Bavarian warning20 July 2004
The Environmental Protection Agency (LfU) of the German feeral state of Bavaria has put a radiation monitoring system into operation to supervise its nuclear facilities. By Siegfried Stuffer and Sören Hartmann
In accordance with the German Atomic Law, the third generation of a remote monitoring system was put into operation to continuously and independently supervise nuclear facilities in the federal state of Bavaria. The main role of the system is to provide information and early warning in case of incidents, accidents or attacks and monitor the impact of any deviation from normal operation.
Radiation monitoring station
After the old network came to the end of its lifetime, a replacement was required employing the newest software, hardware and communication technology and guaranteeing reliability. Prerequisites for the modernisation concept were:
• Failsafe system architecture.
• Stationary and mobile components on the same technological basis.
• Provide full data communication capabilities and remote alarming means without investments into new communication infrastructure.
• Reduce installation and commissioning costs.
• Allow fast and simple deposition of mobile stations.
• Avoid maintenance costs for communication infrastructure.
• Be able to extend and modify the network by configuration means only.
• Be open to the newest standards for communication, networking and data exchange.
• Provide outdoor equipment with proven reliability even under harsh conditions.
The system can be broken down into several functional parts:
• A stationary monitoring part consisting of four monitoring rings around the nuclear facilities. Each contains 12 fixed-position TechniData AGS421-S autarkic (independent) monitoring stations automatically measuring the ambient gamma dose rate and the presence of rain.
• Four additional AGS421-M deployable stations at every facility site for temporary measurements in case of an emergency.
• A monitoring centre for network management, data acquisition, processing, storage, presentation and dissemination.
• An early warning part for analysing the acquired data and information and controlling alarm output devices depending on the detected events.
FIXED MONITORING STATIONS
Fixed monitoring stations are located in monitoring rings around the Isar, Gröhnde, Gundremmingen and FRM II nuclear facilities. Each ring is divided into 12 equal sectors with a monitoring station between two and five kilometres from the facility.
Geographical representations could prove vital in an emergency
The complete functionality of a radiation monitoring station is integrated in one compact housing:
• Two redundant low dose and one high dose Geiger-Müller (GM) tubes for sensitive radiation measuring.
• Rain detector for identification of wash-out effects.
• Data logger for data acquisition, buffering, pre-processing and threshold supervision.
• GSM adaptor for data transmission.
Designed for a power consumption of just 0.2W for the complete system, a solar panel buffered by two sealed batteries supplies the system, including the integrated peripherals, with a stable low voltage. The batteries are able to buffer solar current loss for up to 40 days, guaranteeing continuous operation even during a long cloudy winter.
The fixed stations cover a measuring range for the energy- and temperature-compensated ambient dose rate from 10nSv/h up to 10Sv/h according to recommendations from the International Commission on Radiation Units and Measurements.
This redundancy enhances the reliability and availability of the measurements and allows automatic quality control of the measurements by mutual comparison. Further data validations are made by controlling the required minimum count rates related to the intrinsic background, by assessing unusual steps in the registered counts, and by supervising the high voltage supply of the counting tubes. The measurements can be corrected for tube characteristics, detector background and the contribution of cosmic rays.
As the ambient gamma dose rate increases with onsetting rain, mainly due to radon wash-out from the atmosphere, the detection of local precipitation enhances the accuracy and supports the correct interpretation of gamma monitoring results. Thus, four of the 12 monitoring stations around one nuclear plant are additionally equipped with a rain detector to qualitatively indicate the presence of rain. It is mechanically integrated on the top of the housing and is designed for maintenance-free operation and very low power consumption.
The integrated data logger acquires the measured gamma radiation values, status and rain information every minute, stores them in a ring buffer for up to three days, and pre-processes them by calculating ten-minute, two-hour and 24-hour mean values. The processor continuously controls the status of all main components such as memory, battery and front door access.
All electronics are hermetically sealed to IP68 standards and bathed in a nitrogen atmosphere. Any damage of the shelter can therefore be detected by means of a specific internal sensor. The station is designed for fast deployment and automatic set-up without any commissioning requirements. The unit can also be anchored to an additional base to avoid theft.
The complete unit is specified for rough outdoor usage at temperatures between -40°C and +60°C.
The serial interface, batteries, a status LED and the plug for external charging are accessible through the front door for service and maintenance.
An associated diagnostic and maintenance notebook allows technical staff to have full access to all data and settings of the AGS421 via a serial interface onsite allowing comprehensive system diagnosis, parameter setting, maintenance and accuracy testing.
MOBILE MONITORING STATIONS
The mobile version of AGS421 is based on the same software, electronic and mechanical grounds and is 100% compatible. The mobile version has the following key differences:
• A folding tripod enables installation and commissioning within one minute in order to reduce dosage to workers setting up the monitor.
• The station is supplied by one battery only, guaranteeing continuous operation for 20 days. The battery can be recharged upon return. A solar power supply is available as an option.
• A GPS device automatically records the monitoring position and new coordinates are sent to the centre if it is moved more than 200m.
• The reduced weight of 16kg allows easy transportation.
The stations are designed to operate automatically and maintenance-free and regular visits to the stations are not necessary. But due to quality assurance reasons, a scheduled onsite accuracy test in combination with visual inspection is advised once a year. Since all GM tubes are calibrated in a German secondary standard laboratory, the accuracy test focuses on differences to the initial reference measurements. This is done by mounting a Cs-137 reference source at a fixed geometry and measuring the resulting pulse rates returned from the detectors. The maintenance and diagnostic software supports the automatic test performance and outputs and stores the test reports.
All relevant station parameters and settings can be controlled and modified remotely from the monitoring centre and the correct function of the system’s main components is continuously controlled. Any detected malfunction, for example of the detectors, the battery or the memory units, is immediately reported by means of a spontaneous call to the monitoring centre, so that the station’s status can be analysed remotely and appropriate maintenance activities can be planned.
The core application for network management, data processing, visualisation, reporting and alarm handling is based on TechniData’s NMC-RAD (network management centre for radiological applications). It is a multi-tier client-server concept and allows the implementation of a widely distributed system. Designed on Java and Corba technology, it is platform independent.
The monitoring centre, located at LfU’s main building in Augsburg, is equipped with two redundant Linux servers and six client workstations connected via LAN. The central Oracle database and the components for communication control, event logging, messaging, task management, alarm handling and user management run on the server. The workstations are the front-ends with a Windows based graphical user interface and geographical information system which accesses the server’s database.
Its main tasks are remote control of the monitoring stations, scheduled and manual request for measuring data and status information from the control unit, evaluation, storage and archiving of all data, data presentation, as well as reception and processing of spontaneous alarm messages from the stations.
Data logged by the system is available for analysis at the monitoring centre
Data communication between the monitoring stations and the monitoring centre is done via the GSM cellular telephone network from monitoring stations and ISDN at the centre. There are two types of GSM antennae available, supporting different network coverage conditions.
Communication is regularly performed in master-slave mode: the monitoring centre calls the station and requests data, status information and parameter settings. Since eight ISDN adaptors for data transfer are available at centre site, up to eight stations can be called in parallel. A second mode is used with regular data calls from the monitoring stations to take advantage of contracts with cellular network providers offering free communication time each month.
Particular care was taken to guarantee data transmission even during times of high traffic, such as emergencies. The cellular network provider guarantees priority for the contracted phone numbers. Additionally, SMS messages (text messages) are available as a redundant data channel for data transmission from the stations to the centre or to the operator’s cellular phone.
On any detection of failures, position movement or measurement of values exceeding one of the thresholds (warning and alarm level for one-minute, ten-minute and two-hour values), the monitoring station control unit is able to call the centre spontaneously to transmit measurements and status information. The range events that trigger such calls can be changed locally and remotely from the centre.
The server offers a further line for remote maintenance or remote operation activities.
All network components refer to a top-down synchronised real time base, on which they build or receive data.
At the centre, the server and clients acquire the Coordinated Universal Time (UTC) as real time from an NTP server. All time stamps are stored in the database as UTC, but can be visualised in local time as well.
At the monitoring sites, the real time clock is synchronised by the central server during every data call and the logger uses the synchronised time base for mean value calculation. It can therefore be guaranteed that all aggregated data at all sites refer to the same time intervals and are directly comparable.
Any alarm activity will be triggered by spontaneous calls sent by a monitoring station covering early warning issues. There are two categories of spontaneous calls causing different alarming activities: radiological alarms indicating the exceeding of a threshold; and technical alarms indicating any status change of a component. Information about the event is output on the front-end. As far as enabled, alarm information with timestamp, station-ID and notes on the kind of event is forwarded to external staff via e-mail as well. More details can be gathered from the station’s status report which will be updated when the spontaneous call was received. In case of a radiological alarm, the system switches to intensive mode and reduces the data call intervals.
Siegfried Stuffer and Sören Hartmann, TechniData AG, Munich Office, Hans-Pinsel-Str. 4, 85540 Haar, Germany