Patent Number: 047449390
Section: summary

This invention relates to monitoring neutron fluence by dosimeters and, more particularly, to a method for correcting for "burn-in" or ingrowth of interfering fissioning isotopes during fission rate measurements for neutron dosimetry purposes. Federal regulations require that reactor coolant pressure boundaries have sufficient margin to ensure that the boundary behaves in a non-brittle manner when stressed under operating, maintenance, testing, and postulated accident conditions, and that the probability of rapidly propagating fracture is minimized. These requirements necessitate prediction of the amount of radiation damage to the reactor vessel throughout its service life, which in turn requires that the neutron exposure to the pressure vessel be monitored. Fission neutron monitors are often used in neutron dosimetry, and can provide pivotal fast flux spectral information, such as for light water reactor pressure vessel surveillance. In neutron dosimetry, a fission monitor of charge Z and mass number A is exposed to a neutron beam having and energy spectrum .phi. (t, E) which generally is a function of time t and neutron energy E. During the irradiation, a higher order or larger atomic weight actinide isotope (Z', A') can be created by neutron capture in the (Z, A) isotope of the fission neutron dosimeter. Neutron capture actually produces the isotope (Z, A+1) and subsequently decay processes then create the (Z', A') isotope, with A'=A+1. Consequently, the total number of fissions per unit volume, F.sub.T, observed with this fission neutron dosimeter is given by: EQU F.sub.T =F.sub.Z, A +B.sub.Z', A' where F.sub.Z, A is the number of fissions per unit volume produced in the isotope (Z, A) and B.sub.Z', A' is the number of fissions per unit volume produced in the isotope (Z', A') as it ingrows during the irradiation. Although the quantity F.sub.Z, A is desired, F.sub.T is actually measured. The term B.sub.Z', A' represents a contribution from the higher order actinide (Z', A'), i.e., the so-called "burn-in" effect. In light water reactor pressure vessel surveillance work, this contribution can be non-negligible for a .sup.238 U threshold fission monitor where burn-in effects arise from fission in .sup.239 Pu. In fact, recent analysis shows that the burn-in effect for .sup.238 U can be as high as about 30 percent in light water reactor pressure vessel environments. In light of the above, a method is desired for efficiently and accurately correcting for burn-in effects in fission neutron dosimeters. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for correcting for isotope burn-in effects in fission neutron dosimeters, which method is capable of adaptation to diverse geometries. It is another object of the present invention to provide a method for correcting for isotope burn-in effects in fission neutron dosimeters, wherein relatively small dosimeters are employed that are capable of being used in situ with negligible perturbation of the environment. It is another object of the present invention to provide a method for correcting for isotope burn-in effects in fission neutron dosimeters which is capable of high sensitivity and absolute accuracy. It is another object of the present invention to provide a method for correcting for isotope burn-in effects in fission neutron dosimeters which is capable of quantifying background effects. Finally, it is an object of the present invention to provide a method for correcting for isotope burn-in effects in fission neutron dosimeters which is capable of conducting measurements in extremely high neutron fluences. To achieve the foregoing and other objects of the present invention, and in accordance with the purposes of the invention, there is provided a method for correcting for the burn-in effect in fission neutron dosimeters, wherein two quantities are measured in order to quantify the burn-in contribution, namely P.sub.Z', A', the amount of (Z', A') isotope that is burned in, and F.sub.Z', A', the fissions per unit volume that would be produced from the start of the irridation in a dosimeter made of the (Z', A') isotope. Monitors used to measure these two quantities must experience the very irradiation that the fission neutron dosimeter undergoes, i.e., the same location and flux-time history. To measure the burn-in of the (Z', A') isotope, two solid state track recorder fission deposits are prepared from the very same material that comprises the fission neutron dosimeter and the two are quantified, i.e., the mass and mass density are measured. One of these deposits is exposed along with the fission neutron dosimeter, whereas the second deposit is subsequently used for observation of background, which is any fission track contribution from actinide impurities in the fission dosimeter. The amount of burn-in of the (Z', A') isotope is determined by conducting a second irradiation, wherein both the irradiated and unirradiated fission deposits are used in solid state track recorder dosimeters for observation of the absolute number of fissions per unit volume. The difference between these two absolute solid state track recorder measurements can be used to quantify the amount of burn-in since the neutron cross-section is known. The fissions per unit volume of the (Z', A') isotope can be obtained by using a fission neutron dosimeter prepared specifically for this isotope. The (Z', A') fission dosimeter is exposed along with the original threshold fission neutron dosimeter, so that it experiences exactly the same neutron flux-time history at the same location. In order to determine B.sub.Z', A' from these observations, certain assumptions on the time dependence of the neutron field must hold. More specifically, the neutron field must generally be either: (1) time independent, or PA1 (2) a separable function of time t and neutron energy E. Reactor irradiations can often be carried out at constant power, in which event assumption (1) would be valid. In the case that assumption (1) does not hold, assumption (2) is quite likely to be valid. Moreover, for this case, the reactor power intrumentation can often be used to determine the separable time-dependent behavior of the neutron field.