Fission products, buildup

Hermann, O.W. and Westfall, R.M. (1995) ORIGEN-S Scale System Module to Calculate Fuel Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Association Source Terms,... 

O. W. Hermann and R. M. Westfall, ORIGEN-S A SCALE System Module to Calculate Fuel-Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Associated Radiation Source Terms, Sect. F7 of SCALE A Modular Code System for Performing Standardized Computer Analyses for Licensing Evaluation, NUREG/CR-0200, Rev. 5 (ORNL/NUREG/CSD-2/R5), Vols. 1, 2 and 3 (draft November 1993). Available from Radiation Shielding Information Center as CCC-545. 

The elimination of on-site refueling directly attacks the two greatest proliferation risks of the traditional power reactor accessibility of materials and use of the facility for illicit purposes. Elimination of on-site refueling removes easy access to both fresh and spent fuel from the reactor site. Fissile material is found only inside the reactor, where it is protected by both limitations of physical access and a very intense inherent radiation barrier. The only period where fissile materials might be considered at risk is during transportation and set up, and during early operation where the fission product buildup is limited. Access to fissile materiids and use of the reactor for illicit irradiation is furdier complicated by the lack of physical features and infi astructure to open the reactor vessel. 

Only a few materials have nuclear cross sections suitable for burnable poisons. An ideal burnable poison must deplete completely in one operating cycle so that no poison residue exists to penalize initial U-235 enrichment requirements. It is also desirable that the positive reactivity from poison bumup matches the almost linear decrease in fuel reactivity from fission product buildup and U-235 depletion. A self-shielded burnable poison consisting of Gd203 dispersed in a few selected fuel rods in each fuel assembly provides the desired characteristics. Gd203 depletes as a cylinder with decreasing radius to provide a linear increase in reactivity. The concentration is selected so that the poison essentially depletes in the operating cycle. It is possible to improve power distributions by spatial distribution of the burnable poison. 

Answer Plutonium long term gains continue -ontil the total effects cf U-235 plus plutonl jm burnout plus long term fission product buildup begin to exceed the plutonium buildup beyond this point, at about 600 MWD/T, the pile loses reactivity or.til there is nc net reactivity difference between green and ripe metal at around lOOC MWD/ T. 

Keff l. It provides a measure of excess reactivity available to overcome fission product buildup, fuel burnup, and power defect. 

When voided independently, all core zones except the radial blanket result in a positive reactivity insertion. Even the activation of the OEMs is not sufficient to bring the core to a subcritical state. The sodium void reactivity increases with bum-up due to fission product buildup and Pu quality deterioration. 

L. Robba et al., Fission-product Buildup in Long-burning Thermal Ri artors. Xurleonics 13(12), 30-33 (1955). 

Reduced Power Mode(s) could be used when the spaceship is entering a long period of decreased electrical demand. This potentially conserves fuel, minimizes fission product buildup, and reduces time-at-temperature and creep. More details on Reduced Power Mode options can be found in Sections 3.7 and 8.5.2.3. 

CaCl2> CaF2). The salt wets the oxide fuel and serves as a medium for the reduction. It also dissolves the CaO produced during the reduction and takes up the aklaki and alkaline earth fission products (FP-2) and also the iodine (5). The salt represents the major waste stream in the process and needs to be bled off because of a buildup of FP-2. High salt discard rates are not necessary because the CaO is removed by electrolysis and the calcium is recycled. The variables controlling reduction are discussed more fully later. 

Another case of practical interest in nuclear engineering is the buildup and decay of fission products formed in a nuclear reactor operating at a steady fission rate for a time T and that have been removed from the reactor and aUowed to undergo radioactive decay for an additional time. The schematic diagram for continuous production of the first member of the drain at rate P is... 

The fission product Xe has the largest absorption cross section of all the nuclides in a thermal-neutron flux, and its buildup is especially important in affecting the neutron balance in a thermal reactor. The fission-product decay chain involving the production and decay of Xe is... 

Because the half-life of Te is so short compared to the half-lives of the other members of the chain, Te buildup may be ignored in calculating time variations in the amount of Xe, and the chain is assumed to originate with I, such that yi = 0.0609. The production rate Pj of 1, which is now the first member of a fission-product decay chain, is... 

Another important objective is to follow the changes in reactivity that take place as fissile nuclides are depleted or formed from fertile nuclides, and as neutron poisons are formed through buildup of fission products or burned out through reaction with neutrons. 

Only a few fission-product nuclides have half-lives too long for saturation but too short for the assumption of linear buildup that led to Eq. (8.4). Examples are Ru, Ce, and Pm. A few radionuclides, such as Nb, La, and Pm, have precursors that must be considered in the calculation of activity after a few months of postirradiation cooling. 

The lower end of each tube contains irradiated depleted UOj, the middle portion irradiated mixed depleted UO2 and PuOj, an upper portion irradiated depleted UO2, and the top a plenum to accommodate buildup of fission-product gases. The rod bundles are surrounded by a square or hexagonal stainless steel sheath to the top and bottom of which are attached end fittings to direct sodium flow in the reactor and to facilitate handling outside. Fuel assemblies for the radial blanket are of the same length but contain rods of larger diameter charged initially with depleted UO2. 

The lowering of/(eqn. (19.13)) due to the buildup of fission products and decrease in amount of fissile atoms are the main reasons for fuel replacement. It is obvious diat if ), (fuel) is very large, as is the case for highly iriched fuel, higher amounts of fission products can be tolerated i.e. more energy can be produced from the fuel before/becomes too small. 

In the event that the fan is not operating or the ventilation ducting is decoupled from the fan, there is no mechanism to draw contamination out of the SCBs. Release of hazardous material under these circumstances would occur at a rate determined by diffusion. Evaluations of transport of radionuclides under these conditions indicate that buildup of fission products in Zone 2 of the HCF several hours after the event is less than one microcurie. Transport of these fission products outside the confines of the HCF would be negligible, and dose consequences at 3000 meters would be negligible. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

Calculating, in iteration with the previous step, the changing material distributions in the reactor— taking into account depletion of the fuel, buildup of fission products, control absorber concentrations and distributions, reshuffling of fuel elements, and replacement of used fuel... 

The buildup of other actinides (239, 240, 241, and 242 isotopes of plutonium and Am), some fission products ( " Sm and Sm) as well as the decrease in 2 11 were also calculated. A partial representation of the actinides that are formed during... 

Despite this favorable record, the further development of nuclear power is greatly handicapped in many countries because of public concern over the radioactive products arising in the course of plant operation and the consequences of their possible release to the environment. Energy generation from the neutron-induced fission of heavy atoms is inevitably accompanied by the formation of radioactive nuclides. This is, first of all, the direct consequence of nuclear fission, which leads initially to fission products that are unstable due to an excess of neutrons in the newly formed nuclei. These products are transformed by a sequence of p decays (mainly with associated y emission) to stable end products. Moreover, neutron capture in the heavy atoms of the fuel results in the buildup of nuclei which are heavier than those of the starting element (uranium, plutonium) and which mostly decay — in part, with very long halflives — by a emission. Finally, from elements present in structural and cladding materials, as well as in the coolant, its additives and impurities, additional radionuclides are formed, induced by neutron capture reactions which take place in the intense neutron field inside the reactor pressure vessel. 

There are, however, some deviations from the linear relationship between the element concentrations of fission products, on the one hand, and fuel burnup on the other, partly resulting in a greater than proportional buildup, and partly a... 

With regard to the radionuclide composition of irradiated fuel, there are also deviations from the simple relationship between fission product activity concentrations of longer-lived nuclides and fuel bumup. Similarly to the buildup of the mass concentrations, these deviations are due to the increasing contribution of plutonium fissions to radionuclide production as well as to consumption of long-lived radionuclides by neutron capture in extreme cases, such as with the short-lived... 

In addition to the fission products, actinide nuclides are generated in the nuclear fuel during irradiation. The main starting reaction for the buildup of the transuranium nuclides is neutron capture in the nucleus leading to short-lived... 

The densification data of mixed-oxide fuels shows a tendency to fall near the lower boundary of the scatter band determined for UO2 fuel (see Fig. 3.11.). This can be understood by taking into account the structure of this fuel type, where about 85% of the fuel volume consists of a UO2 matrix whose densification is delayed because of its lower specific burnup. At bumup values beyond 20 MWd/ kg HM, the swelling of the mixed-oxide agglomerates due to the buildup of porosity and solid fission products compensates for further densification of the UO2 matrix. At burnup levels of 40 to 50 MWd/kg HM, the mixed-oxide fuel density is similar to that of UO2 fuel. Fuel transients leading to ramp terminal power values well beyond that of steady-state operation result in virtually identical densities of both UO2 and mixed-oxide fuels, when bumup is taken into account (Goll et al., 1993). 

In principle, two fundamentally different methods can be applied to solve this task. The first one is determination of the residual concentrations of the fissile nuclides after irradiation and calculation of the burnup from the difference between final and initial values. For this purpose, the uranium and plutonium fraction has to be separated from the fission and activation products and from each other (e. g. by extraction chromatography) subsequently, the concentrations of the individual isotopes, in particular of the fissile isotopes, are analyzed by mass spectrometry. Well-established analytical techniques for performing such analyses are available, so that only small error margins are to be expected in the determination of the concentrations of the isotopes under consideration. However, there are two problems that can potentially cause systematic errors. The first one is the well-known question of the accuracy of results which have been obtained as a difference between two numbers, which limits the accuracy at lower burnup values in particular. The second problem is that the fissile nuclides are not only consumed by nuclear fission but by neutron capture as well in order to avoid systematic errors here, the capture-to-fission ratio valid for the particular irradiation conditions has to be taken into account in the calculation of depletion during irradiation. If one recalls the complicated buildup and decay mechanisms of actinide nuclides during reactor irradiation (see Fig. 3.5.), it is obvious that such correction requires complex calculations. On the other hand, the direct determination of the residual concentration of fissile nuclides is not influenced by errors due to inaccuracies in the fission yields of fission products to be measured nor by migration-induced inho-mogenities in the fuel. 

The redox potential of the irradiated fuel material is influenced by the buildup of the fission products. In their review paper, which is strongly influenced by thermodynamic considerations, Assmann and Stehle (1984) stated that the average valency state of the generated fission products is slightly less than the valency state of uranium or plutonium, as a consequence of which the oxygen partial pressure in... 

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. 

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