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NLCTA Safety Audit. Radiation Safety Review W. R. Nelson 9 November 2001. Outline of talk. Quick look at the facility Shielding (acts as a PPS enclosure) Roof (designated a Radiation Area) Beam Authorization Sheet (BAS) Radiation surveys With beam
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NLCTA Safety Audit Radiation Safety Review W. R. Nelson 9 November 2001
Outline of talk • Quick look at the facility • Shielding (acts as a PPS enclosure) • Roof (designated a Radiation Area) • Beam Authorization Sheet (BAS) • Radiation surveys • With beam • X-rays from RF structures (on the roof) • Area Monitoring (OHP) • Unattended operation (RF test only)
Additional Documentation • NLCTA website • http://www-project.slac.stanford.edu/lc/local/Projects/NLCTA/ • See nlcta.htm for various checklists (weekly, daily, etc.) • See Supporting-documentation for memos (e.g., radiation surveys, modifications to requirements for unattended operation, etc.)
Additional Documentation (cont). • ESD/Controls Group (on NT) For example, RF Pre-running Condition 6, the procedure for the NLCTA Radiation Certification[18-29-03] can be found at: W:/PPS Procedures/Certification Procedures/PPS PROCEDURES Latest /PPS Procedures 39-NLCTA/NLCTA RAD3.doc
Radiation Surveys • OHP surveys: klystrons, RF structures • Done on routine basis, results given to operations • Surveys by Radiation Physics with beam • Performed several times as the energy increased • Highest energy was 270 MeV • Special survey done at 65 MeV with penetrations unshielded
Radiation Surveys (cont.) • X-ray surveys were also carefully surveyed on the roof near RF structures with help from OHP
Radiation survey at 270 MeV • Performed on August 17, 1999(270 MeV, 90 mA, 120ns, 10 Hz = 29 Watts) (see memo Rad-survey 270 MeV.pdf on NLCTA web site) • Both neutron (Rem counter) and gamma (ion chamber) data collected • Normal beam operation • Mis-steered beam (two scenarios) • Protection Ion Chamber (PIC) readings recorded
Normal beam operation results • Levels found to be less than 0.2 mrem/h (gamma) and zero neutron • Both around the sides and on the roof • One exception: • Small corner location (F34U) near East PPS Entry • Shield thickness is 4-ft instead of the normal 6-ft • 0.9 mrem/h (gamma) • No neutron radiation
Mis-steered beam results • Highest level found on roof above the source • 1.7 mrem/h (gamma) • 0.3 mrem/h (neutron) • Whereas SHIELD11 code predicted • 3.5 mrem/h (gamma) • 1.3 mrem/h (neutron) • Source most likely not a “standard target”(i.e., 4-inch diameter, 12-inch long Fe cylinder) • Levels around sides still less than 0.2 mrem/h
Protection Ion Chambers (PICs) • Again, PIC levels simultaneously recorded during entire survey (for calibration purposes) • Also, studied PIC history buffers for last 3-4 years (to see if and when they tripped) • Setting trip levels at 50nA would guarantee for worst-case steering • less than 0.6 mrem/h on sides • less than 6 mrem/h on roof • Currently set at 10 nA (NLCTA: Daily BCS Cklist.pdf)
Special survey of roof penetrations • Purpose: determine if local shielding (gravel) is really necessary (at least at this point in time) • The original penetration calculations are “subject to a large uncertainty” (SAD 01-13-07-00 p.7-21) • Survey performed on September 22, 2000 (65 MeV, 109 e/p, 10 Hz = 0.1 Watts) • Steered beam into quad located near penetration • Documented by memo – see NLCTA web site • Rad-survey penetrations 65 MeV.pdf
Protection Ion Chambers (PICs) again • PIC levels simultaneously recorded during entire survey • Largest value was found to be 4.8 nA at PIC-1450 (closest one downbeam of penetration) • With a trip level of 10 nA, this corresponds to (10/4.8)x103rem/h = 0.2 mrem/h above thepenetration, and 5 times smaller away from it
Protection Ion Chambers (cont.) • Based on these numbers, we have removed the penetration shielding restriction from the BAS. • At 200 MeV we will measure again (see Initial Checkout item #2 on BAS).
X-rays from the RF structures (roof) • Somewhere towards the 4th quarter of 1999 it became clear that there was an X-ray source around Station 1 on the roof that needed to be shielded. We saw this because of • Occasional BSOIC trips (very sporadic) • OHP Event Monitoring (TLDs)
X-rays from RF structures (cont.) • As expected, the so-called “flower-petal” mode converter turned out to be the problem, so we decided to make additional measurements • With support from OHP, we placed a matrix of TLDs about a foot away from the coupler • We also placed TLDs inside a stack of Pb foils so we could better understand the spectrum
X-rays from RF structures (cont.) • The Pb-tranmission curve initially resembled a 150-keV spectrum, consistent with that seen earlier by Vlieks et al. (SLAC-PUB-7938 (1998)). • But at larger Pb thicknesses it data flattened out • The spectrum “hardened” (150 250 keV) • 1/8-inch Pb gave a total transmission factor of 0.03 • Although sporadically produced, the integrated dose rate 1-ft from the source turned out to be about 10 mrem/h ** Note: the roof is designated as a Radiation Area
X-rays from RF structures (cont.) • Decided to shield all of such RF structures with boxes made of wood sandwiched around 1/8th inches of Pb • Integrated dose projection (2000 h/y): • 20 rem (unshielded) • 600 mrem (shielded)
X-rays from RF structures (cont.) • Measured (shielded) dose: 0.062 mrem/h, or124 mrem in a 2000 hour working year (to be compared with the 600 mrem calculation) • Reported to Radiation Safety Committee(see NLCTA web site: roof-dosimetry 9 oct01.pdf)
Unattended operation (RF tests only) • During Jan-Feb 2000 the NLCTA staff requested permission to operate klystrons and modulators at the NLCTA in order to process accelerator structures • With the X-ray problem understood, the Radiation Safety Committee agreed to this request, provided that…
Unattended operation (cont.) • …special requirements • the gun was locked off (with some hardware modifications made to the PPS panel) • a designated EOIC would be responsible even with the absence of an operator in the control room • together with some other details (check lists, etc.) (see NLCTA web site: rsc-00-001.pdf)
Extending unattended operation to include weekend running • Request was made on September 20, 2001 to extend the unattended mode of operation to include weekends • This was approved by the Radiation Safety Officer on October 17, 2001 (see NLCTA web site: unattended-mods.pdf) • Good shielding, dosimetry and configuration control was essential to getting this approval
Additional Event Monitoring • The NLCTA physicists believe that they have now significantly reduced the X-ray source strength • Accordingly, we are helping them prove this by installing Event Monitors on the inside and outside each of the wood-Pb boxes • Furthermore, the number of these boxes has been increased from three to nine, as part of a proactive move to make sure that people are not exposed to possible new sources of X-rays