pebbles with TRISO coated particles has been prepared for the HTR-MODULE describing the mechanisms of release of fission products, lit SIEMENS-INTERATOM-1988 (HTR-MODUL Safety Report), Bd 1... 

A possible contamination of the blades and other structures of the gas turbine due to the long term release of fission products, e.g. Cs-137, may in future not be an important issue as before, e.g. in the HHT-project, because the technologies for remote handling and inspection as well as other maintenance conditions have improved. 

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... 

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