Evaluaton of Boron-10 Depletion During Pressurized Water Reactor Operation

Michael Bryson

Department of Nuclear Engineering, University of California at Berkeley

Ne-161 project

Key words: PWR, Boron10

Introduction

Boron 10 (B-10) is a strong neutron absorber and is 19.8 atom % contained in natural boron and depletes by neutron absorption in the core. Boric acid is mixed into the reactor coolant system in order to control long -term core excess reactivity changes within a Pressurized Water Reactors(PWR). In the process of irradiation by neutrons the Boron-10 concentration is depleted by the absorption of the neutrons. The Boron-10 isotopic ratio varies during the core life because of the depletion effect in addition to the boration and dilution operation.

With the depletion of Boron-10, two effects can be observed: (1) the "strength" of the remaining Boron decreases and, (2) a difference between an isotopic and chemical analysis of the Boron concentrate develops. With the decrease in strength of the remaining Boron, it's behavior tends to deviate, since design model predictions assume a naturally occurring abundance of 19.8 atom % is maintained

. The isotopic determination can distinguish between Boron-10 and natural occurring Boron-11, whereas the chemical analysis can not and as a result will indicate a higher amount of Boron. The chemical analysis is used at the San Onofre Nuclear Generation Station (SONGS). I will use the results calculated during the Unit 3 cycle 6 at SONGS. The chemical analysis result will be lower due to significant boration followed by dilution to the original conditions and will cause the isotopic analysis to change. The chemical analysis lowers due to the replenished Boron-10. Replenished Boron is considered, more "powerful" , therefore less is required.

Results During the Unit 3 Cycle 6 U3C6 operation, fourteen RCS boron samples were sent to Teledyne Isotopes for isotopic analysis of B-10. From Table 1, depletion of B-10 clearly is obvious and can be considerable in magnitude. These results bench-mark the accuracy of the B-10 depletion prediction models. Table 2 summarizes the Burn-up interval, atom %, cross section, flux and Boron concentration data for the two cases (from table 1). Figure 1 displays the B-10 a/o for the two cases. Figure 2 and Figure 3 compare the measured, actual isotope data, the 'Y' and 'N' method boron concentrations for the two cases. The two B10 depletion prediction models were used to estimate the theoretical behavior of B10 depletion as a function of core burn-up for SONGS fuel cycle. Figure 4 displays the theoretical behavior of the reduction in B-10 assuming an initial abundance of 19.8 atom%. If no boration occurs, the B10 can reduce to about 16 atom%. Figure 5 shows the theoretical behavior of the difference of depleted verses non-depleted boron values. Appendix A and Appendix B give the predicted Boron-10 depletion models. The two models used are the "Y" and "N" which represent the Yamazaki model and Neymotin model respectively.

Conclusion It is only necessary to consider Boron-10 depletion following extended operation combined with insignificant or no boration. If any plant evolution are to occur, such as shutdown, which would entail significant boration, replenishment of boron reduces or even eliminates B-10 depletion concerns.

From the actual Boron-10 isotopic analysis results that boron samples indicate significant B-10 depletion. The two models used for predicting B-10 depletion were validated by comparison with actual isotopic analysis results.

Reference