Imported: 13 Feb '17 | Published: 11 Oct '16
USPTO - Utility Patents
A device for the electrical heating of a liquid with high thermal flow, according to which the resistor in tubular form and supplied with direct current can indirectly heat the liquid by conduction through an electrically insulating and thermally conductive intermediate element that surrounds it with direct mechanical contact, the assembly consisting of tubular resistor and intermediate element being jacketed by a sheath intended to be immersed, over at least a major part of the length thereof, in the liquid to be heated. The preferred application is the electrical simulation of nuclear fuel rods intended to be assembled in power reactors, of the PWR but also RNR-Na type.
This application is the national stage of International Application No. PCT/EP2012/059187, filed May 16, 2012, entitled, “Electrical Heating Device For Heating Of A Liquid, Method For Producing Same, And Use In The Electrical Simulation Of Nuclear Fuel Rods,” which claims the benefit of priority of French Patent Application No. 11 54336, filed May 18, 2011, the contents of both of which are incorporated herein by reference in their entirety.
The invention concerns a novel device for the electrical heating of a liquid with high thermal flow.
The main application of the invention is the production of a novel device for the electrical simulation of nuclear fuel rods intended to be assembled in assemblies by holding grids and intended to be used in so-called power reactors, and more particularly pressurised water reactors (abbreviated to PWR). As described precisely below, the use of such a device enables the thermohydraulic qualification of fuel rods by detecting the occurrence of the boiling crisis of the liquid in which they are intended to be immersed.
The invention aims in general terms to increase the service life and quality of the existing heating devices with high thermal flow, in particular the qualification of devices for the electrical simulation of nuclear fuel rods currently existing with high thermal flows.
Although described with reference to the main application, the device according to the invention can be used for heating a liquid in which it is immersed.
In order to qualify an assembly of nuclear fuel rods intended to be used in a pressurised water power nuclear reactor (PWR), it is necessary to carry out boiling crisis tests. More precisely, one must be able to detect the occurrence of the boiling crisis. This is because the boiling crisis may be defined in general terms as a significant excursion in wall temperature for a small variation in the thermohydraulic control parameters. It results in concrete terms in the abrupt degradation of the heat exchange between a heating wall and the heat-transfer fluid that surrounds it. Thus, in a PWR reactor, the occurrence of this phenomenon could lead to the rupture of the nuclear fuel rod sheath.
Up to the present time, the devices for electrical simulation of high thermal flow nuclear fuel rods can be termed the direct heating type. This is because the sheath of the device also constitutes the element with the function of a resistor. In other words, the sheath is heated directly.
However, modern nuclear fuel assemblies, in particular those intended to be used in PWR reactors, have more complex designs and higher thermal performance than those of the assemblies already tested, which tends to invalidate the normal qualification procedures in particular with regard to the appropriateness of the devices for electrical simulation of rods assembled in an assembly.
Thus the inventors have analysed that the modern holding grids that form high current diversions may cause rapid degradation of the electrical rod simulation devices of the direct heating type: the contact points created between the protrusions on the holding grid in question and a given rod are liable to generate an overintensity of current and therefore local electrolysis, sources of rapid degradation of the rods. In fact, the inventors considered that the electrical simulation devices of the direct heating type could prevent the reliable qualification of the fuel assemblies.
Moreover, depending on the type of direct heating device necessary, the sheaths with a resistor function must be very thin in order to adapt their electrical resistance to the electrical supplies available, which is highly prejudicial to the service life of the simulation devices in question and to the possibility of reliably qualifying the assemblies.
In addition, the use of a direct-heating electrical simulation device involves electrical insulation thereof that is of high performance and reliable under the same operating conditions, which can take place only with complex technologies and materials that are expensive, but nevertheless fragile, and with a limited service life.
Finally, by definition, direct-heating electrical simulation devices cannot be used to qualify nuclear fuel assemblies intended to be immersed in an electrically conductive heat-transfer agent. However, safety studies on future so-called 4th generation rapid neutron nuclear reactors (RNRs), the functioning of which is ensured with a sodium heat-transfer agent (RNR-Na) will also require boiling tests with high thermal flow sodium. Since sodium intrinsically has high electrical conductivity, direct-heating electrical simulation devices will not be able to be used for said boiling tests for reactors (RNRs).
Other existing electrical simulation devices may be termed the indirect heating type since the resistances or resistors consist of wires supplied with three-phase current.
These devices are not satisfactory since they generate a non-homogeneous thermal flow in a backwash profile transversely to the sheath, that is to say an azimuth backwash profile.
The aim of the invention is therefore to propose a novel device for the electrical simulation of a nuclear fuel rod with high thermal flow that does not have all or some of the aforementioned drawbacks and therefore that is reliable in generating a homogeneous thermal flow and has a long service life, in order to carry out a large number of reliable boiling tests.
Another aim of the invention is to propose a novel device for the electrical simulation of a nuclear fuel rod with high thermal flow that makes it possible to obtain boiling tests at reduced cost and with guaranteed implementation times.
A more general aim of the invention is to propose a device for the electrical heating of a liquid with high thermal flow homogeneous in azimuth, and which is reliable and has a long service life.
To do this, the subject matter of the invention is first of all a device for the electrical heating of liquid, characterised in that it comprises:
In other words, the solution according to the invention consists essentially of all the following:
The electrical heating device according to the invention, which can be termed indirect (the tubular resistor indirectly heating the liquid by thermal conduction through the intermediate element and the sheath), makes it possible to obtain a high thermal flow, up to several MW/m2, with the same metrological qualities that can be found on existing direct heating electrical simulation devices that have a high performance.
In other words, it is possible, by means of the device according to the invention, to qualify nuclear fuel rods intended for power reactors of the PWR type. Furthermore, because of its electrical insulation vis-à-vis the external environment, which enables it to eliminate numerous assembly constraints, it is also possible to qualify nuclear fuel rod assemblies using modern holding grids without any risk of rapid degradation of the tubular resistor, rapid degradation that prevents the finalisation of the boiling crisis tests as may be the case with the direct-heating electrical simulation devices currently existing.
In order to reduce the interface thermal resistances between the various components of the device, the direct mechanical contacts both between the intermediate element and resistor and between the intermediate element and sheath are advantageously tight fits.
It is preferably possible to provide in the device according to the invention one or more mechanical inserts housed inside the resistor, the mechanical insert or inserts being adapted to afford mechanical support of the tubular resistor. In particular, such inserts may advantageously support the tubular wall of the resistor in the case of stretching of the sheath in order to obtain a holding thereof on the whole device. Preferably again, the mechanical insert or inserts is or are made from ceramic, such as magnesia, alumina or zirconia. Ceramic inserts offer both good mechanical strength and high electrical resistivity, which are suitable for implementation of the invention.
Preferably, the tubular resistor is made from Inconel 600 or a 70/30 cupronickel alloy.
Preferably also, the electrically insulating and thermally conductive intermediate element consists of a column of a plurality of pellets pierced at their centre and stacked on one another. Mounting of the insulating element around the resistor is thus facilitated. The pellets of the intermediate element are preferably made from boron nitride or aluminium nitride. In particular, boron nitride has thermal and electrical characteristics that are perfectly suitable for implementation of the invention. Furthermore, manufacturing boron nitride pellets makes it possible to obtain a compact intermediate element.
For anticorrosion purposes, the sheath is preferably made from metal that is non-oxidisable in demineralised water, typically water at 350° C. or even 600° C. in transient regime. Preferably, it is 316L stainless steel or a nickel/chromium/iron alloy, with a proportion of nickel, by weight, greater than 50%, preferably with a composition with at least 72% nickel, 14% to 17% Cr, and 6% to 10% iron (an alloy also called Inconel 600; there may be a few supplementary elements, in small proportions, such as Mn (1% maximum), and/or Cu (0.5% maximum), and/or Si (0.5% maximum), and/or C, and/or S).
The two electrical connections are preferably solid pieces that allow an advantageous insertion of thermocouples in the sheath as explained below. Preferably, these solid pieces are made from cuprous or nickel conductive material or molybdenum if heat treatments at high temperature, particularly an overhardening, prove necessary.
The device according to the invention advantageously constitutes a device for electrical simulation of a nuclear fuel rod.
It then comprises a plurality of thermocouples, advantageously each inserted in a groove formed on the external periphery of the sheath. Thus, unlike the electrical simulation devices according to the prior art, the thermocouples can be positioned with great precision. These are typically N-type with an Inconel 600 sheath.
Another object of the invention is a method for producing a device for the electrical heating of a liquid, said device comprising:
The restraining step also makes it possible advantageously to densify the electrically insulating intermediate element by increasing its thermal conductivity.
Three alternative variants can be envisaged for performing the restraining step:
For the restraining variants by means of hammering or isostatic pressure (hydrostatic expansion), these steps can be performed at least over the length of the sheath, with the electrical connections in place, and an extension can be added to the latter.
Another subject matter of the invention is a method for producing a device for the electrical heating of a liquid, said device comprising:
The thermal spraying techniques used in the context of the invention may be all the techniques of surface deposition of metals for the sheath or ceramic for the intermediate element compatible with the required specifications, such as vapour deposition, liquid deposition, electrolysis, epitaxy, etc.
The sprayed elements may be ground with a very small thickness in order to compensate for their relatively low conductivity. The inventors think that zirconia and alumina are particularly suited to a thermal spraying step and a grinding step.
In order to obtain a device for the electrical simulation of a nuclear fuel rod according to the invention with high homogeneous thermal flow, it is necessary to ensure mainly that the internal maximum temperatures are controlled, linked to the geometry and the materials, and to the interface thermal resistances.
It is thus ensured that the radial distance to be crossed by the thermal flow emitted by the resistor is minimised, and then the choice of the materials making up the resistor, sheath and intermediate element makes it possible to maximise their thermal conduction.
Finally, by virtue of the methods according to the invention, the interface thermal resistances are reduced by radial compression obtained by the restraining of the sheath on the whole device or by the thermal spraying of the various elements.
A plurality of longitudinal grooves are also advantageously produced on the outside diameter of the sheath in order to insert a thermocouple therein, in each one, in order to constitute a device for electrical simulation of a nuclear fuel rod.
The thermocouples thus inserted in the sheath at various azimuthal and longitudinal positions are thus positioned very precisely, typically of the order of 1 millimeter.
It is stated here that the electrical simulation devices according to the prior art (FIG. 1) and according to the invention (FIGS. 2 and 3) must make it possible to detect the occurrence of the boiling crisis, defined as a significant excursion in wall temperature for a small variation in the thermohydraulic control parameters.
It is also stated that, in all of FIGS. 1 to 3, the reference Lt, Ln and Lc designate respectively:
It should be noted that the mounting of the electrical simulation device according to the prior art (FIG. 1) makes provision for immersing and electrical connection whereas the mounting of the electrical simulation device according to the invention (FIGS. 2 and 3) makes it possible to not immerse any electrical connection, which is advantageous since it is not necessary to effect a sophisticated electrical insulation with respect to the external environment.
It is also stated that, in order to carry out the boiling crisis tests, an electrical simulation device according to the invention is arranged in an assembly (not shown) of a plurality of identical devices with holding grids in a vessel (not shown) containing the liquid to be heated, the two electrical connections projecting from the vessel while being isolated therefrom by ad hoc means, and the tubular resistor is fed with direct current. For pressurised water reactors, the liquid to be heated is water. For other applications, the liquid to be heated may be different. Typically, for RNR-Na rapid neutron reactors, the liquid to be heated is sodium.
For these boiling crisis tests the following parameters are imposed on each electrical simulation device:
Likewise, for these tests, the following parameters are imposed on the assembling together of several electrical simulation devices:
The operating conditions of the liquid to be heated referenced Liq in the figures (demineralised water) are as follows:
For reasons of clarity, the same elements of the device according to the prior art and according to the invention bear the same references.
FIG. 1 shows an electrical simulation device according to the prior art, of the direct heating type. The device 1 according to the prior art consists of a resistor 2 in the form of a tube that also serves as a sheath. In other words, the tubular sheath 2 also has the function of electrical resistor, that is to say the part supplied with current to heat the liquid in which the device is immersed. The interior 20 of the resistor/tubular sheath 2 is filled with pressurised nitrogen. Two electrical connections 30, 31 are each fitted in one of the ends of the sheath/resistor 2. One of the connections 30 is the one supplying current: it is pierced at its centre to house the instrumentation thermocouples 4 that extend longitudinally along the axis of the device in the interior 20 of the sheath.
On this side, the impermeability of the pressurised nitrogen on the interior 20 of the sheath 2 is ensured both by the connection 30 itself and by an end plug 5 made from electrically insulating material. The other one 31 of the connections is the current output connection: it is solid and therefore also serves as a sealing plug.
The major drawbacks of the electrical simulation device according to the prior art, of the direct heating type, as shown in FIG. 1 can be detailed as follows:
To remedy these drawbacks while keeping the possibility of achieving high thermal flows, of several MW/m2, necessary for the qualification of fuels and the same metrological qualities as a device 1 according to the prior art, the inventors thought first of all of decoupling the sheath function from the resistor function of the device.
Thus the device 1 according to the invention consists essentially of:
Furthermore, according to the invention, the tubular resistor 2 is supplied with direct current by the connection 30. For applications other than the qualification of nuclear fuel, the electrical supply may be done in single-phase alternating current.
The embodiment in FIG. 2 provides, as an electrically insulating and thermally conductive intermediate element, a column of pellets 6 pierced at their centre, stacked on top of one another, and fitted around the tubular resistor 2 over its entire length and over part of the electrical connections 30, 31.
The embodiment in FIG. 3 provides, as an electrically insulating and thermally conductive intermediate element, a ceramic cladding 22 deposited in liquid form deposited by spraying (thermal spraying) directly on the tubular resistor 2.
Ceramic mechanical inserts 21 are stacked inside the resistor 2 in order to support its wall, in particular when the sheath stretches during a step of restraining by hammering. It is also possible to provide pressurised gas instead of the ceramic inserts 21.
Whatever the embodiment illustrated, in order to reduce the interface thermal resistances between resistor 2/intermediate insulating element 22, 6/sheath 7 as much as possible, a radial compression is carried out on the assembly obtained by a restraining of the sheath 2 on the rest 22, 6; 7 of the device 1. This restraining operation also advantageously makes it possible to densify the electrically insulating intermediate element in the form of stacked pellets 6 by increasing their thermal conductivity.
The measuring instrumentation for performing the boiling crisis tests is provided by thermocouples, preferably eight N-type thermocouples 4 sheathed with Inconel 600 (nickel/chrome/iron alloy, with a composition of at least 72% nickel, 14% to 17% Cr, 6% to 10% iron; there may be a few supplementary elements, in small proportions, such as Mn (1% maximum), and/or Cu (0.5% maximum), and/or Si (0.5% maximum), and/or C, and/or S), each arranged in different axial and azimuthal positions at specified points with a tolerance of ±2 mm.
Advantageously, as shown in FIGS. 2 and 3, each thermocouple 4 is inserted, after the restraining step, directly in a groove formed on the outside diameter of the sheath 7. It is thus possible to position the thermocouples very precisely with the above tolerance.
Sealing elements 5 made from resin are provided around each electrical connection 30, 31 in order to electrically insulate the resistor 2 and its connections 30, 31 from the sheath 7.
By way of example, electrical simulation devices 1 according to the invention are envisaged with the dimensions and materials as follows:
Dimensions of the Device 1 According to FIGS. 2 and 3:
Although described solely as a device for electrical simulation of a nuclear fuel rod for performing boiling crisis tests, the device according to the invention 1 that has just been described in FIGS. 2 and 3 may just as well constitute in more general terms a device for the electrical heating of a liquid, more particularly when a high thermal flow is required, typically up to a few MW/m2.
Thus a device according to the invention may advantageously replace the currently existing heating tubes the surface power of which is limited:
In the case where a device according to the invention is actually used as a heating device, its two ends serve for the electrical supply.