DE1909109A1 - Nuclear reactor with a regulation of the reactivity and power distribution over burnable reactor poisons - Google Patents

Description

Patent attorney

Dr. Erhart Ziegler

6238 Hofheirn / Ts.

Nightingale Path V

691 (24D 934-i'lscher) Ger.er-.l Electric Company, 1 niver ueaa, Dcnenectady, ui, UoA

Nuclear reactor with a control of reactivity and power distribution via burnable reactor poisons,

It is well known "that in fission: large amounts of energy get free. When a fissile isotope like U-235 captures a neutron, the isotope's nucleus decays. This creates on average two fission products with a lower atomic weight but high kinetic energy, as well as several high-energy neutrons. The kinetic energy of the fission products is im Nuclear fuel very quickly converted into heat. This heat can be dissipated by a coolant that comes with the nuclear fuel is in heat exchange, and then the heat absorbed by the coolant can be converted into useful work.

If a nuclear reactor is to be operated with a constant power output, i.e. in a state of equilibrium, the occupation density of those neutrons that trigger fission must remain constant. With every nuclear fission, 1 neutron must remain, which triggers a subsequent fission, so that a chain reaction occurs that is self-sustaining. In order for the equilibrium state to be maintained in a nuclear system, the effective neutron multiplication _{factor k efi>} must be equal to 1, and then one says ^,! 'Let the system be critical' ¹ , The neutron multiplication factor k ~ _f is defined as that

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Ratio of neutron population density to a certain one Point in time at the neutron occupation density at a point in time which is one neutron lifetime earlier.

(The effective neutron multiplication factor k _f . _F is the neutron reproduction factor if one considers the nuclear reactor as a whole. This effective neutron multiplication factor must be distinguished from the local or infinitesimal neutron multiplication factor küa, which defines the neutron reproduction in an infinitely large system, which has the same composition and has the same properties as the narrowly delimited point of interest in the reactor core).

The power rating "of a nuclear reactor depends on the size of the reactor core and on the efficiency of the cooling system. Practically is now the power density in a nuclear reactor often due to the inadequate thermal properties of the used Materials limited.

If the chain reaction in the nuclear reactor is maintained, the nuclear fuel becomes impoverished. This means that the number of fissile cores decreases. . In addition, some will produce the elimination products produced neutron absorber or "reactor poisons ¹ ·. So you venn with a nuclear reactor over a reasonable period of time energy, one must ensure that

u at the beginning there is a certain excess of nuclear fuel, which leads to an initial excess reactivity »This excess

Negative reactivity can be defined as the amount to which the multiplication factor of the unregulated reactor the amount of 1 exceeds.

This excess caused by the surplus of nuclear fuel

Short reactivity requires a regulating or control system of a regulating strength which is sufficient to keep the effective multiplication factor at "l""during reactor operation and to lower this factor to a value below" I " when the reactor

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must be shut down. Usually this contains the tax or control system neutron absorbers or reactor poisons with which the The neutron occupation density in the reactor can be regulated or controlled, and thus also a control or regulation of the Activity of the reactor possible. This is based on the heutron capture by processes that do not lead to nuclear fission. ) The substances for controlling or regulating a reactor can be in different Shapes are used. It is common for the control system to be made mechanical and with a number to equip control or regulation rods, which can be individually retracted or pulled out of the nuclear reactor in order to achieve a single to control or regulate the power level of the reactor and the power distribution in the reactor and, on the other hand, to control the reactor to be able to switch off. In addition, it has already been proposed - / orden to control aer excess reactivity of a nuclear reactor to use burnable reactor poisons of the most varied types and in the most varied of forms. A burnable reactor poison is a neutron absorber whose regular strength (that is to say in this case: its neutron infanp cross section) as a function of decreases with time when exposed to radiation from a neutron flux.

u Through the use of combustible reactor poisons, the scope can The mechanical control of a reactor can be minimized, and it has long been recognized that burnable reactor poisons could open up the possibility of automatically controlling the excess reactivity or to regulate, provided the decrease in excess reactivity during power generation in a reactor with the decrease the regular strength of the burnable reactor poisons can be reconciled. In addition, through a matching distribution the reactor poisons in the reactor core the power distribution within of the core can be improved.

The use of burnable reactor poisons is previously known Literature has already been discussed. In this context

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see, for example, the article by A. Radkowsky, entitled "Theory and Application of Burnable Poisons" in the public pamphlet "Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, Volume 13, Pages 426 to 445, United Nations Publishing House, Geneva, 1958.

In a known nuclear reactor, the reactor core has a heterogeneous structure. This means that the nuclear fuel is filled in pods and forms long rods, and that these fuel rods are arranged at predetermined distances from one another in boxes through which coolant can flow. These boxes with the Fuel rods are called fuel bundles. The reactor core is now built up from a sufficient number of such fuel bundles, which are arranged in a certain geometric scheme are, so that a fission chain can take place in which the nominal capacity of the reactor can be released.

In general, a power reactor is operated cyclically. This means that the reactor is shut down periodically to Refill fuel or to restore the original reactivity of the reactor. During the operating periods the composition of the nuclear fuel changes continuously, and although the total or the effective Reactivity is kept constant, the local reactivity changes very significantly during such an operating period, since the decrease in reactivity due to the fuel burn-up must be compensated for over the entire reactor core. These changes the local reactivity lead to changes in the power distribution inside the reactor core, since the reactivity in the various reactor areas increases the neutron flux within of these areas and since the power generation density within a certain reactor area is the product of the Neutron flux in this area and the concentration of fissile material in this area is proportional.

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The power that can be generated in a nuclear reactor is generally limited by the temperature limits of the materials, which are used in the reactor core at the point of highest power density, if the power density is within the Core is non-uniform, only the area with the highest permissible power density works with the designed nominal value, so that the power averaged over the entire reactor is less than the theoretically achievable power. Practically means such uneven power distribution that the reactor core larger and more expensive must, must, and that for a given nominal power the first fuel input must be larger. To now the Costs for the plant and the first fuel equipment if possible Keeping it small should therefore be the factor that controls the peak power density - (and as the ratio of the maximum power generation density to the average power generation density is defined) - during an operating period like this. be as small as possible.

If you have certain conditions for the end of an operating period pretends, it has been found that the factor describing the peak power density is a minimum if the Power distribution is as stationary as possible during the entire operating period.

The aim of the invention consists in determining a permissible power distribution and the associated reactivity distribution during an operating period by determining the local excess reactivity. For this purpose, burnable reactor poisons are used in an amount, density and in a spatial arrangement that practically completely compensates for the changes in the local excess reactivity during a reactor operating period.

In the embodiment of the invention shown, the reactor core has been divided into zones so that the ideal distribution of the reactor poisons can be approximated by step functions.

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The lowest density of the burnable reactor poisons is chosen so that that self-shielding comes about. The burnable reactor poisons are geometrically arranged in such a way that the burnable heater poisons are depleted with the radiation dose in the reactor. The surface of the reactor poisons, and thus the reactivity, which is regulated by the burnable reactor poison, increases Decide that it corresponds to the decrease in reactivity due to the fuel burn-off. The initial number of atoms which act as keactor poisons is chosen in each zone in such a way that that the initial excess reactivity in the zone of interest is compensated, which at this point of the reactor poison is regulated. The initial number of atoms, aie act as reactor poisons, is chosen so that the reactor poison due to the radiation dose occurring in this zone, which is to be expected during an operating period, practically completely

is needed. In the illustrated embodiment of the invention the burnable reactor poison is arranged cylindrically.

In the following, the invention will be described in detail with reference to the drawings,

Figure 1 shows schematically a nuclear power plant. "* Figure 2 shows a schematic plan view of a reactor core.

Figure 3 is a schematic side view of a fuel bundle.

Figure 4a shows a typical axial power distribution in the stationary State.

FIG. 4b shows a reactivity distribution that corresponds to the power distribution is compatible according to Figure 4A.

? igur 5A shows a typical radial power distribution in the stationary State.

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Figure 5B shows a reactivity distribution that correlates with the power distribution is compatible according to Figure 5A.

FIG. 6 shows the change in reactivity with the burnup.

FIG. 7 shows the relationship between reactivity regulation and burnup for changes in density and the initial one Radius of the self-shielding cylinder made of combustible reactor poisons.

Figure 8a shows the neutron flux distribution for the power distribution according to Figure 5A

Figure 8b shows the neutron flux distribution for the power distribution according to Figure 5B

Figure 9 shows a reactivity distribution; derived from Figure 5B which is to be regulated by the combustible reactor poisons.

FIG. 10 shows the correction factor for the neutron current density as a function of the radius of the reactor poisons.

Figures HA and UB show how the reactor core is divided into zones will.

Figures 12A and 12B show an example of the distribution of the reactor poisons in the fuel rods of a fuel bundle, to be used in the middle of a reactor core.

FIGS. 13A and 1J> Ü show an example of the distribution of the reactor poisons in the fuel rods of fuel bundles which are to be used on the edge of a reactor core.

Figure I ¹ J shows a fuel rod.

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The arrangement according to the invention of burnable reactor poisons can be used in various types of reactor. This arrangement is to be described using a boiling water reactor as shown schematically in FIG. Such a boiling water reactor has a pressure vessel 10. in which a reactor core 11 is contained. The reactor core 11 is immersed in a coolant such as light water. The reactor core 11 has a number of fuel bundles which are arranged at a certain distance from one another. Each fuel bundle consists of a certain number of fuel rods which are arranged in a box at a predetermined distance from one another. Cross-shaped control rods 12 can be individually inserted into the spaces between the individual fuel bundles or pulled out of these spaces so that the reactivity of the reactor core can be controlled mechanically. The drive devices required for this have been designated by 13. A pump Ik has been provided for circulating the coolant through the reactor core. The coolant absorbs the heat generated in the fuel rods and is partially converted into steam. This steam is used to perform useful work, for example in a turbine 16. The exhaust steam from the turbine is condensed in a condenser 17 and pumped back into the pressure vessel 10 as feed water by a pump 18.

FIG. 2 now shows the reactor core 11 from above. In the reactor core 11 are groups of four fuel bundles around each control rod 12 arranged around.

A side view of a fuel bundle 20 is shown in the figure. This fuel bundle 20 has a box 21 of square cross-section, in which a number of fuel rods 22 are held between grid plates 23 and 24. At the bottom of the box 21 a nose 26 is provided, which is equipped with openings through which the cooling water between flows through the fuel rods. The fuel rods 22 can

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nen be constructed from a sleeve in which a number of fuel pills is filled. Typical values for the diameter of the fuel rods are approximately 12.7 mm and their length is typically about 3.65 m.

The rated power of a nuclear reactor is usually determined by the amount of heat dissipation in that area of the reactor core limited, in which the power density is highest. The power distribution must be in Consideration should be given to changes in power distribution due to fuel depletion, the build-up of fission products, which movement of the control rods and is caused by other effects. From an economic point of view, it is now necessary that the power distribution is controlled so that the power density in that reactor core area in which the power density is thermally limited, is kept as small as possible. It is therefore beneficial if the peak power factor (i.e. aas ratio from peak power to average reactor power) during an operating period is kept as small as possible in order to make the total rated power of the reactor as large as possible.

It has already been mentioned that for some given conditions for the end of an operating period the peak power factor can then be kept as small as possible, if the spatial (or three-dimensional) power distribution does not change during an operating period. These conditions in the end of an operating period are dominated by parameters, of which here the fuel burn, the scope of the reactivity control, (the remaining reactor poison and / or the control strength of the control rods) that remains in the core as well as the remaining excess reactivity (if any) should be mentioned.

The power distribution, from the point of view of a possible a small power peak factor during an operating period is therefore a stationary power distribution, determined by the conditions at the end of an operating period

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are. The stationary power distribution may be obtained by an iteration between the power distribution and the influence of the erosion on the local Reaktivitätsfaktor k ^ ₃ calculated based on predetermined conditions at the end of a period of operation. For this purpose, a power distribution is first assumed, and then the local reactivity factor ko ^ is determined under the assumption of certain conditions at the end of an operating period. The reactivity distribution can then in turn be used to calculate the power distribution at the end of an operating period with the aid of the known diffusion theory. A repetition of these processes can then possibly lead to a distribution of the power and the reactivity that are compatible with one another, and the converging solution thus obtained leads to the power distribution that is required to keep the peak power factor as small as possible during an entire operating period.

in FIGS. 4A to 5B are now distributions of the local Reactivity and performance shown, which are mutually compatible are. In these figures, P mean the distribution of the local

borrowed performance as aimed, and k the distribution

the local reactivity

division P is required. The figures UA and

which are typical for generating the performance reports

see axial distributions, for example along a fuel rod, while FIGS. 5A and -5b show typical radial distributions which can be measured in a plane perpendicular to the reactor core. Also in Figures 4B and 5B is the initial one Reactivity k. shown at the beginning of an operating period. At the beginning of an operating period, the excess reaction ; ivity that needs to be regulated, k. - k. The local heaktivi- ; ät now decreases with the radiation dose as the fuel is used up. In order to obtain a stationary reactivity distribution k In order to maintain a steady distribution of power P), the scope of the reactivity regulation or the locality must be used Tax strength so great that ate the local excess reactive

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This tax strength must decrease as well as the local reactivity.

<enn the setpoint of the stationary power distribution P spatial

I is given by the expression P (x, y, z), the margin becomes at any point (x, y, z) within the kernel by the following expression:

1) js (x ₃ y, z ₃ t) = ü _Q (x, y, z) + P _Q (x, y, z, t)

In this expression, E is the burn-up distribution at the beginning of an operating period, P is the power density, and ^:! t "the time from the beginning of an operating period, measured in full power days.

If the burn-up is measured in megawatt days per ton, then P is the power density in megawatts per ton., And ^{: i} t "is the number of days on which the reactor has been operated at power density P.)

If the rate at which the local reactivity decreases for a unit power density is given by the expression 4 k / Δ t (x, y, z, t), the decrease in local reactivity with the burn-up during an operating period can be described by the following expression will:

2) 4 kl (x, y, z _s t) = (? _O (x, y, z) 4k / 4t (x, y, z, t) dt

Assuming that at the end of an operating period the reactor is still critical and that there is no more regulation in the reactor core remains, then the local reactivity is regulated at any point during an operating period or must be controlled, equal to the difference between the loss of reactivity at the end of the operating period and the loss of reactivity at any time during this period.

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The following expression therefore applies:

3) Ak (X ₉ y, z, t) = Ak ₁ (x _s y, z, t ")

(x, y ₃ z, t)

here j \ k mean the entire reactivity control that is present at any point in time, taking into account the control rods and other reactor gif te, A k, the loss of local reactivity, ^r -t " the specified burn-up time and ^; 't' ^{T is} the burn time at the end of an operating period.

The expression (3) therefore defines the relationship between the total burn-up and the regulation of the local reactivity, the must be adhered to in order to maintain the desired stationary power distribution.

Expression (3) is now shown graphically in FIG. In FIG. 6, k mean. the initial local reactivity at time t at the beginning of the burn-up, and k _f the local reactivity after the burn-up time t _f at the end of an operating period. The loss of reactivity A k after the burn-up time t ", ie at the end of an operating period, is k. - k ". The regulation of the local reactivity Λ k, which is required at any point in time, is thus equal to the difference between Δ k at the point in time t “and Δ k- at the point in time t. (In FIG. 6, the relationship between K ₀₀ and E is shown linearly. However, expression (3) also applies if this relationship has any other course).

In order now, according to the invention, during an operating period within the entire reactor core, the desired stationary power distribution To be able to maintain, a) burnable reactor poison is distributed in the core in such a way that at the beginning an operating period sets the desired power distribution, it is also ensured that this power distribution remains practically constant during the operating period, and c) reactor poison is only used in such an amount that the Poison is practically used up at the end of an operating period.

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These results are now, according to the invention, by such a spatial distribution of reactor poisons achieved that the initial Reactivity control strength is comparable to the loss of reactivity that the fuel has during a period of operation suffers, and that the reactor poisons are used in such a concentration and arrangement that they during the burnup be consumed at a rate that is comparable to the rate at which the reactivity of the fuel decreases during an operating period, and that it is completely consumed by the burnup during a full operating period will.

In the case of large power reactors, it has been found that the decrease the local reactivity k ^ in a large area practical runs linearly with the burn. (The only exception to this is a relatively short period of time at the beginning of an operating period, until the equilibrium is established between the formation of new fuel from brood material and the formation of fission products which act as reactor poisons.) The local reactivity k ^ therefore decreases at a practically constant rate.

If one denotes the rate at which the local reactivity decreases as the constant ^IT L ", where 4k / 4t (x, y, z, t) is equal to" L * ¹ , it follows from the relation 2)

l \) H ₁ (x, y, z, t) = P ₀ Lt
and from the relationship 3)

5) k _c (x, y, z, t) = P _o L (t _f - t)

The relationship 5) therefore defines the relationship between the Burning time and the local reactivity control strength that must be maintained for a stationary power distribution, if the local reactivity decreases linearly with the burnup, i.e. if it decreases at a constant rate, like it is shown in FIG. Assuming this condition,

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a) the Hegel strength decrease linearly, i.e. the rate of change of the control strength with the burn-up, must be a constant be that coincides with the constant speed, with which the reactivity decreases, and b) is the required control strength for the local reactivity of the local power density proportional.

wan has found that there is then a decrease in the strength of the rules a practically constant rate if you get burnable Reactor poison used in the form of a self-shielding cylinder. The concept of 'self-shielding' · should here mean that the neutron absorption cross-section ^ and the density of those used as poisons. Atoms are so big that ax incident neutrons are only captured in the outer layers of the cylinder, so that these outer layers of the cylinder are the Shield the inside of the cylinder from the neutrons. When the atoms used as nuclear poison capture neutrons, they will converted into isotopes, the neutron absorption cross-section of which is small. So as the burn continues, the outer layers become of the cylinder becomes more and more permeable to neutrons and the next, more inner layers of the cylinder become exposed to neutrons. This cylinder made of poison acts like a control rod, the radius of which depends on the burn-up shrinks, and its control influence on the reactivity is therefore less and less, since the control strength of the cylinder of Size of its surface is proportional. This is even clearer from the following consideration:

If the length of the poison cylinder is large compared to its diameter, and if its diameter is so small that the neutron flux O outside the cylinder is not disturbed, the neutron flux J at the surface of the cylinder is given by the following expression:

6) J = 0/4 neutrons / cm - see

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, hen 9, the number of atoms per cc with means nohem weutronenabsorptionsquerschiiitt, then the number of these atoms is given per unit y relieving slärii'e η by the following expression:

7) h =

In this expression "r- means the radius of what has not yet been used Reactor poison.

if one assumes that the reactor is completely poisonous for neutrons is black, (if the neutron absorption cross-section is infinitely large., so that a self-shielding comes about, As stated above, the reactor poison is only consumed on the cylinder surface, and the number of it as reactor poison Atoms used that are consumed in time t is practically equal to the number of neutrons that are used during this time fall on the surface of the reactor poison. The following therefore applies:

ö) dh / dt = -2.tr r J dr / dt = 27? -rJ
if you combine the expressions 6) and 8), you get: dr = (-Φ / Hj ) dt

if you start this expression in time from an initial radius r ÜThe r is integrated, so you get *

y) r = r _o - Φχ, / kO
here 0 means the mean neutron flux during the time t.

Expression 9) now shows that the radius of the cylinder that the Contains reactor poison, decreases linearly, namely the product of the time and the flow (St directly proportional to the density of the Reactor poison inversely proportional.

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The reactor poison is then completely used up when the radius of the cylinder has become zero. In addition, the product of flow and time 0t is directly proportional to the burnup E, namely it equals CE, where C is a puncture of the reactor core properties in which, for example, the degree of enrichment of the fuel and the fission cross-section are included. From the Expression 9) therefore results:

10)

0t _b = 4

- ^C l ^E b

Here t mean. the time that is required to completely use up the reactor poison, and E _fe the total burnup that is required for this, for example in megawatt days per ton.

So you can see that the reactor poison in a burn-up interval E ^ is used up, the product of the original cylinder radius r of the poison reactor and the density of the poison reactor ο

is proportional.

The control strength of a cylinder made of reactor poisons and their time dependency will now be discussed. The reactivity that is regulated by the combustible poison, i.e. the quantity k, is practically proportional to the neutron absorption by the reactor poison. The following therefore applies:

k = P.

Herein £ mean the absorption of thermal neutrons by the Poison and 2 * the total absorption of thermal neutrons.

Again, assume that the reactor poison for neutrons is complete is black. If T is the number of cylinders containing poison per unit cross section of the reactor core, is applicable:

TfT

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In this expression, (2Tr r) mean the circumference of the cylinder made of ReaKtorgift, J the neutron flux density, which according to expression 6) equals 0/4, j 0 the neutron flux and £ ~ the total neutron absorption cross-section.

In most large power reactors, the total absorption cross-section is sufficiently constant during the burn-up and depends on the degree of enrichment of the fuel and the ratio in which the breeding material is converted into fissile material. If the total absorption cross-section is regarded as a constant, then the term Tt / 2Z ^ can be used as a constant

/ C _{2 must be} applied. Expression 11) can then be rewritten as follows:

12) k = (1 / Cp) rT

The above is based on the assumption that the cylinders from reactor poison several mean free paths of the thermal neutrons are separated from each other., so that they are from each other can be viewed in isolation. With this assumption and further, already mentioned assumptions (namely that the reactor poison is completely black compared to the neutrons and that the total absorption cross section is constant, one can see from the expression 12) read off that the scope of reactivity, which is regulated or controlled by the reactor poison, is the product of the cylinder radius and the number of cylinders with reactor poisons per unit ■ cross-sectional area of the reactor core is proportional.

From the above already given expressions 9) and 10) it follows that that the radius r of the cylinder with the poison decreases linearly with the burnup. From the expression 12) one can therefore conclude that that the reactivity regulated by the poison also decreases linearly with the burnup.

If you put the expression 10) in the form Q r ^ - C ^ / ^ E ^^ and define C, / 4 as the new constant C, you get:

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If one writes the expression 12) in 'i'ermen of the original cylinder radius r um, one obtains:

14) Tr = C ₂ k _{i #}

nierin means k. the original reactivity that of the burnable Reactor poison is regulated or controlled.

The relationship between the original cylinder radius r, the density Q of the poison and the change; the control or control intensity with the burn-up has been shown graphically in FIG. A largely self-shielding reactor poison in a cylinder with the radius r, and a density ,, leads to an initial control or regulation strength k - ,, and this reactor poison decreases in a burn-up interval Ξ linearly with the burn-up. jiin rod with the same radius, in which, however, the density of the reactor poison is lower, leads to the; slight initial control or regulation strength k .., only the reactor poison is used up in a shorter burn-up interval E. “.

in rod with reactor poison, the diameter of which is smaller and r ρ, and in which the reactor poison with the density Q. -, exists, leads to an initial rule or tax strength that is lower and only k _? amounts to. The reactor poison in such a rod, however, is consumed in the same interval E ,, provided the density is equal to the ratio r ₁ / r ₂ , multiplied by the density.

From the above it can be seen that for a self-shielding reactor poison in cylindrical The arrangement determines the density of the poison in the reactor, the time inside who the poison is consumed. The strength of the reactor

iftes is determined by the size of the cylinder surface (which is a puncture of the cylinder radius) and the number of ylinder with reactor poisons per unit cross-sectional area of the

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Reactor core. The radius, the density, the number can therefore be used ind the distribution of the cylinders with the reactor poisons was chosen in this way be that such a decrease in the control or regulating strength results with the burn-up, that with the decrease in the excess reactivity coincides with the burn-up of the nuclear fuel.

venn according to the invention, the burnable reactor poisons spatially so are distributed so that there is a steady power distribution during an operating period, then both the initial, regulation or control elements caused by the poison

strength k. than on the burn-up interval E, the local Leipi b

power density P proportional. In addition, the local power density P is directly proportional to the local neutron flux 0. So s applies:

15)

16) Tr = CP = CJZ)
whether (

In these two expressions, Cu to C mean “constants of proportionality.

The equations listed above were developed to demonstrate the principles of the invention and will be used subsequently can be applied to a specific embodiment. It should be noted, however, that for calculation In a real reactor, much more accurate analytical methods must be used. As already mentioned, are based aie the equations already given on the following assumptions: a) The presence of the cylinders with reactor poisons is the neutron flow disturbed only insignificantly; b) the cylinders with reactor poisons are so far apart that they can be viewed in isolation and also not mutually within yours Shadows lie; c) the macroscopic effective cross-section and the The densities of the heaktorgotoxins are so great that they are compared to the neutrons are practically black and thus shield themselves;

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and d), the total neutron absorption cross-section of the core remains practically constant during the burn-up. There may be cases in which deviations from these idealized conditions occur, and it may be desirable, the size and the .Direction estimate these deviations in order to obtain the correct correction factors.

For example, equation 6) is based on the assumption that the cylinders containing the poison are so small that they prevent the neutron flux not noticeably disturb. If one considers the neutron flux from a homogeneous, infinite medium on the basis of the well-known diffusion theory calculated, the result is:

17) J = 0/4

4 D / L

Δ +

K ₀ (r / L)

= (0/4) g

Here: D denotes the diffusion coefficient of the thermal neutrons, L the diffusion length of the thermal neutrons, K

id K. Bessel functions, / I the inverse logarithmic derivative of the flow 0 on the surface of the cylinder and r the time-dependent radius of the cylinder.

The term in brackets that ^with:; g '^; now describes the deviation of the neutron flux J from the ideal value 0/4, as given by equation 6). How this term g changes as a function of the cylinder radius is shown graphically in FIG. 10 for representative values of L and D. As you can see, equation 6) leads to an estimate of the neutron flux that is about 50 $ too high for a cylinder with reactor poisons with a diameter of about 1 cm. The larger diameter reactor poison cylinders therefore tend to become impoverished with the burnup faster than the decrease in cylinder diameter.

Cß

On the other hand, equation 8) is based on the assumption that the combustible reactor poison is complete with respect to neutrons is black. A deviation from this assumption means that the neutrons to a certain extent in the cylinder with reactor poison can penetrate, so that the neutron absorption at the beginning of an operating period is greater than at the end of an operating period is. This effect counteracts the influence of larger cylinder radii, which has just been discussed, so that these two effects can cancel each other out.

The neutron absorption cross section of most materials is inversely proportional to the neutron velocity. The self-shielding is therefore lower for higher energetic neutrons, but on the other hand is the fraction of the higher energetic ones Neutrons captured by the reactor poisons are low anyway. ·.

jiin another factor to consider in establishing the above relationships was not taken into account, is the influence of the cylindrical • end faces. In the illustrated embodiment, however, the influence of the end faces of the cylinders with the reactor poisons is relatively small, since the length of these cylinders is very large compared to their diameters,

The distribution of the burnable reactor poison within the entire reactor core is now to be considered. In realized Nuclear reactors change the neutron flux and thus the power density as well as the reactivity regulation within the reactor core both in the axial and in the radial direction. Ideally, the reactor poison should be used to regulate the excess reactivity is arranged in the core, this excess reactivity is balanced out at every point in the reactor core. For example in FIG. 9 a curve k. the initial strength of reactivity control, which the reactor poison on an axial Must apply line through the core. Thus, in order to achieve the distribution represented by the curve k. is shown

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the distribution of the poison would have to be varied continuously. Now you can get the aim of the invention, namely a stationary Power distribution and a complete burn-off of the reactor poison can also be achieved practically by the fact that the ideal Distribution of the reactor poison is approximated by a step function. For this purpose, the reactor core is divided into zones, and into In each zone the poison is arranged in the same way as the central one Properties of this zone corresponds. i <lan therefore assumes that the reactor core consists of a number of elementary volumes, from the mean heutron level and the mean excess reactivity at the beginning of an operating period within such a tlementary volume, the distribution and density of the reactor poison according to those already discussed Determine the basics.

Another factor that needs to be taken into account is the refilling of the nuclear fuel as well as the scheme of what to refill of fuel is pursued if you end up with a Operating period replaces all fuel bundles 20 (Figure 2), all fuel bundles can contain burnable reactor pens, However, mn it is common when reloading the nuclear reactor only replace a quarter of each fuel bundle. In this In the event, the reactor poison can be placed in the new fuel bundle ", which then properly divides in the radial direction Need to become.

In the illustrated embodiment of the invention, the reactor is poison has been distributed in five axial and two radial zones, that is, in ten elementary volumes of the reactor core, as it is shown in the diagram is shown in Figures HA and HB. As already discussed, in FIG. 4 A is a stationary, axial one power distribution P has been shown. The power distribution

development is directly proportional to the neutron flux distribution. In the FIG. 8a therefore shows the steady-state neutron flux distribution 0, which leads to the steady-state power distribution P out Figure 4a belongs. Figure 8B shows the corresponding stationary

909 8 3 8/1028

_23- .9091 0 9

Neutron flux distribution 0 in the radial direction. In the figure SA it has now been shown in dashed lines how the neutron flux distribution 0 is approximated by a step function ₃ , namely in five axial zones la to 5a. The step approximation of the radial neutron flux distribution 0 is in the

S χ

'igur SB has been shown in dashed lines, and this approximation v was carried out in two zones Ir and 2r. One can therefore each of the ten zones that are shown, an axial and assign a radial coordinate, for example the upper

th, outer zone, the coordinates (5a, 2r), as shown in Figures HA and HB.

how to find the mean weutronic flux in each of these zones It is known that the properties of the reactor core can be determined analytically. If you have an initial one for the cylinder with reactor poison Radius r assumes, and if you look at a burn time

. is the density of the poison that is required is, in order to use up the reactor poison completely within this time t, by equation 10), which is as follows can be rewritten:

TSt.

In this expression, "0 means the mean neutron flux in the Zone at full reactor power.

never already mentioned, there is a stationary reactivity distribution in FIG. 4B κ shown, which is necessary to achieve the stationary

ren power distribution P from FIG. 4A is required. the

initial, uncontrolled reactivity of the reactor core is with κ., o been drawn, while k. the difference between the original reactivity and that reactivity means uie from the control or regulating rods of the nuclear reactor for the purpose of changing the power level of the reactor or for start-up or respectively Shutdown is to be controlled or regulated.

909838/102 8

9Ü9109

(The reactivity control range of the control rods is usually on the order of 2% excess reactivity.)

The original reactivity _} which has to be regulated or controlled by the burnable reactor poison is thus the

Difference between k _g and

That difference is in the

FIG. 9 as curve k. shown showing the distribution of the

pi

Reactivity rule strength means. This distribution curve k. can be used just like the neutron flux distribution from FIG. 8A Can be approximated using a step function based on the average reactivity of each zone. This is has been shown in dashed lines in FIG. For each zone you can the mean value of the reactivity and the mean value of the total neutron absorption from the properties of the reactor core be determined analytically in a known manner. The number of cylinders with poisons per unit cross-sectional area of the The reactor core can be determined from equation 11), which can be rewritten as follows:

« ^K pi

I ₉ )

In order to be able to obtain construction data for nuclear reactors, it will generally be necessary to carry out iterative calculations and to base these iterative calculations on the choice of the initial cylinder radius r with the poison, the density J of the poison and the number T of cylinders with the poison. In addition, the assumption that the cylinders with the poison should be self-shielding represents a lower limit for the density of the poison, and this lower limit is material-dependent. The reason for this is that the degree of self-shielding is proportional to the product of the density and the absorption cross-section of the material which is used as a poison in the reactor, and the absorption cross-section is dependent on the material. In order to achieve a certain degree of self-shielding, the density of the reactor poison can be selected to be lower, the higher the neutron absorption cross-section of the re-

909838/1028

Ί 9Ü9109

actuator poison is. "If you consider a certain reactor poison Material or isotope specifies is, as can be seen from equation 10), by specifying a lower limit for the density of the poison at the same time given an upper limit for the initial cylinder radius r.

Depending on the special requirements that are to be met, there are many different arrangements for the cylinders with the reactor poison possible. For example, you can arrange the reactor poison in separate rods or elements, you can put the nuclear fuel Arrange in a circular ring and then insert the cylinder with the poison into the nuclear fuel, you can use rod-shaped 'Rilchen from the reactor poison in the nuclear fuel, and one can also, for example, the reactor poison molecularly in the nuclear fuel disperse.

Various materials can be used as keactor poisons, Only boron, samarium and gadolinium are mentioned here. Gadolinium appears particularly favorable, since its neutron absorption cross-section is particularly high, so that it is already relatively good self-shielding is achieved at low gadolinium concentrations. In addition, the neutron absorption cross-section increases of gadolinium for neutrons, the energy of which exceeds the thermal energy, very quickly, so that the capture of high-energy neutrons by the gadolinium is very low.

A practical embodiment will now be described. In a nuclear reactor containing 464 fuel bundles, burnable reactor poisons were used in 106 fuel bundles. Each fuel bundle contained 36 fuel rods. Gadolinium, which was used in the form of gadolinium oxide GdJJ , was selected as the reactor poison. The powdered gadolinium oxide was mixed with the powdered uranium oxide used as fuel, and from this mixture nuclear fuel pills were made, with which fuel rods were filled. At the low concentrations of gadolinium oxide that were used (between 0.95 and 1.55% by weight), it represented gadolinium oxide

90 98 38/1028

1ÜU9109

and the uranium dioxide ÜO _? represents a solid solution, so that the Gd "CL, was completely molecularly dispersed in the U0", so there were no agglomerates of Gd _? O were formed. The effective radius r of the cylinder with poison was then initially equal to the radius of the

fuel rod j of the nuclear fuel and the reactor poison contained in common. In the example described here, this was Cadius about 6.35 now. ιΊϊό this initial cadius and with aer low density, in which the gadolinium was used, practically a self-shielding of the reactor poison occurred.

In the different zones shown in the figure HA and HB the poison density and the number of poison cylinders must be changed. This was through achieved a change in those fuel rods that poisons the reactor contained, and also by changing the concentration of the reactor poisons within the fuel rods.

In order to achieve the proper distribution of the poisons, two different types of poison fuel bundles have been used. In the middle of the reactor core, where the neutron air density is high, fuel bundles with a relatively high concentration of reactor poisons were used. These fuel bundles are referred to as "H ' ⁴ '" bundles in the following. They are shown in Figures 12A and 12B. In the peripheral areas of the reactor core where the neutron flux is low

it is, fuel bundles were used in which the concentration of the reactor poisons was lower. These bundles are ^{hereinafter referred to as ':} "L ·' bundles and are shown in Figures 13A and 13B.

FIG. 12A shows the axial distribution of gadolinium in three different., Poison-containing rods A, B and C, which are shown in FIG A "H" fuel bundle can be used throughout the figure 12B is a top plan view of such a "!!" bundle showing like the rods with the poison in such a bundle of fuel are arranged radially.

909838/1028

BATH ORIGINAL

_ ₂₇ _ ι yuy ί 09

Figure 13A now shows aie axial distribution of the reactor poison it in the beiuen, υ rods and .c, uie in aen ^{L: i} -Bündeln used ■ jeraeri, währenu in the figure 13B is aargestellt, xfie uie beiaen t.-ibe D and;:, radial in such a ^; -L ^{; i} -ßrennstoffbündel are arranged.

As should be noted that only three different densities of burnable Reactor poison are needed. FIG. 14 shows how the fuel rods according to FIGS. HA and 12A are constructed. she consist of a tubular sleeve 131 j containing nuclear fuel in a number of cylindrical fuel pellets 132.

The distribution of the reactor poison, as shown in FIGS. 11A and 12A, is achieved aadurcn ₅ d.8.Q> the correct number of fuel pills with the appropriate concentration of reactor poisons is poured into the various axial zones of the fuel sleeves 131. Since only three different concentrations of reactor poisons are required, the production of fuels, which contain reactor poison, is very simplified.

Iru present example of the lüo reaktorrriftenthaltenaen -brennstoffbündeln ^l} so-called 6 - ¹ U ¹ "-bundle with a high Reaktorgiftkonzeritration, in the inner radial zone Ir;. LeiciiförinirT are distributed, (see Figures HA and HB) üie restlicnen ^ O fuel bundles are so-called '^' bundles with a low concentration of poison in the reactor, which are distributed equally in the outer rial zone kv.

iilo example should now be specified, such as the number of cylinders with reactive poisons and the concentration in these cylinders in the zone (3a, Ir) (Figures HA and HB) from the equations Ib and 19 can be determined: If the mean weutronenfluf in zone (3a, Ir) at full reactor power 0 = 2.5 χ IU ^ j: eutronen / ciiii: - see fraud, and if an operating period at the rated output should be 1 year, then t is the same

7 2ü 2

ü _s ' ( ^l j χ 3 χ 10 see and 0 t = b, l χ 10 neutrons / cm. if the initial radius of a cylinder with the poison is 0.635 cm

909838/1028 BADORIÖtNAL ' ^?

This is the required concentration of gadolinium with a high neutron capture cross-section, which is present in natural gadolinium with a percentage probability of 30.4 % . The required density Q of natural gadolinium is

J η

therefore carries

The corresponding density of uranium atoms in the UO is about 2.4 χ 10
therefore

■ 28-

the required density of the reactor poison is calculated from equation 18) as follows:

6.1 χ 10 ²⁰ ₂₀

_ = _; __ - _. £ c ^ 2.4 x 10 atoms / ccm

0t,

_ 2.4 χ 10

20th

20th
10 atoms / cc

The weight ratio between Gd ₂ O and UO ₂ is

7.9 x 10
2.4 χ 10

20th

22nd

238 + 2 (16)

or something

Since, in the present exemplary embodiment, the radius of the cylinder with the poison is quite large (0.635 cm), equation 18) leads to an excessively large neutron flow into this cylinder during the beginning of an operating period. The weight fraction · of 2.2 % must therefore be reduced by the factor g from equation 17).

It can be seen from FIG. 10 that the factor g is approximately 0.7 for a radius of 0.6-35 cm. If you now multiply the $ 2.2 by 0.7, you get 1.54 % _s and this value agrees well with the specification of 1.55 % for the zone (3a, Ir) from FIG. 12A.

The number T of cylinders with reactor poisons per square cm of reactor cross-sectional area is required to achieve the catchy control strength k in zone (3a, Ir). (Figure 9) to cause, can from equation 19). For example, if the burn

909838/1028

ι y υ y ί υ y

is supposed to be QlOO megawatt-days per ton in this zone the local reactivity decreases at a rate of about 0.015 / 1000 megawatt-days per ton. The one required at the beginning Strength of rules for reactivity k. is therefore about 4.1 (0.015) = 0.0615. The macropopic cross-section is therefore about ^^ 0.05 cm. From this it follows from relation 19)

T = 2 (0 ^ 0 * 0615) = _3j lx ΙΟ " ³ cylinders / sqcm

In the present example, cylinders with reactor poison are only roughly every fourth fuel bundle is used, and the cross-section of each fuel bundle is about 50 square centimeters. Thus amounts to the number of cylinders with poison in each fuel bundle, in which such cylinders are used, for example

4 (25) (2.5 ¹ O ² O ₅ I χ 10 " ³ ) = 2 cylinders per bundle.

This is the density of cylinders with rectal poison in the, zone 3a, Ir of the present exemplary embodiment, as shown in FIG 12A is shown.

In the embodiment described above, the burnable poison has been arranged cylindrically to cause the control strength to decrease linearly in the same way as the reactivity of large power reactors. However, when the burnable poisons selects a different arrangement or a combination of different arrangements used _a can also curves from other course of which describe a loss of reactivity, be adapted .. If, for example, placing the reactor poison as self-shielded balls, the decrease takes place the Reaktivitätsregelung to Start of an operating period faster than at the end of an operating period. If, on the other hand, the reactor poison is arranged as a self-shielding thin strip or rod, then the reactivity control decreases practically constantly with the burnup, only then there is a prompt decrease in the control strength at the end of an operating period when the reactor poison has been used up

9098 38/1028

y υ y ι υ

»30-

Other possible arrangements are hollow cylinders ¹ or tubes, rods with an elliptical cross section or also rods with flat side surfaces such as rods with a hexagonal cross section. I-ian can also combine different sized parts of burnable reactor poisons with one another to create a given context
between the Aobrand and the Reaktivitätsrepelunr.

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BATH ORIGINAL

Claims (1)

Claims Reactor core for a nuclear reactor with an excess reactivity at the beginning of an operating period, so that a predefined reactor power can be generated during a predefined operating period, characterized in that
that when the reactor core is divided into several volume zones, at least one of these zones contains burnable reactor poison, through which at least part of the excess reactivity is is gelelt that the density of the reactor poison is chosen so that the reactor poison is self-shielding that the reactor poison is spatially arranged so that the decrease in its control strength with
the burn-up is proportional to the decrease in the reactivity of the nuclear fuel with the burn-up. that the initial surface of the poison in each poison-containing volume zone of the excess reactivity to be controlled in this zone
is proportional, and that the number of isotopes used as the poison is chosen so that the poison at the end of a
Operating period has been used up. Reactor core according to Claim 1, characterized in that gadolinium is used as the reactor poison is. 3. reactor core according to claim 1 or 2, characterized in that that "the poison is arranged in a cylindrical shape. 4. reactor core according to claim 1, 2 or 3j thereby
characterized in that the reactor poison with the
Nuclear fuel is mixed. 5. reactor core according to claim 4, characterized in that the reactor poison and the nuclear fuel form a solid solution. 909838/1028 ί y u y ι ο 9 ö. Reactor core according to claim 5 _S, characterized in that the reactor poison is used in the form of its oxide. 7. reactor core according to claims 1, known thereby draws that the density of the reactor poison in each Volume zone containing reactor poison is given as a first approximation by the following equation: 0 t where P is the density of the poison atoms, r is the initial radius of the cylinder with the poison, t is the duration of an operating period and 0 is the mean value of the neutron flux during an operating period, and that the number of cylinders with poison in each zone is given as a first approximation by the following equation: where T is the number of cylinders with poison per unit cross Sectional surface of the volume zone, J [^ the total macroscopic Absorption cross-section for thermal neutrons of all materials present in the volume zone and k. the mean of the initial local reactivity in the zone to be controlled by the burnable reactor poison. 909838/1028