Core fission product volatilization

In particular accident sequences with high-pressure, low-flow conditions the buildup of a significant vapor pressure of volatilized fission products in the reactor pressure vessel over the surface of the overheated core may limit the release rates (e. g. Taig, 1984). Since the release rates used in the codes have mostly been determined in experiments in which vapor pressures did not limit the release, this effect has to be taken into account in order not to overestimate the fission product volatilization rates to be expected in such accidents. 

Typically, the two dominant sources of fission product release from the core are as-manufactured, heavy metal contamination (l.e., heavy metal outside the coated particles) and particles whose coatings fail in service. In addition, the volatile metals (Cs, Ag, Sr) can, at sufficiently high temperatures and long times, diffuse through the SIC coating and be released from Intact TRISO particles. 

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. 

The reactor had operated since 1983, and thus huge amounts of fission products had been stocked in the core and released corresponding to their chemical characteristics (O Table 55.32). Gaseous and volatile elements such as noble gases, iodine, and cesium were released in large amounts. In contrast, only 2-3% of nonvolatile elements such as cerium and zirconium were released. Fission products in the core of the reactor were released for about 10 days following the destruction of the core before the sharp drop of the releases at May 6 O Fig. 55.3). 

The appearance of non-volatile fission products or actinide isotopes in the coolant can indicate the presence of fuel rod defects with a direct contact between the fuel and liquid water. This can occur with large-sized defects, in particular in comparatively cold regions of the fuel rod at the vertical or horizontal periphery of the reactor core. However, any statement in this regard can only be based on radionuclides that are not present in the coolant as a remnant from preceding transients this means that in a PWR Cs or Cs are not appropriate indicators for such fuel rod failures. The requirements are in principle fulfilled by Np, which is a reliable indicator for defects with fuel-to-water contact, as are ruthenium and cerium isotopes, as well. However, because of the complex behavior of these radionuclides in the coolant (adsorption on suspended corrosion products and deposition on primary circuit surfaces), only qualitative assessments can be made, which means that a quantitative evaluation of the number of fuel rods showing... 

Containment spray systems are used in some types of PWR plants, resulting not only in a condensation of steam but also in a washout of airborne radioiodine and other fission products from the containment atmosphere. Normtilly, an alkaline borate spray solution is used, resulting in a shift of the I2 disproportionation equilibrium to the iodate side and a suppression of revolatilization of iodine previously trapped by the spray droplets. In some cases, solutions containing sodium thiosulphate have been used to decompose volatile organic iodides present in the containment atmosphere, but because of its corrosivity this reagent has been largely abandoned. By the addition of boric acid to the spray solution, subcriticality of the reactor core is guaranteed when, after the injection phase, the sump water is recirculated for removal of decay heat from the core. 

This process of melting down of the reactor core inside the reactor pressure vessel is associated with an extensive release of the volatile fission products and a partial volatilization of the other fission products, uranium and the actinides (invessel release). The volatilized fuel constituents are transported by the steam-hydrogen flow out of the core region, together with volatilized fractions of the core structural materials (stainless steels, Ni alloys). 

The quantity, time-dependence and composition of the volatilized substances may significantly influence the further behavior of the radionuclides as well as the progress and the products of chemical reactions occurring in the primary system and in the reactor containment. On the other hand, the volatilization of the fission products from the reactor core is able to affect the further progress of core heatup and degradation, since in the first hour after reactor shutdown the gaseous and volatile fission products contribute approximately 30% of the total decay power of the core. 

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. 

---