Activation Issues for ANU Peter Kasper 17-Sep-2007

Investigating the impact of tunnel activation at MI-14 and MI-39 on the ANU work Penetrations are planned to be installed during the 2008 shutdown The locations correspond to “hot spots” in the tunnel How do we deal with this? – Lead blankets: Low impact but limited efficacy – Removing magnets: High impact on cost, time, and operations

First Step: Find out what we are dealing with – Weekly radiation measurements in order to characterize cool down rates How much can we gain by waiting for things to cool down? Answer: Very little. – Map the radiation fields in the relevant areas How local are the hotspots? Can lead blankets be usefully deployed? What is the average dose rate that a laborer might expect?

Cooldown Measurements: – Measurements taken with LSM at upstream end of all elements … From to From to All readings are at 1 ft. – Work areas are concentrated between and and These are the hottest parts of the tunnel Activated ion pump magnet RR gave – 0.18 mr from Mn 54 (312 days) and Co 58 (71 days)

After 35 days most elements measured had cooled by ~2/3 (consistent with measurements on ion pump magnets) Larger reductions observed downstream of LAM-2 (downstream of the work area

Mapping the radiation: – Measurements taken on a 1 ft-sq. grid 70 ft long section of tunnel at each work site 6 measurements between the inner wall and the beamline elements Measurements take ~2.5 hours (where is Tomlin’s robot when you need it?) – Average dose rates obtained by averaging over all points within ~5 ft. of the major penetrations MI-14: 16.4 mrem/hr MI-39: 7.6 mrem/hr

MI-14 MI-39 Location of big penetrations Max. 100 mr/hr Max. 69 mr/hr

Work Steps: – Excavation and installation of grillage – Drill pilot holes – Cut penetration holes – Install PVC penetrations – Construct forms – Pour and cure concrete – Remove forms – Clean up tunnel – Backfill etc. Work Days 12 3* 8 (2*) 1 (1*) 2 (1*) 1 2* 40 *In tunnel

Tunnel occupancy:- – 9 days spread over 18 work days ( = 3.6 weeks ) Assume 1 tunnel day = 5 hours Assume time is evenly divided between the two sites Total dose – 539 mrem(Quarterly limit: 300 mrem) – 150 mrem/week(Weekly limit: 100 mrem) – 60 mrem/day(Daily limit: 40 mrem)

Repeat measurements in MI-10 with Pb blankets on and 104-2, including gap between them

New measurements :- – Used the same 1 ft grid as the original – Incomplete coverage ft for new measurements ( ft for original) Plots fill missing 15 ft with old measurements – Taken ~10 days after the original New measurements scaled by 1/0.87 to account for extra cool down Factor chosen to obtain agreement over the last 5 ft Consistent with measured cool down rate – Thin point in the Pb coverage at the gap center

Before After Cool down corrected Location of big penetrations Max. 100 mr/hr 35 mr/hr Max. 44 mr/hr Not remeasured

Before - after Location of big penetrations Max. 65 mr/hr Not remeasured Before / after

Tunnel occupancy:- – 9 days spread over 18 work days ( = 3.6 weeks ) Assume 1 tunnel day = 5 hours Assume time is evenly divided between the two sites Total dose with Pb as installed – 357 mrem(Quarterly limit: 300 mrem) – 99 mrem/week(Weekly limit: 100 mrem) – 40 mrem/day(Daily limit: 40 mrem)

Comments Pb blankets gave significant dose reductions Implies that magnets are the dominant source A more extensive installation would easily provide the necessary dose reduction Already close to acceptable as installed Eliminate thin point at gap Cover the entire work area in both enclosures Increasing thickness gives marginal improvement

Caveats Cool down correction Measurements were done after 35 days cool down Rates go up by 20% for a 12 day cool down Have assumed the same workers throughout Dividing the tasks among different individuals will reduce the totals even further Must not allow the tunnels to get significantly hotter