WO1999017299A1

WO1999017299A1 - Fuel assembly for nuclear reactor

Info

Publication number: WO1999017299A1
Application number: PCT/SE1998/001756
Authority: WO
Inventors: Sture Helmersson; Mahdi Majed; Christian JÖNSSON; Henryk Anglart; Gunilla Norbäck
Original assignee: Abb Ab
Priority date: 1997-10-01
Filing date: 1998-09-30
Publication date: 1999-04-08
Also published as: SE9703579L; SE9703579D0; SE510656C2

Abstract

A fuel assembly for a boiling water reactor comprises a plurality of rods (5) arranged in parallel with one another, a plurality of spacers (81, 8b) arranged at separate levels in the fuel assembly and a fuel channel (2). A coolant flows upwards through the fuel assembly and in so doing, changes from a single-phase state to a two-phase state. The fuel assembly may, in the axial direction, be divided into an upper two-phase part (13b) where, during normal operation, bulk boiling prevails, and a lower two-phase part (12b) where, during normal operation, subcooled boiling prevails. The invention is characterized in that at least two of the spacers (8a) in the lower two-phase part have a mutual distance which is smaller than the mean value of the distances between all the spacers in the entire fuel assembly.

Description

Fuel assembly for nuclear reactor

TECHNICAL FIELD

The present invention relates to a fuel assembly for a boiling water reactor which, during operation of the reactor, is arranged vertically in the core and comprises a plurality of fuel rods arranged in parallel with one another in a fuel bundle, a plurality of spacers arranged at separate levels in the fuel assembly for retaining and fixing the fuel bundle, and a fuel channel which surrounds the fuel bundle and which is adapted for connection to a coolant which flows upwards through the fuel assembly and which, in so doing, changes from a single-phase state to a two-phase state.

BACKGROUND ART

A core in a boiling water nuclear reactor comprises a plurality of vertically arranged fuel assemblies, each of which comprising one or more fuel bundles. A fuel bundle comprises a plurality of vertical fuel rods arranged between a bottom tie plate and a top tie plate. The fuel rods are retained and fixed by a number of spacers arranged in spaced relationship to each other along the bundle. The fuel rods comprise a stack of circular-cylindrical pellets of a nuclear fuel arranged in a cladding tube. The fuel bundles or the fuel bundle are/ is surrounded by a fuel channel which is normally formed with a square cross section.

The core is immersed into water which serves both as a coolant and as a neutron moderator. During operation, the water flows from below and up through the fuel assembly, whereby the water starts boiling and part of the water is transformed into steam. In the following description, coolant relates to the water and the steam which flow through the fuel assembly. The fuel assembly may be divided into a single-phase part, where the coolant is in the form of water, and a two-phase part, where the coolant is in the form of both water and steam. The single-phase part is located in the lower part of the fuel assembly and the two-phase part in the upper part of the fuel assembly.

During operation, heat from the cladding surfaces of the fuel rods is transferred to the coolant, which implies that the steam formation takes place at the cladding surfaces . A certain amount of superheating of the water close to the cladding surface is required for steam bubbles to be formed. Further, a certain difference in temperature between the cladding surface and the coolant is required for the con- vective heat transfer at the cladding surface. This implies that the steam formation occurs before the mean temperature of the coolant has reached the boiling temperature, that is, subcooled boiling. As viewed in a cross section through the fuel assembly, the heating of the coolant thus takes place in a non-uniform manner. The coolant nearest the fuel rods boils and generates steam while at the same time the coolant in the spaces between the fuel rods has a temperature which lies well below the boiling temperature. The axial level in the fuel assembly where the coolant, on average, has reached boiling temperature is called bulk boiling.

Because of its lower density, steam is inferior to water as moderator, which means that the percentage of steam in the coolant determines the moderation of the fuel. The smaller the percentage of steam in the coolant, the more neutrons can be slowed down and hence be utilized for generating new nuclear reactions . The percentage of steam in the coolant thus determines the neutron economy of the fuel and hence how efficiently the fuel can be utilized.

A well-known problem which may arise in a boiling water reactor is that the core becomes unstable, which implies that the power in the core starts oscillating, so-called thermo- hydraulic instability. The power oscillations may result in fixed margins for the fuel being exceeded, which in turn may lead to fuel failure. The risk of instability increases the greater the difference in pressure drop is between the two- phase part and the single-phase part. The pressure drop in the two-phase part is much higher than in the single-phase part. To reduce the risk of instability, the pressure drop in the two-phase part must be reduced.

It is known that a spacer influences the flow of the coolant by turbulence arising in the coolant when it passes the spacer. It is also known to form the spacers such that the desired magnitude and direction of the turbulence are achieved. In Swedish patent application 9303583-0, a spacer is shown where the individual spacer cells have been provided with mixing vanes for creating turbulence and hence improving the cooling of the fuel rods.

Increasing the turbulence in the fuel assembly by locating the spacers more closely together is known from, for example, Swedish patent document 9302252-3. The intention of increasing the turbulence in this document is to improve the cooling of the fuel rods.

US patent document 5 164 155A refers to a boiling water reactor which experiences problems with vibrations in the fuel rods caused by the flow of cooling water. To reduce these vibrations, the spacers are located more closely together in the single-phase part of the fuel assembly than in the two-phase part thereof. In this case, the supporting function of the spacers is utilized and not their ability to create turbulence . SUMMARY OF THE INVENTION

The object of the present invention is to suggest a fuel assembly for a boiling water reactor which has improved neutron economy and, in addition, a reduced risk of thermohydraulic instability compared with prior art fuel assemblies .

What characterizes a fuel assembly according to the invention will become clear from the appended claims.

During normal operation, a fuel assembly may be divided, in the axial direction, into three parts, which in this application are designated single-phase part, lower two-phase part and upper two-phase part. The single-phase part extends from the bottom tie plate and about 0.2-0.3 m upwards in the fuel assembly. In the single-phase part no boiling occurs, so the coolant only occurs in aqueous phase. In the lower two-phase part, subcooled boiling prevails, which means that the coolant occurs in two phases and contains both water and steam, but that the mean temperature of the coolant is lower than its boiling temperature. The lower two-phase part extends from the single-phase part to about 0.6-0.8 m from the bottom tie plate. In the upper two-phase part, bulk boiling prevails, that is, the mean temperature of the coolant is equal to or exceeds the boiling temperature. The upper two-phase part extends from the lower two-phase part to the top tie plate . The upper and lower two-phase parts together constitute the two-phase part of the fuel assembly.

According to the invention, at least two, but preferably all, of the spacers are arranged in the lower two-phase part with a distance between them which is smaller than the mean value of the distances between all the spacers in the entire fuel assembly. By the mean value of the distances between all the spacers in the entire fuel assembly is meant the distance between the bottom and top tie plates divided by the total number of spacers increased by one. The purpose of arranging the spacers closer together in the lower two-phase part is to achieve an increased mixing of the coolant between the fuel rods. In this way, an equalization of the temperature of the coolant is achieved, such that the subcooled boiling is reduced, preferably completely terminated, in the lower two- phase part of the fuel assembly. When the boiling is reduced, also the quantity of steam in the fuel assembly is reduced.

A first advantage of the invention is that the moderation increases in the lower part of the fuel assembly as a result of the fact that the percentage of steam is reduced, whereby the neutron economy is improved.

A second advantage of the invention is that the risk of thermohydraulic instability decreases as the pressure drop across the upper two-phase part of the fuel assembly will decrease while at the same time the pressure drop across the lower two-phase part will increase. The stability is determined to a large extent by a quotient between the pressure drop in the upper two-phase part and the sum of the pressure drop in the lower two-phase part and the pressure drop in the single-phase part. The pressure drop in that part of the assembly which comprises the lower two-phase part and the single-phase part increases according to the invention since the turbulence in that part increases in magnitude. When the quotient between the pressure drop in the upper two-phase part and the sum of the pressure drop in the lower two-phase part and the single-phase part decreases, also the risk of thermohydraulic instability decreases.

The above-mentioned advantages may be further reinforced by allowing the spacers in the lower two-phase part to comprise means for increasing the mixing of the coolant between the fuel rods . To achieve the purpose of the invention, it is important to arrange the spacers more closely together in the lower two- phase part and not in the upper two-phase part. If the spacers are arranged more closely together in the upper two- phase part, the quantity of steam will instead increase, which is counterproductive, that is, the neutron economy is deteriorated and the risk of instability increases.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a fuel assembly according to the invention.

Figure 2 shows a horizontal section A-A through the fuel assembly in Figure 1.

Figure 3 shows formation of steam axially in a coolant channel .

Figure 4 shows an example of a spacer intended to be arranged in a fuel assembly according to the invention.

Figure 5a shows a perspective view of a sleeve for a spacer. Figures 5b-5d show the sleeve provided with alternative embodiments of mixing vanes .

Figure 6a shows a fundamental embodiment of a non-supporting spacer structure comprising mixing vanes. Figure 6b shows an alternative embodiment of the spacer structure shown in Figure 6a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a fuel assembly according to a first embodiment of the invention. Figure 2 is a horizontal section A-A through the fuel assembly in Figure 1. The fuel assembly is of boiling-water type and comprises a long tubular container, with rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends, so as to form a continuous flow passage through which coolant flows. The fuel channel 2 is provided with a hollow support member 3 , with a cruciform cross section, which is secured to the four walls of the fuel channel. The support member comprises four hollow wings and a hollow enlarged cruciform centre. The support member 3 forms a vertical channel through which non-boiling water flows upwards through the fuel assembly. The fuel channel 2 with support member 3 surrounds four vertical channel-formed parts 4a-4d, so-called sub-channels, with a substantially square cross section. Each sub-channel comprises a fuel bundle comprising a plurality of fuel rods 5 arranged in parallel. A fuel rod consists of a number of cylindrical pellets of uranium dioxide stacked on top of each other and enclosed in a cladding tube of Zircaloy.

All the fuel rods 5 in the fuel bundle are retained at the bottom by a bottom tie plate 6 provided with openings through which water flows into the fuel assembly, and at the top by a top tie plate 7 which is also provided with openings through which water and steam leave the fuel assembly. The fuel rods are spaced apart from each other by spacers 8a and 8b and are hence prevented from bending or vibrating when the reactor is in operation.

The region between four adjacently located fuel rods in which coolant flows is referred to as cooling channel. Figure 3 shows a cross section B-B through a cooling channel 10 in Figure 2. At the bottom of the cooling channel, the coolant occurs in single phase, that is, in the form of water. This region will be referred to hereinafter as the single-phase region 11a. At a higher level in the cooling channel, the water close to the fuel rods 5a, 5b starts boiling and steam bubbles are formed at the cladding surfaces of the fuel rods.

In the region immediately downstream of the location where the formation of bubbles starts, the bubbles are too small to be torn loose from the cladding surfaces by the water flow. Thus, they adhere to the cladding surfaces and form a thin layer of bubbles. Only at a still higher level in the cooling channel do the bubbles start to become detached from the wall and mix with the water. In the centre of the cooling channel there is a region with water which has a temperature which is lower than the boiling temperature of the coolant.

The mean temperature of the coolant increases with the distance from the bottom tie plate 6 up to a level where bulk boiling sets in. Bulk boiling means that the mean value of the temperature in a cross section through the cooling channel is equal to the boiling temperature of the coolant. In a boiling water reactor the boiling temperature is approximately 286° C. This implies that the steam formation occurs before the mean temperature of the cooling water has reached the boiling temperature. This type of steam formation is called subcooled boiling. The region where subcooled boiling prevails will be referred to hereinafter as the lower two-phase region 12a. The region above the limit to bulk boiling is referred to as the upper two-phase region 13a.

The limit to bulk boiling varies, of course, depending on the power of the reactor, the coolant flow and the subcooling of the coolant at the inlet. During normal operation, the reactor is normally run with a constant power, which means that the limit to bulk boiling lies at a fixed level. During start-up and shutdown of the reactor, the limit to bulk boiling will vary. Figure 1 shows how the fuel assembly may be divided into three parts, one single-phase part lib, one lower two-phase part 12b and one upper two-phase part 13b, depending on in which region these parts are located during normal operation of the reactor. The fuel assembly in Figure 1 has the spacers arranged at varying distances between one another. The spacers 8a in the lower two-phase part are arranged at a distance d₁ from one another. The spacers in the upper two-phase part are arranged at a distance d₂ between them, where d_x < d₂.

425 mm < d₂ < 600 nun d_λ < 400 mm

The mean distance d_av between the spacers in the fuel assembly is calculated as the distance h between the top tie plate and the bottom tie plate divided by the number of spacers n, in this embodiment six spacers, increased by one.

d_av = h/(n+l)

In a fuel assembly according to the invention, d_x < d_av. In a first embodiment of the invention, all the spacers in the fuel assembly are identical. In a second embodiment the spacers 8b in the upper two-phase part and in the single- phase part differ from the spacers 8a in the lower two-phase part in that the spacers 8a in the lower two-phase part are provided with members for increasing the mixing of the coolant between the fuel rods.

Figure 4 shows an example of a spacer 8a which may be used in a fuel assembly according to the invention. The spacer 8a has an orthogonal lattice structure composed of sleeves 14 in which each sleeve is intended to position a fuel rod exten- ding through the sleeve. Figures 5a-5d show examples of sleeves which may be used in the spacer 8a.

Figure 5a shows a sleeve 14a intended to be joined to other similar sleeves into a lattice structure according to Figure 4. The sleeve is provided with two mixing vanes 15a formed as pins extending from the upper edge of the sleeve and angled out from the centre of the sleeve to achieve turbulence of the coolant flowing by and a mixing of the coolant.

Figure 5b shows a sleeve 14b with mixing vanes 15b in the form of tabs punched out of the wall material of the sleeve, the tabs being bent along an axis perpendicular to the longitudinal axis of the sleeve and towards the centre thereof .

Figures 5c and 5d show sleeves 14c and 14d with mixing vanes 15c and 15d arranged in a part of the embossed surface of the sleeve. The mixing vanes are bent around an axis with a certain angle to the longitudinal axis of the sleeve and out from the centre of the sleeve.

The spacers may be formed in many different ways; the lattice structure may, for example, be in the form of cells or crossed strip elements standing on edge.

The spacers may, of course, be both of the type in which the lattice structure is intended to position the rods extending therethrough, and of the type in which the lattice structure does not have any positioning effect but only a turbulence- inducing effect. Figure 6a shows a fundamental embodiment of such a spacer structure 8c. The spacer structure 8c comprises mixing vanes 15 which extend downstream from the downstream edge of the spacer structure. Figure 6b shows an alternative embodiment of a spacer 8d with only turbulence-inducing effect, in which the mixing vanes 15 are arranged extending from the spacer structure 8d and, in the upstream direction, surrounded by the spacer structure 8d. In an embodiment (not shown) , the mixing vanes 15 extend outside and upstream of the spacer structure 8d.

Claims

1. A fuel assembly for a boiling water reactor which, during operation of the reactor, is arranged vertically in the core and comprises a plurality of fuel rods (5) arranged in parallel with one another in a fuel bundle, a plurality of spacers (8a, 8b, 8c, 8d) arranged at separate levels in the fuel assembly, a fuel channel (2) surrounding the bundle and being adapted for connection to a coolant which flows upwards through the fuel assembly and which, in so doing, changes from a single-phase state to a two-phase state, whereby the fuel assembly in the axial direction may be divided into an upper two-phase part (13b) where, during normal operation, bulk boiling prevails, a lower two-phase part (12b) where, during normal operation, subcooled boiling prevails, characterized in that at least two of the spacers (8a) have a mutual distance in the lower two-phase part which is smaller than the mean value of the distances between the spacers in the rest of the fuel assembly.

2. A fuel assembly according to claim 1, characterized in that the distances between the spacers (8a, 8c, 8d) in the lower two-phase part are smaller than the mean value of the distances between the spacers (8a, 8b, 8c, 8d) in the rest of the fuel assembly.

3. A fuel assembly according to claim 1 or 2 , characterized in that the distance between said spacers (8a, 8c, 8d) in the lower two-phase part is smaller than 400 mm.

4. A fuel assembly according to any of the preceding claims, characterized in that said spacers in the lower two-phase part comprise members (15a, 15b, 15c, 15d) for increasing the mixing of the coolant between the fuel rods .

5. A fuel assembly according to claim 4, characterized in that said members comprise mixing vanes arranged on the spacers .

6. A fuel assembly according to any of the preceding claims, in which the fuel rods are arranged between a bottom tie plate and a top tie plate, characterized in that the lower two-phase part is located about 0.6 - 0.8 m from the bottom tie plate.