Fission product release from failed fuel

The importance of corrosion product mass transfer was realized first in the early operation of NRU. Here the solubility of the oxide formed on the aluminum fuel sheathing led to the production of a colloidal alumina floe in the heavy water. The mechanism for its formation, means to control it, and the role it played in transporting uranium and fission products released from failed fuel were studied (55, 56). 

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). 

The WAGR hotbox is an interesting item with regard to its radioactive inventory. It sits above the top core reflector and neutron shield, and, as such, has received little neutron activation. It is, however, the first pennanent reactor component in which the coolant gas came into contact with after passing over the operating fuel pins. It is hence the first site for deposition of fission products which were from time to time released from failed fuel pins. 

Safety is clearly a major consideration and research reactors are designed to fail-safe to prevent fission product release. Reactors operate under a triple containment philosophy. The first container is the cladding of the fuel itself, the second is the swimming pool which is made from heavy, 1.5 m thick, concrete lined with stainless steel. Finally the whole reactor is housed inside a reinforced building that is kept at a slightly sub-ambient pressure and is accessed by an air-lock. 

Individual tube temperatures shall be logged for process control and accountability information. Affluent shall be monitored to determine when fission products from failed fuel elements are being released into the coolant. 

The activation products of the coolant, with the sole exception of N, are not of substantial importance in plant operation in some cases, however, they have to be taken into consideration environmentally following release of off-gas or waste water. The fission products and the fuel activation products represent by far the greatest proportion of the radionuclide inventory in the reactor, from the viewpoint of radioactivity as well as from that of radiotoxicity. However, with the exception of severe accidents (which will be treated in Part C), during plant operation they are reliably confined within the fuel rods, so that only the very small amounts released from failed rods to the primary coolant are of interest in this context. Finally, the activated corrosion products are the origin of the buildup of radiation dose rates at the surfaces of the circuits, which potentially complicate the performance of operational procedures, in particular of inspection and repair work. 

According to Hiittig et al. (1990), the amount of uranium released from defective fuel rods and deposited on in-core surfaces can be assessed from the coolant activity levels of various short-lived fission products such as I, I, Cs, calculating their source strengths under the assumption of a direct and instantaneous release to the coolant. Though the releases of these isotopes from failed fuel rods are quite small (due to their short halflives), such data can provide only an upper limit for the uranium contamination if there are simultaneously fuel rod failures in... 

In general, release of actinide isotopes from failed fuel rods and their subsequent behavior in the primary circuit is very similar to that of certain fission products, e. g. of cerium isotopes. For this reason, the y-emitting cerium isotopes which can easily be measured in the coolant by y spectrometry, can serve as a suitable indicator for early recognition of higher releases of actinides to the coolant. The release behavior of the actinides from failed mixed-oxide fuel rods to the coolant is almost identical to that from uranium fuel rods. This means that in both cases the U Pu... 

The release of fission products from failed fuel rods to the primary circuit... 

III-l. The main source of radiation in a nuclear power plant under accident conditions for which precautionary design measures are adopted consists of radioactive fission products. These are released either from the fuel elements or from the various systems and equipment in which they are normally retained. Examples of accidents in which there may be a release of fission products from the fuel elements are loss of coolant accidents and reactivity accidents in which the fuel cladding may fail due to overpressurization or overheating of the cladding material. Another example of an accident in which fission products may be released from the fuel rods is a accident in handling spent fuel, which may result in a mechanical failure of the fuel cladding from the impact of a fuel element that is dropped. The most volatile radionuclides usually dominate the accident source term (the release to or from the reactor containment). Recommendations and guidance on the assessment of accidents are presented in Section 4 of Ref. [III-l]. 

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. 

Under accident conditions of transport, irradiated fuel pins may fail, with subsequent radioactive release into the package containment system. Data on the fuel fission product inventory, possible failure rate of pin cladding and the mechanism of activity transfer from the failed pin into the containment system are therefore required to enable the package leaktightness to be assessed. 

On-power refueling and a failed fuel detection system allow fuel that becomes defective in operation to be located and removed without shutting down the reactor. This reduces the radiation fields from released fission products, allows access to most of the containment while the reactor is operating and reduces operator doses. 

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