Radiation Shielding and PPS Zones

To enable efficient operation and commissioning, the personnel protection system (PPS) is designed to allow personnel access in one region while beam is in another region. As an

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example, the main linac beam tunnel can be in access while there is beam in the damping ring. It is assumed that all accelerator housings could have radiation levels that exceed the requirements for non-radiation workers. Therefore, the radiation shielding and PPS zones described here are designed for radiation workers.

2.9.4.1 Summary of Regions’ Radiation Requirements

Maximum allowable radiation levels for radiation workers for each region are summarized in Table 2.9.4.1. Radiation shielding and PPS devices must be designed to satisfy these criteria under the ILC beam-loss scenarios.

TABLE 2.9-2

Maximum allowable radiation levels and doses. The one in bold is the lowest and have been used in our design.

(a) Radiation Protection Instructions, DESY, June 2004.

(b) Radiation Safety Instructions, KEK, in Japanese, June 2004.

(c) Radiation Safety System, SLAC, April, 2006.

(d) Fermilab Radiological Control Manual, FNAL, July, 2004.

DESY (a) TESLA KEK (b) SLAC (c) FNAL (d)

Standard 20 mSv/yr 1.5 mSv/yr 20 mSv/yr 50 mSv/yr

Fertile women 2 mSv/month 1.5 mSv/yr 6 mSv/yr 2 mSv/3months

Pregnant women 1 mSv 1.5 mSv/yr 1 mSv 5 mSv

/pregnancy /pregnancy /pregnancy

Operating conditions

Normal 20 µSv/hr 5 µSv/hr

(1mSv/week ) (10 mSv/year)

Mis-steering 20 mSv/event 4 mSv/hr

(20 mSv/year )

System failure 20 mSv/event 250 mSv/hr for

(20 mSv/year ) max. credible beam

(30 mSv/event)

The TESLA TDR cited beam-loss scenarios for the main linac as 0.1 W/m loss for normal operation and 100 W/m loss for 100 hours per year for mis-steering condition.

The SLAC maximum credible beam loss condition is the full beam power of 18 MW. Using these scenarios and the maximum allowable radiation levels, the most stringent criteria comes from the SLAC maximum credible beam condition. This gives a limit of 0.014 mSv/hr/kW loss for the main linac.

The interaction region will be occupied by many experimentalists. Hence tighter radiation design critera have been used so that occupants do not need to have radiation worker training.

For normal operation, the IR hall radiation design limit is 0.5 microSv/h. For the maximum credible incident the limits are 250 mSv/h and 1mSv/event.

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FIGURE 2.9-4. Two designs for passageways between the tunnels that give adequate radiation shielding.

2.9.4.2 Shielding Calculation between Two Tunnels

The linac design has a beamline and a service tunnel separated by 7.5 m. Radiation levels must be low enough in the service tunnel to allow occupancy for repairs when beam is in the beam tunnel. Radiation dose rates were evaluated using the Monte Carlo codes, MARS and FLUKA, and the two tunnel configuration satisfies the radiation dose limit of 0.014 mSv/hr/kW. Here are a few selected results.

• For sections with no penetrations between two tunnels, 4 m of earth provides adequate shielding. This was evaluated by the MARS code and the Jenkins formula with a soil density 1.9 g/cm³ and a 250 GeV electron beam incident on the worst case target: a thick copper cylinder 20 X₀ long and a radius of 1 X₀.

• In sections which have a penetration for waveguides or cables, the radiation near the penetration is above the allowed limit. However, the radiation level falls off rapidly with distance so it is sufficient to fence off the area immediately next to the penetration.

The penetrations are located near the top of the tunnel, well above the personnel passage, so the fencing does not significantly restrict access in the service tunnel. In the calculations, the penetration was assumed to be a 7.5 m long circular hole with a diameter of 48 cm, and no shielding in the penetration. Suitable shielding could potentially lower the radiation levels further.

• Personnel access passages between the two tunnels are located every 500 m along the main linac for emergency egress. Heavy movable shielding doors cannot be used because of the need for a fast escape route. These passages cannot have a direct line-of-sight or the radiation dose in the service tunnel would be unacceptable, but two designs adequately reduced the radiation in the service tunnel below the limit. These are shown in Figure 2.9-4 and described below.

1. A rotated “V” shape passageway gave the lowest dose rate, about 20% of the limit.

In this simulation, the access passageway had a width of 1.2 m, a height of 2 m and the arms of the “V” were angled 10 degrees away from the accelerator tunnels.

The total length of the passage was about 50 m.

2. Another design is a modified crank with an inclined center passageway. The dose rate calculated by the simulation was about 80% of the limit.

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2.9.4.3 PPS Zones

The personnel protection system (PPS) prevents people from being in the accelerator tunnel when beam is on. A system of gates and interlocks turn off the beam before allowing access to the accelerator housing. Access to the service tunnel is not part of the PPS system. The ILC is divided into different regions (PPS zones) with tune up dumps and shielding to allow beam in one region while there is access in another region. The PPS zones are the injectors, DR, main linac and BDS. Entrance gates for PPS zones are monitored and stop the beam when opened.

The ILC PPS zones are long and it would be burdensome to search the full region after each permitted access. To ameliorate this problem, they are divided into multiple search zones separated by fences with gates that are also monitored. The search zones are up to several hundred meters long. For example, in the linac a search zone is 500 m long and is separated by gates midway between each cross tunnel passageway.

Personnel access from a service area (service tunnel, shaft, detector hall etc.) to an accelerator area is controlled by PPS gates, as is the access from one accelerator region (PPS zone) to another accelerator region. Fences, doors, or moving shields are used for these gates and they have redundant gate-closed status switches for PPS monitoring. They are locked to prevent careless access but have an unlocking mechanism for emergencies. Information and communication systems are provided at the gates to show the operational status and allow communication between a person at the gate and an operator granting permission to go through the gate.

There are personnel access passages between accelerator area and service area at the main linac, shafts, alcoves and the detector hall with PPS gates near each end. Since the passageways are used as emergency exits, heavy moving doors are avoided if possible. PPS gates between the accelerator areas and the service areas (including the access passageway) need to restrict the flow of activated air from the accelerator tunnel to the service area.

2.9.4.4 Shielding between PPS Zones

Shielding between PPS zones is designed to allow beam in the upstream zone while people are in the downstream zone. The upstream beam is deflected into a tune-up dump and there are triply redundant beam stoppers between the beam and the accessed region to ensure the beam does not enter the accessed region.