In control of radwaste5 November 2004
Little guidance is available on the design of instrument and electrical systems important to safety in interim storage and final repository facilities for spent nuclear fuel and radioactive waste. Because of this, the International Electrotechnical Commission has issued a draft report on I&C safety systems in these facilities. By Frigyes Reisch and Dan Kristensson
The International Electrotechnical Commission (IEC) has issued a draft report giving guidelines for the instrumentation and control (I&C) systems of interim storage and final repository of nuclear fuel and waste, regardless the origin of the stored material (DTR IEC 62235 Ed 1:2004, Instrument and Control Systems Important to Safety – Systems of Interim Storage and Final Repository of Nuclear Fuel and Waste). The report covers different storage types of facilities such as fuel fabrication plants, nuclear power plants, reprocessing facilities, interim storage facilities, encapsulation facilities and final repositories for operational waste and deep or retrievable repositories for spent nuclear fuel. The technical report also covers storage during transportation. All these facilities contain different nuclear materials such as new fuel, used fuel, operational waste and other miscellaneous radioactive substances and objects.
Many of the instrumentation and electrical system applications for interim storage and final repository of nuclear fuel and waste are similar to those used in other commercial applications. In these cases the standards are already being addressed by other technical committees or by the radiation protection subcommittee of IEC technical committee 45 (nuclear instrumentation). There are, however, two circumstances in which development of additional standards would be appropriate:
- Definition of requirements for systems and equipment that are unique to nuclear waste and spent fuel storage facilities and repositories.
- Identification of requirements and implementation strategies for instrumentation and electrical functions important to safety in these facilities.
Examples of unique systems and equipment include fuel handling and transfer systems, and material control and accountability systems.
Little guidance is available for the design of instrument and electrical systems important to safety in storage facilities and repositories. There are a number of reasons why the existing guidance for nuclear power plant systems important to safety are not directly applicable.
Firstly, the design basis accidents for such facilities are different in nature from those for nuclear power plants. Events in storage and repository facilities tend to be slower to develop and less energetic, consequently guidance prepared for nuclear power plants is not directly applicable. Design and operations to ensure availability of the necessary I&C functions might place more emphasis on the ability to detect and correct I&C failures and less emphasis on very high levels of quality and reliability.
Secondly, functions important to safety in storage and repository systems tend to be simpler than those in nuclear power plants. Often the most important function involves isolation of confinement areas and perhaps maintenance of contamination control zones that provide defence in depth to radiation release. System design guidance should presume (and encourage) simple functionality with the most important functions isolated from more complex control systems.
Thirdly, design basis events in storage and repository facilities do not involve the severe environment conditions associated with nuclear plant design basis events. More practical means for accepting the environmental capability of I&C components are appropriate.
Finally, in some cases, particularly in some repository applications, normal environmental conditions may be quite severe and accessibility for maintenance may be limited. Guidance may be needed on design conditions and system strategies that allow failures to be either corrected or tolerated.
The first three items are also characteristic of many other non-reactor nuclear facilities such as enrichment, fuel fabrication, or reprocessing facilities. The possibility of developing a common set of design guidance for I&C systems important to safety in all such facilities should be investigated.
The only permanently installed instrumentation function for dry cask storage is monitoring of the inter-lid pressure. The pressure sensors may be read periodically by portable instruments or they may be connected to a remote display or alarm system. Periodic radiation surveys of the storage array are conducted using standard plant radiation survey instruments.
At a nuclear power plant the spent fuel pool and fuel building are used to load and unload the dry storage cask. The fuel building I&C functions most important to these operations are the controls for the fuel building crane that is used to move the cask, and the fuel building ventilation isolation system which must respond to limit public consequences of postulated fuel handling accidents occurring during loading or unloading operations.
A number of instrument functions are necessary to support cask loading and unloading. These include the following:
- Radiation monitoring to check on the decontamination of the cask when it is removed from the spent fuel pool.
- Gamma and neutron monitoring to confirm performance of the shielding.
- Vacuum monitoring to confirm evacuation of the cask before backfill with inert gas.
- Water flow monitoring to support manual control of cask cooling water introduced to cool the fuel and cask internals before opening a cask that contains fuel.
- Inert gas leak detection (for example, mass spectrometer) to confirm integrity of the containment seals when the cask is closed.
- Pressure monitoring to monitor and control the pressure of backfill gas in the cask cavity, inter-lid region, and inter-seal region.
- Pressure monitoring to confirm that inter-lid regions, inter-seal, and cask cavity are not pressurised before de-tensioning lid bolts.
STORAGE OF RADWASTE
From the point of view of safety, the adoption of suitable I&C systems in radioactive waste storage installations contributes to the radiation safety in normal operating conditions. Furthermore it helps avoid the occurrence of abnormal conditions and to limit negative consequences for workers, the public, and the environment.
A main area of application of I&C systems and equipment is for the monitoring of the installation with respect to radiation and to other physical parameters which are related to the integrity and to the safety status of the radioactive wastes and of the storage installation in its entirety. Besides the monitored parameters, information on abnormal conditions is provided through particular alarm signals, which are generated when the measured values exceed the allowed values.
Another area for monitoring is in accounting and characterisation for acceptance of the incoming radioactive waste and control of the storage inventory.
The surrounding environment, as a general rule, has to be subject to monitoring in order to ascertain and estimate the radiation levels consequent to the operation of the storage installation.
A further role of I&C is in security systems regarding the detection of non-authorised people.
Radiation monitoring in the storage area is accomplished by means of measurement such as radiation dose rate, airborne activity, and surface contamination. The controlled areas are equipped with fixed, continuously operating instruments for radiation dose rate measurement, with local alarms and readings which can be repeated and synthesised in a central location acting as main station for control and command; additionally, portable or mobile radiation monitoring can be used.
The exit points of controlled areas are equipped with fixed or portable instruments to detect external contamination of workers.
Airborne activity measurement in the storage area is made if there is a risk of radioactive release; this could be the case, for instance, in the storage of irradiated fuel: if some fuel element presents defects in the cladding, fission products could escape from the gap and go to the outside environment. Airborne activity measurement is made on samples collected on the airborne pathway, which can be achieved by using ventilation ducts for the sampling points.
In general it is important that the design of the storage facility be made in such a way that the monitoring, both continuous and periodic, is facilitated. In this regard the installation structures and the storage equipment must ensure the absence of dispersion and release of radioactivity from the storage area. Furthermore a remote location can be used to collect and synthesise information on parameter values, status of equipment, and alarms. The performance of the monitoring instrumentation has to be verified on a regular basis with periodic testing and calibration. In this regard sufficient capability must be guaranteed to access equipment for servicing actions.
Physical parameters are related to the equipment used for the storage process and to the auxiliary systems. As an example, the tanks for bulk storage of liquid waste are equipped with level measurement and with controls and actuation devices for mixing and to transfer the waste to other tanks. Another important application area is the supervision and control of auxiliary systems employed in the storage installation.
In general I&C equipment is integrated with auxiliary systems according to the specific implementation of those systems on a case-by-case basis and according to the particular needs for which they are designed. A need for auxiliary systems is particularly important in installations used for the storage of high-level waste or significant quantities of wastes in liquid, gaseous or solid dispersible state. Special cases may be considered regarding the storage installation for the wet or dry interim storage of irradiated fuel.
Radiation monitoring and I&C requirements can be posed by typical equipment employed in radwaste management like cleaning/ decontamination systems, or movable shield systems.
Regarding use of the operational controls, an example is given by controls associated with lifting or moving equipment of waste containers: these controls are important to guarantee that limits of lift height or speed are not exceeded, in order to avoid or to reduce the risk of waste damage from impacts and collisions in transfer routes. More generally, interlock signals can automatically preclude dangerous or mismatched operations, as an alternative or in addition to other physical or procedural means. In the most critical cases, when a failure of operational controls and interlocks could give rise to events with high consequences, stringent requirements in terms of redundancy and single failure criteria have to be met. An example of this case can be considered to be the main crane of the reactor containment building used for the transfer of the irradiated fuel.
If defective or damaged waste containers give rise to high dose rates, radiation release, and contamination in the working area, their safe handling can be accomplished by the use of control systems that allow remote control from areas protected from the hostile environment. Of course the remotely operated devices themselves can pose specific challenges for their maintenance (repair, calibration, periodic tests) for which dedicated shielded rooms for servicing are required.
Ventilation systems are used if adequate conditions in the storage installation are to be maintained, as well as if there is a need to ensure adequate heat removal or airborne radioactivity removal. The control of the ventilation system must be coordinated with area zoning in the facility, for instance differential pressure measurement between adjacent zones must confirm that the airflow is from lower to higher contamination areas. Other provisions affecting the supervision of the system are the control of accumulation of hazardous substances like flammable, explosive or toxic gas, for instance the production of hydrogen by radiolysis or chemical reaction. With regard to the role of the ventilation system, especially for heat removal from high intensity radiation sources such as irradiated fuel, special requirements can be stipulated in terms of reliability, redundancy/diversity of active components, and behaviour in accident conditions.
Regarding auxiliary systems and connected I&C equipment in radioactive storage installations, consideration should be given to the need for back-up power supply by uninterruptible electrical supplies and batteries to guarantee continuity of electrical sources to relevant loads (like fixed radiation monitoring system) in the case of loss of normal supply; in addition, it should be possible to temporarily replace the installed radiation monitoring system by portable instrumentation in case of loss of normal power supply.
Other auxiliary systems to be provided include: the water drainage system from the storage installation; the lighting system; the water/air supply system; the fire protection system; and the internal and external communication system.
The drainage system is important for avoiding waste container degradation and if there is a risk of criticality events. The drainage system is generally equipped with level measurement and alarm in the water collection points.
The lighting system comprises a normal section and an emergency section supplied by the back-up power supply, which ensures a minimum lighting level that is sufficient for emergency worker intervention.
The fire monitoring and protection system has to limit, inside and outside the storage installation, the risk of release of radiochemical or toxic substances which could be caused by fires, or the risk of fire damage to the installed equipment. Special care should be devoted to fire that can be sustained without oxygen.
For I&C related to the storage installation and the above mentioned systems, care is to be taken to maintain separation and dedication between equipment for monitoring/control and equipment that provides protection functions, which is more important in terms of safety. Similarly, high reliability I&C devices should be used to monitor the status of inaccessible equipment.
Further consideration should be given when the storage installation is in the same site as an operating plant. In this case the I&C and auxiliary systems of the storage installation can share components and support equipment with the existing plant. An example can be the derivation of the electrical supply to the storage installation from the electrical system of the plant. It is important to verify that interfaces and commonalities allow sufficient functional capability and that failure propagation, from/to the storage installation with respect to the plant, is precluded.
Wet storage of spent nuclear fuel is in use, for example in the CLAB facility in Sweden. The receiving building is at ground level and the storage building below the ground level. Spent nuclear fuel arrives in transport casks, which provide radiation shielding and protection against damage. The fuel handling is done under water. The central control equipment contains two computers, one primary computer and a backup. Radiation monitoring is handled by: the stack radiation monitoring (calibrated for Kr-85); the process systems radiation monitoring for surveillance of beta and gamma activity; and the radiation monitoring for certain rooms (calibrated for Kr-85). The most important nuclides in water are Co-60 and Cs-137. The activity is released mainly in the water of the pools and the cooling system. Corrosive and fission products are more or less in ionised form. Water purification is carried out by filters and ion exchangers. Purified water is released to the sea after chemical tests.
The Final Repository for Radioactive Operational Waste (SFR) in Sweden
OPERATIONAL WASTE REPOSITORY
The Final Repository for Radioactive Operational Waste (SFR) is in operation in crystalline rock some 60m below the bottom of the Baltic Sea. Operational wastes are typically packaged and transported in steel drums, fibre drums, fibre boxes, or some combination of these. The only instrument function necessary to support packaging of operational waste is radiation monitoring to verify the surface dose rate.
SPENT FUEL REPOSITORIES
Retrievable repositories for spent nuclear fuel are in the concept stage. Different geographical areas offer different solutions. Large-scale test projects are going on to exploit the possibilities of the best locations and methods.
The deep repository in granite consists of a number of consecutive barriers or subsystems, where the internal subsystems are completely surrounded by the external ones. Innermost in the system i the fuel. Canisters surround all the fuel, with buffer and backfill material in tunnels and shafts.
The geologic repository in volcanic mountain is a large underground excavation with a network of tunnels serving as the emplacement area for spent nuclear fuel and high-level radioactive waste. The engineered barrier system, together with the geological and hydrological properties, ensures that after closure of the repository the waste would be contained and isolated for 10,000 years or more.
For the eventual future storage of long-lived high-level waste in a layer of clay, an underground research laboratory at a depth of 500m in a 150-million-year-old clay formation is under construction in France. The laboratory is planned to be in operation in 2006.
Land carriage of spent nuclear fuel is in packages similar to that for dry storage. Transport containers typically have additional impact absorption devices to protect against possible transport accidents. No instrument applications beyond those for dry storage are provided for transport.
Sea carriage of spent nuclear fuel and radioactive operational waste is accomplished by using specially built vessels which carries spent fuel to the interim storage, and other nuclear waste to the final storage. The vessel meets high safety requirements, with features such as double freeboard and a double hull, and is certified in accordance with international safety standards.
Frigyes Reisch, Royal Institute of Technology/KTH, Nuclear Power Safety, Fatburs Brunnsgata 11, SE-11828 Stockholm, Sweden; Dan Kristensson, Oskarshamn NPP (OKG AB), SE-572 83 Oskarshamn, Sweden