JPH07209458A - Nuclear reactor fuel element - Google Patents

Abstract

PURPOSE:To reduce the rice in temperature of an adjacent covering pipe of fuel element and the emission of radioactivity into the environment, and improve the safety of a nuclear reactor by suppressing the emission of FP gas in a gas plenum when the fuel is broken. CONSTITUTION:A gas plenum vessel 7 is arranged above a fuel pellet 6 in a covering pipe 3, and a check valve formed of a stainless ball 8, a depression 9a and a hole 10a is installed to its bottom part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fast breeder reactor, and also to a fuel element for a nuclear reactor such as a light water reactor.

[0002]

2. Description of the Related Art In a conventional fuel element for a fast reactor, as shown in FIG. 7, in consideration of FP gas (fission product gas) released during fuel life, a gas plenum 1 having a sufficient volume is provided and operated. In consideration of the temperature rise due to an abnormal transient change in the inside, the fuel pellets 6, the blanket pellets 5a, 5b, etc. are provided on the cladding tube 3 having a sufficient thickness by using a material having excellent strength, for example, stainless steel. It was loaded.

[0003]

A fuel element for a conventional fast reactor has a core height (fuel pellet 6 length) of 100 cm, a gas plenum 1 length of 100 cm, and a fuel burnup of 15 a /.
0 (15% of heavy metal atoms undergoing fission), cladding tube 3 outer diameter 7.5 mm, cladding tube 3 inner diameter 6.8 mm, fuel pellet 6 diameter 6.63 mm, 0.2 per 1 nuclear fission
Seven FP gases are generated, and the generated FP gas exists in the gap between the cladding tube 3 and the fuel pellet 6 or in the gas plenum 1. The line power of the fuel element is 300 W / cm. The specific heat of the cladding tube 3 and the fuel pellet 6 is 0.13 cal / g
K, 0.08cal / gK, specific gravity 8g / cc, 1
If the theoretical density ratio of pellets is 1 g / cc and the theoretical density ratio of pellets is 95%, the heat capacity of the fuel element (only the cladding tube and fuel pellets) per unit length is 0.37 cal / cmK (1.55 J /
cmK). From the ratio of the linear output of the fuel element and the heat capacity of the fuel element per unit length, 1 s must be completely removed,
190K temperature rise.

At the end of the fuel's life, 0.0 for each fuel element.
There is 54 mol of FP gas. The volume of the gas plenum 1 is 36.3 cc, and the volume of the gap between the cladding tube 3 and the fuel pellet 6 is 1.8 cc, which is 38.1 cc in total. The temperature of the upper gas plenum 1 is 600 ° C. In this state, the pressure of the gas plenum 1 and the gap is 95 atm.

Assuming that the fuel element for a conventional fast reactor is damaged at the end of its life, when the fuel is damaged, the fuel is discharged until the pressure of the coolant and the pressure are balanced (currently 2 atm). Further, the released FP gas shall flow downstream at the speed of the coolant (5 m / s). FP gas is
It is assumed that 100% FP gas occupies the 6 sub-channels around the broken fuel pin (six 0.71 cm ² flow channel area). Heat transfer between each sub-channel shall be ignored.

About 98% of the FP gas is released into the coolant. The volume of FP gas released is 1.77 l under 600 ° C. and 2 atm. Assuming that 1.77 l of FP gas flows in a flow passage area of 0.71 cm ² at a speed of 5 m / s, the discharge time is 5.0 s. If the heat is not completely removed for 5.0 seconds, the temperature rises by 950K. When the initial temperature of the cladding tube 3 is 700 ° C, the maximum temperature of the cladding tube 3 is about 1650 ° C. In reality, some heat is removed and heat is transferred between the sub-channels, so it will be lower than this value, but stainless steel has sufficient strength at 900 ° C.
Over and over.

Therefore, the conventional fuel element for a fast reactor has the following drawbacks. (1) It is necessary to secure a sufficient design margin for the above reasons, but an excessive design margin reduces the core performance. That is, increasing the wall thickness of the cladding deteriorates the neutron economy,
The reactivity lost with combustion (combustion deficiency reactivity) increases, and the ratio of the extinction rate of the fissile material due to combustion and the production rate of the fissile material produced by the capture reaction (growth ratio) decreases. Increasing the length of the gas plenum increases the pressure loss of the reactor and increases the required performance of the primary system pump. Alternatively, if the coolant flow path is enlarged and the volume ratio of the coolant is increased so as not to increase the pressure loss, the neutron economy deteriorates. The longer fuel elements also increase fuel costs and increase the size of reactor structures, fuel handling systems, new fuel storage facilities and spent storage facilities. Furthermore, the amount of radioactive waste increases.

(2) As the fuel burnup increases, the amount of FP gas released at the time of fuel damage increases, and the effect of FP gas release increases. Conventionally, in the event of FP gas release at the time of fuel damage, since the amount of FP gas release is not large, it has been limited to relatively gentle gas impingement (event in which granular voids occur without expanding gas). On the other hand, as the performance of fuel becomes higher, the amount of FP gas released at the time of fuel damage increases, and a severe gas blanket (a phenomenon in which a gas spreads and a large void is generated) is likely to occur. The gas blanket creates a two-phase flow that can compromise heat transfer characteristics and increase the cladding temperature of adjacent fuel elements (other than broken fuel). As a result, the integrity of the cladding of adjacent fuel elements is compromised, and the likelihood of the event increasing.
On the other hand, if the temperature is raised from the viewpoint of improving the thermal efficiency, the temperature of the cladding tube at the time of fuel failure becomes higher.

(3) Most of the FP gas in the cladding tube is released into the coolant when the fuel fails. Once fuel failure and most of the FP gas in the cladding is released into the coolant. The FP gas in the coolant is isolated from the environment by the sealing function other than the cladding tube, but it also increases the release of radioactivity into the environment, albeit slightly, during normal times, and also seals other radioactivity. If the value decreases, the amount of surrounding radiation increases.

The present invention has been made in view of the above-mentioned circumstances, and solves the above-mentioned drawbacks by disposing a gas plenum container having a check valve in a fuel element.

[0011]

In the fuel element for a nuclear reactor of the present invention, a fuel body is contained in a cylindrical cladding tube whose upper and lower ends are closed by end plugs, and a gas plenum container is arranged above it. A check valve is attached to the bottom of the gas plenum container, and the check valve is mounted in a direction in which FP gas flows into the gas plenum in the gas plenum container during operation. .

Further, in the fuel element for a nuclear reactor of the present invention, a fuel body is housed in a cylindrical cladding tube whose upper and lower ends are closed by end plugs, and a gas plenum container is arranged below it. A check valve is installed on the top of the gas plenum container,
The check valve is characterized in that the FP gas is installed in a direction in which the FP gas flows into the gas plenum in the gas plenum container during operation.

Further, the check valve of the fuel element for a nuclear reactor of the present invention is characterized by being a ball valve.

[0014]

Since the check valve is installed in the gas plenum in the direction in which it flows into the gas plenum, the FP gas is usually fed from the fuel body (blanket pellets and fuel pellets are called fuel bodies in the present invention) due to the pressure difference. Move to Gas Plenum.

However, when the cladding tube is broken, the check valve is closed due to the pressure difference, so that the FP gas in the gas plenum does not come out of the fuel element. As a result, only the FP gas other than the gas plenum is released.

[0016]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The structure of the fuel element for a nuclear reactor of one embodiment shown in FIGS. 1 and 4 is the same as that of the fuel element for a normal fast reactor, and the fuel pellet 6 containing uranium (U) and plutonium (Pu) in the cladding tube 3 and depleted uranium. Blanket pellets 5a, 5b containing the above and shield elements 4a, 4b are loaded, and end plugs 2a, 2b are welded and sealed up and down. The gas plenum 1 is either on top of the upper shield element 4a (FIG. 1),
It is provided on the lower part of the lower blanket pellet 5b (FIG. 4) or both. When the gas plenum 1 is provided above the fuel pellets 6, a gas plenum container 7 made of stainless steel is loaded into the gas plenum 1, and a hole 10a is provided in a concave recess 9a below the gas plenum container 7 made of stainless steel. As shown in FIG. 1, a stainless ball 8 is fitted in the recess 9a. In the present invention, a combination of a stainless ball, a depression and a hole is called a ball valve, and there are various types such as a ball valve and a poppet for the check valve.

FIG. 4 shows an embodiment in which the gas plenum 1 is provided below the fuel pellet 6. In this case, the stainless steel ball 8, the concave recess 9b, and the hole 10b are provided in the upper portion of the gas plenum 1, and the stainless steel ball 8 is supported via the spring 12 so as to fit in the recess 9b. When the gas plenum 1 is provided above and below the fuel pellets 6, the structures of both gas plenums 1 can be combined. In the case of the embodiment shown in FIG. 1, normally,
As shown in FIG. 2, the stainless ball 8 is slightly lifted by the pressure difference, the hole 10a of the recess 9a is opened, and the fission product gas (FP gas) can be transferred from the fuel pellet 6 and the blanket pellets 5a, 5b to the gas plenum 1.

When the cladding tube 3 is damaged, the stainless ball 8 is depressed by the pressure difference as shown in FIG.
0a is blocked and the FP gas in the gas plenum 1 does not exit the fuel element. As a result, only the FP gas other than the gas plenum 1 (about 5% by volume of the gas plenum 1) is released, and the output rise due to voids and the rise in the cladding temperature of the adjacent fuel element due to the two-phase flow and the radioactivity into the environment. Emissions are also reduced.

The fuel element for a nuclear reactor of the present invention shown in FIG.
Normally, although the stainless ball 8 is on the support plate 11, as shown in FIG. 5, the hole 10b of the recess 9b opens due to the pressure difference exceeding the force of the spring 12, and the fission product gas (F
(P gas) is fuel pellets 6 and blanket pellets 5
It is possible to transfer from a, 5b to the gas plenum 1.

Further, when the covering tube 3 is damaged, as shown in FIG.
As shown in FIG. 3, the stainless ball 8 closes the recess 9b 10b by the pressure difference in the opposite direction and the force of the spring 12, and the FP gas in the gas plenum 1 does not come out of the fuel element. As a result, only the FP gas other than the gas plenum 1 (about 5% by volume of the gas plenum 1) is released, and the output rise due to voids and the rise in the cladding temperature of the adjacent fuel element due to the two-phase flow and the radioactivity into the environment. Emissions are also reduced.

In the reactor fuel element of the present invention, the gas plenum length is adjusted so that the volume of the gas plenum container is the same as the volume of the gas plenum 1 of the conventional fast reactor fuel element, and the other conditions are the same. , F released
P gas was 0.0025 mol and was 0.083 l under 600 ° C. and 2 atm. Channel area 0.71 cm
Assuming that ² is flowing at a speed of 5 m / s, the emission time is 0.23 s. If the heat is not completely removed during this period, 44
K temperature rises. When the initial temperature of the cladding is 700 ° C, the maximum cladding temperature is about 740 ° C. Stainless steel is sufficiently lower than around 900 ° C, which has sufficient strength.

[0022]

[Effects of the Invention] Even in the event of abnormal transient changes and accidents, the safety of the reactor core is ensured with a sufficient margin due to the multiple safety equipment in the reactor. However, although it is extremely unlikely to occur, it is conceivable that fuel damage may impair the soundness of the adjacent fuel element under the conservative assumption.

The fuel element for a nuclear reactor of the present invention suppresses the release of the FP gas in the gas plenum at the time of fuel failure, thereby increasing the output due to voids and increasing the cladding temperature of the adjacent fuel element due to the two-phase flow. The amount of radioactivity released into the environment will be reduced, and the safety of fast reactors will be improved.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an embodiment of the first invention of the present invention.

FIG. 2 is an explanatory view showing a flow of FP gas at a normal time of the fuel element shown in FIG.

FIG. 3 is an explanatory diagram showing the flow of FP gas when the fuel element shown in FIG. 1 is damaged.

FIG. 4 is a vertical sectional view showing an embodiment of the second invention of the present invention.

5 is an explanatory diagram showing a flow of FP gas in a normal state of the fuel element shown in FIG.

6 is an explanatory diagram showing the flow of FP gas when the fuel element shown in FIG. 4 is damaged.

FIG. 7 is a vertical cross-sectional view of a conventional fuel element for a fast reactor.

[Explanation of symbols]

 1 Gas Plenum 2a, 2b End Plug 3 Cladding Tube 5a, 5b Blanket Pellet 6 Fuel Pellet 7 Gas Plenum Container 8 Stainless Steel Ball 9a, 9b Recess 10a, 10b Hole 11 Hole 12 Spring

Claims (3)

[Claims] 1. A fuel body is housed in a cylindrical cladding tube whose upper and lower ends are closed by end plugs, and a gas plenum container is arranged above the fuel body. A check valve is provided at the bottom of the gas plenum container. A fuel element for a nuclear reactor, wherein the check valve is mounted, and the check valve is mounted in a direction in which FP gas flows into the gas plenum in the gas plenum container during operation. 2. A fuel body is housed in a cylindrical cladding tube whose upper and lower ends are closed by end plugs, and a gas plenum container is disposed below the fuel container, and a check valve is provided in the upper portion of the gas plenum container. A fuel element for a nuclear reactor, wherein the check valve is mounted, and the check valve is mounted in a direction in which FP gas flows into the gas plenum in the gas plenum container during operation. 3. The fuel element for a nuclear reactor according to claim 1, wherein the check valve is a ball valve.