XV. Fission Product Poisoning XV.1. Xe 135 Buil-Up As we already know, temperature changes bring short-term effects. That is to say, once a power change is produced it is rapidly manifested as a change in reactivity via a change in temperature. But there are also long-term reactivity changes, which occur as a result of variations in the composition of the fuel. These variations are due to the following: (a) Accumulation in the fuel of fission products, which absorb neutrons. (b) U 235 burn-up and the accumulation of Plutonium. Let's analyze the changes in reactivity resulting from point (a). All fission products may be analyzed as poisons, since, to a greater or lesser degree, they all absorb neutrons. But Xe 135 is the most important poison, due to its enormous capture cross section (σ c ), whose value is approximately 3 10 6 barns. This Xe 135 cross section is much greater than the capture cross sections of any of the other fission products. It is also significantly greater than the σ f for U 235 of 582 barns. Therefore, we will consider Xe 135 in particular. Xe 135 BUILD-UP Xe-135 is produced in the fuel in one of two ways: (1) Directly, via fission (about 5% of the Xe-135 produced). (2) Indirectly, via the decaying of I-135, which is a direct product of decay fission of Te- 135. 1
About 95% of the Xe 135 formed is produced in this way. As the half-life of Te 135 is very small compared with the half-life of I, it is considered that the Te 135 formed decays immediately into I 135. Xe-135 disintegrates by two means: (3) Via negative beta decay to Cs 135. (4) Via neutron capture (of a thermal neutron) The Cs 135 and the Xe 136 have low neutron cross sections, and are thus not considered. Summarizing: Figure 1 In order to understand the accumulation of Xe-135, we will use the analogy of a vessel that is filled with water (Figure 2). Let's assume that the two vessels are empty, that is to say, that there is no Xe 135 or I 135 in the fuel. Let's also assume that the reactor has been started up and that it is operating at a constant power level. Let's further suppose that the I 135 is produced directly. 2
Figure 2. Analogy of a vessel filled with water. The accumulation of Xe 135 is represented by the accumulation of water in the lower vessel and I 135 by the water in the lower vessel. When the reactor starts up, I 135 and Xe 135 are produced, thus water will pass to the iodine vessel and some water will pass via line 1 to the Xenon vessel (direct formation of about 5%). At the same time that the level of the iodine increases, the water flows via line 2 to the Xenon vessel (negative beta decay of the iodine). There comes a moment in which the level of the iodine vessel remains constant. It is then said that the I 135 has reached equilibrium concentration (it decays at the same speed that it is formed). Figure 3 demonstrates the accumulation of I 135 over the course of time. 3
Figure 3. Accumulation of I 135 over the course of time Meanwhile, the Xenon vessel is filling by way of the iodine vessel (via line 2) and via direct line 1, and the water is being lost via lines 3 and 4, which represent the removal of Xe 135 by means of decay and neutron capture. Nevertheless, the water flows into the Xenon vessel faster than the rate of the losses, and the level rises. But the faster it rises the faster the fluid will run out through lines 3 and 4. Consequently the Xenon vessel will fill more slowly. In other words, the greater the level of the Xenon vessel the slower the growth of this level. At a certain level, the water will flow out at the same rate that it flows in and the Xe 135 level will remain constant. That is to say, the level of the Xe 135 will continue to grow until its removal (via decay and capture) increases to such an extent that it is precisely balanced with the rate of production. At this point, the Xenon concentration will remain constant. This concentration is known as the Xe 135 equilibrium concentration. The equilibrium concentration of Xe 135 depends on the neutron flux. If, for example, the reactor operates at half-power, only half the amount of I 135 will be produced and the 4
removal of Xenon via decay will also be half. This is equivalent to having the two valves in the previous figure closed halfway, instead of having them completely open. It takes about two days (45 hours) to reach equilibrium. But accumulation in the first part of this time lapse is fairly rapid. In a typical power reactor, the Xe 135 reaches 50% of its equilibrium value in about 10 hours, and 90% in about 24 hours of operation at a constant power level. XV.2. The Xe Reactivity Worth in Equilibrium The atoms of Xe 135 capture neutrons and thus, a reactor that is critical without Xe 135 will be subcritical with it. This reduction in reactivity is called the "Xenon reactivity worth". The presence of the Xenon in equilibrium implies that we must have quite a lot of excess reactivity in the system in order for the regulation system to be able to compensate for the reactivity loss that this signifies. XV.3. Xenon Accumulation During Reactor Shutdown One of the main problem with Xe 135 starts when the reactor is withdrawn from service. Let's assume that the reactor has been operating at full power for some time, and that the Xenon concentration is in equilibrium, when we take the reactor out of service (the control rods drop, for instance). Let s see what happens to the Xenon. Once the flux has fallen to very low levels, we can assume that there is no direct production of Xenon via fission, and that there is no removal of Xenon via neutron capture (supposing that the thermal flow is zero). Taking our previous analogy, to represent withdrawal of the reactor from service, the two valves are closed and the Xe 135 continues to be formed by means of the decaying of I 135 and disappears via negative beta decay to Cs 135. The final result is that Xe 135 is being produced at a faster rate than it is being removed. This result in a growth of the Xe 135 concentration level following the shutdown. But the decay of I 135 will gradually diminish (since there is no production of Iodine). Eventually, the rate of Xenon removal will be equal to the rate of its formation and later, the removal rate will predominate. The result may be observed in Figure 4. The assumption is that power drops directly to zero, but in reality, of course, this is not the case. If a reactor has insufficient excess reactivity in the core to compensate for the Xe peak following shutdown they are in a condition call a Xe precluded startup. The inverse also applies if, for example, power increases are analyzed. The immediate effect is a gain in reactivity, due to the rise in the Xenon burn-up. Meanwhile, more I 135 will start to be produced, and will dominate over the course of time. 5
In a large, loosely coupled reactor such as a power reactor a phenomenon known as xenon oscillations can take place. Xenon oscillations do not normally occur in research reactors as the cores are generally small and closely coupled. XV.4. Samarium-149 Figure 4. Of all stable fission products, Samarium-149 is the most important poison. Figure 5 shows how the neutron cross section is much smaller than that of Xe-135 (4.2 10 4 b at thermal energies). This is produced as a result of the decay of the fission fragment Nd 149. 6
It does not decay as in the case of Xe 135. That is to say, it is stable and the only removal process is via the capturing of a neutron. Sm 149 ACCUMULATION Figure 5. Let's use the same analogy as in the case of Xe 135 and compare the two. 7
Figure 6 At first there are no poisons present. In other words, the vessels are empty. With operation of the reactor, first Pm will start to fill to equilibrium, then Sm will reach equilibrium condition. The loss of ρ as a result of the Sm 149 load is approximately ρ = -5. BUILD UP OF Sm 149 FOLLOWING REACTOR SHUTDOWN Let's see what happens when we close the two valves. The level in the Sm vessel starts to rise over the course of time, reaching its maximum level when the Pm vessel has passed over all of its water. Figure 7 shows the typical Sm 149 transitory. 8
Figure 7. CONCLUSION The equilibrium value of Sm 149 (ρ -5 ) is less than that of Xe 135 (ρ = -27 ). Though smaller, it must, nevertheless, be borne in mind in the design. XV.5. Other Fission Products Xe 135 and Sm 149 are the two most important poisons of all fission products, due to their high neutron cross sections. But there is a considerable number of fission products with smaller cross sections that combine to produce significant changes in reactivity over the long term. These changes are normally considered as reactivity variations resulting from fuel burn-up. 9