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Thermal test driven pressure build-up inside type-B packages containing wet radioactive waste
(2013)
In recent years several German approval procedures for ductile cast iron transport containers containing wet intermediate level waste were conducted. BAM, as one of the German competent authorities, was involved in the complex design assessment work with this specific issue. Thermal analysis is one part of the authority assessment work done by BAM in Germany. The radioactive contents of package designs which were not dried, only drained, consist of saturated ion exchange resin and a small amount of free water. Compared to the safety assessment of packages with dry content, attention must be paid to some more specific points. The most interesting point, however, is the pressure build-up inside the package due to vaporization. This could be caused by radiolysis of the liquid and must be taken into account for the storage period. The inner pressure of the package leads to mechanical loads to the package body, the lid and the lid bolts. Thus, the pressure is the driving force on the gasket system regarding the activity release and a possible loss of tightness. The paper deals primarily with the pressure build-up inside the package caused by the transport regulatory thermal test (30 min at 800 °C) as part of the cumulative test scenario under accident conditions of transport. The pressure build-up is estimated by calculation in a very conservative way regarding conduction and heat radiation. Furthermore the paper discusses a conservative approach for the estimation of the resulting pressure depending on the percentage of water inside the cask. To get trustworthy results without an exact specification of the content, experimental fire tests should be conducted. However, this paper shows the difficulties of assessing casks containing wet content. From the authority assessment point of view, drying of the content could be an effective way to avoid the above described pressure build-up and the associated difficulties for the safety assessment.
The presentation gives an overview over three current fire testing activities for testing transport packages for the transport of high-level radioactive material. Packages for the transport of high-level radioactive material must withstand severe hypothetical accidents. Regulatory test conditions shall cover these severe accident conditions and consist of mechanical tests and a following thermal test. To withstand the mechanical tests heavy weight packages are often designed with impact limiters consisting of wood encapsulated in steel sheets. The thermal test is defined precisely in the IAEA-regulations as a 30 minute fully engulfing 800 °C fire. After the fire phase a pre-damaged impact limiter might continue burning or smoldering and influence the cask thermal behavior with its energy release. The energy transferred from the impact limiter to the cask is of importance for the safety of transport packages. A full-scale fire test with an impact limiter of 2.3 m in diameter and filled with spruce wood was designed and performed. The impact limiter continued burning for 3 days. Energy transfer and temperature measurements were performed. A new test is designed to examine pressure build up and possible mechanical failures in an undamaged impact limiter with a diameter of 1.9 m. The test is designed to measure heat flux from the impact limiter in case of its ignition and burning. To furthermore examine the burning behavior of steel encapsulated wood piles, tests are prepared with wood fire containers and an infrared ignition source. A fire reference package for calorimetric tests was designed. The fire reference package design consists of a closed steel sheet cylinder with a length of 1,500 mm, an exterior diameter of 1,050 mm, and a wall thickness of 10 mm. The cylinder was filled with refractory insulation material and instrumented with thermocouples distributed all over the cylinder. The measured local steel sheet temperatures allow the determination of local as well as overall integral heat fluxes versus time and versus surface temperature. Currently, tests are planned with a new and relatively small fire reference package design. The new design will have a steel sheet cylinder with a length of 182 mm, an exterior diameter of 120 mm. Additionally heat flux sensors will be used in the new fire reference package test design. The propane gas fire will be adjusted with respect to the outcome of the fire tests to meet the requirements of the IAEA-fire.
Packages for the transport of Spent Nuclear Fuel and high active radioactive waste are designed to withstand severe accidents. Specific mechanical and thermal tests are defined in the IAEA Regulations for the Safe Transport of Radioactive Materials in order to cover these hypothetical severe accidents. The thermal test mainly consists of a 30 minute fully engulfing fire. Components such as the package impact limiters can lead to supplementary energy release during the thermal test as they might continue burning after the fire phase. Local heat flux into the package can occur. Measurement of heat flux into the package is of importance to evaluate component temperatures and review their acceptance. The usability of heat flux sensors and temperature measurement equipment has to be tested to apply them in impact limiter fire testing. Further questions arise such as :“Is the infrared camera a useful tool to determine heat flux at the boundary surface?”
Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood, encapsulated by steel sheets. These impact limiters shall ensure the transport cask meets the mechanical and thermal IAEA regulatory test requirements. After damage caused by the mechanical tests the package has to withstand a severe fire scenario. According to the regulations during and following the thermal test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally.
Corresponding to results of the French institute IRSN combustion and smouldering of wood inside the impact limiter occurred during and after the fire test. An additional energy supply from a pre-damaged impact limiter to the cask could be the consequence for the safety assessment of the containment.
BAM started a first test phase to examine the issue of combustion for such kind of package components. The goal was to understand the phenomena under the consideration of relevant regulatory boundary conditions. Several metal buckets were filled with wood and equipped with thermocouples. The test specimens have been prepared with different damage arrangements to take the influence of the mechanical tests into account. This paper shows the experimental setup and the conduction of the tests. The first test shows that pre-damaged metal encapsulations can lead to smouldering of the wood and with this to a supplement energy release after the end of the 30 minute fire. BAM is in the preparation process for a second test phase. A thermal test will take place with a wood filled test specimen weighing about 2Mg.
Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood, encapsulated by steel sheets. These impact limiters shall ensure the transport cask meets the mechanical and thermal IAEA regulatory test requirements. After damage caused by the mechanical tests the package has to withstand a severe fire scenario. According to the regulations during and following the thermal test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally.
Corresponding to results of the French institute IRSN combustion and smouldering of wood inside the impact limiter occurred during and after the fire test. An additional energy supply from a pre-damaged impact limiter to the cask could be the consequence for the safety assessment of the containment.
BAM started a first test phase to examine the issue of combustion for such kind of package components. The goal was to understand the phenomena under the consideration of relevant regulatory boundary conditions. Several metal buckets were filled with wood and equipped with thermocouples. The test specimens have been prepared with different damage arrangements to take the influence of the mechanical tests into account. This paper shows the experimental setup and the conduction of the tests. The first test shows that pre-damaged metal encapsulations can lead to smouldering of the wood and with this to a supplement energy release after the end of the 30 minute fire. BAM is in the preparation process for a second test phase. A thermal test will take place with a wood filled test specimen weighing about 2Mg.
Packages for intermediate level waste (ILW) often contain residual water besides the actual waste. The water either exists as obvious free water or it may be bound physically or chemically, e.g. as pore water.
Water driven gas generation could occur by vaporisation and by radiolysis.
Steam as the result of vaporisation causes an increasing pressure inside a package and can affect corrosion. Vaporisation and condensation processes itself change the thermal behaviour of the content especially during strongly unsteady thermal situations like accident fire situations.
Radiolysis changes the chemical composition of the content which could cause an unexpected interaction, e.g. hydrogen embrittlement. Besides the pressure build-up the radiolysis of water generates hydrogen and oxygen, which can be highly flammable respectively explosive.
The gas generation caused by vaporisation and radiolysis must be taken into account during the design and the safety assessment of a package. Pressure build-up, a changed thermal behaviour and content chemistry, and especially the risk of accumulation of combustible gases exceeding the limiting concentration for inflammability has to be considered in the safety assessment. Approaches to ensure the transportability of stored packages due to radiolysis will be discussed.
Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood, encapsulated by Steel sheets. These impact limiters shall ensure the transport cask meets the IAEA safety requirements. After damage caused by the mechanical tests the package has to withstand a severe fire scenario. According to the regulations during and following the fire test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally.
Due to results of the French institute IRSN it was shown that after the fire test an additional energy supply from a pre-damaged impact limiter should be taken into account. The combustion or smouldering of wood was of interest.
BAM started a first test phase to examine the issue of combustion for such kind of package components. The goal was to understand the phenomena under the consideration of relevant regulatory boundary conditions. Several metal buckets were filled with wood and equipped with thermocouples. The test specimens have been prepared with different damage arrangements to take the influence of the mechanical tests into account. This paper shows the experimental Setup and the conduction of the tests. The first test shows that pre-damaged metal encapsulations can lead to smouldering of the wood and with this to a Supplement energy release after the end of the 30 minute fire. The consequence could be, to consider additional thermal loads of wood filled impact limiters to filled the IAEA regulations, if the conditions of the tests are transferable to the safety analysis of the package design.
Packages for the transport of radioactive material are often equipped with impact limiters consisting of wood. Mostly this wood is encapsulated by steel sheets. The impact limiters are needed to ensure that the transport casks meet the IAEA safety requirements. According to the IAEA safety requirements a package has to withstand consecutively severe mechanical tests followed by a thermal test. The mechanical tests have to produce maximum damage concerning the thermal test. Following this, the impact limiters may have serious pre-damage when the thermal tests begins. The IAEA safety requirements state that during and following the fire test, the specimen shall not be artificially cooled and any combustion of materials of the package shall be permitted to proceed naturally.
Small scale fire tests with wood filled metal drums by BAM and works of the French Institute for Radiological Protection and Nuclear Safety (IRSN) showed that pre-damaged steel encapsulated wooden structures could start smoldering initiated by the thermal test. These processes supply additional energy to the cask which should be considered within the safety assessment of the package.
As not much is known about smoldering processes in encapsulated wooden structures with a reduced oxygen supply the need for a test was identified. To investigate the influence of a smoldering impact limiter concerning the amount of energy supplied to the cask in dependence of the time BAM conducted a large scale impact limiter thermal test. For that, a pre-damaged impact limiter with a diameter of 2,3 m was mounted on a water tank simulating a cask. A complex system of a regulated pump, a heater, a cooler, a slide valve, a flow meter and numerous thermocouples were installed and connected to a control unit to ensure all needed operating conditions.
After a pre-heating compared to typical SNF decay-heat, the 30 min lasting fire phase of the thermal test was started. After that, the expected and initiated smoldering began. The results of the large scale test are presented in this poster. Systematic small scale tests will follow to identify the influence of different parameters, e.g. moisture content and scale effects. The tests took place at BAM Test Site for Technical Safety (TTS) with its various possibilities for mechanical and thermal tests. The results of these tests will have direct influence in the safety assessment of transport cask for the transport of radioactive material