Answer to Question #8941 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

Q

I am looking into criticality monitors and want to know at what dose and/or dose rate the electronics within the system no longer work. Is the dose associated with a criticality event enough to cause the electronics within a criticality monitor to permanently or temporarily stop working? I have found literature online for cosmic radiation and nuclear explosions, but I haven't been able to find anything about doses.

A

Before attempting to answer your question, we should point out that an important function of criticality monitors is to detect and measure radiation levels as they increase during the approach to criticality so that the radiation alarm will sound prior to the actual critical event. Granted, the time in approach to criticality may be very short, depending on the nature of the event. Once the critical event has occurred, the alarms will be sounding to warn people in the area that a possibly dangerous event is underway and that they should leave or not enter the area. Depending on the types of detectors, where the detectors are located, and the size of the event, some detectors may not provide a valid response during the actual excursion, but should provide legitimate readings on either side of the criticality.

The electronic components used in criticality monitors will generally not fail because of integral radiation exposure or dose. Semiconductors used in gamma-sensitive devices can generally sustain integral doses of about 10 kGy before significant failure is likely. Some semiconductor components may be susceptible to fast neutron fluences of roughly 1012 cm-2 (corresponding to a silicon dose of about 0.8 Gy or soft tissue dose equivalent rate of roughly 300 Sv). Given a nominal value of 1015 to 1016 fissions in an accidental criticality event, an integral fluence of 1012 cm-2 is probably unlikely at most monitoring locations. For an isotropic emission from a point source of 1016 fissions, assuming 2.5 neutrons per fission, such a fluence would accrue at about 45 cm from the fissioning source.

Some detector types may fail temporarily under high-dose-rate conditions. For example, many GM-type detectors are limited in exposure-rate range by dead-time problems, often to less than 10 mGy h-1. Under excess exposure rates, such detectors should be designed to fail in a fashion so as to provide a full-scale reading and to activate the alarm if such is intended. Some systems use an ionization chamber to extend the gamma dose rate capability of criticality monitors. GM detectors and other event-type detectors (e.g., scintillators or some neutron detectors) may also be incapable of providing a meaningful response to a very short duration pulse of radiation as is possible in some criticality incidents. Such a short burst of radiation may be recorded as a single event (count). Ion chambers that provide for charge integration may fare better, but they too are subject to possible problems, such as charge recombination, during a high-intensity burst.

At a nominal two meters from a small-volume criticality event involving 1016 fissions, assuming 2.5 neutrons per fission, the expected fast neutron soft tissue kerma (approximation to absorbed dose rate), assuming an average neutron energy of 2 MeV, can be estimated as

K = [(2.5 x 1016 n cm-2 )/(4π(200 cm)2)](3.1 x 10-11 Gy/n cm-2) = 1.54 Gy.

If we assume about 7 gamma rays of 1 MeV each per fission (f) event, we may estimate the soft tissue gamma dose as

D = [(1016 f)(7 gamma/f)(1 MeV/gamma)/(4π(200 cm)2)](0.031 cm2 g-1) (1.6 x 10-13 J MeV-1)(103 g kg-1)(1 Gy/J-kg-1) = 0.69 Gy,

where the factor 0.031 cm2 g-1 is the mass energy absorption coefficient for 1 MeV gamma rays in soft tissue.

We would not expect these integral doses to be high enough to cause monitor failure. The actual dose rates during the event depend on the duration of the critical event. Such durations may be quite short. A criticality duration of one millisecond would imply a neutron dose rate of 1.5 x 103 Gy s-1 and a gamma dose rate of 6.9 x 102Gy 2-1. These rates are beyond the measuring capabilities of most monitoring systems. The hope is that any monitoring system being used would sense the increase in radiation levels as the system being monitored approached criticality, and the alarm would be sounded prior to the actual critical event. The ANSI Standard ANSI/ANS 8.3 (see below) allows a maximum yield of 2 x 1019 fissions for purposes of estimating alarm effectiveness. Such a high, albeit unlikely, yield would increase the above doses and dose rates by a factor of 2,500.

When criticality monitors are placed into service they are often located in areas, and at distances from potential events, where sufficient sensitivity exists and where the detectors are unlikely to be overexposed to the point of catastrophic failure. Various guides and regulations such as ANSI Standard ANSI/ANS 8.3, Criticality Accident Alarm System, 10 CFR 70.24, and 10 CFR 76.89 provide recommendations regarding criticality monitor system requirements.

If you have specific requirements and a particular monitoring system in mind, you should be able to get additional information from the manufacturer. Good luck.

George Chabot, PhD, CHP

Answer posted on 16 March 2010. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.