JP2013217832A - Fast reactor core and fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a coolant density decline reactivity coefficient even in the case of high burn-up, to turn an absolute value of a Doppler reactivity coefficient to a sufficiently large value, and to suppress the decline of safety intrinsic when a temperature rises.SOLUTION: A fast reactor core includes a plurality of fuel assemblies 10 each having a plurality of first fuel rods 20a and at least one second fuel rod 20g. Each of the first fuel rods 20a has a first cladding tube and first reactor core fuel housed inside the first cladding tube, and further includes at least one of a lower deceleration material and an upper deceleration material in a solid shape provided on a lower side and an upper side of the first reactor core fuel, respectively. Each of the second fuel rods 20g includes a second cladding tube, second reactor core fuel housed inside the second cladding tube, and a lower blanket and an upper blanket provided on a lower side and an upper side of the second reactor core fuel, respectively.

Description

  The present invention relates to a fast reactor core and its fuel assembly.

  Fast reactors have high fuel proliferation and can effectively use uranium resources, but the coolant void reactivity coefficient, which is an important safety feature, is positive, that is, the coolant density lowering reactivity coefficient is positive. The absolute value increases as the degree of burnup increases.

  Here, when the coolant density ρ decreases by Δρ and the multiplication factor k increases by Δk, the coolant density decrease reactivity coefficient is given by (Δk / k) / (Δρ / ρ), and Δk Is positive, the coolant density reduction reactivity coefficient is positive.

  As with light water reactors, one of the fast reactor design conditions is that negative feedback reactivity is required when the core temperature increases, that is, it has inherent safety.

  Here, the intrinsic safety is a safety function that is based on natural physical principles and functions even without external power or power. Specifically, there are those caused by natural circulation of liquid or natural air circulation in the atmosphere, evaporation of water, thermal expansion or gravity drop of the substance, thermal radiation in the substance or accumulated energy.

  The feedback of reactivity is referred to as positive reactivity feedback when the fission increases as a change from the reference state, and negative reactivity feedback when the fission decreases.

  When the core temperature is increased, the reactivity of the coolant density lowering becomes positive, while the expansion effect of the fuel and the Doppler effect of the fuel work and these are generally negative. For this reason, even in a general fast reactor, the overall reactivity is kept negative.

  However, in a high burnup core such as a core without fuel replacement, the absolute value of the coolant density reduction reactivity coefficient increases and the absolute value of the Doppler coefficient due to the Doppler effect decreases. Negative reactivity feedback is decreasing.

  Here, the Doppler coefficient is given by T · dk / dT when the multiplication factor increases by dk when the temperature increases by dT. When dk is negative, the Doppler coefficient is negative.

  In Patent Document 1, for the purpose of increasing the absolute value of the Doppler reactivity coefficient, the fuel assembly includes a normal fuel rod and a hydride rod. In the hydride rod, the position of the hydride is approximately 1 from the upper end of the core to the core height. A technique is disclosed in which only the upper part of the core centering on the position of / 4 is disclosed. However, this technique has a problem that the growth rate decreases because the spectrum of neutrons in the core becomes soft.

Japanese Patent No. 3021283

  Unlike light water reactors that use water with a high neutron moderation capacity as a coolant, fast reactors use sodium with a low neutron moderation capacity as a coolant to cause the next fission without reducing the energy of the neutrons generated by fission. This is a nuclear reactor that increases the growth of plutonium.

  This is because the higher the energy of neutron, the more difficult the neutron absorption reaction occurs, and the fuel proliferation reaction increases. In the fast reactor, the continuous operation period and the fuel use period can be greatly increased by the proliferation of the fuel, but it is necessary to consider the coolant reactivity especially in the core design compared to the light water reactor.

  In particular, the point that the reactivity at the time of voiding or density reduction of the coolant is positive except for the small reactor is different from the light water reactor.

  In particular, in a high burnup core such as a core without fuel replacement, the absolute value of the coolant density lowering reactivity increases due to the high burnup and the absolute value of the Doppler effect decreases. The absolute value of the negative reactivity at decreases. Further, it has been found that, in the event of any accident in which the core temperature rises abnormally, it is important to design the reactivity of the coolant density lowering as small as possible in order to reduce the influence at the time of the accident.

  FIG. 19 is a graph showing changes in the reactivity coefficient of the conventional core depending on the burnup.

  Here, the fuel assembly constituting this core has 169 fuel rods, and each fuel rod has a core fuel of 70 cm in height and upper and lower blankets of 50 cm in height above and below the core fuel, respectively. The gas plenum is arranged on the upper part of the upper blanket.

  The core fuel is zirconium alloy U-10 wt% Zr of enriched uranium, and the upper and lower blankets are zirconium alloy U-10 wt% Zr of deteriorated uranium.

  FIG. 19 shows a core with no refueling in an operation cycle of 60 years, with a burnup of a fast reactor coolant density reduction reactivity coefficient and a Doppler coefficient of about 200 GWD / t on the core average and about 70 GWD / t on the blanket average. The comparison between the initial stage and the end of combustion stage is shown. It can be seen that at the end of combustion, the coolant density lowering reactivity increases and the absolute value of the Doppler coefficient decreases.

  This core has a high burnup equivalent to 2.5 times in the core and three-fourths in the blanket as compared with the average burnup of the fast prototype reactor “Monju” about 80 GWD / t.

  In the following, the reactivity coefficient at the end of combustion in FIG. 19 is compared with “Monju”.

  Compared with the “Monju” Doppler coefficient of − (0.0057 to 0.0076) T · dk / dT, the core Doppler coefficient is −0.0031 T · dk / dT, which is about half as small. It has become.

Moreover, when the reactivity coefficient of the coolant density reduction of the core is converted to the same definition as the coolant temperature coefficient of “Monju”, it becomes about 1 × 10 ⁻⁵ Δk / k / ° C., the maximum value of “Monju” being 14 × Compared to 10 ⁻⁷ Δk / k / ° C., it is about 7 times.

  The two cores have different specifications, such as different fuel compositions, and not all of the above reactivity coefficient differences are due to differences in burnup, but it is clear that burnup is one of the main factors. ing.

  FIG. 20 is a graph showing the change in the reactivity change at the time of temperature rise depending on the operation period. A curve indicated by a broken line T indicates an increase in reactivity due to a decrease in coolant density, a curve indicated by an alternate long and short dash line D indicates a decrease in reactivity due to a Doppler effect, and a curve indicated by a solid line S indicates a total change in reactivity. . When the operating period is over 50 years, the total reactivity change has turned positive.

  When the degree of burnup is increased in this way, the inherent safety at the time of temperature rise tends to decrease due to an increase in the coolant density decrease reactivity coefficient and a decrease in the absolute value of the Doppler reactivity coefficient.

  Therefore, the present invention suppresses the coolant density reduction reactivity coefficient even when increasing the burnup, and makes the absolute value of the Doppler reactivity coefficient sufficiently large so that it is unique to the temperature rise. The purpose is to suppress a decrease in safety.

  To achieve the above object, a fast reactor core according to the present invention includes a plurality of first fuel rods each having at least one first fuel rod and a plurality of second fuel rods extending in the vertical direction and parallel to each other. Each of the first fuel rods includes a first cladding tube that extends in a vertical direction, and a first core fuel that is contained in the first cladding tube and contains a fissile material. Furthermore, a solid lower moderator containing a neutron moderator material provided in the first cladding tube and below the first core fuel, and housed in the first cladding tube and the first cladding tube. Each of the second fuel rods includes a second cladding tube extending in a vertical direction, and at least one of a solid upper moderator including a neutron moderator material provided on the upper side of one core fuel. The fissile material contained in the second cladding tube A second core fuel, a lower blanket contained in the second cladding tube and provided below the second core fuel and containing a fissile material parent material, and housed in the second cladding tube. And an upper blanket including a fissile material parent material provided on the upper side of the second core fuel.

  The fast reactor fuel assembly according to the present invention includes at least one first fuel rod and a plurality of second fuel rods extending in the vertical direction and parallel to each other, and each of the first fuel rods is vertically A first cladding tube extending in the direction, and a first core fuel contained in the first cladding tube and containing a fissile material, and further housed in the first cladding tube and the first cladding tube. A solid lower moderator containing a neutron moderator provided on the lower side of the core fuel, and a solid state containing a neutron moderator contained in the first cladding tube and provided on the upper side of the first core fuel. And each of the second fuel rods includes a second cladding tube extending in a vertical direction and a fissile material housed in the second cladding tube. The second core fuel and the second core fuel stored in the second cladding tube A lower blanket provided below the second core fuel and containing a fissile material parent material; and contained in the second cladding tube and provided above the second core fuel and containing a fissile material parent material. And an upper blanket.

  According to the present invention, even when a high burnup is achieved, the reactivity coefficient for reducing the coolant density is suppressed, and the absolute value of the Doppler reactivity coefficient is set to a sufficiently large value. A decrease in safety can be suppressed.

1 is a horizontal sectional view of a fuel assembly constituting a first embodiment of a fast reactor core according to the present invention. 1 is an elevational sectional view of a fuel assembly constituting a first embodiment of a fast reactor core according to the present invention. 1 is an elevational sectional view of a first fuel rod in a fuel assembly constituting a first embodiment of a fast reactor core according to the present invention. FIG. 3 is an elevational sectional view of a second fuel rod in the fuel assembly constituting the first embodiment of the fast reactor core according to the present invention. 1 is a vertical sectional view schematically showing a first embodiment of a fast reactor core according to the present invention. It is a table | surface which shows the core characteristic of 1st Embodiment of the fast reactor core which concerns on this invention. It is a graph which shows the change by the burnup of the reactivity coefficient of 1st Embodiment of the fast reactor core which concerns on this invention. FIG. 5 is an elevational sectional view of a first fuel rod in a fuel assembly constituting a second embodiment of a fast reactor core according to the present invention. FIG. 6 is an elevational sectional view of a first fuel rod in a fuel assembly constituting a third embodiment of a fast reactor core according to the present invention. FIG. 6 is an elevational sectional view of a first fuel rod in a fuel assembly constituting a fourth embodiment of a fast reactor core according to the present invention. FIG. 10 is an elevational sectional view of a second fuel rod in a fuel assembly constituting a fourth embodiment of a fast reactor core according to the present invention. FIG. 6 is an elevational sectional view schematically showing a fourth embodiment of a fast reactor core according to the present invention. FIG. 10 is an elevational sectional view of a first fuel rod in a fuel assembly constituting a fifth embodiment of a fast reactor core according to the present invention. It is a horizontal sectional view showing a 1/6 system of composition of a 6th embodiment of a fast reactor core concerning the present invention. FIG. 10 is an elevational sectional view of a first fuel assembly constituting a sixth embodiment of a fast reactor core according to the present invention. It is an elevation sectional view of the 1st fuel rod which constitutes a 6th embodiment of the fast reactor core concerning the present invention. FIG. 10 is an elevational sectional view of a second fuel assembly constituting a sixth embodiment of a fast reactor core according to the present invention. It is an elevation sectional view of the 2nd fuel rod which constitutes a 6th embodiment of the fast reactor core concerning the present invention. It is a graph which shows the change by the burnup of the reactivity coefficient of the conventional core. It is a graph which shows the change by the operation period of the reactivity change at the time of a temperature rise.

  Hereinafter, embodiments of a fast reactor core and a fuel assembly according to the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a horizontal cross-sectional view of a fuel assembly constituting a first embodiment of a fast reactor core according to the present invention.

  The fuel assembly 10 has 169 fuel rods 20 of the same outer shape extending in the vertical direction and arranged in parallel with each other in a staggered pattern. The 169 fuel rods 20 are surrounded by a radially outer periphery of a mantle portion 11 having a regular hexagonal cross section called a trumpet tube.

  There are two types of fuel rods 20, that is, 13 first fuel rods 20 a and the remaining 156 second fuel rods 20 g. Specifically, as shown in FIG. 1, the first fuel rods 20a are arranged in a rotational symmetry of 60 degrees so as not to be adjacent to each other and to be mixed with the second fuel rods 20g. Note that the number of the first fuel rods 20a is not limited to 13, and may be set so that the effect can be exhibited in design.

  FIG. 2 is an elevational sectional view of the fuel assembly constituting the present embodiment.

  The fuel rod 20 is supported from below by the support member 15. Further, the fuel rod 20 is surrounded by the mantle portion 11 on the outer periphery in the radial direction. A lower shielding body 13 is provided on the inner side of the outer jacket portion 11 and below the fuel rod 20. Similarly, an upper shielding body 14 is provided on the upper side of the fuel rod 20.

  The lower part of the mantle 11 has a circular shape with a reduced outer periphery so that it can be inserted into a core support plate (not shown). In this portion, an entrance nozzle 12 is provided, and a liquid metal such as a coolant such as liquid sodium flows into the fuel assembly 10 from the entrance nozzle 12 and rises outside the fuel rods 20.

  FIG. 3 is an elevational sectional view of the first fuel rod in the fuel assembly constituting this embodiment.

  The lower moderator 31, the core fuel 22, and the upper moderator 32 are arranged from the lower side in the state of being accommodated in the cladding tube 21 extending in the vertical direction, and the upper space of the upper moderator 32. Is a gas plenum 26 that accommodates gas, which is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  FIG. 4 is an elevational sectional view of the second fuel rod in the fuel assembly constituting the present embodiment.

  The lower blanket 23, the core fuel 22 and the upper blanket 24 are arranged in the vertical direction from the lower side in a state of being accommodated in the cladding tube 21 extending in the vertical direction. It is a gas plenum 26 that contains the product gas. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  The height of each core fuel 22 of the first fuel rod 20a and the second fuel rod 20g is, for example, 70 cm, and the lower moderator 31, the upper moderator 32, and the second fuel rod of the first fuel rod 20a. The height of each of the 20 g lower blanket 23 and the upper blanket 24 is, for example, 50 cm.

  Each core fuel 22 of the first fuel rod 20a and the second fuel rod 20g is a zirconium alloy U-10 wt% Zr of enriched uranium. The lower blanket 23 and the upper blanket 24 of the second fuel rod 20g are, for example, zirconium alloy U-10 wt% Zr of deteriorated uranium.

  The height of each of the lower moderator 31 and the upper moderator 32 of the first fuel rod 20a is, for example, 50 cm, which is the same as the height of the lower blanket 23 and the upper blanket 24 of the second fuel rod 20g.

The material of the lower moderator 31 and the upper moderator 32 of the second fuel rod 20a is a neutron moderator, for example, zirconium hydride ZrH _1.7 .

As for zirconium hydride, a metal compound of deuterium D such as zirconium deuteride ZrD _1.7 can be used as a neutron moderator instead of hydrogen.

  In this case, since deuterium D has a smaller deceleration effect than hydrogen H, the effect of improving the reactivity is smaller. However, a phenomenon in which low energy neutrons are generated by the neutron moderator and the output of the adjacent blanket or core fuel increases, that is, an output spike can be suppressed.

  Further, the moderator is not limited to zirconium hydride but may be titanium hydride, calcium hydride, or the like as long as it is a material having a small absorption cross-sectional area and a moderating effect and suitable for loading into the core.

  FIG. 5 is an elevational sectional view schematically showing the present embodiment, and shows the entire core 100.

  On the outer side in the circumferential direction, a radial shield region 7 is disposed, and surrounds the core fuel region 1 from the circumferential direction. A lower blanket region 2 is disposed below the core fuel region 1, and an upper blanket region 3 is disposed above. A lower shield region 5 is formed below the lower blanket region 2, a gas plenum region 8 is formed above the upper blanket region 3, and an upper shield region 6 is disposed above the lower shield region 5.

  Here, the lower shield region 5, the lower blanket region 2, the core fuel region 1, the upper blanket region 3, the gas plenum region 8 and the upper shield region 6 surrounded by the radial shield region 7 constitute the core 100. A plurality of fuel assemblies 10 are formed.

  Therefore, the core fuel region 1 is a constituent element of the fuel assembly 10 in the range of the core fuel region 1. Similarly, in the lower blanket region 2, the components of the fuel assembly 10 in the range of the lower blanket region 2 are the components thereof, and the first fuel rod is the same as the lower blanket 23 of the second fuel rod 20g. The lower moderator 31 of 20a is also a constituent element. Similarly, in the upper blanket region 3, the components of the fuel assembly 10 in the range of the upper blanket region 3 are the components, and the first fuel rod is the same as the upper blanket 24 of the second fuel rod 20g. The upper moderator 32 of 20a is also a component.

  That is, as a feature of the core 100 constituted by the fuel assembly 10, in the lower blanket region 2 and the upper blanket region 3, the second fuel rod 20g including the axial blanket and the first fuel including the neutron moderator It is a feature that the bar 20a is mixed.

  Next, the effect of this embodiment configured as described above will be described.

  FIG. 19 is a graph showing a change in the reactivity coefficient of the conventional core depending on the burnup as already described. In the initial stage, the coolant density reactivity is mainly contributed by the core, but the contribution of the axial blanket increases to the same extent as that of the core by combustion, and more than doubles in the entire core. The increase in the reactivity of the core is due to a change in fuel composition due to combustion, that is, a change in composition from uranium to plutonium and an increase in fission products.

  The increase in the reactivity of the axial blanket is caused by the same cause, but in the early stage, for example, in the case of depleted uranium, the reactivity effect is small because fissionable material is hardly present at about 0.2% by weight.

  On the other hand, the absolute value of the Doppler coefficient has a small contribution of the axial blanket in the initial stage, but the contribution of the core has decreased and the contribution of the axial blanket has increased toward the end of combustion. This is an influence from both sides because uranium 238 having a large Doppler effect is reduced in the core and low energy neutrons contributing to the Doppler effect are absorbed by accumulation of plutonium and fission products.

  On the other hand, the axial blanket has the same effect, but rather the increase in output due to the generation of plutonium works in the direction of increasing the Doppler effect. However, the reduction of the Doppler effect in the core portion accompanying the increase in the burnup is dominant, and the Doppler coefficient decreases with combustion in the entire core.

  Based on the above fact, this embodiment loads the neutron moderator only on the axial blanket. Neutron moderators are known to enable reduced coolant density reduction reactivity and increased absolute value of Doppler reactivity.

  However, as the energy of neutrons decreases, the proliferation of the core decreases at the same time. On the other hand, it is at the end of combustion of the core as shown in FIG. Considering both, it is possible to minimize the deterioration of the core proliferation by loading the neutron moderator only on the axial blanket that contributes to the end of combustion and not loading the entire core.

  FIG. 6 is a table showing the core characteristics of the present embodiment. The average burnup is 200 GWd / t in the core, 70 GWd / t in each of the upper axial blanket and the lower axial blanket, and is a high burnup core with a continuous operation period of 60 years.

  In this embodiment, the coolant density reduction reactivity coefficient at the end of combustion of the core is reduced by 25% compared to the conventional core, the absolute value of the Doppler coefficient is increased by 68%, and the feedback reactivity coefficient when temperature rises is negative. And the inherent safety has increased.

  The cause of the decrease in the coolant density reduction reactivity coefficient and the increase in the absolute value of the Doppler coefficient is apparent from the improvement effect of loading the neutron moderator on the axial blanket part compared with the conventional core.

  FIG. 7 is a graph showing a change in the reactivity coefficient according to the burnup according to the present embodiment.

  The horizontal axis represents the reactor operation period (years), and the vertical axis represents the total reactivity change due to the increase in reactivity due to the decrease in coolant density when the temperature rises and the decrease in reactivity due to the Doppler effect. It is shown in the same unit.

  The three curves correspond to those obtained by changing the loading ratio of the neutron moderator as a parameter. The curve indicated by the solid line A is 0% neutron moderator loading, the curve indicated by the broken line B is 3% neutron moderator loading, and the one-dot chain line C is 8% neutron moderator loading Is shown.

  Here, the ratio of the loading amount of the neutron moderator is the total amount of the total blanket fuel contained in all the fuel rods 20 and the total amount of the total neutron moderator in each of the lower blanket region 2 and the upper blanket region 3. The ratio of the amount of neutron moderator.

  Specifically, in the present embodiment, since there is one type of fuel assembly 10 for the entire core 100, the number of first fuel rods 20a and the number of second fuel rods 20g in the fuel assembly 10 This is the ratio of the number of the first fuel rods 20a to the total number. Since the number of the fuel rods 20 in the fuel assembly 10 is 169 and the number of the first fuel rods 20a is 13, in this case, the ratio of the loading amount of the neutron moderator is about 8%. .

  As shown in FIG. 7, in order to prevent the total reactivity from becoming positive even if the reactivity error at the time of temperature rise is taken into consideration, the necessary loading amount of the neutron moderator blanket is 3% or more.

  In addition, when the neutron moderator is loaded, the fuel loading of the blanket is reduced by that amount, and the increase in low-energy neutrons reduces the proliferation. The reactivity decreases, and the same 60-year refueling-free operation becomes impossible.

  In order to avoid this, it is necessary to increase the fuel loading amount by increasing the diameter of the fuel rod and maintain the proliferation ability. However, this increases the fuel loading amount and deteriorates the economy.

  Analytical calculations show that the fuel loading to maintain growth is increased by approximately 4% per 1% neutron moderator loading on the blanket. In view of this, it is appropriate that the upper limit of loading of the neutron moderator is approximately 10% from the viewpoint of economy.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Second Embodiment]
The present embodiment is a modification of the first embodiment, and instead of the 13 first fuel rods 20a that the fuel assembly 10 in the first embodiment has, 13 first fuel rods 20a. A fuel rod 20b is provided.

  FIG. 8 is an elevational sectional view of the first fuel rod 20b in the fuel assembly constituting the second embodiment of the fast reactor core according to the present invention.

  The second fuel rod 20g in the present embodiment is the same as that in the first embodiment (FIG. 4). In the first fuel rod 20b in the present embodiment, a lower moderator 31, a core fuel 22, and an upper blanket 24 are arranged from the lower side in the vertical direction inside the cladding tube 21 extending in the vertical direction. The upper space of the blanket 24 is a gas plenum 26 that contains a gas that is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  That is, the difference between the first fuel rod 20b in the present embodiment and the first fuel rod 20a in the first embodiment is that an upper blanket 24 is provided in place of the upper moderator 32, and the lower moderator is provided. The moderator is provided only at 31.

  The upper blanket 24 in the present embodiment is the same as the upper blanket 24 in the first embodiment.

  Zirconium hydride is obtained by occluding hydrogen in zirconium, and it is known that dissociation of hydrogen proceeds at a high temperature of about 700 ° C.

  The temperature of the coolant around the neutron moderator rises to around 500 ° C. because the temperature of the lower shaft blanket is about 400 ° C. or less and the core fuel is just cooled at the upper shaft blanket position.

  These temperatures are not high enough to cause dissociation, but by loading zirconium hydride only at the lower shaft blanket position where the temperature is low, the dissociation rate can be suppressed even if the temperature becomes high during an accident. can do.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Third Embodiment]
FIG. 9 is an elevational sectional view of the first fuel rod 20c in the fuel assembly constituting the third embodiment of the fast reactor core according to the present invention.

  The first fuel rod 20c in the present embodiment includes a lower moderator 31, a lower shock absorber 34, a core fuel 22, an upper shock absorber 35, and a lower moderator 31 from the lower side in the vertical direction inside the cladding tube 21 extending in the vertical direction. An upper moderator 32 is disposed, and the upper space of the upper moderator 32 is a gas plenum 26 that contains a gas that is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  This embodiment is a modification of the first embodiment, in which the vertical dimension between the core fuel and the moderator adjacent to the core fuel is ensured.

  At the end of the core fuel in contact with the neutron moderator, there are many low energy neutrons due to the moderating effect of the adjacent neutron moderator. The lower the neutron energy, the larger the cross-sectional area of the nuclear reaction, and the more likely the reaction takes place, so there is a tendency for an output peak to occur at the end of the core fuel in contact with the neutron moderator. When the peak of this output is large, there is a possibility of exceeding the allowable output.

  The mean free path of neutrons in the fast reactor is approximately 5 cm, and if the neutron moderator is arranged 5 cm or more away from the end of the core fuel, the generation of output peaks can be suppressed.

  In this embodiment, the vertical height of the lower cushioning material 34 and the upper cushioning material 35 is 5 cm or more, for example, 7 cm. As a result, the lower moderator 31 and the core fuel 22, and the core fuel 22 and the upper moderator 32 are arranged at a distance of 5 cm or more.

  The material of the lower cushioning material 34 and the upper cushioning material 35 is metallic zirconium.

  In the present embodiment, since the lower moderator 31 and the core fuel 22, and the core fuel 22 and the upper moderator 32 are arranged at a distance of 5 cm or more, the generation of the output peak as described above can be suppressed.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Fourth Embodiment]
FIG. 10 is an elevational sectional view of the first fuel rod 20d in the fuel assembly constituting the fourth embodiment of the fast reactor core according to the present invention. FIG. 11 is an elevational sectional view of the second fuel rod 20e in the fuel assembly constituting the present embodiment.

  This embodiment is an embodiment applied to a core called an axially inhomogeneous core in which core fuel is divided into two vertically and an internal shaft blanket is provided inside the core.

  The fuel assembly 10 has two types of fuel rods, a first fuel rod 20d and a second fuel rod 20e.

  As shown in FIG. 10, the first fuel rod 20 d is disposed in the cladding tube 21 extending in the vertical direction, and from the lower side in the vertical direction, the lower moderator 31, the lower core fuel 22 a, the intermediate moderator 33, and the upper core. A fuel 22b and an upper moderator 32 are arranged, and an upper space of the upper moderator 32 is a gas plenum 26 that accommodates a gas that is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  As shown in FIG. 11, the second fuel rod 20e is disposed inside the cladding tube 21 extending in the vertical direction, and from the lower side in the vertical direction, the lower blanket 23, the lower core fuel 22a, the intermediate blanket 25, and the upper core fuel 22b. In addition, an upper blanket 24 is disposed, and an upper space of the upper blanket 24 is a gas plenum 26 that accommodates a gas that is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  FIG. 12 is an elevational sectional view schematically showing the present embodiment.

  On the outer side in the circumferential direction, a radial shielding body region 7 is arranged. A lower blanket region 2 is disposed below the lower core fuel region 1a, an intermediate blanket region 4 is disposed between the lower core region 1a and the upper core region 1b, and an upper blanket region 3 is disposed above the upper core region 1b. ing.

  A lower shielding body region 5 is formed below the lower blanket region 2, a gas plenum region 8 is formed above the upper blanket region 3, and an upper shielding body region 6 is disposed above the lower shielding region. .

  Here, the lower shield region 5, the lower blanket region 2, the lower core region 1 a, the intermediate blanket region 4, the upper core region 1 b, the upper blanket region 3, the gas plenum region 8 and the upper shield surrounded by the radial shield region 7. The body region 6 is formed by the fuel assembly 10.

  In the present embodiment, the first fuel rod 20d and the second fuel rod 20e, which are components of the fuel assembly 10, are the components.

  Therefore, the lower blanket region 2 includes the lower blanket region 2, the intermediate blanket region 4, and the upper blanket region 3 in the lower moderator 31 of the first fuel rod 20d, respectively. The middle moderator 33 and the upper moderator 32 are mixed with the lower blanket 23, the intermediate blanket 25, and the upper blanket 24 of the second fuel rod 20e.

  A conventional axially inhomogeneous core is a core made of a fuel assembly including only the second fuel rods 20e without the first fuel rods 20d. This axially inhomogeneous core has advantages such as high proliferation in the internal shaft blanket, small change in reactivity due to combustion, and flattening of the maximum neutron flux in the core.

  In this embodiment, a moderator is mixed in an intermediate blanket region 4 corresponding to a conventional internal shaft blanket.

  By mixing the first fuel rods 20d and the second fuel rods 20e, it is possible to improve the reactivity change when the temperature of the core rises.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Fifth Embodiment]
FIG. 13 is an elevational sectional view of the first fuel rod 20f in the fuel assembly constituting the fifth embodiment of the fast reactor core according to the present invention.

  The fuel assembly 10 in the present embodiment has one type of the first fuel rod 20 f for the fuel rod 20. The first fuel rod 20f in FIG. 13 has a lower mixed material 41, a core fuel 22 and an upper mixed material 42 arranged in the vertical direction from the lower side in the cladding tube 21 extending in the vertical direction. The upper space of the mixed material 42 is a gas plenum 26 that contains a gas that is a fission product. The lower end of the cladding tube 21 is sealed with a lower end plug 27, and the upper end is sealed with an upper end plug 28.

  The lower mixed material 41 and the upper mixed material 42 correspond to a mixture of the blanket fuel of the first fuel rod 20a and the moderator of the second fuel rod 20g in the first embodiment.

  The blanket fuel of the first fuel rod 20a and the moderator of the second fuel rod 20g at the same ratio as the number ratio of the first fuel rod 20a and the second fuel rod 20g in the first embodiment By using the mixture of the lower mixed material 41 and the upper mixed material 42 in the present embodiment, the same effects as in the first embodiment can be obtained.

  Moreover, since there is only one type of fuel rod, the administrative burden in assembling the fuel assembly 10 is reduced.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Sixth Embodiment]
FIG. 14 is a horizontal sectional view showing a 1/6 system of the configuration of the sixth embodiment of the fast reactor core according to the present invention.

  In the core region, a first fuel assembly 10b, a second fuel assembly 10c, an adjustment control rod 51, a reactor stop control rod 52, and a radial shield 60 are arranged as core components. Each is a regular hexagonal shape having the same external dimensions in a plan sectional view, and adjacent and opposing surfaces are arranged in parallel to each other. Further, the position shown by X is the center of the core, and the core components in the range excluding the shown X are arranged around this position.

  FIG. 15 is an elevational sectional view of the first fuel assembly 10b constituting the present embodiment.

  The first fuel assembly 10b is different from the fuel assembly 10 in the first embodiment only in the fuel rods to be stored. That is, the fuel assembly 10 in the first embodiment has the first fuel rod 20a and the second fuel rod 20g, but the first fuel assembly 10b in the present embodiment is the first fuel rod 20a. Only the fuel rod 20a is provided.

  FIG. 16 is an elevational sectional view of the first fuel rod 20a constituting the present embodiment. The configuration of the first fuel rod 20a is the same as that of the first fuel rod 20a in the first embodiment.

  FIG. 17 is an elevational sectional view of the second fuel assembly 10c constituting the present embodiment.

  The second fuel assembly 10c is different from the fuel assembly 10 in the first embodiment only in the fuel rods to be stored. In other words, the fuel assembly 10 in the first embodiment has the first fuel rod 20a and the second fuel rod 20g, but the second fuel assembly 10c in the present embodiment is the second fuel rod 20a. It has only fuel rods 20g.

  FIG. 18 is an elevational sectional view of the second fuel rod 20g constituting the present embodiment. The configuration of the second fuel rod 20g is the same as that of the second fuel rod 20g in the first embodiment.

  The first fuel assembly 10b and the second fuel assembly 10c configured as described above are arranged in the core 100 as shown in FIG.

  Similar to the number ratio of the first fuel rods 20a and the second fuel rods 20g in the fuel assembly 10 in the first embodiment, in the present embodiment, the first fuel assembly 10b and the second fuel assembly By setting the body number ratio with the body 10c, the same effect as in the first embodiment can be obtained.

  Further, in the present embodiment, since both the first fuel assembly 10b and the second fuel assembly 10c are composed of one type of fuel rod, the first fuel assembly 10b and the second fuel assembly 10c In each assembly, the administrative burden on the arrangement of fuel rods inside the assembly is reduced.

  As described above, according to the present embodiment, even when the burnup is increased, the coolant density lowering reactivity coefficient is suppressed and the absolute value of the Doppler reactivity coefficient is set while suppressing the decrease in proliferation. It can be set to a sufficiently large value, and as a result, a decrease in inherent safety when the temperature rises can be suppressed.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention.

  For example, although the example in the case of enriched uranium as a core fuel has been shown, it may be plutonium or a mixture of uranium and plutonium.

  Moreover, although the example in the case of depleted uranium as a blanket fuel was shown, natural uranium may be sufficient, and the case of thorium or thorium and depleted uranium, or thorium and natural uranium may be sufficient.

  Furthermore, although the example in the case where the form of the fuel is a metal fuel has been shown, an oxide fuel, a nitride fuel or a carbide fuel may be used.

  In the first embodiment, the moderator region is provided in the upper and lower blanket regions of the first fuel rod, and in the second embodiment, the moderator region is provided in the lower blanket region of the first fuel rod. Alternatively, it may be provided only in the upper blanket region. In the fourth embodiment, the moderator is provided in the lower, middle, and upper blanket areas. However, the moderator is provided in any one of these, or in any two of these. An area may be provided.

  Moreover, you may combine the characteristic of each embodiment. For example, a single core may be configured by combining the sixth embodiment using a fuel assembly that uses one type of fuel rod and the other embodiment using a fuel assembly that uses two types of fuel rods. . In addition, the first fuel rods of the respective embodiments may be mixed as necessary to constitute one fuel assembly.

  Further, the core is not limited to the two types of fuel rods, and may be partially a reactor core combined with a fuel assembly that uses one type of fuel rods of the mixed material according to the fifth embodiment.

  Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention.

  These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 ... Core fuel area 1a ... Lower core area 1b ... Upper core area 2 ... Lower blanket area 3 ... Upper blanket area 4 ... Middle blanket area 5 ... Lower shielding body area 6 ... Upper shield region 7 ... Radial shield region 8 ... Gas plenum region 10 ... Fuel assembly 10b ... First fuel assembly 10c ... Second fuel assembly DESCRIPTION OF SYMBOLS 11 ... Outer part 12 ... Entrance nozzle 13 ... Lower shielding body 14 ... Upper shielding body 15 ... Support member 20 ... Fuel rod 20a ... 1st fuel rod (moderator) Replaced with
20b ... 1st fuel rod (only the lower part is replaced with a moderator)
20c ... 1st fuel rod (with buffer area)
20d ... first fuel rod (replacement with moderator by axially inhomogeneous core)
20e ... second fuel rod (for axially heterogeneous core)
20f ... 1st fuel rod (mixed type)
20g ... second fuel rod (conventional type)
21 ... cladding tube 22 ... core fuel 22a ... lower core fuel 22b ... upper core fuel 23 ... lower blanket 24 ... upper blanket 25 ... middle blanket 26 ... gas plenum 27 ... Lower end plug 28 ... Upper end plug 31 ... Lower moderator 32 ... Upper moderator 33 ... Intermediate moderator 34 ... Lower cushion 35 ... Upper cushion 41 ..Lower mixed material 42 ... Upper mixed material 51 ... Control rod for adjustment 52 ... Control rod for reactor stop 60 ... Radial shield 100 ... Core

Claims (13)

A plurality of first fuel assemblies each having at least one first fuel rod and a plurality of second fuel rods extending in the vertical direction and parallel to each other;
Each of the first fuel rods includes a first cladding tube extending in a vertical direction, and a first core fuel contained in the first cladding tube and containing a fissile material, and further, the first fuel rod A solid lower moderator containing a neutron moderator material provided on the lower side of the first core fuel, and an upper side of the first core fuel contained in the first cladding tube And at least one of a solid upper moderator containing a neutron moderator,
Each of the second fuel rods is accommodated in a second cladding tube extending in the vertical direction, a second core fuel contained in the second cladding tube and containing a fissile material, and in the second cladding tube. A lower blanket including a fissile material parent material provided on the lower side of the second core fuel, and a fissile material parent housed in the second cladding tube and provided on the upper side of the second core fuel. An upper blanket containing the substance,
A fast reactor core characterized by that. The first fuel rod is disposed between the upper moderator and the first core fuel, and includes a third core fuel containing a fissile material, the first core fuel, and the third core fuel. Further comprising either an intermediate blanket containing a fissile material parent material or an intermediate moderator containing a neutron moderator material disposed between the core fuel and
The second fuel rod is disposed between the upper blanket and the second core fuel, and includes a fourth core fuel containing a fissile material, the second core fuel, and the fourth core fuel. And an intermediate blanket disposed between and comprising a fissile material parent material,
The fast reactor core according to claim 1.   The buffer portion is provided in at least one of the first core fuel and the lower moderator, and between the first core fuel and the upper moderator. The fast reactor core according to claim 2.   The fast reactor core according to claim 3, wherein the height of the buffer portion is at least 5 cm. A plurality of first fuel assemblies each having at least one first fuel rod and a plurality of second fuel rods extending in the vertical direction and parallel to each other;
Each of the first fuel rods includes a first cladding tube extending in a vertical direction, and a first core fuel contained in the first cladding tube and containing a fissile material, and further, the first fuel rod A solid moderator containing a neutron moderator and a lower mixed material containing depleted uranium provided under the first core fuel, and housed in the first clad and Having at least one of a solid moderator and an upper mixed material containing depleted uranium and including a neutron moderator material provided on the upper side of the first core fuel;
Each of the second fuel rods is accommodated in a second cladding tube extending in the vertical direction, a second core fuel contained in the second cladding tube and containing a fissile material, and in the second cladding tube. A lower blanket including a fissile material parent material provided on the lower side of the second core fuel, and a fissile material parent housed in the second cladding tube and provided on the upper side of the second core fuel. An upper blanket containing the substance,
A fast reactor core characterized by that.   6. The fuel rod according to claim 1, wherein 3% to 10% of all fuel rods in the plurality of first fuel assemblies are the first fuel rods. The described fast reactor core.   6. The method according to claim 1, further comprising a plurality of second fuel rods each having a plurality of first fuel rods extending in parallel to each other and having no moderator. The fast reactor core according to any one of the above.   The total number of first fuel rods included in the plurality of first fuel assemblies in the entire core is 3% to 10% of the total number of fuel rods included in the fuel assemblies in the entire core. The fast reactor core according to claim 7, wherein the core is fast reactor.   9. The core fuel includes at least one of enriched uranium oxide, metal or carbide, mixed oxide of plutonium and depleted uranium, mixed metal or mixed carbide. The fast reactor core according to any one of the above.   The fast reactor core according to any one of claims 1 to 9, wherein the neutron moderating material includes at least one of zirconium hydride, zirconium deuteride, titanium hydride, and calcium hydride.   The fast reactor core according to any one of claims 1 to 10, wherein the fissile material parent material contains at least thorium. Comprising at least one first fuel rod and a plurality of second fuel rods extending in the vertical direction and parallel to each other;
Each of the first fuel rods includes a first cladding tube extending in a vertical direction, and a first core fuel contained in the first cladding tube and containing a fissile material, and further, the first fuel rod A solid lower moderator containing a neutron moderator material provided on the lower side of the first core fuel, and an upper side of the first core fuel contained in the first cladding tube And at least one of a solid upper moderator containing a neutron moderator,
Each of the second fuel rods is accommodated in a second cladding tube extending in the vertical direction, a second core fuel contained in the second cladding tube and containing a fissile material, and in the second cladding tube. A lower blanket including a fissile material parent material provided on the lower side of the second core fuel, and a fissile material parent housed in the second cladding tube and provided on the upper side of the second core fuel. An upper blanket containing the substance,
A fast reactor fuel assembly characterized by that. The first fuel rod is disposed between the upper moderator and the first core fuel, and includes a third core fuel containing a fissile material, the first core fuel, and the third core fuel. Further comprising either an intermediate blanket containing a fissile material parent material or an intermediate moderator containing a neutron moderator material disposed between the core fuel and
The second fuel rod is disposed between the upper blanket and the second core fuel, and includes a fourth core fuel containing a fissile material, the second core fuel, and the fourth core fuel. And an intermediate blanket disposed between and comprising a fissile material parent material,
The fast reactor fuel assembly according to claim 12.

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