Pressure fission product retention

The AHTR appears to have excellent safety attributes. The combined thermal capacity of the graphite core and the molten salt coolant pool offer a large time buffer to reactor transients. The effective transfer of heat to the reactor vessel increases the effectiveness of the RVACS and DRAGS to remove decay heat, and the excellent fission product retention characteristic of molten salt provides an extra barrier to radioactive releases. The low-pressure, chemically nonreactive coolant also greatly reduces the potential for overpressurization of the reactor containment building and provides an important additional barrier for fission product release. The most important design and safety issue with the AHTR may be the performance and reliability of the thermal blanket system, which must maintain the vessel within an acceptable temperature range. 

The ceramic fuel and the multiple coating of the fuel kernels results in a micro pressure vessel capable of maintaining the integrity of the fuel and guaranteeing fission product retention in conditions much more severe than postulated accident conditions. 

Design Basis Accidents Fission product retention Coated fuel particles Pressure essel unit Passive Passive/Active - Inherently safe fission product retention - Automatic closure fail-safe isolation valves - Unfiltered release below permissible limits (normal and disturbed operation) - Further reduetion by filtered venting of reactor building in case of disturbed operation... 

Water Air Ingress Severe Accidents Fission product retention Cooling water systems Same systems and design features as tor Design Basis Accidents Active/Passive Passive/Achve - Automatic isolation - valves - Low-temperature, low-pressure coolers - See design Basis Accidents - No accident with fuel heatmg above permissible limits... 

Fission Product Retention Coated particle Pressure vessel unit Passive Passive/active Inherently safe fission product retention within the fuel element For minimizing purposes Valves fail safe Further mitigation by filtered venting of the reactor building In case of non-isolatable leakages unfiltered release of coolant to environment via stack, no exceeding of limiting values of Art. 28.3 StrSchV. 

The most efficient matrix for retention of actinides and fission products is the uraninite mineral. However, it has been shown that other matricies such as apatite, clay minerals, zirconium silicates, and oxides (Fe, Mn) may also be important in the retention of fission products and actinides. For example, Pu was stored in apatite (Bros et al. 1996) and chlorite (Bros et al. 1993) in the core of the reactor 10. In the core of the reactors, between uraninite grains, 20-200 (j.m-sized metallic aggregates containing fissiogenic Ru, Rh, and Te associated with As, Pb, and S were found. These aggregates also exist in spent fuels of water-pressured type reactor plants, suggesting their analogy with spent fuels. 

The situation for the AHTR, shown in Fig. 4.2, has the potential to be more favorable. The molten salt coolant offers additional fission product containment features and may reduce some licensing barriers. Key features are the low-pressure coolant and gas-trapping system and the retention of the most important radiotoxic fission products in the salt. With a low-pressure coolant, a true containment building is a more practical option than for a high-pressure, gas-cooled system. 

Pressure vessel unit provided with isolation valves (retention of fission product inventory in primary helium at normal operation) and with safety valves (for overpressure protection, release into confinement)... 

For BWRs, the radiologically representative design basis accidents have not been defined as precisely as for PWRs. However, the possible events are phenomenologically quite similar (with the exception of the steam generator tube rupture accident, which does not apply for BWRs) and there are no significant differences to be expected in radionuclide behavior. Consequently, equivalent requirements are applied to the design of both PWR and BWR plants. This means that in most cases the radiochemistry principles discussed in what follows will apply mutatis mutandis to boiling water reactors as well. In the event of a loss-of-coolant accident, the BWR pressure suppression pool represents an effective retention system the behavior of the fission products in this pool will be discussed in Section 7.3.2.4. 

In PWR plants equipped with both a cold-leg and a hot-leg injection of the emergency coolant, a fraction of the fission products released from the failed fuel rods will be washed down by the downward water flow. Thus, it will be transported back to the water phase inside the reactor pressure vessel and, finally, to the containment sump water. Since the extent of this type of retention of fission products depends strongly on the contact time between the steam flow and the downward flow of the liquid emergency coolant, it is only difficult to quantify. It can be assumed that Csl (and other iodides) will be trapped almost completely in the water phase for this reason, a 90% retention of the halogens and alkalis and a 99% retention of the so-called solid fission products has been assumed in the German Storfall-Berechnungsgrundlagen . For the h fraction in the steam flow a similar degree of washout can be expected experiments performed under conditions similar to those in the relevant LOCA period have yielded h washout fractions of about 92% at 25 C and about 96% at 85 °C water temperature (Kabat, 1980). 

Cesium and iodine atoms which are released from fuel specimens into a high-temperature steam-hydrogen environment are thermodynamically unstable and will be rapidly converted into species that are stable under these conditions. Since the chemical form of iodine in particular will considerably influence its transport and retention behavior within the reactor pressure vessel and the primary system, it is important to know the kinetics of these conversion reactions. A kinetics assessment of the most essential reactions (Cronenberg and Osetek, 1988) has shown that for extremely low concentrations of iodine and cesium in steam (e. g. mole ratio I H2O < 10" ), the predominant form of iodine is HI and that of cesium is CsOH. This is due to the fact that because the concentrations of iodine and cesium are so dilute, the elements are much more likely to collide and react with H2O and H2 than with each other. Low concentrations of iodine and cesium increase the time for thermochemical equilibrium to be established for their reaction products. For mixtures which are so dilute in fission products, the reaction times may approach tens of seconds or longer, so that for high effluent rates the environmental conditions may change (e. g. by transport into the next control volume showing other conditions) before thermochemical equilibrium has been achieved. Under such conditions, certain limitations caused by reaction kinetics may exist. 

The retention of fission products by scrubbing the steam flow in a BWR pressure suppression pool is based on the exchange of matter between gas and liquid phases. Similar scrubbing processes are also at work in other accident situations, such as in a PWR steam generator tube rupture event (see Section 6.2.3.), as well as in the Venturi scrubbing process during controlled depressurization of the containment after a core melt accident (see Section 7.3.4.4.). 

The degree of retention of some of the fission products in the debris bed from the upper and lower plenum of the reactor pressure vessel is shown in Table 7.21. 

Already the first measurements performed in the containment about one day after the onset of the accident showed that the major fraction of fission product iodine was plated out in the sump water, while only a very smaU fraction was airborne in the containment atmosphere. Taking the different voliunes of both phases into account, an integral iodine partition coefficient of about 2 1(F was calculated from these data (Pelletier, 1980). The pH of the siunp water was about 8.6 (due to sodium hydroxide solution which was automatically injected into the containment sump to improve iodine retention in the liquid phase) the value of the partition coefficient is consistent with the data obtained in the CSE experiments, when the lower pH of the sump water in these experiments is taken into account. This high value indicates that in the TMI-2 accident the bulk of the fission product iodine was released from the primary circuit to the containment in the form of an iodide compound and not as elemental I2. This assumption is consistent with the observation made later on that only about 1% of the iodine present in the sump water was in the form of iodate it is also consistent with the redox conditions in the reactor pressure vessel which were mentioned above For such an H2 H2O... 

Investigations of the behavior of fission product tellurium in the TMI-2 accident showed that only about 4% of the tellurium inventory of the reactor core was released from the reactor pressure vessel (Vinjamuri et al., 1984), i. e. an amount which is very small compared to the iodine release. About 2.4% of the tellurium inventory was detected in the sump water as dissolved species, 0.88% attached to the solid material suspended in the sump water, 0.42% on the ion exchangers of the purification systems, 0.17% on the surfaces in the containment and 0.086% in the primary coolant. An essential fraction of the tellurium released from the reactor core was retained in the upper plenum of the reactor pressure vessel. This low tellurium release strongly supports the results of laboratory experiments concerning tellurium retention on non-oxidized Zircaloy and steel surfaces. 

Accident management measures can help ensure an early control of the state of the plant and the retention of fuel and fission products in the reactor pressure vessel and in the primary circuit with a high degree of effectiveness even if the events exceed the design basis. In the absence of accident management measures, limitation of fission product release and the prevention of long-term contamination would not be achieved for certain low probability scenarios. 

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