Reactor poisons

The second control mechanism is the soluble reactor poison boric acid [10043-35-3] Natural boron contains 20% boron-10 [14798-12-0] ... 

Verify the statement that the reactor poison 135Xe reaches a maximum 10h after shutdown of a high flux (> 1014 n/cm2s) reactor. 

Fuel reprocessing has three objectives (a) to recover U or Pu from the spent fuel for reuse as a nuclear reactor fuel or to render the waste less hazardous, (b) to remove fission products from the actinides to lessen short-term radioactivity problems and in the case of recycle of the actinides, to remove reactor poisons, and (c) to convert the radioactive waste into a safe form for storage. Fuel reprocessing was/is important in the production of plutonium for weapons use. 

In a reactor, the energy per fission, including the energy of the delayed neutrons and of the fission products, is 200 MeV. To produce 1 MW thermal energy, 3.1 x 1016 fissions per second are required. If the half-life of the fission product is short compared with the duration of operation of the reactor, its activity comes into equilibrium when creation by fission equals radioactive decay. Assuming a constant level of power for a duration of Tsecs, the activity is 3.1 x 104/(1 — exp—AT) TBq per MW. Some fission products themselves absorb neutrons (the socalled reactor poisons) and for them the calculation of activity is more complicated. Figure 2.2 shows the combined activity of 1 g of fission products formed in an instantaneous burst of fission and also from 1 g of fission products formed over a period of a year (Walton, 1961). The activity from a short burst decays approximately as t-1 2. 

FIGURE 38 MW distribution of polymers (HLMI of 10 g (10 min) density of 0.946 g mL ) made with bis(triphenylsilyl) chromate on silica treated with AlEt2OEt. Adding traces of 02 to the reactor poisoned the catalyst and broadened the MW distribution. 

FIGURE 75 The addition of CO to the reactor poisons Cr/silica catalyst and lowers the polymer elasticity because of diminished LCB levels, here indicated by a rise in the MSV ratio. 

Wl. Walker, W. H. The Effect of New Data on Reactor Poisoning by Non-Saturating Fission Products, Report AEC1 2111, Nov. 1964. 

Cl) The loading Includes all fuel and integral fuel-target columns in the active zone of the reactor. Poison materials charged In separate columns or capable of Insertion or discharge separate from adjacent fuel are considered separately as poison columns In the loading as defined above. 

The third reactor poison considered in this analysis is Sm, which appears as the radioactive beta-decay product of Pm (47-hr half-life). The Pm is itself the beta-decay product of Nd (1.7-hr half-life), as well as being a direct fission product. The decay process is given by... 

What two characteristics of Xenon-135 result in it being a major reactor poison/neutron absorber, compared to other fission products ... 

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. 

It is important to note that carbon monoxide (which is iso-electronic with an ethylene molecule) is often used as the reactor poison compoimd because it is a reversible poison that is slowly consumed by the ethylene polymerization catalyst and is incorporated into the growing polymer chain. Carbon monoxide will coordinate with the polymerization active site by displacing the ethylene. Because carbon monoxide is incorporated much more slowly than ethylene into the growing polymer chain, the polymerization process is essentially stopped until the carbon monoxide can be consumed by the catalyst. These reactions are shown in Figure 5.14. 

The PC serves as an expert system. For example, computation of xenon buildup after reactor shutdown with varying operational histories and determination of available time for startup before the reactor poisons out can be easily performed by shift staff themselves without the need for elaborate calculations to be performed by reactor physicists. 

Statistics on FPV s undeh Conditions of 1% Reactor Poisoning eou a 500-Mw Reactor 1000 ppm 150 tons of Bi... 

Group Typical concentrations Approximate reactor poisoning, % Removal rate, g/day Weight fraction of total fission products produced... 

The FPN group, minus Mo, represents 11 a/o of the total fission products. With practically all the Mo out of solution, a 400-day residence time gives an FPN concentration of 177 ppm with a reactor poisoning effect of about 0.8% for a 500-Mw reactor. To maintain that concentration, the fuel would have to be proce.ssed at the rate of only 9.2 gal/day, assuming complete removal of the FPN s. The size of the batches, and therefore the frequency of processing, would be determined by economic factors. Processing would begin probably after 400 days of full-power operation. 

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