REFUELLING AND/OR STORAGE NEUTRON-ABSORBING RODS

Abstract

A nuclear reactor is provided. The reactor including: plural fuel rods containing fissile material; plural control rods, each made of a first neutron-absorbing material, the control rods being inserted between the fuel rods to reduce the rate of a fission reaction of the fissile material and put the reactor in a shutdown state, but being operable to move in and out of the reactor to vary the rate of the fission reaction when the reactor is critical and generating useful power; and plural refuelling and/or storage rods, each made of a second neutron-absorbing material different to the first material, the refuelling and/or storage rods being inserted between the fuel rods to further reduce the rate of the fission reaction and maintain the shutdown state.

Claims

1. A fuel assembly for a nuclear reactor having plural, individually extractable and replaceable fuel assemblies holding fuel rods of the reactor, and having plural control rods, each made of a first neutron-absorbing material, which are insertable between the fuel rods to reduce the rate of a fission reaction of fissile material contained within the fuel rods to put the reactor in a shutdown state, and operable to move in and out of the reactor to vary the rate of the fission reaction when the reactor is critical and generating useful power; wherein the fuel assembly includes: plural of the fuel rods containing fissile material; and at least one refuelling rod, made of a second neutron-absorbing material different to the first material, the refuelling rod being inserted between the fuel rods to further reduce the rate of the fission reaction and maintain the shutdown state.

2. The fuel assembly of claim 1, wherein the refuelling rod is not operable to survive the intense radiation flux or the high temperatures present in an operating or critical nuclear reactor.

3. The fuel assembly of claim 1, wherein the refuelling rod is made of a borated material.

4. The fuel assembly of claim 3, wherein the borated material is borated steel.

5. The fuel assembly of claim 1, wherein the fuel assembly comprises a locking mechanism for mechanically locking the refuelling rod within the fuel assembly.

6. A nuclear reactor including: plural fuel rods containing fissile material, the fuel rods held in plural, individually extractable and replaceable, fuel assemblies of the reactor, and wherein: at least one of the fuel assemblies comprises the fuel assembly of claim 1.

7. A method for reducing a rate of fission during shut down of a nuclear reactor including plural fuel rods containing fissile material, the method comprising the steps of: inserting plural control rods, each made of a first neutron-absorbing material, between the fuel rods (201) to reduce the rate of a fission reaction of the fissile material and put the reactor into a shutdown state; and inserting (302) plural refuelling rods, each made of a second neutron-absorbing material different to the first material, between the fuel rods to further reduce the rate of the fission reaction and maintain the shutdown state.

8. The method of claim 7 further comprising: removing a reactor vessel head of the reactor, thereby exposing the fuel rods with the reactor; mechanically locking the refuelling rods in place; refuelling the reactor; and unlocking and removing the refuelling rods.

9. The method of claim 8, wherein the refuelling is performed without introducing a neutron-poisoning solution to coolant water of the reactor.

10. The method of claim 8, wherein the fuel rods are held in plural fuel assemblies of the reactor, at least one of the fuel assemblies each containing one or more of the refuelling rods, and wherein in the mechanically locking step the one or more refuelling rods are mechanically locked in place within their respective assemblies, the method further comprising steps, between the steps of mechanically locking, and unlocking and removing, of: extracting a fuel assembly containing one or more of the refuelling rods from the reactor, and transferring it to a storage pool; and returning the extracted fuel assembly from the storage pool to the reactor.

11. The method of claim 10, wherein the step of refuelling includes refuelling the extracted fuel assembly while in the storage pond.

12. The method of claim 8, wherein the fuel rods are held in plural fuel assemblies of the reactor, at least one of the fuel assemblies each containing one or more of the refuelling rods, and wherein in the mechanically locking step the one or more refuelling rods are mechanically locked in place within their respective assemblies, the method further comprising a step, between the steps of mechanically locking, and unlocking and removing, of: extracting a fuel assembly containing one or more of the refuelling rods from the reactor, and transferring it to a storage pool; wherein the step of refuelling includes transferring a replacement fuel assembly from the storage pool to the reactor, the replacement fuel assembly replacing the extracted fuel assembly.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0042] FIG. 1 is a schematic diagram of a PWR;

[0043] FIG. 2 shows schematically a fuel assembly for the reactor of FIG. 1; and

[0044] FIG. 3 is a flow diagram of a method of refuelling the reactor of FIG. 1.

DETAILED DESCRIPTION

[0045] FIG. 1 is a schematic diagram of a PWR 10. An RPV 12 containing fuel assemblies is centrally located in the reactor. Clustered around the RPV are three steam generators 14 connected to the RPV by pipework 16 of the pressurised water, primary coolant circuit.

[0046] Coolant pumps 18 circulate pressurised water around the primary coolant circuit, taking heated water from the RPV to the steam generators, and cooled water from the steam generators to the RPV.

[0047] A pressurizer 20 maintains the water pressure in the primary coolant circuit at about 155 bar.

[0048] In the steam generators 14, heat is transferred from the pressurised water to feed water circulating in pipework 22 of a secondary coolant circuit, thereby producing steam which is used to drive turbines which in turn drive an electricity-generator. The steam is then condensed before returning to the steam generators.

[0049] FIG. 2 shows an example layout of a fuel assembly 200, which is formed of a 17×17 grid of rod guides. The grid is held together by metal banding (not shown). The grid of rod guides contain: fuel rods 201, control rods 202, refuelling and/or storage rods 203, and an instrumentation rod 204. Any control rod may be replaced, during a refuelling or storage operation, by a refuelling and/or storage rod 203. Further, not all rod guides within a fuel assembly need be filled. For example, the rod guide for one or more control rods 202 and/or refuelling and/or storage rods 204 may contain no rod. The instrumentation rod 204 typically contains one or more sensors, e.g. a temperature sensor, a radiation flux sensor, etc. In preferred examples, any given fuel assembly 200 would contain either control rods 202 or refuelling and/or storage rods 203 and not both. For example, all of the positions shown in FIG. 2 indicated as control rod positions could be used to house refuelling and/or storage rods 203. The opposite is also true.

[0050] The control rods 202 are operable to move in a direction which is in and out of the plane of FIG. 2, so as to present varying depths to the surrounding fuel rods 201 and thereby control the rate of the fission reaction in a manner known in the art. In particular, when the reactor is critical and generating useful power, the control rods can be moved to vary the rate of the fission reaction in real time, can also be fully inserted to put the reactor in a subcritical shutdown state. In contrast, the refuelling and/or storage rods 203, when present, are immobilised and do not move in and out of the fuel assembly in the same manner as the control rods, as discussed in more detail below. A role of the refuelling and/or storage rods is to ensure that substantially no fission reaction occurs during a refuelling operation or storage operation occurs i.e. the reactor is safely maintained in the subcritical shutdown state, as discussed in more detail below.

[0051] The fuel assembly may comprise a locking mechanism for mechanically locking the refuelling and/or storage rod within the fuel assembly. The locking mechanism may comprise for example, a cap or locking nut operable to be secured over each such rod to immobilise the refuelling and/or storage rod in the fuel assembly during refuelling operations and storage. Alternatively the locking mechanism could be separate to the fuel assembly and operable to be inserted into the fuel assembly to immobilise the refuelling and/or storage rod in the fuel assembly during refuelling operations and storage.

[0052] Typically, the control rods 202 are formed of a neutron-absorbing first material which meet the following criteria: (i) capture neutrons, and thereby moderate the rate of a fission reaction; (ii) survive the intense radiation flux present in an operating or critical nuclear reactor; and (iii) survive the high temperatures present within such a reactor. For example, control rods 202 can be made from AglnCd, Hf, B.sub.4C, or combinations thereof.

[0053] In contrast, the refuelling and/or storage rods 203 can be formed of a different, neutron-absorbing second material which only needs to satisfy the criteria (i) above. For example, the refuelling and/or storage rods may be formed from a borated material, such as borated steel or a borinated polymer. In one example, the refuelling and/or storage rods are formed from Borated Stainless Steel (BSS) as is known for use in fabricating fuel storage racks. The BSS would typically contain 0.6% by weight natural boron, with the remainder of the chemical composition being in common with normal stainless steel i.e. a mixture of iron, chromium, and nickel. In embodiments, the borinated material may comprise between 0.3% and 12% wt of boron; or between 0.4% and 6%; or between 0.5% and 2%; or between a range formed from any of the aforesaid endpoints.

[0054] A method of refuelling a reactor having fuel assemblies such as that shown in FIG. 2 is illustrated in FIG. 3. In a first step, the control rods 202 are inserted to put the reactor in a subcritical shutdown state. In a next step, 301, the reactor pressure head is removed so as to expose the fuel assemblies contained within the reactor. Next, in step 302, n refuelling and/or storage rods 203 are introduced to m fuel assembles. The value of n will be determined by the level of suppression desired to safely prevent the reactor going critical, and would depend (amongst other factors) on the neutron capture cross-section of the material forming the refuelling and/or storage rods 203. The value of m will depend both on the value of n and also on the number of free rod guides within the reactor as a whole. Whilst step 302 is performed before step 301 in this example, it is possible to reverse the order i.e. first introduce n refuelling and/or storage rods to the m fuel assemblies and subsequently remove the reactor pressure head. The refuelling and/or storage rods further reduce the rate of fission occurring.

[0055] After the refuelling and/or storage rods 203 are introduced, they are immobilised in place in step 303. This can be performed, for example, by securing a cap or locking nut over each such rod. This ensures that, unlike the control rods 202 discussed previously, they cannot be inadvertently retracted from the fuel assembly in which they are located. This provides an additional safety margin not provided by a control-rod-only core. The control rods, in comparison, may be mounted to a moveable arm so as to allow them to be moved in and out of the core with relative ease.

[0056] After the refuelling and/or storage rods 203 are introduced, the reactor can be refueled in step 304. Optionally, a step may also be performed of moving fuel assemblies within the reactor so as to balance any subsequent fission reaction.

[0057] After the step of refuelling and, when performed, the step of moving the fuel assemblies, each refuelling and/or storage rod 203 is dis-immobilised (e.g. by removing the cap or locking nut) and removed in step 305.

[0058] The refuelling of the reactor can either be performed by replacing any given fuel rod within a fuel assembly, or, preferably, by replacing entire fuel assemblies. If the fuel assemblies are to be refueled, this may be performed in-situ within the reactor. Alternatively the fuel assembly(s) may be extracted from the reactor to a storage pool where fuel rods are replaced. Indeed, another option (and the preferred option) for accomplishing refuelling of the reactor is to swap over extracted fuel assemblies with replacement fuel assemblies held in the storage pool, i.e. the replacement fuel assemblies from the pool are transferred to the reactor to take the place of the extracted fuel assemblies, which can then be stored pending subsequent processing.

[0059] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.