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Performance Criteria for Fixed Monitoring Instruments - doe-std-1128-98_ch10067
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Surface Contamination Monitors - doe-std-1128-98_ch10069


DOE-STD-1128-98
Airborne Contamination Monitors. Airborne contamination monitors,
normally CAMS (see Section 3.5.1), should meet the following criteria
according to ANSI N317 (ANSI, 1980a). The primary purpose of any
CAM is to detect the presence of airborne radioactivity and activate an
alarm to warn personnel in the area so that actions can be taken to
minimize personnel exposures. The goal for any CAM should be to
perform this function as quickly as possible and at the lowest detectable
level of radioactive airborne concentration. The quantity of airborne
radioactivity that will result in an alarm within a given time interval is
defined in units of DAC-h for a particular radionuclide and is a function
of the nuclide's airborne concentration in DACs, the sampling rate, the
lower limit of detection of the instrument, and the time needed for the
alarm to occur. Mishima et al. (1988) provides guidance on each of these
functions.
The minimum detection level of 239Pu, in terms of derived air
concentration (DAC), should be 8 DAC-h at the point of sampling in the
presence of nominal amounts of naturally occurring alpha-emitters such
as radon and thoron and their decay products. (No guidance is provided
on what a "nominal" amount is, however.) The operating range should be
at least 100 minimum detection levels (i.e., up to 800 DAC-h for 239Pu).
Instrument error should not exceed 20% of the reading over the upper
80% of the operating range. The reproducibility of the system for any
given measurement should be within 10% at the 95% confidence level
for a mid-scale or mid-decade reading. The instrument should be capable
of operating with less than a 5% change in calibration over the ambient
temperature range expected. The instrument should be equipped with an
adjustable alarm set point (audible and visible alarms) that can be set at
any point over the stated range. The air flow rate should be indicated and
adjustable. Voltage and frequency variations of 15% within design
values should result in reading variations of no greater than 5% at the
minimum detection level.
ANSI N42.17B (ANSI, 1987b) provides additional performance criteria
for air monitors used to detect plutonium. This standard provides
specifications for general criteria (sampler design, units of readout, alarm
threshold, etc.), electronic criteria (alarms, stability, response time,
coefficient of variation, and line noise susceptibility), radiation response,
interfering responses (radiofrequency, microwave, electrostatic, and
magnetic fields), environmental criteria (temperature, humidity, and
pressure), and air-circuit criteria. More detailed specifications are
provided in ANSI N42.17B than in ANSI N317 (ANSI, 1980a);
however, the environmental criteria and the limits of variation are not as
restrictive as those in ANSI N317. With respect to accuracy, ANSI N317
requires less than 20%, and ANSI N42.17B requires 40% at the 95%
confidence level. For the environmental criteria, ANSI N317 requires
that the readings change less than 5% under ambient conditions, while
ANSI N42.17B gives a 15% limit of variation. As discussed previously,
criteria from ANSI N42.17B are more applicable because they are
supported by instrument testing.
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