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)

The NLCTA Shielding (PPS) Enclosure

The NLCTA Roof (Radiation Area)

Beam Authorization Sheet (BAS)

Beam Authorization Sheet (cont.)

Beam Authorization Sheet (cont.)

Additional Documentation

NLCTA website– http://www-project.slac.stanford.edu/lc/local/Projects/NLC

TA/

– 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

Penetration study (cont.)(0.1 W mis-steering loss)

Condition LocationNeutron(rem/h)

Gamma(rem/h)

Total(rem/h)

Normal loss Above hole 7 80 87

Mis-steering Above hole 7 96 103

Mis-steering 3-ft Off hole 1-2 17 18-19

SHIELD11calculation

Same 1 13 14

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)x103 rem/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)

NLCTA Area MonitoringSpring01 & Summer01 (6 months)

Location Dose (mrem)

S side, outside wall by Faraday cup, on floor 4

Operator's Office 0

BLDG 237, on deck 0

Roof, above beam dump 2

Roof, above Farady cup, on BSOIC (near Station 1) 4

Maze entrance, beam dump end 0

Inside ESB, east of klystrons, on S wall of NLCTA 1

West End PPS 0

Outside wall, S of beam dump 1

N side, outside wall by Faraday cup, on BSOIC 2

Bldg 225, Inside on wall 0

Bldg 104C, Next to Door, on wall 2

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