Nuclear Pressurised Water Reactor Transformer Individual Rod Position Indicator

Abstract

An important function of the control and instrumentation for a nuclear reactor is the ability to display and control all the rod positions within the reactor core. This is an important function as the reactor operators use the control rods to control the nuclear chain reaction throughout the life of the reactor: from initial cold condition start-up, through periods of sustained long-term reactions and finally when the reactor has completed its life cycle. Specific functions of the control rods position indication system include showing the positions of individual control rods in a control rod group, the position of each rod group and the required control rod position versus the actual control rod position. As well as being important to the operation of a nuclear reactor, the control rod position indication system is important to its efficiency. The associated costs for a nuclear power plant are relatively high, but the efficiency of the reactor can be increased either by increasing the life of the reactor core or using less fuel for the same period of use by managing the positions of the reactor control rods. Control rod position is therefore directly related to the efficiency of the reactor through life and considered an area of where improvements can be made. The control rod position is determined using two methodologies, known as direct measurement and inferred measurement. The inferred measurement uses a digital counter which increments and decrements as the control rod moves up and down, and which displays a high-resolution position measurement. However, the counter is reset every time the control rod is released during a scram condition. On engagement of the control rod following a scram condition, the counter has no previous position knowledge and restarts counting from the zero position of the control rod. Hence the inferred method does not provide a repeatable rod position measurement. The direct measurement method determines the actual position of the control rod by detecting the position of the driveshaft which is mechanically fixed to the upper section of the control rod. By detecting the driveshaft position, the control rod position can be accurately determined, and the nuclear reactor controlled to increase overall through-life efficiency reducing cost of operation. The direct measurement control rod position is achieved by using three system components: the indicator (also known as a transducer), the instrumentation to indicator cable and the instrumentation electronics. Generally, the indicator and the cable are exposed to a harsh environment: mostly high radiation, temperatures and pressures. In addition, within the nuclear reactor the highly radioactive water known as the primary coolant is encapsulated within the metallic boundaries of the reactor to shield the surrounding atmosphere from it. For a Pressurised Water Reactor (PWR) plant the indicator is placed within a metallic boundary known as a Probe Tube (PT). The driveshaft of the control rod envelops the PT as it rises and exposes the PT as it declines. The PT is designed into a reactor system so a measurement of the driveshaft can be obtained thus measuring the control rod position. Without such a breach of reactor boundary there is no other current methods to measure the control rod position. Detecting the driveshaft position accurately is difficult as the detecting technology has to penetrate the material of the PT. The technology most widely used is a form of magnetic sensing that has a long term pedigree of use within many worldwide applications. In particular, the use of electro-magnetic sensing leads itself to nuclear applications. Electro-magnetic sensing has been used reliably for many years in nuclear reactors, especially for control rod position detection. However, improving the accuracy of determining the position of a driveshaft has been an ongoing challenge that has been met with limited success. This thesis investigates a novel method of

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alexander Smith (Supervisor)