Fission product poisons

On September 13, 1944, the Hanford Site started the B Reactor. For approximately 1 hour all went well, but the reactor malfunctioned as a result of fission product poisons. On December 17, 1944, the Hanford Site D reactor was started and the B reactor was repaired and restarted. Large-scale plutonium production was under way. On February 25, 1945, the Hanford F Reactor was started. With these three reactors operating simultaneously, the theoretical plutonium production capacity was approximately 21 kilograms per month. 

In order for the-reactor to satisfy these requirements, careful consideration had to be given to the minimum quantity of fissionable material which would be necessary at the flux level desired, the amount of foreign matter to be inserted, the need to reactivate the reactor within a few hours or less after shutdown and overcome fission product poisoning, temperature effect, and depletion of fissionable material, to name the most important. 

Way, K., Fission Product Poisoning,, Lecture i , Clinton Laboratories Training Program, M-3690, April 14, 1947. 

Review of Weqairements. It has been shown in Chap. 4 that the k of the MTR is very much larger than that of the early uranium-graphite reactors and that considerable excess reactivity will, b used in order to handle losses due to fuel depletion, experiments, fission product poisons, etc. Any control system designed for>such a reactnr not only must be able to maintain the machine at any given power but, when necessary, smst also be able to overcome quickly all the excess reactivity. 

The processes (7) and (8) are evidently linear this is because there are so few neutrons relative to the number of atomic nuclei present, even at high flux levels, that the effect of neutron-neutron interactions is neghgible. The assumption that the pt and qi are time-independent is, however, known to be valid only at low power levels ( zero power reactors ). At higher power levels, thermal expansion and Doppler broadening of resonance bands tend to reduce reactivity [7, pp. 339-344] fission product poisoning and fuel depletion also affect the matrices P and Q, but on a longer time scale. 

While the actinide region has achieved much recent attention, (n,y) reactions throughout the periodic table are important for the analytic tool of neutron activation. Cd is also important for reactor control as the reaction Cd(n,y), with cr = 2.0 x 10 b, is used to control the reactor neutron flux and hence the multiplication factor k in reactor design. The reaction Xe(n,y) with cr = 2.6 x 10 b is a prominent fission-product poison that creates problems in the operation of nuclear reactors by consuming neutrons unproductively. 

However, power reactors require significant amounts of reactivity (i.e., well above the amount needed to go prompt critical if added suddenly) that must be provided by movable control absorber devices (or removable poison dissolved in the piimaiy coolant) under the direction of a licensed operator and following jq>proved procedures during reactor start-up and the transition to equilibrium full-power operation. This positive reactivity is needed to compensate for losses associated widi increased core temperature, reduced coolant density including bubble void formation, and equilibrium fission product poison loads, especially Xe. Consequently, it is only possible to limit the amount of reactivity that could theoretically be inserted to small, intrinsically safe values when the reactor is already in the normal full-power operating mode with all movable control devices very near their maximum withdrawal positions (and when the dissolved poison concentration is close to zero). 

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 ... 

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... 

Plgijire 6.3<2.2 is a plot of Fission Product poisoning and neptuniuui holdup in N Reactor and is identical to Figure C-9 of Wif JlkOS V0L2. 

Answer "Transient" is a term used to describe the potential behavior of the reactivity in a pile under changing conditions such as fuel depletion, fission product poisoning, and temperature effects. Thus a "reactivity transient" implies that the pile reactivity is not settled or established, but is shifting and changing continuously according to the pile conditions at any particular time. This variation in pile potential reactivity must be continually balanced by rod insertion or withdrawal. 

A fission product poison can be produced directly as a fission product or as a decay product of another fission product. The fission yield (y) of a fission product multiplied by fissions/second is the number of atoms of fission product produced per second directly from fission. The rate of production by decay of another fission product is equal to the rate of decay of the parent nuclide. 

The fission product poison Xenon-135 is removed from an operating reactor by ... 

Excess reactivity, in the form of fuel loading, is added to a reactor to compensate for fuel burnup during a cycle, fission product poisons, experiments, and reactivity coefficients associated with moderator temperature and coolant voids. 

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. 

Fission Product Poisons Reed Robert Burn December 1988... 

Two fission fragments are produced in every fission. Some of these nuclei and their progeny have substantial neutron absorption cross sections. Their appearance in the reactor tends to reduce the multiplication factor, For that reason, these nuclei are known as fission product poisons,... 

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. 

The most important fission product poison is xenon-135. Xenon-135 is formed directly as a fission product and by decay of iodine-135. Xenon-135 is lost by radioactive decay and by neutron absorption to become xenon-136 which is called burnout. 

Xenon balance in the reactor is illustrated in Figure 8.1. Since iodine-135 decays to become xenon-135, its balance in the reactor must also be considered, Iodine-135 is formed by decay of tellurium-135 and as a fission product and, in turn, decays to become xenon 135. The half life of tellurium-135 is 19 seconds in comparison to 6.7 hours for iodine-135, For all practical purposes it can be assumed that all iodine-135 is formed directly by fission, For this reason a fission product yield fraction for iodine-135, Vt, is given in Table 8,2 when tellurium-135 is, in fact, the fission product. Tables 6.2 and 8.3 provide fission product yields, half-lives, and decay constants for those isotopes involved with fission product poisoning. 

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