RU2065627C1 - Fuel assembly of pressure-tube reactor - Google Patents

Abstract

FIELD: nuclear power engineering; fuel assemblies of pressure-tube reactors RMBK. SUBSTANCE: fuel assemblies of pressure-tube reactor are filled with fuel composed of uranium and erbium oxides with concentration of the latter being between 0.3 and 0.8%; this provides for removing additional absorbers from core and charging it with more fuel, increasing degree of fuel burnout, reducing fuel transportation, storage, refueling, and burial costs, reducing maximal linear load on fuel elements, and decreasing energy release variation through reactor height and radius. EFFECT: reduced fuel component leveling cost and maintenance charges, improved safety of reactor.

Description

 The invention relates to nuclear energy, namely: to fuel assemblies (FA) of RBMK channel nuclear reactors.

 One of the important reasons that led to the catastrophic consequences of the Chernobyl accident was the large positive value of the steam reactivity coefficient αΦ4-4.5β .. Drainage of a significant part of the channels led to the acceleration of the reactor using instant neutrons.

Known fuel assembly channel nuclear reactor containing fuel elements filled with fuel from uranium oxide [1]
As a result of measures to improve the safety of the RBMK reactor, the value was reduced to 1β, mainly due to the installation of additional boron steel absorbers (DP) in 80 working channels of the core instead of fuel assemblies.

An increase in the number of absorbers in the core led to a decrease in the fuel burn-up depth by 25% and to an increase in the fuel component of the reduced costs by almost 30%
The deterioration in the economic performance of channel nuclear reactors has put on the agenda for the search for more economical options, in which the already achieved value of the steam reactivity coefficient would be preserved or even reduced.

A fuel assembly of a channel nuclear reactor was proposed comprising fuel elements filled with uranium oxide fuel with enhanced enrichment [2]
As calculations have shown, the transfer of RBMK, for example, from fuel, the initial enrichment of which is 2.2% for fuel with an initial enrichment of 2.4%, reduces fuel costs by 15% and reduces the value of av to 0.5 b.

However, with an increase in the degree of enrichment of uranium oxide, high fuel costs are still maintained due to the need to preserve DP in the active zone, the subcriticality of the stopped poisoned reactor is reduced, and the maximum linear load on the fuel elements increases q $\genfrac{}{}{0ex}{}{\text{max}}{\text{l}}$ by 5% and for a homogeneous lattice even by 7%
The aim of the invention is to further reduce the fuel component of the above costs and reduce operating costs, increase the safety of a channel nuclear reactor and increase the duration of the campaign.

 The technical result, which is achieved by using the invention, is to remove additional absorbers (DP) from the core and increase the amount of fuel in it, increase the degree of fuel burnup, store it, reload and dispose of it, reduce the maximum linear load on the fuel elements and reduce unevenness energy release along the height and radius of the reactor.

This is achieved by the fact that in the fuel assembly of the channel nuclear reactor containing fuel elements filled with fuel from uranium oxide, erbium is additionally introduced into the fuel, the concentration of which is selected from the interval 0.3-0.8%
The loading of RBMK fuel assemblies into the fuel of which erbium (or its oxide) is added from uranium oxide in an amount of 0.3-0.8% reduces the steam reactivity coefficient to a level at which additional absorbers are not required. At the same time, the fraction of neutron capture in water decreases, which leads to a decrease in reactivity growth in the case of dehydration of the core.

This is due to the fact that erbium at o, 47 eV has a resonance in the absorption cross section. In RBMK, the average temperature of graphite is 200 ^° C higher than the average temperature of water; therefore, when dehydrated, the temperature of the neutron gas rises and the neutron spectrum shifts toward this resonance. The proximity of the erbium resonance to the thermolization region provides an additional negative component of the steam reactivity effect, which allows us to abandon the DP in the RBMK core. Removing the DP from the core increases not only the fuel load of the reactor (due to the placement of fuel assemblies instead of the fuel assemblies in the technological channels), but also reduces the unevenness of energy release, which, in turn, allows increasing fuel enrichment and, thereby, increasing the fuel burnup in the core and therefore reduce the amount of spent fuel. The latter - in connection with the current problem of the processing and disposal of radioactive waste from nuclear plants, it can somewhat relieve the severity of this important problem.

 In addition, the presence of a burnable absorber in fresh fuel assemblies leads to a significant decrease in their power and reactivity introduced during overloading. These circumstances, along with more even than in the prototype. The picture of energy release in the core greatly simplifies the procedure of zone overloads and control of energy release in it, increases the safety of existing RBMKs without modification of fuel assemblies.

 At the same time, the placement in the technological channels of the RBMK core instead of DP fuel assemblies increases the urangraphite ratio, and since in the resonance energy region, if the water density decreases, the slowing ability of the medium decreases, this leads to an additional decrease in the steam reactivity coefficient.

 It should be borne in mind that at an erbium concentration of less than 0.3%, the effect of erbium on the vapor reactivity coefficient αΦ practically disappears, and to maintain the values achieved in the prototype, the need again arises to be placed in the active zone of the DP, and when the erbium concentration in the fuel is greater than 0, 8% of the economic performance of the reactor becomes lower than that of a reactor with a zone containing DP, due to unproductive capture of neutrons.

 The aforementioned results are also affected by the location of erbium in the fuel assembly, since two competing factors interact in the case of local placement of erbium. The influence of the spatial redistribution of the neutron flux on the dehydration effect is enhanced and the effect on the change in the neutron spectrum is weakened. But since the latter factor plays the main role for erbium; therefore, for example, the installation of erbium rods in fuel assemblies does not give advantages in the burnup depth and does not allow to reduce costs. But if you put erbium in the fuel, then, as the calculations showed, the fuel burnup depth increases.

The use of other burnable absorbers in RBMK seems problematic, although it is known to use reactors such as boron, gadolinium and hafnium, for example, burnable absorbers in light-water hull reactors. However, studies have shown that the placement of these burnable absorbers in RBMK fuel assemblies does not lead to the expected results. In particular, when the hafnium content in the claddings of fuel rods is more than 1%, the burnup depth in comparison with the DP loading not only does not increase, but even decreases, and when the gadolinium is evenly distributed, the coefficient of uneven energy release along the height K _z increases due to the uneven burning of gadolinium, which leads to to an increase in the maximum linear load q $\genfrac{}{}{0ex}{}{\text{max}}{\text{l}}$ on the fuel rod. The same problems arise when using boron. In addition, gadolinium and boron quickly burn out. On the one hand, this limits their influence on the steam reactivity coefficient αΦ, and on the other hand, it requires a high initial load of the burnable absorber to reduce the maximum linear power of the fuel assemblies, which leads to significant unproductive losses of neutrons at the beginning of the campaign.

 Studies have also shown that the technical order is also affected by the placement of absorbers. For example, heterogeneous (and especially extra-fuel) placement of gadolinium is more advantageous than homogeneous. However, if we take into account that when choosing the optimal option for the placement of the absorber, it is necessary to take into account that applying a thin coating from the absorber is difficult technologically, and its presence in the central rod of the fuel assembly makes it difficult to use neutron flux sensors, it becomes clear that the question of the possibility of using other than erbium burnable absorbers in RBMK is still open.

 Therefore, only when loading RBMK fuel assemblies containing fuel from uranium oxide with erbium, the concentration of which is selected from the interval 0.3-0.8%, the fuel component of the core can be increased. the degree of fuel burn-up has been increased, the cost of transporting fuel, its storage, handling and burial has been reduced, the maximum linear load on the fuel elements has been reduced, and the unevenness of energy release along the height and radius of the reactor has been reduced.

Claims (1)

 A fuel assembly of a channel nuclear reactor containing fuel elements filled with fuel from uranium oxide, characterized in that erbium is added to the fuel, the concentration of which is selected in the range of 0.3 0.8%