A Ventilation Concept for Activated Air

Transcription

1 Laboratory Note DESY D3 104a March 2001 A Ventilation Concept for Activated Air in the TESLA Tunnel A. Leuschner, B. Racky Deutsches Elektronen-Synchrotron DESY Abstract The beam delivery systems of the Linear Collider TESLA are expected to contain several components causing considerable beam losses. As a worst case example which can be scaled to other scenarios, a 100 kw collimator is investigated from the point of view of air activation. The impact on both the public and the DESY site were calculated for an open and a closed ventilation system. With respect to the public it seems that the installation of a high chimney can be avoided for both options. For the DESY staff no access restrictions will be necessary from the point of view of activated air.

2 1 Introduction In the main accelerator tunnel of the Linear Collider TESLA the beam delivery collimation system represents the region with the largest permanent beam loss. This collimator section is located 1 km from the interaction point and is about 100 m long. In the laboratory note [DL99] the air activation, the soil and ground water activation as well as the residual radioactivity caused by this collimator system were calculated by means of the Monte Carlo code FLUKA [FFS94] for the worst case of 100 kw beam loss. Here the air activation data from [DL99] are taken to investigate the impact on both the public and the DESY members working in the tunnel on maintenance days. A ventilation and exhaust system is foreseen in each of the 9 halls along the TESLA tunnel, so that the distance between two release points is not more than 5 km if no bypass is active. Two simple operation modes of the ventilation system are compared: 1. Closed system: The ventilation system is switched off during accelerator operation, air will only be blown out at monthly maintenance days. 2. Open system: The ventilation system is switched on during accelerator operation, a continuous air stream passes through the tunnel and is blown out permanently. A new German radiation protection law (Strahlenschutzverordnung) is expected to be released soon to fulfill the requirements of European right [EU96]. A preliminary version is existing [Bun00] where new limits for doses and activity concentrations are given. According to the new regulations the annual dose limit for staff members is 20 msv and the dose limit for the general public is 0.3 msv per year due to the release of radioactive air or water. [Bun00] is giving limits for nuclide specific activity concentrations for air leaving any radiation area without special permission.these limits depend on the exhaust rate. Here the limits for 6 10 m /h are used, corresponding to a ventilation speed of 0.8 m/s. The calculated activity concentration of a nuclide divided by its limit has to be for the sum of all nuclides. These evaluations are performed to compare the two ventilation concepts. Furthermore, doses due to the activated air are calculated for DESY workers who enter the tunnel for an 8 hour shift immediately after shut-down. 2 Closed system The calculations for this ventilation mode are based on the following assumptions: The ventilation system is switched off during one month of accelerator operation (720 h). During this time an air volume of 2000 m in the 100 m long collimator section is being activated. 2.1 Impact on the public At the beginning of the shut-down the air is ventilated with a speed of 0.8 m/s, so that the active volume from the collimators reaches the next exhaust station after 5000 s and is then released 1

3 & & & within 130 s. The maximum activity concentrations at release are given in table 1 for all relevant nuclides based on the FLUKA calculations reported in [DL99]. The mean value of the annual release was calculated, assuming 7 accelerator duty cycles of one month, followed by at least one shut down day (operation time: 5000 h per year). The ratios of the calculated annual mean values divided by the limits from [Bun00] are given in column 6 of table 1 for each nuclide. The sum of all ratios shows, that the activity release is only about 0.1 % of the activity concentrations that can be blown out without a permission required. The highest contributions are made by Be and Ar. Be is a product of spallation reactions due to high energetic radiation whereas Ar is produced by low energetic neutrons only. 2.2 Impact on DESY staff The maximum activity concentrations after 720 h of accelerator operation without ventilation are given in table 2. Then the dose of a worker due to the stay in activated air for one shift ( h) starting immediately after shut down (no delay time) is calculated, using nuclide specific reference concentration derived from [EU96] and [Bun00]. The reference concentration corresponds to a dose rate Sv/h. To be conservative it is assumed, that the air is not exchanged during the shift. Taking into account the time behavior of the activity concentration the simple formula for the dose due to the nuclide i is: (*),+ #!" $% '& - %/.0 (1) With the exponential radioactive decay %1 2 & the integral of equ.(1) can be calculated to!" 78 90:;< with :;"1= >% 9A@CBD (2) 2 E& # :;F G78;90:;< (3) In order to check how the formula equ.(3) works the two limits are considered. Firstly, for short living nuclides with half lives much shorter than one shift the dose is independent of its duration. E& : for 8H : (4) And secondly, for long living nuclides with half lives much longer than one shift the dose is given by the simple proportion IJ E& # for 8KL:; (5) The doses for one shift calculated by equ.(3) are listed in table 2, column 5. for all relevant nuclides. The sum of all contributions is NMPO Sv per shift, which is about the same as 80 Sv ( = 20 msv 8 h / 2000 h). These calculations are very conservative, because there will be a certain delay time before the active air volume is entered and with the ventilation switched on the active air will be blown out after 1.4 h. 2

4 3 Open system The calculation for this ventilation mode is based on the following assumptions: During accelerator operation fresh air is driven through the tunnel with a velocity of 0.8 m/s. For an arbitrary activation length of 6 m the irradiation time is 7.5 s and the corresponding air volume amounts to 120 m. Note, that the activity concentrations are independent of the choice of the activation length once the air velocity is fixed. 3.1 Impact on the public The activated air from the collimator is transported permanently to the exhaust station at the next kryo shaft and is blown out after a delay time of 1.4 h. The maximum activity concentrations at the outlet are given in table 1 for all relevant nuclides based on the FLUKA calculations reported in [DL99].The annual mean values of the activity concentrations at the release point are given in table 1 for an accelerator operation time of 5000 h as well as the ratios of the mean values to the limits from [Bun00]. The ratios sum up to 0.26 for all nuclides, which is below the limit of 1.00, but about a factor of 240 higher than for the closed system. The main contribution is Ar with a ratio of Impact on DESY staff Here the dose equivalent due to the stay in activated air for one shift (8 h) starting immediately after shut down is calculated with the maximum activity concentrations given in table 2. To be conservative the air flow is assumed to be stopped at shut down. The doses listed in table 2, column 7 were calculated with equ.(3). The sum of the dose contributions is QDSRUT Sv, which is small compared to the 80 Sv and about a factor of 20 lower than at the closed system. 4 Conclusions The planning goals for the air ventilation concept in the main TESLA tunnel with respect to radiation protection aspects are: V to keep the doses to the public due to the release of radioactive air well below the limit of 0.3 msv per year, V to achieve, that the release of radioactivity to the environment is small enough that according to [Bun00] even no permission for the release would be required, V to keep the personal doses due to activated air of staff members working in the tunnel on maintenance days as low as possible. As a key value of a limit the annual dose limit (20 msv) weighted by the shift duration, which is 80 Sv, can be used. 3

5 The maximum and annual mean activity concentrations at the TESLA air exhaust station with the shortest distance to a tunnel section with 100 kw beam loss were evaluated for two ventilation concepts. A closed system with no air exchange during accelerator operation was compared with an open system with a permanent input of fresh air and release of activated air. For both concepts the nuclide specific activity concentrations at the outlet are below the limits, for the open system 26 % of the limit is reached, for the closed system even only 0.1 %. So the closed system would cause much lower doses due to the release of radioactive air to the public than the open system. The other advantage of the closed system is that it provides a damping effect in case of large unexpected beam losses in the vicinity of a release station. In case the open system has advantages with respect to the accelerator operation, a compromise of the two concepts may be possible like an open system with bypassed release stations closed to beam loss points. The limits for nuclide specific activity concentrations at the exhaust stations we used are defined in a way, that a person staying in this air permanently cannot receive a higher dose than 0.3 msv. So for any suggested ventilation concept it seems not to be necessary to build a high chimney to achieve that the doses to the public are below 0.3 msv. The dose per shift for a worker in the tunnel is about 50 Sv for the closed system and 3 Sv for the open system. These doses are maximum values, assuming no ventilation at shutdown and no delay time before entering the area after termination of the accelerator operation. Taking the ventilation into account the activated air passes by any location within only a few minutes. So the doses are expected to be much smaller than the maximum values given here. Note that the dose rates due to activated material in the vicinity of the collimator [DL99] are in the order of 10 msv/h which is a severe radiation protection issue. References [Bun00] Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit. Entwurf Strahlenschutzverordnung. BMU, [DL99] [EU96] H. Dinter and A. Leuschner. Induced radioactivity and dose rates in the vicinity of a collimator at the Linear Collider TESLA. Laboratory note DESY D3 104, September EU. Richtlinie 96/26/EURATOM des Rates vom 13. Mai 1996 zur Festlegung der grundlegenden Sicherheitsnormen für den Schutz der Gesundheit der Arbeitskräfte und der Bevölkerung gegen die Gefahren durch ionisierende Strahlungen. EU, [FFS94] A. Fasso, A. Ferrari, and P.R. Sala. Designing electron accelerator shielding with FLUKA. In 8th Intern. Conf. Radiation Shielding, Arlington,

6 W Table 1: Activity concentrations of tunnel air released to the environment. Closed system Open system Nuc. Half- Limit max. at mean ann. ratio max. at mean ann. ratio life X/ outlet 2YZ release YZ [YZ <9\ X] outlet YZ release ^YZ YZ _9Z X/ = >% [Bq/m ] [Bq/m ] [Bq/m ] [Bq/m ] [Bq/m ] H 12 a ` `]a ` ` `]a Be 53 d ` ` ` ` ` c C 20 m ` `6b ` d C 6 ka ` `6b ` ` `6b N 10 m ` `6b ` a d O 2 m `Fe ` ` b `6e `Fe ` %fe Cl 37 m ` `]a ` Cl 56 m f ` `]a% ` % Ar 35 d ` ` ` ` ` Ar 1.8 h ` ` ` sum of the ratios ` ` Column 3: Column 4: Column 5: Column 7: Column 8: Column 6,9: Act.conc. which can be released without permission for an exhaust rate of 6 10 m /h. Act.conc. after an 720 h activation in the collimator section and transport to the exhaust station (delay time 1.4 h). Annual mean value at the exhaust station for 7 run periods per year (5000 h). Act.conc. after activation in the collimator section and transport to the exaust station (delay time 1.4 h). Annual mean value at the exhaust station per year (5000 h). Ratios of the annual mean values and the limits of col.3. Table 2: Doses due to activated air for workers in the tunnel (one 8 h shift). Closed system Open system Nuclide Half-life Reference Act.conc. Dose ( ) Act.conc. Dose ( ) = >% act.conc. at per shift at per shift $% shut down shut down [Bq/m ] [Bq/m ] [ Sv] [Bq/m ] [ Sv] H 12.3 a 2 10a ` Be 53.3 d 1 10a ` % d C 20.4 m d C 5730 a ` 0.00 N 9.96 m a a O 2.03 m %f e Cl 37.2 m Cl 56.0 m 2 10a Ar 35.0 d 1 10e ` 0.00 Ar 1.83 h sum of the doses Column 3: Column 4,6: Column 5,7: Reference activity concentration for a dose rate of 10 Sv/h. Act.conc. at shut down after 720 h operation. Dose equivalent for one 8 h shift (to be compared to 80 Sv). 5