An innovative method for on-power radiometry of end-shields of nuclear power plants

Every lndian PHWR reactor calandria is sandwiched within a pair of shield on either side. These shields are perpendicular to the coaxial axis of calandria and are called end-shields. These provide shielding from leakage radiation from reactor core in escaping out to Fuelling Machine vault, thereby significantly reducing the dose rates in the vaults. This has got a direct impact on radiation field in accessible areas. By maintaining low dose rates in accessible areas, the individual and collective doses of radiation workers can be effectively controlled well within the stipulated limits. Thus, it is of utmost importance to ensure adequacy of shielding provided by end-shields. In this context, a limited radiometry exercise is executed after filling of end-shields with steel balls and prior to their installation at designated place. This exercise provides limited inputs along the periphery of end-shield due to limited strength of radiation source, its handling provisions and dose constraints to the individual. In order to ascertain an in-depth analysis of shielding adequacy on-power, different methodologies have been adopted and have certain limitation in precisely pinpointing the affected area/location besides limitation on number of locations that can be monitored at a single stretch. To overcome these important anomalies, a computer based setup has been indigenously designed. The setup essentially comprises of a radiation monitor with wide energy, measuring, temperature and humidity range; a custom designed 25 m long compatible cable with suitable connectors; a laptop with additional cooling arrangement; a configurable interfacing software; thermal shielding for the detector and tying/fixing provisions. The radiation monitor after being properly shielded for thermal impacts is installed on the head of Fuelling Machine. It is connected through long cable to a laptop kept at Fuelling Machine service area with due cooling provisions (as temperature in the area will rise on retraction of roll-on-shield). The software is then configured to collect important radiological data viz. dose, dose rate, maximum and minimum dose rates, etc. for a specific period before auto-resetting of the system to collect another such data set. These data sets are recorded in a database on the laptop and are later used to map radiological information on the end-shields. To start-with, the main control room master clock and the laptop clocks were synchronized and the fuelling machine was moved to various identified locations on end-face. Simultaneously, the main control room master clock timings were noted w.r.t. locations of fuelling machine on end-face. Subsequently, the detailed sequential database collected from laptop was correlated with the recorded timing and locations. This information was then plotted in 2D/3D to visualize a precise radiation dose rate profile on end-face. These measurements were conducted effectively at low, moderate and full power of the reactor on several occasions to obtain baseline information. As a result of these exercises, a shielding weakness in the upper portion of one of the end-shield could be timely detected; its location was precisely identified and was addressed with negligible dose consumption. Such timely on-power measurements not only provide us with the confidence on effective implementation of equipment/systems, but also allow an early detection and addressal of a problem with almost no dose impact for the future. This setup can be integrated with robotic arms, manipulating devices, etc. for radiation dose rate profile measurement of highly active equipment/components. (author)