110 Measured Distribution of Neutrons Inside

110

MEASURED DISTRIBUTION OF NEUTRONS INSIDE CONTAINMENT OF A PWR

H. M. Butler, W. F. Ohnesorge and J. A. Auxler Insustrial Safety and Applied Health Physics Oak Ridge National Laboratory- Oak Ridge, TN 37830

ABSTRACT

Fast neutron dose rates inside the containment shell of an operating pressurized water reactor were measureds using threshold detectors. The threshold detector system also provided crude spectral results. Results indicated a highly moderated spectrum and dose rates that ranged from 0.30 to 39^ Rem/hr.

Fast neutron dose rates inside the containment shell of an operating pressurized water reactor were measured. The measurement was made with a type of threshold detector unit which contained sources of fissile material.1 Fission fragments emanated as a result of neutron capture in the fissile material damaged thin poly-carbonate film which had been placed next to the sources.2 Etching and subse- quent "spark counting" of the damaged areas provided a measure of the neutron fluence to which the units were exposed.

The unit contains three foils, one each of a38U, 23 7Np, and a39Pu, which, when exposed simultaneously pro- vides some spectral information. The plutonium foil has a 'very broad neutron capture reaction cross section (begin- ning at approximately 1 KeV) resulting in fission while the neptunium fissions upon capture of neutrons having energies above ^750 KeV and uranium fissions with neutron energies exceeding 1.5 MeV. The threshold detectors also contained bare and cadmium covered gold foils for use in thermal neutron detection, and sulphur pellets for use in detection of higher energy neutrons (with energies exceeding ^2.5 MeV).

The survey was made by positioning eighteen of these units in pairs at nine stations inside the containment shell. A plot of their locations is shown in Figure 1. All of the locations, except two, were at the elevation of the entry hatch for personnel into the containment shell with the remaining two detector locations being just above the reactor pressure vessel flange gap in the reactor cavity. The Ill

four detectors exposed just above the reactor pressure vessel flange gap were given 4.5 hours of exposurea while the remaining fourteen were given 21 hours of exposure. The measurements were made with the reactor operating at full power (829 MWe).

ORNL-DWG.78-12133

Fig- 1- Plot shotting location of threshold detectors Inside the reactor containment shell. 112

The results of the survey are shown In Table 1. As indicated, dose equivalent rates at the nine locations varied from 290 mrem/hr to 394 rem/hr. The two highest dose equivalent rates were, as predicted, at the lower elevation in the reactor cavity directly above the reactor pressure vessel flange gap. The data from each pair of threshold detector units were found to agree well. For example, when the results from each pair were averaged, the maximum difference between the mean and extreme for any pair was l6%. The average difference between the mean and extreme for all pairs was 9%. The energy spectrum peaked considerably lower than that observed for unshielded fission reactions with greater than 90$ of the neutrons having energies below 750 KeV. The dose information from the plutonium foil dominated that from the uranium and neptunium and in most cases was 80$ or greater of the dose determined from summation of all the foils. As a result of this finding, the relative "softness" of the spectrum, a quality factor of 7-5 was used in determining dose equivalent rates.

ORNL-DWG 78-12900

Table 1. Results of Survey

Average Absorbed Average Dose Location Dose Rate Equivalent Rate Rad/hr Rem/hr

.04 30 2 .098 n 3 .064 48 4 .041 31 5 .039 29 6 1.096 8.2 7 52.5 394 8 .781 5.9 9 -131 1.0 113

These measurements confirm that a major source of neutrons inside the containment shell of a pressurized water reactor exists in the pressure vessel flange gap, hot leg area. They also confirm that the energy spectrum is "soft" and the majority of the neutrons are below 750 KeV. A quality factor less than 10 may be used in the determination of dose equivalent rates.

REFERENCES

1. G. D. Kerr and T. D. Strickler, Health Physics 12 1141 (1966).

2. K. Becker, Solid State Dosimetrys CRD Press, Cleveland, 1973-