SE1650775A1 - Westinghouse Electric Sweden AB, -, 721 63 Vastergs, SE - Google Patents

Description

15 20 25 30 35 such a long time and has to be replaced earlier. ln order to take care of the excess reactivity, usually so called burnable absorbers are used, for example Gd in the form of GdgOß.

Traditionally, the enrichment of the nuclear fuel has been constant in the axial direction (i.e. the same enrichment in the whole fuel rod).

However, it is known that the enrichment may vary in the radial di- rection (for example fuel assemblies located at the outer periphery of the core may have a lower enrichment). lt is also known that fuel rods may have so-called blanket zones (for example at the ends of the fuel rods), where no nuclear fuel is located. lt is also known that the amount of burnable absorber may vary both axially and radially across the core.

When the nuclear fuel has been used over for example one fuel cycle, fissile material (U-235) has been consumed. The consumption of the fissile material is not constant in the axial direction. Therefore the concentration of the fissile isotope(s) in the nuclear fuel (the “enrich- ment”) may vary in the axial direction in nuclear fuel that has been used over one or more fuel cycles.

Different designs have been suggested for the initial core in a nuclear reactor.

US 4,324,615 describes an initial core arrangement of a nuclear re- actor. The fuel assemblies in the central region of the reactor core are grouped into a plurality of fuel units each of which being consti- tuted by four adjacent fuel assemblies. The fuel assemblies in each fuel unit are replaced one by one at each fuel replacement cycle with a new fuel assembly. The initial core is so arranged that each fuel unit has one gadolinium containing high enrichment fuel assembly and three low enrichment fuel assemblies containing no gadolinium.

This arrangement permits a stable reactor operation substantially equivalent to that obtained in the state of equilibrium (equilibrium core), even with an initial core. 10 15 20 25 30 35 EP 1647994 A2 describes an advanced initial core fuel configuration for improving the fuel management efficiency and thus economics for a nuclear reactor. A method of implementing such an initial core in- volves providing a plurality of fuel assemblies having different aver- age enrichments of uranium 235 and arranging the fuel assemblies in an initial core configuration structured to emulate a known equilib- rium reload cycle core at least in terms of spatial reactivity distribu- tion. The resulting average enrichment within the initial core ranges from below about 1.0 percent weight of uranium 235 to about 5.0 percent weight of uranium 235.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved initial core for a nuclear power boiling water reactor. An object is thereby to obtain a very precise design of the initial core. A further object is to reduce the need for using burnable absorbers in the initial core.

An object is thereby to be able to extend the length of a fuel cycle in the nuclear reactor without the need for a relatively large amount of burnable absorbers.

The above objects are achieved with the method defined in claim 1.

Since the enrichment distribution is varied in the axial direction, an initial core is obtained, which has an enrichment distribution which is very similar to that of the equilibrium core. Therefore, a very precise design of the initial core is provided. Since the enrichment distribu- tion of the initial core also axially is similar to that of the equilibrium core, the need for burnable absorbers is reduced. The traditional method of using a relatively large amount of burnable absorbers that varies axially is rougher than and therefore not as accurate as the variation of the enrichment according to the present invention. Since the enrichment distribution in the initial core is very precise, a very precise control of the axial power shape is obtained, and the fuel can be used for a relatively long operation cycle. Furthermore, since the 10 15 20 25 30 35 enrichment distribution in the initial core is similar to that of the equi- librium core, the control of reactivity oscillations between fuel cycles is improved, and the reactor operation is simplified. lt should be noted that a burnable absorber may also be used when the present invention is implemented. However, since the enrichment is varied as explained above, a lower amount of burnable absorber can be used for controlling the excess reactivity during the first oper- ation cycle.

Also the following should be noted.

The boiling water reactor is preferably a light water reactor.

The initial core means the first core of a nuclear reactor, when the reactor is first taken into operation. Alternatively, the initial core can refer to a situation when all the fuel in the nuclear reactor is being replaced by new fuel.

The equilibrium core is a concept well known in this technical field, and refers to the core of the nuclear reactor when the nuclear reactor has been used a plurality of fuel cycles, often at least 4 or 5 fuel cycles, and the core therefore has reached an equilibrium state.

A fuel cycle refers in this document to the time during which the ar- rangement of nuclear fuel in the core is normally unchanged. After each fuel cycle part of the nuclear fuel in the core is replaced by new nuclear fuel, and nuclear fuel may also be rearranged in the core between each fuel cycle. A fuel cycle may for example last 1-2 years.

By enrichment is meant the concentration of the fissile isotope(s) in the nuclear fuel. For uranium fuel this is often expressed as the pro- portion, in weight percent, of U-235 in the uranium. ln this document, we use the concept “enrichment” also for the nuclear fuel that has been used one or more fuel cycles in the nuclear reactor. ln this case the enrichment is thus the proportion of the fissile isotope(s) (U-235 for uranium) that remains in the fuel. 10 15 20 25 30 35 By nuclear fuel enrichment distribution in the equilibrium core is meant the distribution of the concentration of the fissile isotope(s) in the nuclear fuel in the equilibrium core, i.e. the proportion of the fis- sile isotope(s) in the nuclear fuel in different parts of the equilibrium core.

Similarly, by the nuclearfuel enrichment distribution in the initial core is meant the distribution of the concentration of the fissile isotope(s) in the nuclear fuel in the initial core, i.e. the proportion of the fissile isotope(s) in the nuclear fuel in different parts of the initial core.

When we mention the enrichment distribution in the axial direction for a fuel assembly, we mean (in case the fuel rods in the fuel assembly have fuel with different enrichment or different enrichment distribu- tion), the distribution of the average enrichment for the fuel in the fuel rods in the fuel assembly. This means that for each level (perpendic- ular to the axial direction) the average enrichment for the fuel assem- bly is taken into account, and the distribution of the enrichment is thus how this average enrichment varies in the axial direction of the fuel assembly.

When it is said that the nuclear fuel enrichment distribution for differ- ent fuel assemblies for the initial core is such that the enrichment distribution in the equilibrium core is at least approximately obtained, we mean both that the total quantity of enrichment in the fuel assem- bly (or categories of fuel assemblies) in the initial core is similar to a corresponding fuel assembly (or category of fuel assemblies) in the equilibrium core and also that the axial distribution of the enrichment is similar to that of the axial enrichment in the corresponding fuel assembly or assemblies in the equilibrium core. However, this does not mean that it is necessary to exactly mimic the enrichment distri- bution in the corresponding fuel assembly or fuel assemblies in the equilibrium core. For example, the fuel assemblies used during one or more fuel cycles may have “enrichment spikes” close to the top and bottom of the fuel assembly (for example due to neutron leakage 10 15 20 25 30 35 during operation), which it may not be necessary to mimic when de- signing the initial core. However, the idea of the invention is that gen- erally the axial enrichment distribution obtained in the fuel assembly or the fuel assemblies in the equilibrium core is taken into account and the corresponding fuel assembly or fuel assemblies in the initial core have an enrichment distribution that approximately corresponds to the enrichment distribution of the corresponding fuel assembly or fuel assemblies of the equilibrium core.

According to one manner of carrying out the method of the invention, in step a) the axial enrichment distribution is determined for at least 3, preferably at least 4, different categories of fuel assemblies in the equilibrium core. lt is of course also possible to determine the enrich- ment distribution for all fuel assemblies in the equilibrium core. How- ever, since many of the fuel assemblies in the equilibrium core have a similar enrichment distribution, it may simplify the design to con- sider categories of fuel assemblies.

According to a further manner of carrying out the method, the differ- ent categories of fuel assemblies in the equilibrium core concern fuel assemblies that have undergone different number of fuel cycles. Fuel assemblies that have undergone the same number of fuel cycles have a similar enrichment distribution. lt is therefore beneficial to consider these fuel assemblies as one category.

The number of different fuel cycles during which the nuclear fuel has been used herein also includes O fuel cycles (i.e. fresh fuel that has not yet been used during a fuel cycle). One of the mentioned catego- ries may thus be fresh fuel.

According to a further manner of carrying out the method, in step b) enrichment distributions are determined for at least 3, preferably at least 4, different categories of fuel assemblies in the initial core. lt simplifies the design of the initial core if categories of fuel assemblies are considered instead of all individual fuel assemblies. 10 15 20 25 30 35 According to a further manner of carrying out the method, the enrich- ment distribution for the different categories of fuel assemblies in the initial core is determined such that the enrichment distribution of each category is at least approximately similar to the enrichment distribu- tion in a corresponding category in the equilibrium core. By consid- ering the fuel assemblies for the initial core as belonging to different categories corresponding to categories of fuel assemblies in the equilibrium core, the design of the initial core is facilitated.

According to a further manner of carrying out the method, the deter- mined initial core comprises at least the following categories of fuel assemblies: a first category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 0 fuel cycles, i.e. fresh fuel, a second category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 1 fuel cycle, a third category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 2 or 3 fuel cycles. This alternative constitutes a simple and advantageous man- ner of categorizing the fuel assemblies for the initial core. lt should be noted that the fuel assemblies in the equilibrium core that have undergone two or three fuel cycles may be considered together as one category. Alternatively, these fuel assemblies may be considered as two different categories: one category that has undergone two fuel cycles and one category that has undergone three fuel cycles. ln case they are considered as two different categories, then it is ben- eficial if also the initial core is designed with two different categories of fuel assemblies: one corresponding to fuel assemblies that have undergone two fuel cycles and one corresponding to fuel assemblies that have undergone three fuel cycles. 10 15 20 25 30 35 According to a further manner of carrying out the method, the deter- mined initial core also comprises the following category of fuel as- sembly: a fourth category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 4 or 5 fuel cycles. lt is a further advantage to have such a fourth category of fuel assemblies. Similarly to the explanation above, it is possible to con- sider the fuel assemblies that have undergone four or five fuel cycles as one category or as two categories.

According to a further manner of carrying out the method, in step b) the enrichment distribution in the axial direction is determined for at least some fuel assemblies such that there are at least three different enrichments at three different levels in the axial direction in the fuel assembly. By having at least three such levels, the approximation of the enrichment distribution in the corresponding fuel assemblies of the equilibrium core can be made quite accurate. lt may also be an advantage not to have too many different levels of enrichment in the axial direction, since too many levels makes the production of the nuclear fuel more complicated. For example, three to five such levels may be suitable.

Each of the at least three different enrichments may for example form at least 15%, preferably at least 20 %, of the length of the fuel zone of the fuel assembly in the axial direction (by fuel zone is here meant the part of the fuel assembly that contains nuclear fuel).

According to a further manner of carrying out the method, each of the fuel assemblies in at least two of the different categories in the initial core has at least three different enrichments at three different levels in the axial direction in the fuel assembly. ln this manner it is possible to quite accurately approximate the enrichment distribution of the equilibrium core. 10 15 20 25 30 35 According to a further manner of carrying out the method, each of the fuel assemblies in at least the second and third categories in the ini- tial core has at least three different enrichments at three different levels in the axial direction in the fuel assembly. The second and third categories may correspond to fuel assemblies that have undergone a number of fuel cycles, such that the axial distribution of the enrich- ment in the equilibrium core varies to a substantial degree. lt is there- fore an advantage that the corresponding categories of fuel assem- blies in the initial core have at least three different levels of different enrichment in the axial direction.

According to a further manner of carrying out the method, all the fuel assemblies in at least one of the categories of fuel assemblies in the initial core have substantially, or exactly, the same enrichment distri- bution. ln this way, the design of the initial core is simplified.

According to a further manner of carrying out the method, for each of the categories of fuel assemblies in the initial core it is the case that all the fuel assemblies in the category have substantially, or exactly, the same enrichment distribution. Thereby the design of the initial core is further simplified.

According to a further manner of carrying out the method, each of the categories of fuel assemblies in the initial core comprises at least 50 fuel assemblies. By having a quite large number of fuel assemblies in each category, the design of the initial core is further simplified.

The invention also concerns a method of operating a nuclear power boiling water reactor with an initial core of nuclear fuel. This method comprises: establishing the nuclear fuel for an initial core in a nuclear power boiling water reactor in accordance with the method of any one of the above described examples, arranging the thereby established nuclear fuel in the initial core of the nuclear reactor, 10 15 20 25 30 35 10 starting the operation of the nuclear reactor and operating the nuclear reactor to produce energy. This method thus provides an ad- vantageous manner of operating the nuclear reactor with an initial core. The method has advantages corresponding to those described above.

According to one manner of carrying out the method of operating a nuclear power boiling water reactor with an initial core of nuclear fuel, step a) comprises carrying out measurements on an equilibrium core in a nuclear energy boiling water reactor in operation. By carrying out measurements on an equilibrium core in a nuclear energy boiling wa- ter reactor, a very accurate determination of the enrichment distribu- tion in the equilibrium core can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows schematically the enrichment distribution of different categories of fuel assemblies in the equilibrium core and in the initial core according to an embodiment of the present invenüon.

Fig. 2 shows a schematic flow chart of a manner of carrying out the present invention.

DESCRIPTION OF EXAMPLES OF THE INVENTION With reference to Fig. 1 and Fig. 2, a manner of carrying out a method according to the present invention will now be described.

First, nuclear fuel enrichment distribution in an equilibrium core for a nuclear power boiling water reactor is determined, preferably by us- ing a computer. There are computer programs known to a person skilled in the art that can be used for determining this. Preferably, measurements are carried out on an equilibrium core in a nuclear 10 15 20 25 30 35 11 BWR. These measurements may include measurements of the neu- tron flux in different parts of the core. The measurement values are input into the computer program in order to determine the enrichment distribution for the equilibrium core. ln the computer program, the design of the core of the nuclear reactor which is to be started is taken into account (e.g. type and number of fuel assemblies used).

When determining the enrichment distribution in the equilibrium core, also the enrichment distribution in the axial direction is determined, at least approximately.

Preferably, the different fuel assemblies in the equilibrium core are considered in different categories. For each category a typical, or average, fuel assembly is determined.

Fig. 1 illustrates on the vertical axis the core height in percent and on the horizontal axis the enrichment of U-235 in weight percent for different categories of fuel assemblies. The enrichment relating to the equilibrium core is shown in broken lines and the enrichment relating to the initial core is shown in continuous lines. ln this example four different categories in the equilibrium core are considered. The curve (line) 10 illustrates the (average) enrichment distribution for fuel assemblies that have not undergone any fuel cy- cle, i.e. fresh fuel.

The curve 12 illustrates the (average) enrichment distribution for fuel assemblies that have undergone one fuel cycle.

The curve 14 similarly illustrates the (average) enrichment distribu- tion for fuel assemblies that have undergone two or three fuel cycles.

According to the present example, these fuel assemblies are thus considered as one category.

The curve 16 shows the (average) enrichment distribution of fuel as- semblies that have undergone four or five fuel cycles. According to 10 15 20 25 30 35 12 this examples, these fuel assemblies are thus considered as one cat- egory.

According to the present example, the fuel assemblies of the equilib- rium core are thus considered as constituting four different catego- ries.

Next, the enrichment distribution for different categories of fuel as- semblies for the initial core is determined. This is done in the follow- ing manner.

Also for the initial core, the fuel assemblies are, according to this example, considered as constituting four different categories, corre- sponding to the four categories described above in the equilibrium core. The enrichment distribution in each category of fuel assembly for the initial core is determined such that the enrichment distribution of the corresponding category in the equilibrium core is at least ap- proximately obtained. With reference to Fig. 1, this is done as follows.

For a first category of fuel assemblies, the enrichment distribution is similar to that of fresh fuel. The enrichment distribution is illustrated by the curve (line) 20.

For a second category of fuel assemblies, the enrichment distribution is determined according to the curve 22. As can be seen, this curve shows that there are three different enrichments 22a, 22b and 22c at different levels in the axial direction in the fuel assembly. With this enrichment distribution, the enrichment distribution of the curve 12 for the equilibrium core is approximated, i.e. the enrichment distribu- tion of fuel assemblies that have been used during one fuel cycle. lt should be noted that the curve 12 includes some enrichment peaks P1, P2 at the top and at the bottom, which it is not desired to use for the initial core. Consequently, the enrichment distribution of the curve 22 does not include such peaks.

Furthermore, the initial core also has a third category of fuel assem- blies with an enrichment distribution as illustrated by the curve 24. 10 15 20 25 30 35 13 This enrichment distribution corresponds to the enrichment distribu- tions of fuel assemblies in the equilibrium core that have undergone two or three fuel cycles. As can be seen, the enrichment illustrated by the curve 24 has three levels 24a, 24b and 24c with different en- richments. Also in this case the enrichment peaks at the top and bot- tom of the curve 14 are not used for the enrichment distribution 24.

The initial core also has a fourth category of fuel assemblies. The enrichment of the fuel assemblies in this category is illustrated by the curve 26. The enrichment distribution of this category in the initial core corresponds at least approximately to the enrichment distribu- tion of fuel assemblies in the equilibrium core that have undergone four or five fuel cycles. lt should be noted that in this case, the en- richment in the equilibrium core for the fuel assemblies in this cate- gory is quite low. lt has therefore been found appropriate, according to this example, to use a constant axial enrichment for the fuel as- semblies in this category for the initial core. lt should be noted that according to this example, all the fuel assem- blies in a certain category of the initial core has the same axial en- richment distribution. ln the shown example, the fuel assemblies in the second and third categories of the initial core have three different enrichments at three different levels of the fuel assemblies. However, it is of course pos- sible to vary the axial enrichment distribution more than so, in order to even more closely approximate the corresponding enrichment in the equilibrium core.

Each of the categories of fuel assemblies in the initial core has a large number of fuel assemblies, for example over 100 fuel assem- blies.

When the enrichment distribution for the different fuel assemblies in the initial core have been determined, as explained above, nuclear fuel assemblies with this enrichment distribution are produced. The 10 15 14 thus produced nuclear fuel assemblies are arranged in the initial core of a nuclear reactor. lt should be noted that the different categories of fuel assemblies are distributed according to a pattern in the initial core of the nuclear reactor. Since such patterns of arranging the nuclear fuel is known to a person skilled in the art, this will not be described more closely in this document.

When the fuel assemblies have been arranged in the core of the nu- clear reactor, the operation of the nuclear reactor may start. The nu- clear reactor is then operated to produce energy.

The present invention is not limited to the examples described herein, but can be varied and modified within the scope of the following claims.

Claims (15)

10 15 20 25 30 35 15 Claims

1. A method of establishing the nuclear fuel for an initial core in a nuclear power boiling water reactor, comprising the following steps: a) determine, at least approximately, for different fuel assem- blies in an equilibrium core, the nuclear fuel enrichment distribution, b) determine the nuclear fuel enrichment distribution for differ- ent fuel assemblies for the initial core, such that when these fuel as- semblies are arranged in the nuclear reactor in the initial core, the enrichment distribution determined in step a) is at least approxi- mately obtained, wherein when performing step a) the enrichment distribution in the axial direction is at least approximately determined for at least some fuel assemblies in the equilibrium core, and wherein when performing step b) an enrichment distribution in the axial direction is determined for at least some fuel assemblies, such that the enrichment varies in the axial direction.

2. The method of claim 1, wherein in step a) the axial enrichment distribution is determined for at least 3, preferably at least 4, different categories of fuel assemblies in the equilibrium core.

3. The method of claim 2, wherein the different categories of fuel assemblies in the equilibrium core concern fuel assemblies that have undergone different number of fuel cycles.

4. The method according to any one of the preceding claims, wherein in step b) enrichment distributions are determined for at least 3, preferably at least 4, different categories of fuel assemblies in the initial core.

5. The method of claims 2 or 3 in combination with claim 4, wherein the enrichment distribution for the different categories of fuel assemblies in the initial core is determined such that the enrichment distribution of each category is at least approximately similar to the enrichment distribution in a corresponding category in the equilibrium core. 10 15 20 25 30 35 16

6. The method of claim 5, wherein the determined initial core com- prises at least the following categories of fuel assemblies: a first category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 0 fuel cycles, i.e. fresh fuel, a second category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 1 fuel cycle, a third category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 2 or 3 fuel cycles.

7. The method of claim 6, wherein the determined initial core also comprises the following category of fuel assembly: a fourth category in which the enrichment distribution corre- sponds at least approximately to the enrichment distribution of fuel assemblies in the equilibrium core that have undergone 4 or 5 fuel cycles.

8. The method of any one of the preceding claims, wherein in step b) the enrichment distribution in the axial direction is determined for at least some fuel assemblies such that there are at least three dif- ferent enrichments at three different levels in the axial direction in the fuel assembly.

9. The method of claim 8 in combination with claim 4, 5, 6 or 7, wherein each of the fuel assemblies in at least two of the different categories in the initial core has at least three different enrichments at three different levels in the axial direction in the fuel assembly.

10. The method of claim 9 in combination with claim 6 or 7, wherein each of the fuel assemblies in at least the second and third catego- ries in the initial core has at least three different enrichments at three different levels in the axial direction in the fuel assembly. 10 15 20 25 17

11. The method of claim 4, 5, 6, 7, 9 or 10, wherein all the fuel assemblies in at least one of the categories of fuel assemblies in the initial core have substantially, or exactly, the same enrichment distri- buüon.

12. The method of claim 11, wherein for each of the categories of fuel assemblies in the initial core it is the case that all the fuel as- semblies in the category have substantially, or exactly, the same en- richment distribution.

13. The method of claim 4, 5, 6, 7, 9, 10, 11 or 12, wherein each of the categories of fuel assemblies in the initial core comprises at least 50 fuel assemblies.

14. A method of operating a nuclear power boiling water reactor with an initial core of nuclear fuel comprising: establishing the nuclear fuel for an initial core in a nuclear power boiling water reactor in accordance with the method of any one of the preceding claims, arranging the thereby established nuclear fuel in the initial core of the nuclear reactor, starting the operation of the nuclear reactor and operating the nuclear reactor to produce energy.

15. The method of claim 14, wherein step a) comprises carrying out measurements on an equilibrium core in a nuclear energy boiling wa- ter reactor in operation.