Fission products poisoning ratio

Some of tlie materials whicli act as neutron poisons can be altered by reactor-system design these include fission-product poisons, core-tank material, conlaminaiils such as II2O, and additives such as the cupric ion. J he effect of the. e on lireeding ratio i.s discassed below. 

Computations have been performed for several spherical reactors using an Oracle eode [31] for two-region, time-dependent, thorium breeder systems. The variation with time of the breeding ratio and the concentrations of 11- , Pa, and fission-product poisons were... 

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... 

The fission-product poison level in the blanket cannot be determined without first knowing the blanket processing cycle. As a first approach, the breeding ratio for the hot clean conditions was used to determine the cycle time from which the RP in the blanket was calculated as described previously. The relative poison levels determined on this basis were as follows ... 

The values of conversion ratio and AA o/Ami ratio calculated in this parametric study arc for hot, clean reactor conditions, and they are used for comparative purposes only. The effects of fission-product poisons, control rods, and Pa losses have not been included. 

Results of time study. The. study was carried to 2000 days of full-power operation. The mass of fuel and the buildup of Pa are shown in Fig. 21-29, and the buildup of fi.ssion product poisons (other than Xc and Sm j along with breeding ratio are graphed in Fig. 24-30. The fission-product poisons vary in an almost linear manner for burnups corresponding to 2000 to (iOOO days. Other calculations have indicated that extrapolations U epresentod by dashed lines on Figs. 24-29 and 24-30) to 5840 days, the expect( d life of the plant, are rcasoualile. 

Figure 2.18 shows the contribution of individual nuclides to the poisoning ratio as a function of time, starting with fresh, unirradiated fuel at time zero. The poisoning ratio of Sm builds up very quickly to 0.0113, the fission yield at mass 149, and then increases more gradually because of additional Sm production by neutron capture in nuclides of mass 147 and 148. Other nuclides of this chain that make appreciable contributions to the poisoning ratio include Pm, Pm, Sm, " Sm, and Sm. The overall poisoning ratio, the sum of the contributions of individual nuclides, is shown in Fig. 2.19. 

In the operating reactor, it is assumed that the total poisoning ratio for all hi -cross-section fission products from U and the U caused to undergo fission by fast neutrons from U, q u, has the same value as in the reference design ... 

In addition, fission products come from other fissile species such as Pu and Pu, each of which has its own total poisoning ratio q for high-cross-section fission products formed from that species and from caused to undergo fission by fast neutrons from it ... 

Nuclide Subscript Absorption cross section, Ufl, b Neutrons produced Ratio of capture to fission cross section, a Poisoning ratio of high-cross section fission products, Q... 

To simulate release of materials from the damaged core the model CORSOR-M, corrected by the surface to volume ratio, has been selected. Volatile fission products (Cs, I and Te) follow the same release pattern from the fuel nevertheless, Te is retained by the non-oxidized zircalloy. Less volatile fission products (Ba, Sr) leave the fuel only at temperatures above 2200 K, while the fuel itself is only released at temperatures above 2400 K. Other materials, such as control rod neutron poisons (Ag, In, Cd), clad (Zr, Sn) and structural components (Fe, Cr, Ni) are also released at high temperatures. 

The Liquid Metal Fuel Reactor offers the opportunity for continuous removal of hssion products from the fluid fuel by chemical and physical processing. By this procedure the poisoning effect of the fission products may be kept to a low level, and thus make possible a good breeding ratio in this thermal reactor. In this chapter, the various chemical and physical processes for removing the fission products are discussed. 

---