GB2097821A - Carbon deposition inhibition - Google Patents

Abstract

An operational problem of Advanced Gas Cooled Nuclear Reactors is the formation of carbonaceous deposits on the surfaces of the corrosion resistant heat conducting containers for the nuclear fuel. The heat transfer characteristics of the containers are thereby adversely affected. In the invention, the containers are fabricated from a Cr-containing steel and have a surface oxide layer which is substantially free from Fe and Ni provided by annealing in an atmosphere of low oxygen partial pressure such as an atmosphere of hydrogen saturated with water. Test experiments have shown that such containers exhibit far less tendency to be subject to carbonaceous deposition than equivalent containers whose oxidised surfaces contain Fe and/or Ni.

Description

SPECIFICATION Carbon deposition inhibition This invention relates to the inhibition of carbonaceous deposition on steel surfaces.

In operation of an Advanced Gas-Cooled Reactor, which is known in the art and is referred to herein as the "AGR", nuclear fuel is enclosed in a corrosion resistant heat conducting tube (or clad), e.g. in the form of a stainless steel container (or fuel can), and heat generated by the fuel is extracted therefrom by passing carbon dioxide gas (the "coolant") about the clad. The gas is then passed to a steam turbine and the heat energy contained in the gas converted into electrical energy. In order to control the radiolytic oxidation of graphite components in the reactor, methane is added to the carbon dioxide at a pre-determined level and carbon monoxide allowed to build up to a required concentration. Water is formed, also from radiolytic oxidation of methane, and its concentration may be controlled by conventional fixed bed driers.However, under certain combinations of methane and carbon monoxide concentration, carbonaceous deposition can occur on the surface of the clad thereby deleteriously affecting its heat transfer characteristics.

An example of a material used in the fabrication of the clad is a stainless steel, usually a Cr-bearing austenitic stainless steel, for example stabilised by Nb. A particular example of such a steel is the socalled "20/25" steel which contains approximately 20% Cr, 25% Ni, .6% Mn, .6% Si, .04% C, about 0.7% Nb and the balance Fe, wherein the proportions are by weight. A further example is the so-called "20/26 TiN" steel which has the same composition as the above "20/25" steel with the exception that the Nb is replaced by Ti and the C by N. it has now been found that the abovementioned carbonaceous deposition may be reduced substantially or even prevented completely by providing steel containers constituting the clad with a surface which is substantially free from Fe and Ni.Thus, this invention provides a container for nuclear fuel for use in a nuclear reactor which is fabricated from a Cr-containing steel and has a surface oxide layer which is substantially free from Fe and Ni.

Such containers have been found, in test experiments which are described hereinafter, to exhibit far less tendency to be subject to carbonaceous deposition than equivalent containers whose oxidised surfaces contain Fe and/or Ni.

Clearly the relative proportions of oxides within the oxide layer changes as one moves from the outer surface of the steel constituting the container to the interior thereof. It has, however, been found in specific cases by using the technique of X-ray photoelectron spectroscopy in conjunction with ion beam sputtering that depletion of Fe and Ni may extend to a depth of approximately 500 nm.

The surface oxide layer which is substantially free from Fe and Ni may be produced for example, by annealing the container or the steel to be used in fabricating the container in an atmosphere of low oxygen partial pressure such as an atmosphere of hydrogen saturated with water at 200 C. It has been found that the temperature of such an anneal and the length of time of the anneal may be important factors in generating a surface oxide layer which is substantially free from Fe and Ni. For example, in the case of the abovementioned "20/25" steel, it has been found necessary to anneal at a temperature in excess of 7000C and for a prolonged period of time e.g.

greater than 4 hours in order to generate such a surface oxide layer. Alternatively, annealing at 8000C for 2 hours or at 9000C for 1 hour have each also been found suitable in this respect. It should be noted, however, that other methods may be useful for generating such a surface oxide layer, for example, using a 50:1 (volume volume) mixture of CO/CO2 though use of these gases may introduce the problem of carburisation. However, the ease with which such a surface oxide layer can be produced on exposure to H2/H20 depends upon the previous history of the steel. In this respect, prior cold working of the steel can be beneficial.

Also, it may be possible to generate a surface oxide layer which is substantially free from Fe and Ni in situ on containers already in use in a nuclear reactor but which initially lack such a layer. For example, steel fuel containers in an AGR may be treated with a hydrogen-enriched gas in order to generate such a layer. Further, a minimum concentration of hydrogen may be included in the coolant to ensure that the surface oxide layer of the steel remains substantially free of Fe and Ni.

The latter two procedures are, of course, dependent on the temperature of the steel during use exceeding that required to produce a surface oxide layer which is substantially free from Fe and Ni. Those parts of the steel which do not attain such temperatures will not acquire such a layer unless they have been subjected to a pretreatment such as described above.

The containers of this invention may carry coatings which are known in the art for inhibiting carbonaceous deposition when the surface oxide layer provides a compatible surface for subsequently applied coatings such as SiO2 coatings applied by sol-gel technology (see UK Patent Application Publication No 2 023 453A) or by plasma activated vapour deposition. Thus, the presence of an underlying oxide layer substantially free from Fe and Ni on the container would provide additional protection against carbonaceous deposition in the event of the coating becoming cracked, dislodged or otherwise removed therefrom.

It is believed that the present invention may be applied to Cr-containing steels other than those specifically mentioned herein, for example those steels containing a lower proportion of Cr, for example down to 5% Cr by weight. It should also be mentioned that there is tentative evidence that the presence of Mn in the steel may be valuable in stabilising the surface oxide layer.

Finally, there is evidence that a container for nuclear fuel according to this invention may be more resistant to the problem of oxide spallation than a container lacking surface oxide layer substantially free from Fe and Ni.

The invention will now be particularly described by way of example only as follows. Reference will be made to the accompanying drawings in which Figure 1 shows the atomic composition of surface layers according to the invention as a function of thickness of the layer, and Figures 2 and 3 show the same relationship but for surface layers which are not according to the invention. Comparative experiments are included below. These are not examples of the invention as will be apparent from the context.

EXAMPLE 1 Specimens of "20/25" steel (as defined herein) were heat treated at 8000C for two hours in hydrogen saturated with water at 200 C. The surface composition of a sample of the treated steel was measured by means of X-ray Photoelectron Spectroscopy (XPS), which measures the composition of approximately the top nm of the sample. The sample was then progressively eroded by means of ion beam sputtering and XPS measurements taken after each succeeding erosion. The results are shown in Figure 1 from which it can be seen that the surface oxide layer of the sample steel was substantially free from Fe and Ni to a depth of approximately 400 nm.

Specimens of the steel heat treated as above, were then exposed to an equivalent CO2/CH4 (3 :1) RF plasma for 4 hours at 6500C in order to stimulate carbon deposition under ionizing conditions. A comparison of weights before and after treatment in the plasma gives an indication of the extent of carbonaceous deposition.

Experiment A The procedure of Example 1 was repeated with the exception that the heat treating was carried out in dry hydrogen at 9300C. The results of the XPS measurements are shown in Figure 2.

Experiment B The procedure of Example 1 was repeated with the exception that the heat treating was carried out in CO2 at 8000C. The results of the XPS measurements are shown in Figure 3.

Results from Example 1 and Experiments A and B Referring to Figures 1 to 3, it will be seen that the product of Example 1 possesses a surface oxide layer which is substantially free from Fe and Ni whilst each of the products of Experiments A and B do not. Thus, Example 1 constitutes an example of the present invention whilst Experiments A and B do not constitute examples of the present invention. The weight of carbon deposited in each case is summarised in the table below: Example Weight of Carbon (or Experiment) Deposited (mg) 1 than .1 A 4 B 0.7 The above results clearly show that the products of Experiments A and B have each acquired much heavier carbonaceous deposits than the product of Example 1.

EXAMPLE 2 "20/25" steel nuclear fuel containers in the form of 1.5 cm diameter tubing and containing UO2 fuel pellets were used in a Prototype Advanced Gas Cooled Nuclear Reactor for periods of up to three years. A first batch of the containers which had previously been annealed at 9300C in dry hydrogen was then exposed in the reactor core to a hydrogen-enriched coolant (350 vpm CH4; 4% vpm H20; 400-600 vpm H2; balance CO2) at 700-8000C for approximately 6 months and then subjected, together with a second batch of containers which also had been annealed at 9300C in dry hydrogen but not previously been in the reactor, to a coolant comprising 700 vpm CH4; 0.250.5% CO; 600 vpm H20; 100 vpm Hz; balance CO2 for approximately 1 5 months.The latter coolant was known, from previous experience, to give rise to significant carbonaceous deposition onto the steel surfaces in the reactor.

The second batch of containers was found to have acquired a carbonaceous deposit as measured by installed thermocouples and subsequently verified by post-irradiation examination. The first batch, in contrast, was found to have remained free of any significant carbonaceous deposition. However, the deposit on the second batch was removed when it was reinserted into the reactor and exposed to the abovementioned H2-enriched coolant.

Subsequently, both the first and second batch resisted carbonaceous deposition in the reactor when exposed to the abovementioned coolant known to give rise to carbonaceous deposition.

These results, when taken in conjunction with the results from Example 1 , point to the conclusion that exposure of the steel in the reactor to the H2/H20 containing coolant can cause a modification in the surface composition of the steel which renders it much less likely to acquire carbonaceous deposits.

Claims (10)

1. A container for nuclear fuel for use in a nuclear reactor which container is fabricated from a Cr-containing steel and has a surface oxide layer which is substantially free from Fe and Ni.

2. A method of making a container as claimed in claim 1 which includes annealing the steel under conditions such as to form the surface oxide layer which is substantially free from Fe and Ni.

3. A method as claimed in claim 2 wherein the conditions include an atmosphere of sufficiently low oxygen partial pressure to form the surface oxide layer which is substantially free from Fe and Ni.

4. A method as claimed in claim 3 wherein the atmosphere comprises hydrogen saturated with water.

5. A method as claimed in claim 4 wherein the steel is cold worked before annealing.

6. A method as claimed in claim 3 wherein the atmosphere comprises a mixture of CO and CO2.

7. A method as claimed in any of claims 2 to 6 wherein the steel is annealed when in the form of the container.

8. A method as claimed in claim 7 wherein the container is situated in a nuclear reactor.

9. A method as claimed in any of the preceding claims wherein the steel is provided with an additional coating for inhibiting carbonaceous deposition.

10. A method of making a container as claimed in claim 1 substantially as described herein with reference to Example 1 or to Example 2.