Regulatory Guide 5.27

Special Nuclear Material Doorway Monitors

ML003740050
Person / Time
Issue date:	06/30/1974
From:	Office of Nuclear Regulatory Research
To:
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RG-5.27
Download: ML003740050 (12)

June 1974 U.S. ATOMIC ENERGY COMMISSION

REGULATORY

GUIDE DIRECTORATE

OF REGULATORY

STANDARDS

REGULATORY

GUIDE 5.27 SPECIAL NUCLEAR MATERIAL DOORWAY MONITORS

A. INTRODUCTION

Paragraph (b), "Exit Requirement," of § 73.60, "Ad ditional Requirements for the Physical Protection of Special Nuclear Material at Fixed Sites," of 10 CFR Part 73, "Physical Protection of Plants and Materials," requires that individuals exiting from material access areas be searched for concealed special nuclear material (SNM). This guide describes means acceptable to the Regulatory staff for employing SNM doorway monitors to comply with that requirement.

B. DISCUSSION

Special nuclear material doorway monitors provide an efficient, sensitive, and reasonably unobtrusive means of searching individuals exiting from a material access area for concealed SNM. With proper installation and opera tion, gram quantities or less of SNM can be detected with a high level of reliability while maintaining a low false alarm rate. I. Theory of Operation The doorway monitor is composed of a detector unit(s), associated electronics, and alarm logic. The detector unit(s) is sensitive to the radiations which emanate from the SNM and responds to these radiations (usually gamma rays) by generating current pulses. These pulses are amplified, filtered, and fed to alarm logic v'hich interprets the number (or rate) of pulses in some period of time, for example, one second. The alarm logic may be either a digital or analog system; in either case, if the number (or rate) of pulses exceeds a set level, an alarm condition ensues.2. General Characteristics Typically, the detectors of a doorway monitor are NaI(Tl) scintillators or solid or liquid organic scintilla tors. Geiger-Mueller detectors have also been used in this application, although the lower intrinsic efficiency of these detectors renders them less suitable than scintilla tion detectors.

Detectors are arranged such that a detection area is defined by a plane perpendicular to the line of passage of individuals through the doorway monitor. Various arrangements of the detectors are possible;

however, specific placement of detectors is usually dictated by the need to eliminate dead spots. Some commercially available doorway monitors are equipped with an automatic background updating system. The automatic background updating system periodically monitors and averages the background.

A doorway monitor equipped with an automatic back ground updating system is also provided with a treadle pad or beam-break system to indicate that the sensitive area is occupied.

When the sensitive area is occupied, the radiation level detected by the doorway monitor is compared with the mean background.

If the level is "significantly" greater than the mean background, an alarm condition ensues. Significance is usually deter mined by comparing the radiation level when the sensitive area is occupied with the mean background plus some multiple of the square root of the mean back ground* *The square root of the mean of a Poisson-distributed quantity is the unbiased estimate of the standard deviation of that quantit

y. USAEC REGULATORY

GUIDES Copies of published guides may be obtained by request indicating the divisions desired to the US. Atomic Energy Commission, Washington, D.C. 20545, Regulatory Guides are issued to describe and make available to the public Attention:

Director of Regulatory Standards.

Comments and suggestions for methods acceptable to the AEC Regulatory staff of implementing sPecific parts of improvements in these guides ae encouraged and should be sent to the Secretary the Commission's regulations, to delineate techniques .ned by the staff in of the Commission, US. Atomic Energy Commission, Washington, D.C. 20545, evaluating specific problems or postulated accidents, or to provide guidance to Attention:

Chief, Public Proceedings Staff. applicants.

Regulatory Guides are not substitutes for regulations and compliance with them is not required.

Methods and solutions different from those set out in The guides ae issued in the following ten broad divisions:

the guides will be acceptable if they provide a basis for the findings requisite to the imuance or ontinuance of a permit or license by the Commission.

1. Power Reactors 6. Products 2. Reaseerd enid Test Reactors

7. Transportation

3. Fuels and Materials Facilities

8. Occupational Health Published guidas will be revised periodically, as appropriate, to accommodate

4. Environmental and Siting 9. Antitrust Review comments amd to refloct new informeaion or experience.

5. Materials and Plant Protection

10. General Thus the condition for an alarm can be written as G > B + n, where B is the mean background, G is the radiation level with the sensitive area occupied, and n is a multiplier, usually an integer between 4 and 10.* The value of n directly affects the false alarm rate, and the combination of B and n affect the sensitivity.

Although the automatic background updating system allows unattended use of the doorway monitor, for technical reasons, the system may be less effective in certain situations.

Techniques to prevent this are des cribed in the Regulatory Position.

Doorway monitors not equipped with an automatic background updating system either must rely on the inherent sensitivity of the instrument and low back ground variation to ensure that concealed SNM will be detected or must be attended so that a measurement of background can be taken immediately prior to use and the alarm level set to maintain the desired sensitivity to concealed SNM. As with the doorway monitor equipped with an automatic background updating system, a doorway monitor not so equipped compares the activity with the detection area occupied with some set value, usually the measurement of background taken just prior to use. In any case, the expression above can be used as a condition of alarm by replacing B + nr'Wwith the set level. Whether or not a doorway monitor is equipped with an automatic background updating system, high back ground activity will decrease sensitivity.

Measuring activ ity for longer periods will somewhat compensate for high background;

however, longer measurement periods will make use of the doorway monitor less convenient.

C. REGULATORY

POSITION 1. Minimum Qualifications for SNM Doorway Monitors a. General (1) SNM Doorway monitors should be used in conjunction with a metal detector and should be installed in a passageway (see Regulatory Guide 5.7, "Control of Personnel Access to Protected Areas, Vital *Note that, in general, for a count rate system, the condition for afarm should be modified to account for the response time of the instrument as follows: G > B + n1]0l -e-t/7) where t is the counting time and r is the time constant of the instrument.

If, as should be the case, t/,r > 5 the added factor is essentially unity.Areas, and Material Access Areas,") in such a manner that objects cannot be passed over, around, or under the detection area. .(2) The detector elements should be designed and positioned so that detection sensitivity is as uniform as possible over the detection area; in no case should any areas where SNM is not detectible be permitted.

(3) Power, sensitivity, and other controls of the doorway monitor should be tamper-safed when unat tended. (4) Signal lines connecting alarm relays to the alarm monitor should be supervised.

(5) Some doorway monitors may require an indi vidual to occupy the detection area for a specified time, longer than a normal walking pace would provide. If this is the case, the doorway monitor should be provided with a treadle pad and a "clock" device to assure that the detection area is occupied for the requisite time. An aural and visual indication should be given if an individual being searched does not occupy the area sufficiently long. b. System Specification

(1) Plutonium-239.

A doorway monitor used to detect plutonium should be capable of detecting a minimum of 0.5 gram of plutonium-239 encased in a minimum of 3 mm of brass at a 90% confidence limit. The false alarm rate should be less than 0.1% (Appendix C). (2) Uranium-233.

A doorway monitor used to detect uranium-233 should be capable of detecting within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of purification*

a minimum of 1 gram of uranium-233 containing between 7 and 10 ppm of uranium-232 encased in a minimum of 3 mm of brass at a 90% confidence limit. The false alarm rate should be less than 0.1% (Appendix C). (3) Uranium-235.

A doorway monitor used to detect uranium-235 should be capable of detecting a minimum of 3 grams of uranium-235 contained in uranium enriched to 20% or more in the uranium-235 isotope encased in a minimum of 3 mm of brass at a 50% confidence limit. The false alarm rate should be less than 0.1% (Appendix C). 2. Use of Doorway Monitors In general, doorway monitors should be used in locations of minimum background and minimum back ground fluctation.

If circumstances dictate use of a doorway monitor in an area of high background, sufficient shielding should be provided to maintain necessary sensitivity.

a. Attended Doorway Monitor (1) If the doorway monitor is attended during use, it need not be equipped with an automatic

• Purification means removal of all decay products.5.27-2 background updating system, although such capability is preferred.

(2) Prior to each use* of a doorway monitor not equipped with an automatic background updating system, a measurement of background should be taken, and the alarm threshold should be set to the proper value listed in Table I for the measured background and the proper n value as determined in Appendix B. Each individual to be checked should, in turn, enter the detection area and be required to remain sufficiently long for the device to operate properly.

During use, the background should be checked and the alarm threshold reset at least each 15 minutes.

(3) With the individual being checked in the detection area, an alarm should sound if the activity in the detection area exceeds the alarm threshold T, as such a situation would indicate the presence of SNM. (4) The doorway monitor should be equipped with a high-background alarm which will sound if the measurement of background exceeds the appropriate maximum permissible background level listed in Table II as determined in Appendix C. The doorway monitor should not be used during such periods of high back ground. b. Unattended Doorway Monitor (1) If the doorway monitor is unattended, an automatic background update system should be incor porated into the doorway monitor electronics and alarm logic. The control circuitry, if possible, should be located at the central alarm station (or other monitoring point). (2) Door interlocks and closed-circuit TV in combination with beam breaks, motion detectors, and/ or treadle pads, should be employed to: (a) Indicate to the person manning the central alarm station that an individual has entered the secure access passageway and/or is approaching the doorway monitor, (b) Allow observation of the individual ap proaching the doorway monitor, (c) Preclude a slow approach to the sensitive area of the doorway monitor, and *By use is meant an individual or several individuals, each, in turn, being checked for SNM by the doorway monitor. The maximum period between threshold sets while the doorway monitor is in use is determined by the stability of local background and may necessarily be more frequent than every 15 minutes.(d) Activate the electronics and alarm logic when an individual is within the detection area and initiate operation of the doorway monitor.

(3) An alarm should sound in the central alarm station if, when occupied, the activity in the detection area exceeds the internally set threshold level (the mean background plus some multiple times the square root of the mean background), as such a situation would indicate the presence of SNM upon the individual being checked.

(4) The doorway monitor should be equipped with a high-background alarm which will sound if the average background at the location of the doorway monitor exceeds the appropriate maximum permissible background level listed in Table II as determined in Appendix C. The doorway monitor should not be used during such periods of high background.

Other moni tored exits should be used. 3. Testing and Calibration a. Testing Doorway monitors should be tested by passing an appropriate source of the amount and isotope specified in Regulatory Position C.l.b. through the doorway monitor no less frequently than once per day.* In addition, a functional performance test should be carried out at least once per week. An acceptable functional performance test procedure is discussed in Appendix A of this guide b. Calibration Doorway monitors should be calibrated with a source of the amount, configuration, and variety of SNM to be detected (e.g., 0.5 gram Pu in 3 mm of brass). Calibration should be carried out according to a proce dure such as that in Appendix B. c. Operating Instructions Operating instructions should be posted near the doorway monitor,*

if attended, or at the monitoring point if the doorway monitor is unattended.

The instructions should clearly indicate the procedure for use of the doorway monitor and the procedure for setting thresholds, if appropriate.

In addition, the operating instructions should indicate what corrective action is to be taken and who is to be notified in the event of a malfunction.

• Doorway monitors used to search for concealed U-233 should be tested according to §6 of Appendix A.5.27-3 TABLE I ALARM THRESHOLD

T B n= 4 5 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 6 140 164 .187 211 234 257 279 302 324 347 369 392 414 436 458 480 502 524 546 568 589 611 633 655 676 698 720 741 763 784 806 827 849 870 892 913 935 956 977 999 1020 1041 1063 1084 1105 5.27-4 7 8 150 175 199 223 247 271 294 317 341 364 387 409 432 455 477 500 522 545 567 590 612 634 656 678 700 722 744 766 788 810 832 854 876 898 920 941 963 985 1007 1028 1050 1072 1093 1115 1137 160 186 211 236 260 285 309 333 357 380 404 427 451 474 497 520 543 566 589 611 634 657 679 702 724 747 769 792 814 836 859 881 903 925 948 970 992 1014 1036 1058 1080 1102 1124 1146 1168 170 197 223 249 274 299 324 348 373 397 421 445 469 493 516 540 563 587 610 633 657 680 703 726 749 771 794 817 840 863 885 908 930 953 975 998 1020 1043 1065 1088 1110 1132 1155 1177 1199 180 208 235 261 287 313 339 364 389 414 439 463 488 512 536 560 584 608 632 655 679 702 726 749 773 796 819 842 866 889 912 935 958 981 1003 1026 1049 1072 1095 1117 1140 1163 1185 1208 1230 9 190 219 246 274 301 327 353 379 405 431 456 481 506 531 555 580 604 629 653 677 701 725 749 773 797 820 844 868 891 915 938 961 985 1008 1031 1055 1078 1101 1124 1147 1170 1193 1216 1239 1262 10 200 230 258 286 314 341 368 395 421 447 473 499 524 550 575 600 625 650 674 699 724 748 772 797 821 845 869 893 917 941 965 988 1012 1036 1059 1083 1106 1130 1153 1177 1200 1223 1247 1270 1293 TABLE I (Cont'd) ALARM THRESHOLD

T B n = 4 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 1126 1339 1550 1760 1970 2179 2388 2596 2804 3012 3219 3426 3633 3840 4047 4253 4459 4665 4871 5077 5283 5483 5694 5899 6105 6310 6515 6720 6925 7130 7335 7539 7744 7949 8153 8358 8562 8767 8971 9175 9379 9584 9788 9992 10196 5 1158 1373 1587 1800 2012 2224 2435 2645 2855 3065 3274 3483 3692 3900 4108 4316 4524 4732 4939 5146 5354 5561 5767 5974 6181 6387 6594 6800 7006 7212 7418 7624 7830 8036 8242 8447 8653 8858 9064 9269 9474 9680 9885 10090 10295 5.27-5 9 6 1190 1408 1624 1840 2055 2268 2481 2694 2906 3117 3329 3539 3750 3960 4170 4379 4589 4798 5007 5216 5424 5633 5841 6049 6257 6465 6672 6880 7087 7295 7502 7709 7916 8123 8330 8537 8743 8950 9156 9363 9569 9775 9932 10188 10394 7 1221 1442 1662 1880 2097 2313 2528 2743 2957 3170 3383 3596 3808 4020 4232 4443 4654 4864 5075 5285 5495 5705 5914 6124 6333 6542 6751 6960 7169 7377 7586 7794 8002 8210 8418 8626 8834 9042 9249 9457 9664 9871 10079 10286 10493 8 1253 1477 1699 1920 2139 2358 2575 2792 3008 3223 3438 3653 3866 4080 4293 4506 4718 4931 5143 5354 5566 5777 5988 6199 6409 6620 6830 7040 7250 7460 7669 7879 8088 8297 8507 8716 8924 9133 9342 9550 9759 9967 10176 10384 10592 1285 1512 1737 1960 2182 2402 2622 2841 3059 3276 3493 3709 3925 4140 4355 4569 4783 4997 5210 5424 5636 5849 6061 6273 6485 6697 6909 7120 7331 7542 7753 7964 8174 8385 8595 8805 9015 9225 9435 9644 9854 10063 10273 10482 10691 1316 1546 1774 2000 2224 2447 2669 289C 3110 3329 3548 3766 3983 4200 4416 4632 4848 5063 5278 5493 5707 5921 6135 6348 6562 6775 6987 7200 7412 7625 7837 8049 8260 8472 8683 8894 9106 9317 9527 9738 9949 10159 10370 10580 10790

TABLE II MAXIMUM PERMISSIBLE

BACKGROUND

a = 0, Pa = 50%B n= 4 100 120 140 160 180. 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 5 67 83 100 117 134 151 168 186 203 221 238 256 274 292 310 328 346 364 382 400 418 436 455 473 491 510 528 546 565 583 602 620 639 657 676 695 713 732 750 769 788 806 825 844 863 G 61 76 92 108 124 141 157 174 191 208 225 242 259 277 294 312 329 347 365 382 400 418 436 454 471 489 507 525 543 562 580 598 616 634 652 671 689 707 725 744 762 780 799 817 835 6 55 70 85 100 116 131 147 163 180 196 213 229 246 263 280 297 314 331 348 365 383 400 417 435 452 470 488 505 523 541 558 576 594 612 629 647 665 683 701 719 737 755 773 791 809 5.27-6 7 50 64 78 93 107 123 138 153 169 185 201 217 233 249 266 282 299 316 332 349 366 383 400 417 434 451 468 486 503 520 538 555 573 590 607 625 643 660 678 695 713 731 748 766 784 9 10 8 46 59 72 86 100 114 129 144 159 174 190 205 221 237 253 269 285 301 317 334 350 367 383 400 417 433 450 467 484 501 518 535 552 569 586 603 621 638 655 673 690 707 725 742 760 42 54 67 80 93 107 121 135 150 165 179 194 210 225 240 256 272 287 303 319 335 351 367 384 400 416 433 449 466 482 499 516 532 549 566 583 600 617 633 650 667 685 702 719 736 38 50 62 74 87 100 113 127 141 155 170 184 199 214 229 244 259 274 290 305 321 337 352 368 384 400 416 432 448 464 481 497 513 530 546 563 579 596 613 629 646 663 679 696 713 TABLE II (Cont'd)MAXIMUM PERMISSIBLE

BACKGROUND

B G n = 4 1000 1200 1400 1600 1000 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 881 1069 1258 1448 1638 1829 2020 2212 2404 2596 2789 29S2 3175 3368 3561 3755 3949 4143 4337 4531 4725 4919 5114 5309 5503 5698 5893 6088 6283 6478 6673 6868 7064 7259 7455 7650 7846 8041 8237 8433 8628 8824 9020 9216 9412 5 854 1039 1225 1412 1600 1789 1978 2167 2357 2548 2738 2929 3121 3312 3504 3696 3888 4081 4273 4466 4659 4852 5045 5238 5432 5625' 5819 6012 6206 6400 6594 6788 6982 7176 7371 7565 7760 7954 8149 8343 8538 8733 8928 9122 9317 a -'0, Pa = 50Z 6 827 1009 1193 1377 1563 1749 1936 2124 2312 2500 2689 2878 3068 3258 3448 3638 3829 4020 4211 4402 4593 4785 4977 5169 5361 5553 5745 5938 6130 6323 6516 6709 6902 7095 7288 7481 7674 7868 8061 8255 8449 8642 8836 9030 9224 7 802 981 1161 1343 1527 1710 1895 2081 2267 2453 2640 2828 3016 3204 3392 3581 3770 3960 4149 4339 4529 4719 4910 5100 5291 5482 5673 5864 6055 6247 6438 6630 6822 7014 7206 7398 7590 7782 7975 8167 8360 8553 8745 8938 9131 8 777 953 1131 1310 1491 1673 1855 2039 2223 2407 2593 2778 2964 3151 3338 3525 3713 3900 4088 4277 4465 4654 4843 5032 5222 5411 5601 5791 5981 6172 6362 6552 6743 6934 7125 7316 7507 7698 7889 8081 8272 8464 8656 8348 9039 5.27-7 9 753 926 1101 1278 1457 1636 1816 199D 2180 2363 2546 2730 2914 3099 3284 3470 3656 3842 4029 4216 4403 4590 4778 4966 5154 5342 5531 5719 5908 60'97 6286 6476 6665 6855 7045 7234 7425 7615 7805 7995 8186 8376 8567 8758 8949 10 730 900 1073 1247 1423 1600 1778 1958 2138 2318 2500 2682 2865 3048 3232 3416 3600 3785 3970 4155 4341 4527 4713 4900 5087 5274 5461 5648 5836 6024 6212 6400 6588 6777 6965 7154 7343 7532 7721 7911 8100 8290 8479 8669 8859 TABLE II (Cont'd)MAXIMUM PERMISSIBLE

BACKGROUND

B G n 4 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 57 72 87 103 119 135 151 168 185 201 218 235 252 270 287 304 322 339 357 374 392 409 427 445 463 480 498 516 534 552 570 588 606 624 642 660 679 697 715 733 751 770 788 806 824 5 51 65 80 95 110 126 141 157 173 189 206 222 239 255 272 289 306 323 340 357 374 391 409 426 443 461 478 496 513 531 549 566 584 602 619 637 655 673 690 708 726 744 762 780 798 6 46 59 73 87 102 117 132 147 163 178 194 210 226 242 258 275 291 308 324 341 357 374 391 408 425 442 459 476 493 511 528 545 562 580 597 614 632 649 667 684 702 720 737 755 773 S= 1.3, Pa = 90%7 42 54 67 81 95 109 123 138 153 168 183 198 214 229 245 261 277 293 309 325 342 358 374 391 407 424 441 457 474 491 508 525 542 559 576 593 610 627 644 661 679 696 713 731 748 9 8 38 49 62 75 88 101 115 129 143 158 172 187 202 217 233 248 263 279 295 311 326 342 358 374 391 407 423 439 456 472 489 505 522 539 555 572 589 605 622 639 656 673 690 707 724 5.27-8 34 45 57 69 81 94 107 121 135 149 163 177 191 206 221 236 251 266 281 297 312 327 343 359 375 390 406 422 438 454 470 487 503 519 535 552 568 585 601 618 634 651 668 684 701 10 31 41 52 64 76 FS 100 113 127 1L0 154 167 181 196 210 224 22S 254 268 283 298 313 329 344 359 375 390 406 421 437 453 469 485 500 516 532 549 565 581 597 613 630 646 662 679 TABLE II (Cont'd)MAXIMUM PERMISSIBLE

BACKGROUND

G n= 4 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 5600 5S00 6000 6200 6400 6600 6800 7000 7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200 9400 9600 98-00 843 1027 1212 1398 1536 1773 1962 2151 2340 2530 2720 2911 3101 3292 3484 3675 3867 4059 4251 4443 4636 4828 5021 5214 5407 5600 5793 5987 6180 6373 6567 6761 r9955 7148 7342 7536 7731 7925 8119 3313 8508 8702 8897 9091 9286 5 816 997 1180 1363 it48 1734 1920 2107 2294 2482 2670 2859 3048 3238 3427 3617 3807 3998 4188 4379 4570 4761 4953 5144 5336 5528 5720 5912 6104 6296 6488 6681 6874 7066 7259 7452 7645 7838 8031 8225 8418 8611 8805 8998 9192 6 790 968 1148 1329 1512 1695 1879 2064 2249 2435 2622 2808 2996 3183 3371 3560 3748 3937 4126 4316 4505 4695 4885 5075 5266 5456 5647 5S38 6029 6220 6411 6602 6794 6985 7177 7369 7561 7753 7945 8137 8329 8521 8714 8906 9099 5.27-9 a = 1.3, P= = 90%7 765 940 1117 1296 1476 1657 1839 2022 2205 2389 2574 2759 2944 3130 3317 3503 3691 3878 4066 4253 4442 4630 4819 5007 5196 5386 5575 5765 5954 6144 6334 6524 6715 6905 7096 7286 7477 7668 7859 8050 8241 8433 8624 8815 9007 8 741 913 1088 1264 1441 1620 1800 1980 2162 2344 2527 2710 2894 3078 3263 3448 3634 3319 4006 4192 4379 4566 4753 4940 5128 5316 5504 5692 5831 6070 6258 6447 6636 6826 7015 7205 7394 7584 7774 7964 8154 8345 8535 8725 8916 9 718 887 1059 1232 1407 1584 1761 1940 2119 2300 2481 2662 2844 3027 3210 3393 3577 3762 3946 4131 4317 4502 4688 4874 5061 5247 5434 5621 580G 5996 6184 6371 6559 6747 6936 7124 7313 7501 7690 7879 8063 8257 8447 8636 8826 10 695 861 1030 1201 1374 1548 1724 1900 2078 2256 2435 2615 2795 2976 3158 3340 3522 3705 3888 4072 4256 4440 4624 4809: 4994 5180 5365 5551 5737 5923 6110 6296 6483 6670 6857 7044 7232 7419 7607 7795 7983 8171 8360 8548 8737 APPENDIX A PROCEDURE

FOR TESTING SNM DOORWAY MONITORS FOR FUNCTIONAL

PERFORMANCE

Doorway monitors should be tested by employing a test source of the same isotope of SNM the doorway monitor is used to detect as follows: 1. With the detection area unoccupied, measure and record background.

2. Determine alarm threshold T from Table I (see Appendix B). 3. Place a test source in the detection area of the doorway monitor. The test source should be such that the activity in the detection area slightly exceeds the T level.* The doorway monitor should go into an alarm condition if operating properly.

4. Remove test source to its original location and measure background once again. If the measurement of *The test source may be improvised by partially shielding the calibration source.background taken immediately after the test varies by more than 2vN from the background taken before the test, the test should be repeated, for such a difference indicates nonrandom fluctuations of the background or equipment malfunction.

The probability of such an occurrence due to the randomness of the background is less than 8%. 5. A calibration source (Appendix B) should be carried repeatedly to various places within the detection area of the doorway monitor in simulation of actual use to verify that the SNM can be detected everywhere within the detection area and to assure proper operation of treadle pads, beam-break, or similar devices if the doorway monitor is so equipped.

6. Doorway monitors used to search for concealed uranium-233 should be tested with each uranium purification run, but no more frequently than daily. The test source should be freshly purified uranium-233 (within four hours of removal of decay products).

5.27-10

APPENDIX B PROCEDURE

FOR DETERMINING

MAXIMUM PERMISSIBLE

BACKGROUND

Background should be measured over several work shifts to determine the setpoints for alarm threshold (with or without automatic background updating)

to assure that normal operation will be minimally affected by alarms due to high background.

The doorway monitor should be calibrated with the amount of the appropriate isotope specified in Regula tory Positions C.l.b(1), (2), or (3) of this guide (e.g., 0.5 gram plutonium-239 in 3 mm of brass). The calibration procedure described below is essen tially a means of determining maximum permissible background for effective operation of the doorway monitor.

1. Place a calibration source variously about the detec tion area and take readings to determine the least sensitive point. This location of minimum sensitivity should be maintained as the calibration point. 2. Measure background over several workshifts to deter mine the periods of high background and the range of background.

A minimum of 20 measurements should be taken. The variance of the background is given by N Var B 1 (Bi-B)2 (1) where N is the number of measurements, B is the mean of the background measurements, and Bi is the ith background measurement.

3. With the calibration source at the calibration point, the mean gross counts G should be determined during a period of high background to establish the upper operating range of the doorway monitor. Table II lists various maximum permissible background levels for a given value of G for values of the parameters n and a. During periods when the background exceeds the value found from Table II, the sensitivity of the doorway monitor will generally be below that specified as minimum in Regulatory Position C.l.b of this guide. The parameter n is a function of the background variation and the permissible false alarm rate and is calculated from n >3.1 (var B) 1/2 (2)Generally, n is taken as an integer. If n satisfies the above expression, the expected false alarm rate (Appen dix C) due to background fluctuations should be less than 0.1%. Larger values of n will decrease the expected false alarm rate; however, the maximum allowable background for a given G will also decrease.

4. For doorway monitors equipped with automatic background updating systems, the alarm threshold is Counts > B + nVii, (3)where n should be derived from expression

(2), and the instrument set accordingly.

5. The high-background alarm should be set at the B value given in Table II for the measured G and calculated n values. 6. For doorway monitors not equipped with an auto matic background update, the value of n determined above should be employed in the use of the doorway monitor according to Regulatory Position C.2.a. *The false alarm rate is estimated by the probability that an observation of a quantity distributed normally about some value X will exceed X by n(Std. deviation of X). The factor 3.1 limits the false alarm rate to 0.1%, while the factor (Var B/B)1/2 compensates for observed deviations in the background distribution from Poisson.5.27-11 APPENDIX C DETECTION

CONFIDENCE

LIMITS, THRESHOLDS, AND MAXIMUM PERMISSIBLE

BACKGROUND

With a calibration source at the calibration point, the condition for an alarm is, in general, g> T = B + m/Va' B (1) where g is a single measurement of G (the mean radiation level with the source), B is the mean back ground, and m is some multiplier.

The detection confidence limit is the probability that with the calibra iton source at the calibration point, any single determin ation of G will exceed a threshold T, i.e., the above inequality will be satisfied.

For any given probability Pa., there exists a value a such that g > G- (2) with a probability of Pa, where g is any single measure ment of the quantity G. Hence the condition for a detection confidence limit of Pais G -& > T = B + m,,/'r B. (3) For a given value of G, solving (3) gives the maximum permissible B at which the doorway monitor will detect the source with a confidence Pr- For Pa = 90%, a = 1.3, and for Pa = 50%, a = 0.The condition for a false alarm* is written as b = B + j a-'"B> T (4)where b is a single measurement without the source and 0 is a number corresponding to a false alarm probability Pp. For P = 0.1%, 0 = 3.1. Hence the necessary condition for maintaining a false alarm rate below Pp is b = B + -"B < B + m ar. (5) However, the doorway monitor actually compares b with B + ni, hence the condition on n becomes orV < rn'VaTr = nvff (6) or n>0 ( Va B ) 1/2 (7)In Table I threshold values of T were determined by substituting equation (6) into equation (3): T = B + nv'r The values of B in Table II were then calculated for a = 1.3 and a = 0 from equation (3) assuming that, for the determination of G, the background should be reasonably stable and therefore

\/Var G -\ *False alarm means an alarm condition generated by statistical fluctuations in the background radiation or by instabilities of the electronics which appear as background fluctuations when the detection area is occupied.5.27-12