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Gas-cooled graphite-moderated reactors
Published in Kenneth Jay, Nuclear Power, 2019
Xenon-135 absorbs neutrons thousands of times more strongly than almost any other isotope. It is not a primary product of fission but is formed by radioactive decay, after a few hours, from iodine-135. After a few more hours the xenon-135 removes itself from affecting the chain reaction by decaying to long-lived caesium-135, which is a weak neutron absorber. Xenon-135 is removed also by capturing a neutron, which converts it into xenon-136 with a small neutron absorption. The rate at which this second process goes on depends on the neutron flux in the reactor; so too does the rate at which xenon-135 is formed. Consequently, when a reactor is brought up to power, xenon-135 forms and builds up, after a few days, to an equilibrium concentration (when the rate of removal is equal to the rate of formation) the value of which depends on the neutron flux. This equilibrium concentration of poison tends to reduce the reactivity; the reactor has therefore to be built with enough extra reactivity to make up for this reduction when it occurs.
Nuclear Fission and Nuclear Energy Production
Published in Robert E. Masterson, Introduction to Nuclear Reactor Physics, 2017
Xenon-135 is important in the reactor control theory because it has the highest thermal neutron absorption cross section of any nuclear material known. In other words, it literally soaks up thermal neutrons like a sponge (see Chapter 5). The amount of each fission product produced from the fission of a heavy nucleus is called its yield, and the yield is represented in most nuclear engineering books by the symbol Y. (In other books, the fission product yield is represented by the Greek symbol γ.)
Nuclear Fission and Nuclear Chain Reaction
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
Xenon-135 is important in reactor control theory because it has the highest thermal neutron absorption cross section of any nuclear material known. In other words, it literally soaks up thermal neutrons like a sponge (see Chapter 5). The amount of each fission product produced from the fission of a heavy nucleus is called its yield, and the yield is represented in most nuclear engineering books by the symbol Y. (In other books, the fission product yield is represented by the Greek symbol γ.)
Bifurcation Analysis of Spatial Xenon Oscillations in Large Pressurized Heavy Water Reactors Using Multipoint Reactor Kinetics with Thermal-Hydraulic Feedback
Published in Nuclear Science and Engineering, 2021
Abhishek Chakraborty, Suneet Singh, M. P. S. Fernando
The irradiation of fuel (generally UO2) in a nuclear reactor is accompanied with the generation and accumulation of fission products. Fission products absorb neutrons that lead to decrease in reactivity due to the reactor operation. The decrease in reactivity is normally referred to as the reactivity load due to fission products. Because of their large absorption cross section, some fission products such as 135Xe, 149Sm, and 105Rh saturate within a few days and are called saturating fission products. Other fission products with smaller cross sections continue to build up for a long time and are classified as nonsaturating fission products. Xenon-135 is the most important of all the fission products from the reactor operation point of view because of its large cross section, yield, and manner in which it is produced and removed. The thermal absorption cross section of 135Xe is very large (~106 b), and the bulk of the removal at full power is due to absorption. Thus, at high flux levels, a relatively large amount of 135I in the steady-state condition of the reactor acts as a reservoir for the production of xenon.
Demonstration of the Advanced Dynamic System Modeling Tool TRANSFORM in a Molten Salt Reactor Application via a Model of the Molten Salt Demonstration Reactor
Published in Nuclear Technology, 2020
M. Scott Greenwood, Benjamin R. Betzler, A. Lou Qualls, Junsoo Yoo, Cristian Rabiti
Figure 14 presents the concentration of tritium, a selected neutron precursor group with a relatively short half-life (group 5) and 135Xe as a function of position in the reactor model. The values used for normalization are given in Table X. The tritium in the loop decreases between the inlet of the reactor to the inlet of the core and once again from the outlet of the core to the outlet of the reactor. This decrease is due to diffusion of tritium into the graphite of the axial reflectors. These reflectors have a significant amount of surface area for transfer. The tritium builds up as the fuel salt passes through the core as a function of the power profile. The tritium decays slowly (t1/2 = 12.3 yr), so the impact from decay as it moves around the core is not noticeable in Fig. 14. As the salt passes through the PFL heat exchangers, tritium passes through the tube walls to the PCL and ultimately to the environment. The precursor builds up in the core and then quickly decays as the salt moves through the core. Depending on the half-life of the precursor and the salt flow rate, the return concentration to the core will vary from zero to some larger, significant value. Xenon-135 builds up in the core and decays very little as it moves around the loop (t1/2 = 9.2 h). However, the off-gas system is connected to the PFL at approximately 10 m. The decrease in xenon concentration that occurs due to the long holdup period in the charcoal adsorber bed in the off-gas system can be readily observed. The pump bowl is at this position, which has the associated separation process previously described.