Samarium Poisoning

Calculations of the reactivity gains in a reactor due to plutonium buildup usually assume instantaneous formation of plutonium. OSie fact that the formation of plutonium-239 is delayed throu di the 2.33 beta-decay of neptunium-239 makes it necessary to correct the calculated reactivity gains. 

This is most conveniently done by assigning a reactivity loss for the neptuniwa 

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Samarlum-1 9 also contributes significantly to the fission product poisoning This Isotope Is stable and Is a product of the radioactive decay of promethium-l49 Sanarium poisoning reaches an equilibrium value at about 100 MMD/T eaqposure. The poisoning at equilibrium Is 0.65 k. After reactor shutdovn> the samarium poisoning will increase due to the decay of the precursor promethium-1 9 ... 

Answer Although a potent absorber of neutrons, samarium is a minor problem compared with that of xenon poisoning. Promethium-1 9 is one of the fission products of lj235 and decays on a two-day half-life to samarium-1 9 which is a stable nuclide with a large absorption cross-section (66,000 barns) for thermal neutrons. As the samarium poison accumulates, the chance for burnout by neutron capture increases, so the pile reactivity absorbed by samarium reaches a maximum or saturated value of around 600 c-mk under steady operating conditions. 

Samarium-149 poisoning reaches an equilibrium value of about 6.5 x 10 Ak/k at 100 lOT/T exposure. After each reactor shutdown, the samarium poisoning increases due to the decay of its precursor promethium-149. The equilibrium value is regained by the time an additional exposure of 100 MWD/T is reached after each startup. The equilibrium samarium is shown in Figure C-9 as a function of fuel exposure,... 

The xtmon and samarium poisons in the (mre are determined as described for the blanket. 

SAFETY PROFILE Poison by intravenous and subcutaneous routes. When heated to decomposidon it emits acrid smoke and irritating fumes. See also SAMARIUM and RARE EARTHS. 

The compound Sm,Co - SmO, is a permanent magnet. Samarium is used also as a burnable poison in nuclear reactors. Finally, its compounds are used as phosphors for television screens, catalysts, and ceramic capacitors. 

The complete samarium transient will be given In Appendix XXX Volume IX Other fission product poisonings are Included In the long term effects. 5 2.3 Reptunlum Holdup... 

Answer Xenon and Samarium have large absorptive cross sections for neutrons, thereby act as poisons within the reactor. 

Samarium is another fission product that is a strong poison. It is very much like Xenon, except that l JLoes not decay, and what you have at shutdown does not decrease as the outage goes on. Samarium c es from Fromethl hn, just as Xenon comes from Iodine. During an... 

Capture of neutrons in other than fuel material derivation of K effective (six factor formula) control elements reflector poisons (xenon and samarium buildup and decay). 

Poison addition to the reactor is the final general factor affecting neutron multiplication. Poisons can be in the form of boron in control rods xenon and samarium fission products or any absorbing nucleus that is introduced into the reactor. Poisons increase the denominator in the thermal utilization factor making the overall value of f decrease. In the case of resonance absorbers, resonance escape probability also decreases. 

The excess measure of extra fuel lo needed to be critical in experiment insertions, f defect during operation, fission product poison, perpetual equilibrium be compared to the ten day samarium-149 is not cons the core s excess multip core fuel loading. 

As xenon-135 and samarium-149 are formed in a reactor, they reduce the multiplication factor by decreasing the thermal utilization factor, f, Since the formation of fission product poisons is a direct function of the fission rate, as power level changes the amount of poison present in the reactor also changes. Control system reactivity insertions such as rod motion and chemical shim must be made to compensate for fission product reactivity. 

For a U -fueled reactor, the reactivity associated with the equilibrium poisoning is about 0.45 %, independent of the flux level of the reactor. After a shutdown, the samarium concentration will increase, since none of it is being burned out, while it is still being produced by the decay of promethium. The variation of samarium reactivity as a function of time after shutdown is shown in Fig. 3.15 for several values of the pre-shutdown flux. For a flux of 2 x 10 " n cm" s the reactivity held by rises... 

The cluster system of reactivity compensation is used to compensate for temperature and power reactivity effects, reactivity margins for core poisoning by xenon-135 and samarium-149, operating margins to change reactivity during reactor power changes, and to provide core sub criticality under reactor shutdown. 

It is interesting to see the work by Matsuzaki and Yasuda [18], who investigated Cr poisoning using different combinations of electrolyte and cathodes as electrolyte, they selected YSZ and samarium-doped ceria (SDC), and LSM and LSCF were selected as cathodes. As shown in Fig. 2.5, a potential drop from Cr poisoning is largest and most rapid for LSM/YSZ, whereas LSCF/... 

Firstly, it is the operational effects arising because of the reactor thermal power itself as also the temperature rise in the various regions of the core. Operational reactivity effects also include poisoning due to fission products like Xenon and Samarium. Secondly, it is the reactivity effect due to the various experimental/irradiation assemblies present in the reactor core. 

Fission-product poisons in the core. The level of fission products, FP, other than xenon and samarium, in the core is determined by the chemical processing cycle for the core fuel solution. The steady-state value of FP poisons in the core should be established by an economic balance between the value of improved breeding ratio and increased chemical processing costs. The relationship between the core processing cycle, Tc, and the rela-ti e poison, IIP, in the core may be expressed as... 

Since only a relative comparison was needed, all calculations were made with a spherical core and complete 3-ft spherical blanket. The xenon poison fraction was taken as 0.01, and the samarium steady-state value was computed for each region in each case. 

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