Fission poisoning effect

The xenon poisoning effect is well known in the field of nuclear engineering as the effect that prevented early reactors from rapid startup after shutdown. In addition to Xe being created in high relative yields as an independent yield fission product in the fission of U and Pu, it is also created in high amounts by a chain-yield fission product from decay of Te and I via ... 

Answer The great majority of fission products have relatively small absorption cross sections and therefore are rather negligible in poison effects. Transmutation nuclei such as U-236, U-237, Pu-2-+0, and Pu-2 1 and numerous other fission products may be formed in quantities s-jfficient to affect pile reactivity, but the exact cuar.titites and cross sections of any one of these types of nuclei are not large. 

Answer The most important fission product to reactor operation is xenon-135 because of its large absorption cross-section - about 3.2 x 10 barns - for thermal neutrons. The effects of the xenon poison transients influence reactor operation in many ways. For example, although the saturated poison effect at equilibrium operation may cause a reactivity loss of 2 to 2-l/2 k (2000-2500 c-mk) in the usual power ranges, the decay of this amount of poison in the shutdown pile will represent a total swing of 4000 to 5000 c-mk from the initial xenon-free pile at startup. This delayed action effect in xenon formation and decay is the major cause for the scram transient, minimum downtime, and turnaround problems encountered in the operation of a hi power reactor. A detailed discussion on the effects of xenon poisoning on pile reactivity may be found in Chapter IV of this series. 

Iodine - (Also noted as M) - Iodine-135 is formed from the fission process in a reactor and decays, with a 6.68 hour half-life to xenon-135. Since the Xe has a considerable poison effect due to its large capture cross section, the 1-135 is significant as a source for Xe-135 and must, therefore, be considered in all reactivity calculations, particularly during start-up. 

The first two chapters serve as an introduction to the basic physics of the atom and the nucleus and to nuclear fission and the nuclear chain reaction. Chapter 3 deals with the fundamentals of nuclear reactor theory, covering neutron slowing down and the spatial dependence of the neutron flux in the reactor, based on the solution of the diffusion equations. The chapter includes a major section on reactor kinetics and control, including temperature and void coefficients and xenon poisoning effects in power reactors. Chapter 4 describes various aspects of fuel management and fuel cycles, while Chapter 5 considers materials problems for fuel and other constituents of the reactor. The processes of heat generation and removal are covered in Chapter 6. 

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. 

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. 

Table 2.15 gives direct fission yields y [B3], effective thermal-neutron absorption cross sections a and half-lives (cf. App. C) for radioactive decay that are used below to evaluate the poisoning ratio for this chain. Effective cross sections were calculated from cross sections for 2200 m/s neutrons and for neutrons of higher energy from cross-section data given by Bennett [B3], applied to the neutron spectrum of a typical pressurized-water reactor. 

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

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. 

These longer-term effects—the rise in temperature, the burnup of fuel, production of neutron-absorbing fission products, etc.—tend to lower the reactivity. Thus considerable positive reactivity must be built in to the fresh, clean and cold reactor. This in turn is contained by absorber rods, to be withdrawn as the effect develops (and perhaps burnable poisons ). One of these effects, the production of xenon, certainly played a part in the Chernobyl accident and is detailed in Appendix 6.1 to this section. 

The above applies to the stable fission products. There may be, however, numerous radioactive fission products (in addition to the Xe ) which have large cross sections. Their effect has been estimated by Miss Way on the basis of a statistics of the cross sections and she finds that those with an odd number of neutrons are most dangerous. All elements may give substantial contributions, the hfetime of which is an hour or more. Only those with a lifetime of more than a day have been surveyed to date and Miss Way estimates that they may cause a loss in efficiency of 3 % if the purification is carried out once a day. If this is done it is imlikely that the corrosion products will contribute much to the poisoning. 

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. 

Central reactivity worths were calculated in spherical. geometry by void replacement. Effective delayed-neutron fractions were determined both by subtraction of delayed-group spectra from the fission-source distribution and by the standard diffusion-theory perturbation method. Prompt-neutron lifetimes were calculated by 1/v poison and by perturbation methods. 

These data show that the K-effective from any one calculation may vary from the actual value by a significant amount, b fact, another paper on a similar subject also illustrates how this pitfall can occur when the fission density distribution does not adequately account for the distribution of fissile material. The system discussed here illustrates a situation where the fission density distribution is strongly influenced by the presence of a lumped neutron poison. This can also contribute to uncertainties in the calculated K-effectives. The user can most easily recognize the potential for errors of this kind by comparing the relative fission densities in localized sections of the system with the fissile material distribution. The optimum condition would occur when the two distributions are the same, hi fact, this should be considered any time the fissile material is distributed in an unsymmetrlc or complex manner. This also must be considered any time there are lumped-neutron absorbers in the system. 

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... 

The problems of the third category deal with phenomena that cause long-term variations in the neutron population. These include the production and decay of the principal fission fragments that appear as poisons in the reactor. The effect of these substances upon the reactivity of the system is examined and their practical significance noted. This treatment is concluded with a study of the use of burnout poisons for controlling these long-term changes in reactivity. 

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. 

Core analysis includes determination of reactor core neutron flux profiles, determination of the conditions for reactor criticality, the effects of fuel burnup and fission product poisons on core flux and core temperature profiles based upon heat generation rate, coolant flow rate, and core inlet temperature. 

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