Patent Number: 054250641
Section: summary

The present invention relates to measurement of the liquid flow in for instance a boiling water reactor, and more particularly to a liquid flow meter wherein a turbine rotor is used, and to a nuclear reactor equipped therewith. An important objective of the present invention is to provide more suitable measurement of a coolant flow in a reactor vessel of a nuclear reactor. Nuclear reactors generate heat through fission of radioactive elements such as uranium isotopes (U-233, U-235) and plutonium isotopes (Pu-239, Pu-241) which are contained in fuel bundles in the reactor core. In boiling water reactors (BWRs) this heat is used to convert liquid water into steam. The steam is conveyed to a steam turbine which can drive a generator to produce electricity. The water not converted to steam is recirculated through the core. Such a nuclear reactor is not equipped with means for generating a forced water flow through the core. The power output of the reactor must vary in order to meet varying loads. As reactor power increases, a greater coolant flow is required. An insufficient coolant flow can damage the fuel bundles because an isolating layer of steam can form around the fuel pins in the bundle. The isolating layer impairs heat transfer and causes an increase in the temperature in the fuel pin. The high temperature and the steam layer cause rapid oxidation of the cladding of the fuel pins. As a result of oxidation the cladding becomes brittle and subject to breakage. A resulting breach of the cladding exposes fissile fuel to the coolant. The fuel can then leach out into the coolant, which in turn can enter the fuel pin and further erode the fuel. The consequences may be loss of fuel and increased contamination of reactor components with radioactive fuel. In order to prevent such fuel loss and contamination the flow of the coolant at the fuel bundles must be monitored so that the correct steps can be taken when the coolant flow becomes too low. A method of monitoring the coolant flow is to measure pressure drop across a lower grid plate of the core. The measured pressure drop can then be converted to a flow rate measurement. However, in reactors with a low pressure drop, such as natural circulation boiling water reactors (NCBWRs), the accuracy of the pressure drop method of monitoring coolant flow is limited. Natural circulation boiling water reactors, which rely on convection rather than pumping for coolant circulation, offer significant advantages with respect to reactor economy, reliability and safety. Another method of monitoring coolant flow is to use electromagnetic pulses. A metal turbine rotor is arranged in the inlet of a fuel bundle. The liquid water flowing through the inlet causes the rotor to turn. The rotation of the rotor generates magnetic pulses. An electrically conductive coil close to the rotor converts the magnetic pulses into electrical pulses. The rate at which the pulses are generated corresponds with the rotation speed of the turbine. The coolant flow rate can be calculated from the turbine rotation speed. For this purpose the coil must be placed within a few millimeters of the turbine rotor in order to receive a pulse strong enough to enable the pulse generation rate to be determined. Because of the close proximity of the coil it has to be placed within the fuel bundle itself. Signal leads carrying the electrical pulses of the coil to a control room must then be attached to the fuel bundles. The leads make the elements difficult to handle, which complicates refuelling operations. The leads extending from the elements are prone to damage and are easily tangled, which can be inconvenient. For these reasons turbine flow meters have remained restricted to experimental bundles and are not used routinely. What is required is an improved system for measuring coolant flow through fuel pins in NCBWRs and other reactors with a limited pressure drop. Such a system must preferably not interfere with fuel bundle replacement. In addition, the system must be relatively immune to damage. The invention is based on the discovery that close to, and namely directly beneath, the core the order of magnitude of the neutron and/or gamma radiation field during operation is still less such that with a turbine assembly a sufficiently large radiation field modulation can be obtained to perform an accurate flow measurement. In accordance with the present invention a radiation detector detects radiation field modulations resulting from the rotation of a turbine motor induced by a liquid flow. The turbine rotor can comprise a spatially inhomogeneous distribution of nuclear-active material which absorbs neutrons and/or generates nuclear radiation after neutron activation. A nuclear reactor according to the invention comprises a reactor vessel having received therein a core comprising fuel bundles containing fissionable material and control rods, and which is immersed in water, a water inlet and a steam outlet and a measuring device disposed close to the core for measuring the water flow in the core, which measuring device comprises a turbine assembly having a rotor which is rotatably mounted in a housing and which contains at least one radiation field modulating material, at least one radiation detector for converting the radiation field modulation into a detector signal modulation, and a converter coupled to the radiation detector for converting the detector signal into a value corresponding to the water flow. The turbine rotor can be placed at the coolant inlet of a fuel bundle where the flow of coolant sets the rotor into rotation. The nuclear-active material is preferably arranged along only a portion of the circumference of the turbine rotor so that rotation of the rotor causes regular modulation of the radiation field. The modulations can be detected at a distance of several centimeters, whereby the nuclear radiation detector can be placed outside the fuel bundle. A converter, such as a frequency analyzer, translates the detector output signal into a flux oscillation rate that corresponds to the rotor revolution speed. The rotor revolution speed yields the coolant flow rate. In the context of this application the designation "nuclear-active" comprises both materials which absorb nuclear radiation and materials which emit such radiation, as long as the absorption or the emission effects a detectable modulation in the detected radiation field. Nuclear-emitting material can be chosen such that it will be made radioactive in the core by capture of thermal neutrons produced by operation of the reactor itself, or the nuclear-emitting material can be made radioactive prior to installation in the reactor. By selecting a material that becomes radioactive upon neutron capture in the reactor core, the user can choose the time of activation suited to his purposes. Selection of a source material that emits gamma rays immediately upon neutron capture, which are called prompt gamma rays, ensures that emission of radiation will occur upon exposure of the core to thermal neutrons. Alternatively, a source material that emits delayed gamma rays after neutron capture can also be selected if delayed in-core activation is acceptable. Materials with a relatively high neutron capture cross section must be used to ensure production of sufficient gamma rays for detection. A core material with delayed emission can also be preactivated by making use of another reactor or a particle accelerator prior to installation in the reactor core. This method ensures a source radiation suitable for immediate detection after installation in the reactor. Alternatively, a nuclear-active material that absorbs neutrons can be chosen. A strong neutron absorber embedded in the rotor can depress the local neutron field close to the turbine, which causes modulations in the neutron field as the rotor rotates. A neutron-sensitive detector can detect the modulations. The turbine rotation rate and thus the coolant flow rate can be calculated from the modulation of the neutron field. As neutron-absorbing material can be used 113-Cd and/or 176-180-Hf because of the high thermal neutron absorption coefficient, when the nuclear reactor is operating at 50-100% of its capacity. As material emitting gamma radiation through neutron activation can be used 110-Ag when the nuclear reactor is operating at 50-100% capacity, or 45-Sc and/or 59-Co when the nuclear reactor is operating at low power or is shut down. These materials have favourable neutron activating properties and optimal gamma decay schemes. The turbine assembly can simultaneously contain the different types of radiation field-modulating materials, for instance as a composite material such as cadmium/indium/silver optionally coated with a layer of nickel. Penetration of gamma and neutron fields can be much greater than that of the magnetic fields generated by rotor movement as in the prior art. Accordingly, the radiation detector can be placed at a greater spatial distance from the turbine assembly than magnetic field detectors. In practical conditions the detector can be placed centimeters from the radiation source in contrast to the millimeters required by the flow meter operating with a magnetic field. The greater detector range enables placing of the detector outside the fuel bundles; it can be placed in the reciprocating in-core probe, an instrument assembly already incorporated in many reactor cores. Although the use of nuclear radiation to measure the coolant flow can pose problems in some environments, these drawbacks are minimal in a reactor core. The present invention therefore provides a coolant flow meter which is suitable for operation in a low pressure drop environment such as in a NCBWR. The flow meter further makes use of sufficiently remote detection that no leads are required from the fuel bundles. The fuel bundles can thus be exchanged without manipulating or damaging flow meter leads. These and other aspects and advantages of the present invention will be elucidated in the following description with reference to the drawings.