Investigation report. Radiation resistance of fire detectors under the influence of radioactive gamma radiation

Transcription

1 Radiation resistance of fire detectors under the influence of radioactive gamma radiation Page 1 ST FIR/PRM1

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3 Contents 1. Introduction Fire detectors tested Trial description Trial design Energy distribution and volumetric heat generation rate of -radiation Execution of the trials Trial results Discussion of results Basics Evaluation and conclusions Operating time as influenced by -radiation Appendix

4 1. Introduction Reliability and safety requirements are particularly important for early recognition of fires in nuclear systems. Here automatic fire detectors are also used in areas in which they are subjected to radioactive radiation. The type and dosage levels of radiation at the location of the detector can lead to changes in the response of the fire detector as well as damage to individual components. Taking into account the location factors in each case, the results of these investigations have a significant impact on the selection of fire detector types and their operating time when developing new fire detector systems and with regard to maintenance measures for existing systems. The present investigation shows how the sensitivity and the quiescent value of various MAGIC.SENS fire detectors are influenced by varying degrees of gamma radiation at different dose rates. 2. Fire detectors tested Fire detectors with GLT- and LSN-technology, and with optical, thermal and chemical sensors were tested. Trial 1 Detector designation Type Sensors OT 300 GLT optical + thermal OT 400 LSN optical + thermal OTC 410 LSN optical + thermal + chemical Trial 2 Detector designation Type Sensors OT 300 GLT optical + thermal OT 400 LSN optical + thermal OTC 410 LSN optical + thermal + chemical 4

5 3. Trial description 3.1. Trial design The trials were performed in the tank system at the technical facility of the Jülich Research Centre. Here medium burn up fuel assemblies positioned vertically in storage capsules, are used for specific gamma radiation as required. Supervision of sample containers in radiation tank The fire detectors were lowered into the radiation tank in a sample container with a diameter of 330 mm, and were positioned between the fuel assemblies and supplied and checked via pipelines. The surrounding medium consisted of air at a temperature of C. Sample containers 5

6 3.2. Energy distribution and volumetric heat generation rate of -radiation In order for the quality of the gamma radiation to be evaluated, the energy distribution and volumetric heat generation rate were determined for medium burn up fuel assemblies (55%). Figure 1 shows the volumetric heat generation rate and the photon energy averaged with the volumetric heat generation rate across the entire energy range as a function of the decay time. The emission rate (photons/sec) decreases erratically during the first two weeks. The next phase over a period of 16 months is characterised by an average negative slope. Thereafter, the trend is relatively flat, attributable to the gamma activity in durable fission products. The average energy of the gamma radiation decreases rapidly during the first three months. After that the trend is almost constant at an average energy of 0.6 MeV. Emission rate in s 1 1.0E E E+15 Volumetric heat generation rate: photons per s Energy in MeV 1.0E+14 Average energy of gamma radiation E Time in months Fig. 1: Volumetric heat generation rate and average energy in gamma radiation of burnt up fuel assembly depending on the decay time Figure 2 illustrates the energy distribution in gamma radiation with different decay times. Accordingly, after only two weeks, no high energy photons (> 3,5 MeV) are emitted. As the decay time increases, so the energy spectrum shifts towards lower energies due to the remission of high energy gamma radiation. This results in a slight decrease in average energy in the -photons (Fig. 1). The fire detectors were exposed to an energy spectrum of MeV during the trials. On average, the quantum energy amounted to 0.7 MeV. The absorbed dose was measured using an ionisation chamber with direct display. The neutron field is negligibly low, which means that no activation takes place. After radiation the probes can easily be handled. 6

7 Emission rate in s 1 1.0E E E E E E+07 Gamma radiation spectrum at discharge after 2 weeks after 2 months after 3 months after 3 years 1.0E Energy in MeV Fig. 2: Energy distribution in gamma radiation of burnt up fuel assembly (55% burn up) 3.3. Execution of the trials In trial 1, all fire detectors were exposed to gamma radiation with an average absorbed dose rate of 25 Gy/hr for a period of 20 hrs 35 min. The cumulative absorbed dose amounted to 515 Gy. The second trial was executed at an average absorbed dose rate of 15 Gy/hr. A cumulative absorbed dose of 105 Gy was achieved for all LSN-fire detectors over an exposure time of 7 hrs. The GLT-fire detectors remained in the radiation tank for 17 hrs. The cumulative absorbed dose amounted to 255 Gy. The sensitivity of all detectors was measured in an aerosol channel in accordance with EN 54-7 before and after radiation. Sensitivity measurement of the chemical sensors in the OTC 410-detectors was carried out using a test gas concentration of 30 ppm carbon monoxide (CO). In addition to measuring the sensitivity, the quiescent value of the optical component was recorded for all LSN-detectors. One component of the detectors was in operation during radiation (test mode). 7

8 3.4. Trial results The measurement results are recorded in tables 1 6. During exposure to radiation the noise in the optical component increases marginally (from approx. +/ 2 digits to approx. +/ 5 digits). The quiescent value is higher after exposure to radiation, but far below the limit value that would trigger a contamination warning. In trial 1, the first fire detector was defective after a radiation duration of 5 hrs ( cumulative absorbed dose of 125 Gy). In total, defects occurred in 2 of 6 OT 300-detectors, 4 of 6 OT 400-detectors and 1 of 6 OTC 410-detectors. The failure in the OT 300-detectors was caused by defective operation amplifiers. The detectors were unable to produce an optical alarm, but could still produce a temperature alarm. The LSN- chip was defective on each of the four affected OT 400-detectors. The FET-transistors could be switched, although the LSN-chips were no longer responding. In the case of the defective OTC 410-detector, the current consumption of the EEPROM increased and the alarm -LED lit up. In trial 2 (cumulative absorbed dose of 255 Gy or 105 Gy), no defects were recorded in the OT 300 and the OT 400-detectors. A higher current consumption occurred in two of the six OTC 410-detectors in the EEPROM ( 200 A). Where the detectors were started with a higher current, they became functional again. The first defect occurred after a radiation duration of 5 hrs ( cumulative absorbed dose of 75 Gy). In summary, the results of the second trial show that defects occurred in 0 of 6 OT 300-detectors, 0 of 6 OT 400-detectors and 2 of 6 OTC 410-detectors. The sensitivity of the optical sensor increases by approximately 5 to 20% as a result of the radiation. The sensitivity of the chemical sensor increases slightly (approximately 7%) for an absorbed dose of 515 Gy and on average remains unchanged for an absorbed dose of 105 Gy. 8

9 MAGIC.SENS OT 300 Trial 1: Average dose rate: 25 Gy/hr Duration of radiation 20 hrs 35 min Cumulative dose: 525 Gy Serial number before radiation after radiation Serial number Sensitivity of O part in db/m Sensitivity of O part in db/m Error/comment OP defect/t alarm intact OP defect/t alarm intact Trial 2: Average dose rate: 15 Gy/hr Duration of radiation 17 hrs Cumulative dose: 255 Gy Serial number before radiation after radiation Sensitivity of O part in db/m Sensitivity of O part in db/m Error/comment

10 MAGIC.SENS OT 400 Trial 1: Average dose rate: 25 Gy/hr Duration of radiation 20 hrs 35 min Cumulative dose: 525 Gy Serial number before radiation after radiation Quiescent value Sensitivity of 0 part in db/m Quiescent value Sensitivity of 0 part in db/m Error/comment LSN chip defective * LSN chip defective * LSN chip defective * LSN chip defective Trial 2: Average dose rate: 15 Gy/hr Duration of radiation 7 hrs Cumulative dose: 105 Gy Serial number Quiescent value before radiation after radiation Sensitivity Quiescent value of 0 part in db/m Sensitivity Error/comment of 0 part in db/m * * * * Detectors were operational (test mode) during radiation. 10

11 MAGIC.SENS OTC 410 Trial 1: Average dose rate: 25 Gy/hr Duration of radiation 20 hrs 35 min Cumulative dose: 525 Gy before radiation after radiation Serial number Quiescent Sensitivity Sensitivity Quiescent Sensitivity value of 0 part in db/m of C part in db/m value of 0 part in db/m Sensitivity of C part in db/m Error/ Comment * EEPROM defective * Trial 2: Average dose rate: 25 Gy/hr Duration of radiation 7 hrs Cumulative dose: 105 Gy before radiation after radiation Serial number Quiescent Sensitivity Sensitivity Quiescent Sensitivity Sensitivity Error/ value of 0 part in db/m of C part in db/m value of 0 part in db/m of C part in db/m Comment * EEPROM defective * EEPROM defective * Detectors were operational (test mode) during radiation. 11

12 4. Discussion of results 4.1. Basics In principle, the question may be asked as to whether the trials performed with a gamma radiation source meet the conditions of use in nuclear systems and to what extent statements regarding the effect on electronic devices are representative. In scientific discussions and trials the types of radiation relevant for electronic devices are limited to gamma radiation and neutron radiation. Neutron radiation occurs only in containment with a direct view of the reactor, without reflections, and only during reactor operation. In the utility rooms, which are in the majority, only gamma radiation is available. The safety regulations of the KTA ( Kern-Technischer Ausschuss Nuclear Safety Standards Commission) and the IEEE (USA) standard prescribe only gamma radiation for testing devices Evaluation and conclusions The trials showed that gamma radiation can damage the ICs over a longer period of time. In particular, these were the LSN-chip with an absorbed dose of more than 125 Gy and the EEPROM in the OTC 410 with an absorbed dose of more than 75 Gy. Due to their construction, the GLT-fire detectors (OT 300) were significantly less sensitive to radiation. Because defects were also not found in the temperature component of the OT 300 detector for a cumulative absorbed rate of 515 Gy in trial 1, it can be concluded that the T 300 GLT-detector (thermal sensor) is suitable for areas of high radiation (up to 515 Gy). No defects were detected for a cumulative dose of 255 Gy in OT 300-detectors. OT 400- detectors also showed themselves to be resistant to radiation of 105 Gy. This result can be assumed for O 400 and T 400-detectors due to the similarity in construction. In all fire detectors tested, sensitivity either remained the same or increased Operating time as influenced by -radiation Using the results from different absorbed dose rates (trial 1 and trial 2), an estimate of the expected lifespan in known conditions for use can be made. For this, the type, dose rate and cumulative dose of radiation present at the detector location must be known. According to radiation protection regulations, the absorbed dose rate in the controlled zone of nuclear systems is between 7.5 Sv/hr and 3 msv/hr. 12

13 The results of the trials enable an estimate to be made of the operating time in which no malfunctions are to be expected in controlled zones. This is based on the maximum radiation exposure, which is also the limit value for identifying restricted areas: For OT 400: t E = D V D E = 105 Gy 3 mgy/hr = 35,000 hrs = 4 years D V 255 Gy For OT 300: t E = = = 85,000 hrs = 9.7 years D E 3 mgy/hr with t E operating time D V absorbed dose from trial 2 (no defects) D E expected absorbed dose rate at installation location In containment, the maximum ion dose is around 70 na/kg and the absorbed dose in the range of approx. 10 mgy. An operating time can also be estimated for this area: For OT 300: t E = D V = D E 255 Gy 10 mgy/hr = 25,500 hrs = 2.9 years 5. Appendix Radiological values Values Unit Relation Old unit Energy W ev (electron volt) 1 ev = 1.602x10 19 J Activity A Bq (Becquerel) 1 Bq = 1 sec 1 1 Ci (Curie) = 37x10 9 Bq Absorbed dose D Absorbed dose rate D Ion dose J Gy (Gray) 1 Gy = 1 J/kg 1 rd (Rad) = 0.01 Gy Gy/sec, Gy/hr C/kg (Coulomb/kg) Equivalentdose Sv (Sievert) 1 Sv = 1 Gy D e (for -radiation) 1 C/kg = 1 As/kg 1 R (Röntgen) = mc/kg 1 rem = 0.01 Sv 13

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