Fission products escape

As above noted, in a breeder system the neutron losses must be reduced to a minimum and it has been found that a substantial portion of these losses is due to neutron absorption in gaseous fission products such as xenon formed within the reactive solution 10. These fission products escape within the high pressure chamber 24 and are swept therefrom by an inert gas such as helium pumped through inlet and outlet conduits 35 and 36. Additional gaseous fission products are released within the low pres-... 

The cladding tubes contain the fission products released from the pellets. Ehiring the life of the fueL less than 0.5% of the tubes may develop pinhole-sized leaks through which some fission products escape. 

The release of fission products to the plant environs d.U require a sequence of events In which (l) some portion of the fuel Is over-heated/ releasing fission products (either as a result of Inadequate cooling or excessive heat generation rate), (2) the primary loop Is ruptured to admit fission products Into the building spaces and (3) the fission products escape from the building to the atmosphere. 

The 105-KU Basin currently contains 3,821 sealed (MK I and II) canisters filled with N Reactor fuel. The 1,773 MK I canisters include 777 aluminum canisters and 996 stainless steel canisters. The MK I and II canisters provide the primary barrier for the fission products escaping from the damaged fuel assemblies, and the basin cooling water provides a secondary barrier to the potential release of radioactive materials to the environment. Because the fuel has been stored in sealed canisters in this basin the contamination of the basins s cooling water has been substantially less than that experienced in the 105-KE Basin where the fuel is stored in open-top canisters. 

Fission products escape to the environment through the safety valves of the broken steam generator. The secondary side of the broken steam generator is not dry when fission product release begins. Therefore, radioactive aerosols could be retained in this pool. After pressure vessel breach the release of fission products go directly to the containment. 

Fuel matrix - Fission products, bound in a ceramic matrix, may escape only by slow diffusion or melting of the matrix. 

Selected fission products in the Chernobyl reactor core and their estimated escape into the environment... 

Table 32.16 Selected Fission Products in the Chernobyl Reactor Core, and Their Estimated Escape into the Environment... 

Since transport by water is virtually the only available mechanism for escape, we will be predominantly concerned with the chemistry of aqueous solutions at the interface with inorganic solids - mainly oxides. These will be at ordinary to somewhat elevated temperatures, 20-200 C, because of the heating effects of radioactive decay during the first millennium. The elements primarily of interest (Table I) are the more persistent fission products which occur in various parts of the periodic table, and the actinides, particularly uranium and thorium and, most important of all, plutonium. 

This paper deals mainly with the condensation of trace concentrations of radioactive vapor onto spherical particles of a substrate. For this situation the relation between the engineering approach, the molecular approach, and the fluid-dynamic approach are illustrated for several different cases of rate limitation. From these considerations criteria are derived for the use of basic physical and chemical parameters to predict the rate-controlling step or steps. Finally, the effect of changing temperature is considered and the groundwork is thereby laid for a kinetic approach to predicting fallout formation. The relation of these approaches to the escape of fission products from reactor fuel and to the deposition of radon and thoron daughters on dust particles in a uranium mine is indicated. 

In the new designs, if coolant were lost, the nuclear chain reaction would be terminated by the reactor s negative temperature coefficient after a modest temperature rise. Core diameter of the modular units would be limited so that decay heat could be conducted and radiated to the environment without overheating the fuel to the point where fission products might escape. Thus, inherent safety would be realized without operator or mechanical device intervention. 

The release of 131I and other fission products in reactor accidents has been considered in the previous chapter. In the Windscale accident, the temperature in the fire zone reached an estimated 1300°C and 8 tonne of uranium metal melted. Over 25% of the 1311 in the melted fuel escaped to atmosphere. In the Chernobyl accident, the fuel was U02, the temperature exceeded 2000°C, and about 25% of the total reactor inventory of 131I was released to atmosphere, as vapour or particulate aerosol. In the Three Mile Island accident, 131I remained almost completely in the reactor coolant. The activities of 131I released in reactor accidents, including that at Chernobyl, have totalled much less than the activities released from weapons tests (Table 2.3). 

In order to prevent corrosion of the fuel and escape of fission products, the fuel is tightly enclosed in fuel rods. Good heat transfer and low neutron absorption are important properties of the cladding. Generally, the fuel rods are assembled to fuel elements to make their exchange easier. 

The behavior of volatile fission products is largely unknown. Brief literature references to iodine and ruthenium are contradictory. It is likely that elemental iodine is the stable species in the melt (16 ), and that some will be volatilized. Possible process modifications to guarantee a unique path for ruthenium have not been considered. The rare gases should escape because of the elevated temperature crystal modification. However, experience with the voloxidation process suggests that this release may not be complete. The behavior of both Kr-85 and tritium must thus be investigated. 

The total yearly production of neutron-activation tritium in the PWR coolant is 400 Ci, as shown in Table 8.10. Another source of tritium in the coolant is fission-product tritium that diffuses through the fuel cladding and escapes through pin-hold penetrations through the cladding. Estimates of the amount of fission-product tritium reaching the coolant in LWRs with zircaloy fuel range from 0.2 to 1 percent of the total fission-production tritium produced within the fuel. 

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. 

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