Fission product and actinide release

The activity of fission products and actinides released per year can be obtained from V... 

Overall Scenario A release rates are shown in Figure 26. From the dumping in 1981, initially, no active material is expected to appear due to the hull and bitumen barriers. When it does start, the initial fission product and actinide release is less than 0.0001 GBq-a at the year 2105 when the fission product and actinide inventory in the corroded left board SG starts to appear. Corrosion of the outer surfaces of the activated RPVs by that time contributes about 8 GBq-a. ... 

Figures 29 and 30 show the peak generated as the containers on the pontoon are finally breached in the year 2305. Five hundred GBq of fission products and 1600 GBq of actinides are immediately released to the Kara Sea from the cracks and porosity of the damaged fuel. In the following year, the rate of release reverts to the calculated corrosion rate of the oxide fuel fission product and actinide release rates are 1.7 GBq-a and 5.7 GBq a -, respectively. The fuel slowly corrodes away and the activity of the fuel itself decreases in the year 3300, the release rates for fission products and actinides are 0.05 GBq-a and 2.7 GBq a, respectively. The fuel is finally corroded away by the year 4570.

The RPVs within the RC corrode away at the year 2700 and their cladding by the year 2795. This exposes the remaining material left in the thermal shields to corrosion at the full BCR and the release rate jumps to 6.0 GBq-a. The rate falls away gradually for the final 250 years until all the activated material is corroded away by the year 3050. From Section 3.4.3.1 (Fission product and actinide release), there is still an appreciable amount of fuel material left to corrode away after the activated steels have corroded away. 

Beslu, P., Leuthrot, C., Brissaud, A., Harrer, A., Beuken, G. Solid fission products and actinides release and deposition inside PWR s primary circuits. Proc. 5. Intemat. Meeting on Thermal Nuclear Reactor Safety, Karlsruhe 1984, (KfK-3880) p. 1387-1397 Bishop, W. N. Iodine spiking. Report EPRI NP-4595 (1986)... 

One of the key processes here is the dissolution of the spent nuclear fuel matrix in groundwater liberating radioactive fission products and actinides. Without this process no radioactivity will be released to the biosphere. 

Corrosion and fission products appear in dissolved ionic form and "precipitated" in the crud, d nding on the chemistry and water conditions. Most of the corrosion products giving rise to induced activity enter with the feed water. The dominating activated corrosion products are Cr, Mn, Fe, Co, Co, Zn, and Sb, and the dominating fission products are H, Cs, and Cs. Other fission products and actinides are released... 

Radiological (contamination) Nitrogen release in SCB causes overpressure and contamination of Zone 2A canyon or Zone 2. Ventilation system failure. Process fission products and actinides as loose contamination in SCB. 

Once sea water has entered the core region of RPVs with SNF, corrosion of the cylindrical fuel pins will occur, allowing release of fission products and actinides. The release rate will depend on the size of the fuel pin the volume of fuel alloy released per year will be given by... 

The first barrier to sea water ingress is the bitumen which covers the whole SGI. This bitumen is assumed to degrade linearly in 100 years from full to zero effectiveness, modelled by the bitumen factor k(, = O.Olt (t 100 years). The fuel, and hence the fission products and actinides, is surrounded above and below by at least 300 mm of solidified Pb-Bi coolant, and multiple layers of SS. Using the most pessimistic estimates of corrosion rates and thicknesses, the minimum time for water ingress to the fuel via corrosion directly from above or below is over 40 000 years. This timescale is much longer than that of ingress through the EPR tubes and ECTs hence, this method will be assumed to be the primary means of release and is studied in more detail below. 

The reactor cores ninety per cent of the total fission product and actinide inventory is contained in the two cores, which will corrode initially from the EPR channels, and later from the outside of the cores as the thermal shields corrode away. Activation products will also be released at the same rate from the Pb-Bi coolant and structural material in each core. 

The left board SG contains 10% of the total fission product and actinide inventory. Once the bitumen and Furfurol(F) have degraded, sea water can attack the exterior of the SG and then reach the damaged fuel pins inside, allowing release of fission products and actinides. The sea water will also attack the Pb-Bi coolant in the SG, releasing activation products. A diagram of the SG, showing the weld sites and SNF, is shown in Figure 11. 

FIG. 19. Fractional release rate of fission products and actinides from corrosion via the emergency protection rod (EPR) channels of the cores of the liquid metal reactors of submarine factory... 

The damaged fuel from the N2 PWR is assumed to comprise UO pellets, 5% enriched, 4.5 mm diameter, 10 g-cm density, carried in Zr-Nb alloy cladding. As soon as the fuel is in contact with sea water, an immediate release of 20% of the fission product and actinide inventory is assumed from the fuel grain boundaries. The remaining fission products and the actinides are released through dissolution of fuel grains at a pessimistic rate of 30 x 10 g cm d [22]. For fuel pins with a 4.5 mm diameter and 10 g-cm density, this equates to a constant dissolution rate of 0.0011 mm-a. Once exposed to seawater, the fuel will take some 2250 years to corrode away. 

However, because of the difficulties and lack of information in estimating the rates of radionuclide release from grain boundaries, the model assumes an instantaneous release from the fuel surface and grain boundaries once sea water comes into contact with the fuel. As a conservative estimate, this is assumed to account for 20% of the total fission product and actinide inventory in the fuel. Similarly, since the extent of damage to the Zr-Nb alloy cladding around the fuel is unknown, the model assumes that cladding is ineffective as a barrier to sea water and radionuclide release. 

By the year 2180, fission products and actinides from the damaged and undamaged reactor cores join the release stream and the total release rate is of the order of 5 GBq-a. By the year 3000, the rise in release rate of the SNF and thermal shields of the reactors, caused by the expanding circles of corrosion exceeds the fall due to decay, and the release rate rises until the year 5200 when the ECTs merge to form an annulus. The release rate then varies as the shields external to the ECTs corrode away by the year 6800 and the left board SG loses all its SNF by the year 7500. By this stage, the release rate has fallen to 0.07 GBq a. ... 

As indicated earlier. Figure 28 shows the overall release for Scenario A. The initial release is dominated by the activation product material from the RC, with the peak between the years 2035 and 2055 on the order of 22 GBq-a. Release rates drop until the year 2305 when Containers A and B holding the damaged SNF open and the fission product and actinide peak of 2100 GBq-a dominates the release in that year. 

Molten salt is a secondary barrier to prevent radionuclide releases to the environment (fission products and actinides dissolved in salt)... 

III-2. Account needs to be taken of the possibility of radioactive material accumulating on and being released from air filters or components of the liquid waste treatment system after accidents. In comparison with the radiation emanating from fission products and actinides, activation products are usually of minor importance. 

The release of a significant fraction of the stored radioactivity requires the breaching of multiple barriers. In the first place, most of the fission products and actinides are embedded in the fuel pellet matrix, and large-scale escape would only occur if the fuel were to melt. The fuel is enclosed in zircaloy cladding and the core is enclosed within the pressure vessel which forms part of a sealed primary circuit. Finally, the whole of the primary arcuit is enclosed within a containment structure which is specifically designed to minimize release of radioactivity to the environment. 

Nuclear reactors and reprocessing plants are constructed and operated in such a way that the radioactive inventory is confined to shielded places. Only limited amounts of radionuclides are allowed to enter the environment. The amounts of T and produced in nuclear reactors vary with the reactor type, between about 10 and 10 Bq of T and about lO Bq of per GWg per year. Tritium is released as HTO and about one-third of the is in the form of " C02. Under normal operating conditions, very small amounts of fission products and radioelements are set free from nuclear reactors and reprocessing plants. In this context, the actinides and long-lived fission products, such as °Sr, Tc, I, and Cs, are of greatest importance. 

Figure 28 shows the overall release for Scenario A. Figures 29 and 30 break this down to illustrate the fission product and the actinide releases separately. 

The SNF (after a cooling period to allow for decay of short-lived radionuclides) is chopped up and dissolved in nitric acid. The gasses emitted in the process are treated to avoid their release to the environment. The solution is filtered to separate the insoluble residues and sent to the solvent extraction stage in which the uranium and plutonium are extracted into the organic phase (usually TBP in a hydrocarbon solvent) and the fission products and minor actinides remain in the aqueous phase. The radioactive fission products may then be treated as high-level-waste while the uranium and plutonium are then separated from each other by selective back-extraction. 

In the case when defective fuel rods are present in the reactor core, the BWR reactor water contains the other fission products and the activation products released from the fuel in concentrations well below those of fission product iodine. This applies as well for fission product cesium, which is retained on the ion exchangers of the reactor water cleanup system with a decontamination factor of about 100. As far as it is known, cesium in the reactor water is present as the Cs ion, whereas large proportions of most of the polyvalent fission products and of the actinides are attached to the corrosion product particles suspended in the water as yet, there is no detailed knowledge on the chemical state of these elements (i. e., adsorbed to the surfaces or incorporated into the Fe203 lattice). It was reported that the strontium isotopes as well as Np appear in the reactor water in the dissolved cationic state, while Tc was found in the reactor water as a dissolved anionic species, most likely Tc04 (Lin and Holloway, 1972). According to James (1988), discrete fuel particles were not detected in the BWR reactor water. 

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). 

Nuclear power reactors operate by converting energy released by the fission of actinides such as and Pu into electrical energy. This process produces fission products and reduces the fissile content of the fuel until eventually further fission becomes uneconomical and the spent fuel is removed from... 

Fission products of uranium and other actinides are released to the environment during weapons production and testing, and by nuclear accidents. Because of their relatively short half-lives, they commonly account for a large fraction of the activity in radioactive waste for the first several hundred years. Important fission products are shown in Table 3. Many of these have very short half-lives and do not represent a long-term hazard in the environment, but they do constitute a significant fraction of the total released in a nuclear accident. Only radionuclides with half-lives of several years or longer represent a persistent environmental or disposal problem. Of primary interest are °Sr, Tc, and... 

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