Radiological protection system application in the radioactive incident of Acerinox.

Transcription

1 Radiological protection system application in the radioactive incident of Acerinox. J.T. Ruiz, J.M. Campayo Logística y Acondicionamientos Industriales S.A. C/ El Palleter,13, Valencia. Spain. jt.ruiz@lainsa.com; j.campayo@lainsa.com In 1998 occurred an accidental fusion of a Cs-137 source in the stainless steel factory that ACERINOX has in the South of Spain. It has been estimated that the melted source activity was more than 4500 GBq. This communication is about actions that the Radiological Protection Technical Unity (UTPR) of the firm LAINSA carried out during the recovery operations of the affected facilities by the contamination and the posterior actions directed fundamentally to the radiological characterization of the waste materials. The detected maximum radiation levels at first were between 1 and 2 msv/h. The maximum activity of the wastes (smoke dust, soils, sludges, etc), before beginning the decontamination operations it was approximately at 2000 Bq/g. The swiftness in the intervention made in Acerinox demonstrated the response capacity and coordination between the specialised private firm LAINSA and the official institutions in charge of the radiological protection, nuclear safety and radioactive wastes handling, to solve with efficiency and safety one of the most important contamination radioactive incidents which have ever happened in Spain. 1. INTRODUCTION The ACERINOX Plant contamination was due to an accidental fusion of a Cs-137 source, which was probably melted with scrap which is used as raw material in stainless steel manufacturing. It has been estimated that the melted source activity was more than 4500 GBq. After notifying Acerinox the incident to the Spanish Safety Council (CSN) the Radiological Protection Technical Unity staff (UTPR) of LAINSA went to the affected facilities to take in charge the preliminary radiological assessments. Immediately the following radiological protection measures were taken to avoid the undue exposition to ionised radiations: - Inventory of the contaminated areas. - Radiological inspections of cloack-rooms and work clothes of the staff which worked normally in the affected areas. - Delimitation, beaconing and signalling of the contaminated areas. - People accesses, materials and vehicles control and restriction to the affected areas or its proximities - Samplings in the depurators gases outputs, in order to verify that there were not emissions to the exterior As result of these first actions it has been elaborated a radiological map of the affected areas. It confirmed the lack of contamination outside the facility and in the rest of the manufacture plant. Furthermore the external and internal contamination measurements carried out on Acerinox people and contractors which were in contact with the radioactive material showed Caesium levels very under the equipments detection limit. Decontamination works took five months, and more than man-hours were necessary to perform the whole work (20 % corresponding to radiological protection activities). The total collective dose was about 60 man.msv.

2 2. PERFORMANCE PLAN The recovery operations for the affected facilities began immediately, even before the formal approval from CSN of a Performance Plan. The fundamental aim, was recovering the contaminated facilities taking all the necessary measures to warrant the Radiological Protection for the workers, the public and the environment. According to the Performance Plan approved by the CSN the final state of the facilities should be such that: The maximum permissible dose in any area of the factory did not exceed the value of 1 msv per year. The derived values from surface contamination were such that they did not exceed 4 Bq.cm 2 (for beta-gamma emitters), in those areas where its measurement was possible. Due to the size of the facility and the great number of the contaminated areas it was not easy to establish a strict and unique access. Thus, in the first moments, those areas with higher dose rates and requiring greater movements of people were identified. The measures adopted were based on two general approaches: Immediate intervention: action to remove radioactive material, decontaminating the zone, remove systems, equipment, etc., and Isolation of these areas, by establishing alternative access and routes. Since the first moment, it has been established an organisation of human and technical media, as in situ as in the central offices of LAINSA. The human organisation of the Radiation protection Service consisted basically in: 1 Radiation Protection Manager of UTPR (full-time) in situ 2 Radiation Protection Officers. Radiation Protection auxiliary technicians in variable number according to the activities development. Furthermore, personal protection equipments, particularly of respiratory protection, which were necessary due to the high risk of internal contamination presented in the closed areas. 3. CONTAMINATED AREAS The contamination fundamentally fixed in the steel fabricating smoke dust, affected the gases extraction system conducts of the Electrical furnace nº 1, where it circulates, and the filtration system which is common for the Electrical Furnaces Nº1 and Nº 2 as shows figure 1. The average values of dose rate in the more contaminated areas ranged between 20 and 200 µsv/h, reaching maximum values of 2 msv/h in some specific areas. Table 1 shows a summary of the initial contamination levels of the main systems. The objective established for the final state of the facilities had to fulfil two requirements; the production of the Steel Works had to continue and it was necessary to cope with the radiological protection principles.

3 Table 1. Initial contamination of the main elements. Element Average dose rate (msv/h) Maximum dose rate (msv/h) Electric Arc Furnace nr. 1 and gas 0,5 1,8 ducts extractions. Natural coolers and stark arrester. 0,020 0,050 Bag filter Depuration Systems nr. 0,05 0,1 1 and 2 Silos A and B 0,03 0,1 Therefore, in the first phase decontamination was limited to clearing line no.1, allowing normal production to continue on the other clearing line. In that phase most of the very low activity contaminated wastes were generated. Next decontamination of the electrical Furnace no.1 was undertaken, followed by the Bag filter no.2 and silos. In these phases, less smoke dust wastes were extracted but metallic wastes, refractory bricks of the furnace, etc., were generated. Dry decontamination techniques (vacuum cleaning, grinding, etc.) were used to avoid the generation of liquid wastes that would have been difficult to treat in that facility.

4 4. WORKS DEVELOPMENT The works were divided in two phases. The first phase goes since the incident is produced until all the wastes of activity superior to 70 Bq/g were disposed by ENRESA. The second one begins with the waste materials characterisation between 1 and 70 Bq/g and the dismantling of depuration system exchangers and finalises with the removal by ENRESA of all the wastes with activity superior to 10 Bq/g in February The restoration process has been carried out, in its turn in various steps: 1. Decontamination of the evacuation systems common for both furnaces. 2. Depuration line nº 1 decontamination. 3. Furnace nº 1 decontamination and its different components. 4. Depuration line nº 2 decontamination 5. Silos decontamination, systems exteriors and soils. The dry decontamination techniques were used (aspiration, manual extraction, electrical reaming, whetting, sanded, etc.) with the aim of avoiding the liquid wastes generation of difficult management in the factory. However it was carried out a cleaning of two coolers by means of high pressure water, which produces 35 m 3 of liquid wastes, which later had been treated by means of reverse osmosis with efficiency. The special work conditions, being in the open air and the facilities, equipments and affected components quantity, conditioned the radiological protection staff action. On the following the main activities which were carried out from the radiological point of view are described Emissions control at the exterior Continuous isokinetic samples were taken in the gases evacuation systems to control the emissions level at the atmosphere. The measurements results are showed in the figure 2, where it can be appreciated that the emissions are very under the LDCA for the Cs-137 (2 Bq/l). This control was maintained till it finalised the smokes depuration system decontamination Radiological Survey of Decontamination Works The radiological protection criteria are summed up in the Table 2. The radiological information of the work areas was known and registered in the corresponding Radiation Permit Work (RPW) before carrying out any intervention, in order to know and evaluate the radiological risks and in consequence adopting the adequate radiation protection measures. First of all it was carried out environmental contamination measures, in order to determine which type of respiratory protection it must use. Following the radiation levels were measured in order to establish the necessity or not to control times and the DLD dosimeters use.

5 Table 2. Radiation protection criteria Individual dose Constraints: 0.3 msv per day; 1 msv per week; 3 msv per month. ALARA surveys: If estimated collective dose is higher than 10 man.msv. Use of electronic dosimeters: Works with dose rates greater than 30 µsv/h. Control of time exposure In an ambient dose rate higher than 150 µsv/h. Environmental contamination control. Before and during the execution of the works with risk of producing dust. With values included between 3,75 % and 37,5 % of the LDCA it will be used a face mask. If the environmental contamination is superior to 37,5 of LDCA, then it will be used fancirculated air or improving the ventilation conditions. Control of surface contamination Surface contamination limit in zones in which the measurement is feasible: < 4 Bq/cm 2. Surface contamination measurements were always done if it was possible, not only to establish the initial conditions but to avoid not wanted cross contaminations. These measures was done periodically during the works carrying out, and according they changed the radiological conditions. The radiological control of the decontamination process is shown in figure 3.

6 4.3. Accesses controls and periodical controls The accesses to the contaminated areas (till three simultaneously) were controlled by the radiation protection staff, which carried out the control labour of persons, materials and wastes entries/exits. The bounded areas in work sites and in wastes storage areas were target of periodical control, in order to warrant that the established perimetric sites fulfilled the established radiation protection conditions. Figure 4 shows a general view of the contaminated areas.

7 4.4. Occupational Exposure All the personnel involved in the decontamination operations of ACERINOX were classified as Professionally Exposed Workers (PEW), and therefore they fulfilled the basic requirements of training, dosimetric control and medical control. Furthermore all the workers had a specific training about radiological risks of the works they were going to do. Figure 5 shows a total and monthly collective dose summary, with data obtained of the TLD official dosimetry. The total collective dose was about 60 man.msv for the 5 months period. The average individual dose was 0.6 msv and the maximum individual dose was 3.5 msv. It was not necessary establishing DLD dosimetry for all the workers, due fundamentally to the radiation levels which did not justify this measure. It has been established DLD dosimetry for the critical groups, which should have work in areas with superior levels to detected average values. Table 3 shows the results of the operational dose for some critical tasks. Forty percent of the collective dose was assigned to the decontamination operations related to electric arc furnace and gas ducts extraction, where doses rates were the highest. The next critical group consisted of individuals dedicated to the wastes segregation and conditioning (23 % of the total collective dose). In this case, the number of people and the spent time were more important than the dose rates.

8 As far as the internal dosimetry was concerned, two programs of monitoring were set up (whole body counter). The first, a few days after the start of works to verify the suitability of the adopted protection measures. The second, at the end of the work to confirm the absence of contamination. In all cases the results were less than the recording level (0, 5 msv). Table 3. Collective dose for some critical tasks CRITICAL TASK DOSE (man.msv) Electric arc furnace and gas ducts 16,1 extraction Natural Coolers 3,1 Bag filter nr.1 5,3 Bag filter nr. 2 2,3 Scaffolding installation and stripping 3,4 Silos 0,5 Waste Handling 9, Wastes characterisation The wastes produced were put into two types of containers. The smoke dust was put into 1 m 3 big-bags, whereas metallic, plastic wastes, paper, etc., were put into 220 litres drums. Each one of the package was identified, labelled, it measured its radiation level and it has been registered its containing, weight, size, origin, specific activity, and so on. Furthermore, the big-bags should be perfectly closed, in order to avoid an eventual dispersion of its containing at the exterior.

9 In the wastes removal to the temporary storage areas, radiological protection standards had been applied about transport in the radioactive facilities interior. The packages were always enclosed with the presence of radiation protection staff, which guarantee the strict fulfilment of these norms. A very important percentage of these wastes were target of spectrometric measures by means of samplings with special tools. In this way the specific activity was determined, it was useful not only to characterise the waste, but also to evaluate the systems decontamination process and the system depuration as consequence of the not contaminated dust flow. Table 4 shows a percentual distribution of wastes according to its typology. Table 4. Distribution of wastes. Type of waste Quantity (%) Smoke dust 91 Fibre cement panel 4 Refractory brick 2 Pressable waste (plastics, paper, 2 etc) Metallic wastes 1 Table 5 shows the total weight and activity corresponding to each type of waste that had to be disposed in ENRESA facilities. Table 5. Balance of radioactive wastes Type Units Weight (Kg) Activity (GBq) Smoke dust (bigbags) l drums Total Liquid waste treatment As result of the cooling circuit cleaning of the contaminated furnace, it had been produced approximately 40 m 3 of contaminated liquid wastes which present a concentration of a Cs-137 activity of approximately 200 Bq/g. The chosen treatment technique was the reverse osmosis. The obtained results showed retention values for Cs superior to a 99%. From the initial 40m 3, it has been declassified 36 m 3 by means of membranes treatment. The declassified liquid volume with a residual activity below to 1 Bq/l was then emptied in a neutralisation plant with a flow of 246 m 3 /h. The concentrated liquid had been spilled at a destructive distillation, collecting the steam flow in a liquid bath. By means of this process it has been reduced from 4m 3 to 1m 3 the concentrated liquid, it has been inertized in a cement matrix and calcium hydroxide, in order to be handled later as solid waste.

10 5. CONCLUSIONS The participation of UTPR of LAINSA in the incident of ACERINOX guaranteed the application of the radiological protection norms during the facilities regenerating operations, relying on sufficient technical and human means for an emergency situation of this importance. The adopted radiological measures in the works have been adequate and efficient as it has not been produced decontamination staff contaminations, and neither intern contamination of them. So the external doses have been maintained at levels reasonably low. The radioactive incident happened in ACERINOX in May of 1998 did not suppose undue risks of ionised radiations exposure and neither the workers, or for the public or the environment. The immediate intervention carried out by ACERINOX, has showed the response capacity and the coordination which exists in Spain with an unprecedented incident in our country. The works planning and development, we consider that it has been the adequate to raise a problem posed with efficiency, harshness and professionalism, considering under pressure of time and the ACERINOX request to maintain operative the rest of the Iron Mill. 6. REFERENCES Ruiz, José T; Curiel, M..Incidente radiactivo en Acerinox: Historia de una Intervención. Sociedad Nuclear Española. Granada Ruiz, J.T; Campayo J.M. Control radiológico de las operaciones de descontaminación en Acerinox. Sociedad Nuclear Española.Granada Ruiz, J.T.; Campayo J.M. Radiological Protection in the Radioactive Incident of ACERINOX in Spain. European ALARA Newsletter. Issue 8 April 2000.