TECHNICAL FIELD

The present invention relates to method for reducing the proportion of fast neutrons leaking out of a reactor core in a boiling water reactor. The reactor core comprises a plura¬ lity of fuel assemblies and is surrounded by a reactor vessel. Water flows upwards through the fuel assemblies and part of the water is thereby transformed into steam. A fuel assembly comprises a number of vertical channel-formed parts, so-called sub-channels, to accommodate fuel bundles. A fuel bundle comprises a plurality of fuel rods arranged between a bottom tie plate and a top tie plate. Openings are provided between adjacent sub-channels such that water and steam may flow between the sub-channels.

The invention also relates to a device for improving the neutron moderation at the upper part of a fuel assembly.

The invention also relates to a fuel assembly of the kind described above, comprising a device for improving the neutron moderation at the upper part of the fuel assembly.

BACKGROUND ART

A reactor core in a boiling water reactor comprises a plurality of vertical fuel assemblies of substantially square cross section. The fuel assemblies are arranged in a regular lattice. The core is surrounded by a moderator tank as well as by a pressure-maintaining reactor vessel. The core is immersed into water which serves both as coolant and as neutron moderator, that is, it slows down the neutrons released during the nuclear reaction. The space between the moderator tank and the reactor vessel is water-filled.

A fuel assembly comprises a plurality of vertical fuel rods, each one containing a column of pellets of a nuclear fuel arranged in a cladding tube. The water flows upwards through the fuel assembly whereby part of the water is transformed into steam. The higher up in the fuel assembly, the larger the proportion of steam in relation to the proportion of water. As moderator, steam is inferior to water, which means that the moderation deteriorates at the upper part of the fuel assem¬ bly. Fast neutrons are neutrons which have not be slowed down by the neutron moderator and therefore still have a high energy. For a boiling reactor, the proportion of fast neutrons leaking out of the core is higher at the upper part of the core than at the lower part thereof because of the inferior moderation at the upper part of the fuel assembly.

It is a well-known problem in reactors that the material in the reactor vessel becomes brittle after having been exposed to a critical dose of fast neutrons, which entails a risk of cracking of the reactor vessel. A certain amount of cracking in the moderator tank has also been noted, which may also have been caused by too high a dose of fast neutrons. This is above all a problem in older reactors, where the dose of fast neutrons to which the reactor vessel has been exposed during its life starts to approach the critical dose. The material in older reactor vessels is also more sensitive than the material in newer reactor vessels. Different attempts have been made to reduce the neutron flux in those parts of the core which are located nearest the reactor vessel. One way is to use, in these parts of the core, fuel which is partially spent. Another way is to replaced the fuel rods in those fuel assem¬ blies which are located nearest the reactor vessel by rods of a non-fissile material, for example stainless steel or zirco¬ nium. These two ways reduce the number of fast neutrons by reducing the number of generated neutrons in the vicinity of the reactor vessel. In the latter case, also a certain neutron scattering from the steel or zirconium rods is obtained.

SUMMARY OF THE INVENTION

One object of the invention is to suggest a method for redu¬ cing the proportion of fast neutrons leaking out of the reac- tor core, hence reducing the proportion of fast neutrons reaching the reactor vessel and the moderator tank. The object is fulfilled with as small a reduction of the active core, that is, that part of the core which comprises fuel, and with as high a utilization of the fuel as possible.

Another object of the invention is to provide a device which reduces the proportion of steam at the upper part of a fuel assembly, thus improving the neutron moderation at the upper part. An improved neutron moderation leads to a reduction of the proportion of fast neutrons leaking out of the fuel assembly.

Still another object of the invention is to suggest a fuel assembly for a boiling water reactor, which fuel assembly has an improved neutron moderation at its upper part.

What characterizes the method, the device and the fuel assem¬ bly according to the invention will become clear from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic picture of the composition of a boil¬ ing water reactor.

Figure 2 shows an example of a known fuel assembly in a verti¬ cal section through the line C-C in figure 3.

Figure 3 shows the same fuel assembly as Figure 2 in a view perpendicular to a horizontal plane through the line A-A in Figure 2.

Figure 4 is a schematic picture of a fuel assembly according to the invention in a vertical section through the line D-D in Figures 5 and 8.

Figure 5 shows a first embodiment of a fuel assembly in Figure 4 in a view perpendicular to a horizontal plane through the line B-B.

Figure 6 is a perspective view of a device for reducing the proportion of steam at the upper part of a fuel assembly according to the invention.

Figure 7 shows the volumetric proportion of steam (α) in a sub-channel as a function of the distance (1) from the inlet section of the fuel assembly.

Figure 8 shows a second embodiment of a fuel assembly in Figure 4 in a view perpendicular to a horizontal plane through the line B-B.

Figure 9 is a schematic picture of a third embodiment of a fuel assembly according to the invention.

Figure 10 shows a horizontal section of part of a reactor core comprising fuel assemblies according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reactor shown in Figure 1 comprises a reactor core 1 containing one or more vertical fuel assemblies, control rods 2 which are used, inter alia, for stopping and starting the reactor, a moderator tank 3 which surrounds the core, steam separators 4 which separate the steam formed in the reactor from the water in the reactor. The reactor is surrounded by a pressure-maintaining reactor vessel 5. The reactor vessel is filled with water to a level some distance above the reactor core.

The fuel assembly 6 illustrated in Figures 2 and 3 is enclosed in a fuel channel 7 of substantially square cross section. The fuel channel is provided with a hollow support member 8 of cruciform cross section, which is secured to the four walls of the fuel channel. The fuel channel with the support member surrounds four vertical channel-formed parts 9a - 9d, so- called sub-channels, with an at least substantially square cross section. The four sub-channels each includes a fuel bundle 15a - 15d. Each fuel bundle comprises fuel rods 10 arranged in a symmetrical lattice of 4x4 rods. The fuel rods are kept in their positions by a number of so-called spacers 11, placed between a top tie plate 12 and a bottom tie plate 13. 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.

Moderator water and cooling water flow into the fuel assembly from below through an inlet section 14. The spaces between the fuel rods within each sub-channel are flown through by water, as is the cruciform channel 25 formed by the support member. The support member 8 is provided with a number of axially distributed openings 26 allowing water and steam to flow between the sub-channels.

Figure 4 shows a vertical section through the line D-D in Figure 5, which shows a first embodiment of a fuel assembly 20a according to the invention, and Figure 8, which shows a second embodiment of a fuel assembly 20b according to the invention. The fuel assembly in Figure 5 has four sub-channels 9e, 9f, 9g and 9h. To reduce the number of generated neutrons in the fuel assembly, the fuel bundle has been removed from the sub-assembly 9h and replaced by a top plug 16h. The top plug is arranged at the upper part of the fuel assembly and the function thereof is to restrict the upward water flow in the sub-channel and instead force the water to flow into adjacent sub-channels 9e and 9g, through the openings 26a,

between the plugged sub-channel and the adjacent sub-channels, which are arranged upstream of the top plug.

The top plug is formed so as to block those openings 26b, between the plugged sub-channel and adjacent sub-channels, which are arranged downstream of the top plug. In this way, the water flow is increased at the upper parts of the adjacent sub-channels. The increased water flow provides a reduced proportion of steam, which results in an increased neutron moderation and hence a reduced number of fast neutrons. The proportion of steam is increased somewhat in the lower parts of the adjacent sub-channels, which does not matter so much since the proportion of steam is low there. The plugged sub¬ channel 9h contains no steam but only water and hence consti- tutes a good moderator for fast neutrons arriving from the other sub-channels in the fuel assembly as well as from other surrounding fuel assemblies.

Figure 6 shows the top plug 16h, which comprises a tubular part 27, one end of which is provided with a bottom part 17 and the other end of which is open. The top plug has a sub¬ stantially square cross section. The cross section substan¬ tially corresponds to the cross section of that sub-channel in which the top plug is arranged. The bottom part is provided with a number of openings 18 allowing a certain flow of water to take place through the top plug. The bottom part serves as a strong restrictor of the upward water flow in the sub¬ channel. The task of the tubular part is to block the openings between the sub-channels downstream of the bottom part, thus preventing water and steam from penetrating into the upper part of the plugged sub-channel from adjacent sub-channels. The top plug may be made from, for example Zircaloy or stain¬ less steel. The length of the plug is optimized to obtain the best effect with respect to reduction of the steam content. To attain improved moderation in the upper part of the fuel assembly, the length of the plug should be smaller than half the length of that sub-channel in which it is arranged.

Figure 7 shows the result of calculations made, on the one hand, on the unplugged fuel assembly 6 shown in Figures 2 and 3, which has one fuel bundle in each sub-channel, and, on the other hand, on the first embodiment of the plugged fuel assem- bly 20a shown in Figures 4 and 5, where a fuel bundle is remo¬ ved and replaced by a top plug. Figure 7 shows in the form of a diagram how the steam content α (the proportion of steam per volume) varies with the distance 1 from the inlet section of the fuel assembly for a sub-channel. Curve F shows the steam content versus the distance from the inlet section of the fuel assembly for the four sub-channels 9a, 9b, 9c, 9d in the unplugged fuel assembly.

Curve E shows the steam content versus the axial length for that sub-channel 9f which is furthest away from the plugged sub-channel 9h in the plugged fuel assembly 20a. Curve G shows the steam content versus the distance from the inlet section of the fuel assembly for those sub-channels 9e and 9g, one side of which adjoins the plugged sub-channel 9h, in the plugged fuel assembly 20a. The diagram shows that the steam content is reduced heavily at the upper part of those sub¬ channels, one side of which adjoins the plugged sub-channel. The steam content increases somewhat in the sub-channel located furthest away from the plugged sub-channel. In the plugged sub-channel there is no steam, that is, the steam content is zero for all distances 1. In those cases where two plugged channels are used, the steam content at the upper part of the fuel assembly is, of course, reduced still further.

Figure 8 shows a second embodiment of a fuel assembly accor¬ ding to the invention. The fuel assembly has four sub-channels 9e, 9f, 9g and 9h. To reduce the number of generated neutrons in the fuel assembly, two fuel bundles are removed from the sub-channels 9h and 9e, and each removed fuel bundle is replaced by a top plug 16h and 16e. The top plug is arranged at the upper part of the fuel assembly. In a further embodi¬ ment it is possible to have three plugged sub-channels.

Figure 9 shows a third embodiment of a fuel assembly 21 accor¬ ding to the invention. In one or two sub-channels all the fuel rods are removed and at least some of them are replaced by rods 22 of a non-fissile material, for example stainless steel or Zircaloy. In these sub-channels, a top plug 16i of the same kind as the one shown in Figure 6 is arranged. At least certain of the openings 18 are arranged such that the non- fissile rods are able to run through these openings.

Figure 10 shows a horizontal section of part of a reactor core comprising plugged fuel assemblies according to the first 20a and the second 20b embodiment of the invention. The object of the invention is to reduce the dose of fast neutrons to which the reactor vessel 5 and the moderator tank 3 are exposed. Fuel assemblies with one or more plugged sub-channels are arranged in the positions which are located furthest out in the core, that is, nearest the moderator tank and the pressure vessel. Depending on the distance of the sub-channels to the vessel walls, fuel assemblies are arranged with one or two plugged sub-channels. The fuel assemblies are turned such that the plugged sub-channels are facing the moderator tank. The plugged sub-channels increase the amount of moderated water between the fuel and the reactor vessel, increases the modera¬ tion in the unplugged sub-channels, and reduces the generation of neutrons in the immediate vicinity of the reactor vessel. For certain positions furthest out in the core where the dis¬ tance to the vessel walls is sufficiently large, unplugged fuel assemblies 6 are arranged, that is, fuel assemblies with one fuel bundle in each sub-channel.

The removed fuel bundles may be assembled so as to constitute one whole fuel assembly and be loaded into a suitable position in the core, or they may be assembled into a fuel assembly with two or three fuel bundles according to the invention and be used in the peripheral parts in the core.