Coincidence loss

The cause of this difficulty therefore resides within the counter itself. The difficulty is described by saying that the Geiger counter has a dead time, by which is meant the time interval after a pulse during which the counter cannot respond to a later pulse. This interval, which is usually well below 0.5 millisecond, limits the useful maximum counting rate of the detector. The cause of the dead time is the slowness with which the positive-ion space charge (2.5) leaves the central wire under the influence of the electric field. This reduction in observed counting rate is known as the coincidence loss. 

The observed counting rate can be increased to account for coincidence losses by adding to the counting rate a factor , given by... 

Construct a graph similar to that shown in Figure 3-42. This graph may be used to correct all the experimental data obtained for coincidence losses as long as the same Geiger-Miiller tube is always used. 

Detector coincidence loss (detector dead time). 

Guss et al (1988) measured data at four wavelengths about the copper K edge, i.e. 1.2359 A, 1.3771 A, 1.3790 A and 1.5416 A. Of particular note in the data reduction was the need for an empirical method for coincidence losses to be used, as the analytical dead time correction for the area detector was unreliable at the upper end of the range of counting rates. As a result, merging f -factors within the data set at each wavelength were reduced by 20-60%, for the empirical versus the analytical coincidence method of data correction. 

Usually LOCA events are examined assuming coincident loss of the in-house power supply. Failure to start one diesel will then be a typical assumption of a single failure. This assumption leads, in turn, to failure of one train of the ECCS pumps. 

Summing of events, either a result of coincident emission of gamma rays in the decay chain of the nuclide of interest or of random coincident emissions, can lead to significant losses or potential additions to an otherwise clean peak (De Bruin and Blaauw 1992 Becker et al. 1994). While coincidence losses are not an issue for comparator NAA, calibration, and/or computational correction must be applied (Debertin and Helmer 1988 Blaauw and Celsema 1999) to arrive at true peak areas for other methods of calibration. 

Wyttenbach, A. (1971). Coincidence losses in activation analysis, J. Radioanal. Chem., 8, 335-343. 

Total loss of electrical power accident has been studied in the Temeh n Accident Analyses in the part 15.2.6 - Coincident Loss of On-site and External AC Power to the Station. This accident is classified as an event of ANSI class II. From the performed analyses, the safety limits corresponding to this class will not be violated. Resolution of this transient will be included in EOP. 

The steam generator tube rupture accident examined is the complete severance of a single steam generator tube. The accident is assumed to take place at power with the reactor coolant contaminated with fission products corresponding to continuous operation with a limited number of defective fuel rods within the allowance of the Technical Specifications (see Chapter 16 of Reference 5.6). The accident leads to an increase in contamination of the secondary system due to leakage of radioactive coolant from the reactor coolant system. In the event of a coincident loss of offsite power, or a failure of the condenser steam dump, discharge of radioactivity to the atmosphere takes place via the steam generator power-operated relief valves or the safety valves. 

A loss of off-site power should be assumed coincident with any extreme DBEE if a direct or indirect causal relationship cannot be excluded. Particularly, for DBEEs that are expected to affect the entire site and, therefore, to give rise to a potential for a common cause failure mode, a loss of off-site power should be combined with the DBEE. For other events, a loss of off-site power should be assumed if the location of the transmission lines or the switchyard is such that the direct effects on them of the DBEE could cause a loss of off-site power. For external events such as ship collisions and internal events such as fire or anticipated operational occurrences, a coincident loss of off-site power should be assumed if the event could be expected to result in an unplanned turbine trip or reactor trip that would increase the potential for grid instability. 

However, at high-count rate the probability of the loss of counts from a y-ray peak owing to the coincidence between two y-ray pulses detected at the same (or nearly the same) instant becomes important and has a significant effect on the accuracy of the measurement. Such losses cannot be accounted for by dead-time correction. It is important to realise that the coincidence loss depends not on the count rate at the multi-channel analyser, which might be low if a biased amplifier is used to select a region of interest, but on the total y-ray count rate at the detector. Since coincidence losses are rate dependent, samples and standards should be of comparable intensity or errors will result. 

Wyttenbach has shown that it is possible to correct for coincidence losses mathematically using the equation ... 

Coincident loss of onsite and external (offsite) a.c. power to the station. 

Experiment LP-02-6 simulated a double-ended offset shear of a commercial PWR main coolant inlet pipe . The principal boundary conditions were (a) coincident loss of offsite power, (b) PWR nominal primary pump coastdown, (c) initial core power representative of USNRC licensing limits in a commercial PWR, and (d) US minimum emergency core coolant (ECC) injection. The cladding thermocouple temperature response was similar to that in LOFT L2-3 as shown in Figure 7. The early return to saturation conditions occurred at the same time, as had been expected. 

This analysis, which is performed to support reactor restart, shall consider postulated fire events when off-site power is available as well as when there is a coincident loss-of-off-site power. (See SER section 6.3.1, "Electrical Power Systems," for the definition of on-site and off-site power.)... 

The effects of a postulated accident on other systems should be considered in the analyses. None of the Action Plans addressed this specific requirement. However, flooding of AC/DC pumps and motors, is included in the consideration of the cooling water LOCA effects on the process water LOPA. SER Section 6.3.2 addresses environmental effects of postulated accidents. SER Section 6.3.1 addresses the loss of normal electric power event scenarios. WSRC should analyze coincident Loss of Normal Power in conjunction with some DBEs in Chapter 15. The effects of postulated accidents on other systems is an open item. 

---