NUCLEAR RADIATION DOSIMETER USING STRESS INDUCED BIREFRINGENCE CHANGES IN FIBER OPTIC CABLES

The present invention relates to devices and methods for measuring neutron fluence at a pre-selected location which is positioned in a nuclear power plant. The devices and methods include passing neutrons through a fiber optic cable. The fiber optic cable has disposed therein a neutron sensitive material which is capable of absorbing the neutrons to produce a gas. The gas results in a build-up of pressure in the fiber optic cable which causes a change in the optical stress birefringence pattern. This change is measured and used to determine the amount of gas in the fiber optic cable, the number of neutrons absorbed by the neutron sensitive material and subsequently, the neutron fluence at the pre-selected location. In particular, the devices and methods of the invention are effective without the need to employ a radioactive material.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to devices and methods for measuring neutron fluence in a reactor vessel and/or components and/or structures related thereto. These devices and methods include the use of fiber optic cable. Further, these devices and methods exclude the use of a radioactive material. For example, the devices are constructed without employing a radioactive material. The invention measures neutron fluence by measuring the change in the optical stress birefringence pattern produced in the fiber optic cable.

In general, in accordance with the invention, a fiber optic cable is installed in a pre-selected location where neutron fluence measurement is desired. Typically, a plurality of fiber optic cables is installed. The fiber optic cables had a pre-determined length. The length of the plurality of fiber optic cables can be the same or different. The pre-selected location includes, for example, the reactor vessel and/or related components and/or structures in a nuclear power plant. In certain embodiments, related components and structures include the containment building and the equipment position therein. Further, in certain embodiments, the neutron fluence in the reactor vessel can be measured from a location external to the reactor by installing the fiber optic cables of the invention in a related component or structure positioned outside of the reactor vessel, such that the amount of neutrons passing out of the reactor vessel is measured.

The fiber optic cables include a hollow cavity with at least one or more cores located within the cavity. The one or more cores contain, e.g., are at least partially filled with, a neutron sensitive material. Neutrons which are present in the pre-selected location pass through the fiber optic cable and are at least partially absorbed by the neutron sensitive material positioned therein. This interaction of the neutrons with the neutron sensitive material produces a build-up of gas within the cavity of the fiber optic cable which causes a change, e.g., increase, in pressure therein. The change in pressure in the cavity of the fiber optic cable produces a change of the stress distribution in the fiber optic cable. By measuring the change in the stress optical birefringence pattern produced by the change in pressure in the cavity of the fiber optic cable, the amount of gas can be deduced and therefore, the number of neutrons absorbed in the neutron sensitive material. This will, in turn, allow the total neutron exposure of the pre-selected location to be determined.

Suitable fiber optic cables for use in the invention can be selected from those known in the art. As described above, the fiber optic cables are hollow. Thus, an outer surface, e.g., cylindrical in shape, forms an inner cavity. Further, the length of the fiber optic cables can vary. The inner cavity includes the one or more cores. The inner cavity and the one or more cores extend throughout the length of the fiber optic cable.

Suitable neutron sensitive materials for use in the invention can be selected from those known in the art. As described above, the neutron sensitive material is effective to absorb neutrons. In certain embodiments, the neutron sensitive material is lithium-6. This interaction of the neutrons and the neutron sensitive material causes the production of a gas which results in a build-up of pressure in the cavity of the fiber optic cable. In certain embodiments, hydrogen atoms, helium atoms or a mixture thereof can be produced.

It is known in the art that the optical birefringence pattern obtained from the fiber optic cable can change in a predictable manner as a function of applied stress. In certain embodiments of the invention, the change in the birefringence pattern as a function of the change in the stress distribution in the cable can be readily determined using white-light interferometric techniques. For example, analysis by Wojtek J. Bock and Waclaw Ubanczyk entitled “Multiplexed system of white-light interferometric hydrostatic pressure sensors based on highly birefringence fibers” (Proc. Of SPIE, Vol. 2838/243, 1996) utilizes such technique and recites that fiber sensing employing white-light interferometric techniques offers several significant advantages such as the possibility of absolute measurements (with no initialization problem), the possibility of multiplexing a number of single-point sensors into a larger measuring system, and a lower noise level than coherent systems. Thus, the optical birefringence pattern obtained from the fiber optic cable prior to the interaction of the neutron sensitive material and neutrons (and the build-up gas and pressure) is different than that obtained following such interaction.FIG. 1Ashows a birefringence pattern for a fiber optic cable having Sensor1and Sensor2at atmospheric pressure. The birefringence pattern shown inFIG. 1Ais prior to the fiber optic cable being installed in a location wherein there is the presence of neutrons. This is demonstrated inFIG. 1Bwhich shows the birefringence pattern for the fiber optic cable shown inFIG. 1Awith the exception that the birefringence pattern shown inFIG. 1Bis following installation in a location having neutrons present. The change in the birefringence pattern is caused by the change in the stress distribution in the cable which is caused by the change in the pressure applied to the cable. InFIG. 1B, Sensor1and Sensor2are each at a pressure of 24 MPa and 14 MPa, respectively. Thus,FIG. 1Arepresents the “before” pattern andFIG. 1Brepresents the “after” pattern.

When the fiber optic cable is installed in a location such that neutrons pass through the fiber optic cable, a change in the stress distribution across the cross section of the cable results in a change in the associated birefringence pattern. The change in the birefringence pattern can be used to determine an accurate numerical change in the applied stress distribution. A change in the internal pressure in the cable can be readily and accurately used to determine the number of neutrons interacting with the neutron sensitive material.

In certain embodiments, this invention employs the activation of a controlled amount of the neutron sensitive material, such as Li-6, substantially uniformly packed into one or more hollow axial cores in a length of a fiber optic cable to produce a change in the internal pressure in the cable core as the Li-6 interacts with neutrons passing through the cable length. The change in pressure can be used to determine the number of neutrons interacting with the core material.

For the purpose of demonstration, the following description relates to installation or introduction of fiber optic cables in the reactor vessel, however, this process is equally applicable to installation of the fiber optic cables in a related component or structure. A plurality of fiber optic cables having pre-selected length(s) (i.e., pieces) is installed in strategic locations throughout the reactor vessel and in the space between the reactor vessel and the reactor vessel support structure. The number of fiber optic cables employed can vary and can depend on the size and configuration of the particular component and/or structure wherein the neutron fluence is being measured. The fiber optic cables are installed when the nuclear reactor plant is in shutdown mode. The fiber optic cables remain in the reactor vessel during the following operating cycle and then are subsequently extracted during the next scheduled refueling outage.

Prior to introduction or installation in the reactor vessel, an optical birefringence pattern is obtained from the fiber optic cables. Following extraction, another optical birefringence pattern is obtained from the fiber optic cables and compared to the original optical birefringence pattern obtained from the fiber optic cables prior to installation in the reactor vessel. An assessment is made as to the change in pattern and as a result, the number of neutrons that passed through the reactor vessel is determined.

FIG. 2is a graph of gas pressure (Pa) versus fractional percentage of lithium-6 reacted. The plot demonstrates the change (i.e., increase) in pressure inside the cavity of the fiber optic cable for a given amount of reacted lithium-6 as a result of neutron absorption.

While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.