JPH0640139B2 - Fuel assembly with partial length gadolinia applied to pressurized water reactor - Google Patents

Description

The present invention relates to a fuel assembly with a partial length gadolinia for a pressurized water reactor (hereinafter referred to as PWR).

[Prior Art] Reactor reactivity control is carried out by two independent methods, namely, control rod cluster operation and adjustment of boron concentration in the primary coolant. In addition to these control methods, a burnable poison containing a substance (boron) with a large thermal neutron absorption cross section is used in order to make the moderator temperature coefficient negative at the time of excess reactivity control and high-temperature output operation in the early core life. I'm using a stick.

Further, for the same purpose as above, from the viewpoint of waste treatment of burnable poison rods, fuel economy, etc., UO ₂ fuel pellets mixed with Gd ₂ O ₃ having a large thermal neutron absorption cross section are mixed with UO.
₍₂ ) Fuel rods (hereinafter referred to as gadolinia-containing fuel rods) filled with fuel pellets are also being used. However, this gadolinia-containing fuel assembly is intended for use in a replacement core (core after the second cycle), and fuel rods with gadolinia uniformly distributed in the axial direction over the active fuel length (hereinafter referred to as full-length gadolinia-containing fuel rods). (Hereinafter referred to as “fuel assembly”) (hereinafter, referred to as full-length gadolinia-containing fuel assembly).

[Problems to be Solved by the Invention] When a full-length gadolinia-containing fuel assembly is used in a replacement core, the change in the axial power distribution due to combustion is relatively small due to the small number of loading bodies. . That is,
Fig. 4 shows a graph of a typical example of axial fuel calculation when a full length gadolinia-containing fuel assembly is used in the replacement core. The axial offset [(upper half core power-lower core half power) / ( Power of the upper half of the core + power of the lower half of the core), which is a measure of the degree of distortion of the axial power distribution]
Has a small fluctuation range, and F _z (the axial peaking factor, which is the maximum value of the relative average output in the axial direction) has not become large, so there is no nuclear problem.

However, in the case of the initially loaded core, from the viewpoint of keeping the moderator temperature coefficient negative, the amount of fuel assemblies with full-length gadolinia used is large, and due to the difference in coolant temperature between the upper and lower ends of the core, A large negative reactivity feedback is applied, and combustion progresses from the negative side (about -10%) in the axial offset at the beginning of life, so that the gadolinia (Gd) on the lower side burns faster than the upper side, and the axial output distribution Changes greatly. In Figure 5 shows the variations caused by the combustion of the axial offset (A.O.) and _{F z.}

The swing range of the axial offset is large and F _z is also large, which may cause a deviation from the control condition of the maximum core power density.

As the axial power distribution control method of PWR, usually, a target value of axial offset is set every month, and output distribution control is performed so that the axial offset is within the target value ± 5% range. If it largely fluctuates, it is necessary to change the target value frequently, which is not preferable for core management.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a fuel assembly containing a partial length gadolinia suitable for use from the time of initial loading core.

[Means for Solving the Problems] As shown in FIG. 1, a fuel assembly containing a partial length gadolinia of the present invention has gadolinia-free fuel pellets (UO ₂ fuel pellets with enriched uranium) in upper and lower portions 1b and 1c, as shown in FIG. The central portion 1a is composed of fuel rods filled with fuel pellets containing gadolinia (hereinafter referred to as fuel rods with gadolinia containing partial length).

X is the length of the fuel rod upper portion 1b filled with gadolinia-free fuel pellets, Y is the length of the fuel rod lower portion 1c filled with gadolinia-free fuel pellets, and is the fuel effective length that is the length of the fuel rod filled with fuel. The fuel rod central portion 1a is filled with gadolinia-containing fuel pellets. Since the fuel pellet portion without gadolinia has higher reactivity than the fuel pellet portion with gadolinia, when the length of X becomes longer than the length of Y, the axial offset shifts from the negative side to the positive side.

The ratios of X and Y are such that the axial offset (A.
O. ) Is near 0%, that is, + 3%> A. O. > 3%.

Therefore, the structure of the fuel assembly containing the partial length gadolinia of the present invention has UOs of predetermined lengths on the upper and lower portions of the fuel in the axial direction.
_Two fuel pellets are filled, and a gadolinia-containing fuel pellet which is a UO ₂ fuel pellet in which Gd ₂ O ₃ is mixed is filled in the central portion.

Maintaining X and Y from 3% to 13% of the effective core length is effective for keeping the axial offset near zero and also reducing the axial peaking factor F _z .

[Operation] UO ₂ that does not include gadolinia in the axial upper and lower parts of the fuel rod
Since the fuel pellets are filled, if the upper and lower lengths are set so that the axial offset at the beginning of the core life is close to 0%, the output in the upper half of the core and the output in the lower half of the core become almost equal. The gadolinia in the upper and lower halves of the core burn almost evenly, and the difference in gadolinia survival rate in the upper and lower halves due to combustion becomes relatively small. This reduces changes in the axial power distribution due to combustion. That is, runout width of the axial offset is reduced, F _z becomes small, even smaller core maximum linear power density.

Embodiments Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Using a one-dimensional diffusion calculation code, the fuel length X without gadolinia above the fuel and the fuel length Y without gadolinia below the fuel rod shown in Fig. 1 were used as parameters for the initially loaded core with a cycle length of 16 months. As a result of the examination, the size of the fuel rod with a partial length gadolinia suitable for the initially loaded core
Shown in the figure. In this case, X = 27.0 cm, Y = 16.2c
m, (= 364.8 cm). FIG. 3 shows changes in the axial offset and Fz with burnup in this case. In the figure, the broken line is the case of the fuel rod with full length gadolinia, the solid line is the case of the fuel rod with partial length gadolinia of this example, and the wt% of gadolinia (Gd ₂ O ₃ ) contained in the fuel pellet with gadolinia is 6%, Gd ₂ O ₃ wt% in the case of the conventional full length gadolinia-containing fuel rod is also 6
%. As shown in FIG. 3, when the fuel rod containing the partial length gadolinia was used, F _z , A. O. Both shakes are less.

[Effects of the Invention] According to the present invention described in detail above, it is possible to suppress the change in the axial power distribution due to combustion, and it is possible to solve the problem of deviation from the core maximum line power density limit. Further, the swing range of the axial offset (A.O.) becomes small, and the core management becomes easy.

[Brief description of drawings]

FIG. 1 is an explanatory view of a fuel rod with a partial length gadolinia according to the present invention, FIG. 2 is a dimensional diagram of a fuel rod with a partial length gadolinia according to an embodiment of the present invention, and FIG. F _z , A. When the fuel rod with gadolinia is used in the initial core O. FIG. 4 is a graph of burnup vs. Fz, A.F. showing a typical example in which a conventional full length gadolinia-containing fuel rod is used in a replacement core. O. Graph of burnup vs. cycle, No. 5
The figure shows the F _z , A.A. When a conventional full length gadolinia-containing fuel rod is used in the initial core. O. It is a graph of burnup. 1a ... central part of fuel rod, 1b ... upper part of fuel rod, 1c ... lower part of fuel rod.

Claims (1)

[Claims] 1. A axial upper portion of the fuel rods over the length X and the lower portion of the length Y filled with UO ₂ fuel pellets, the center portion is a UO ₂ fuel pellets mix Gd ₂ O ₃ gadolinia containing In a fuel rod filled with fuel pellets,
X and Y are in the range of 3% to 13% of the effective core length, and X is longer than Y, and the difference between X and Y is (power of upper half of core-power of lower half of core) / (core A fuel assembly with a partial length gadolinia applied to a pressurized water reactor characterized in that an axial offset defined by (upper half power + lower core half power) is adjusted so as to be maintained near zero.