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INVESTIGATION ON USING NEUTRON COUNTING TECHNIQUES FOR ONLINE BURNUP MONITORING OF PEBBLE BED REACTOR FUELS

ZHAO, ZHONGXIANG
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1096914339

Abstract Details

2004, PhD, University of Cincinnati, Engineering : Nuclear and Radiological Engineering.
Modular Pebble Bed Reactor (MPBR) is a high temperature gas-cooled nuclear power reactor. In addition to its inherently safe design, a unique feature of this reactor is its multi-pass fuel circulation in which the fuel pebbles are randomly loaded and continuously cycled through the core until they reach their prescribed End-of-Life burnup limit. Unlike the situation with a conventional light water reactor, depending solely on computational methods to perform in-core fuel management for MPBR will be highly inaccurate. An on-line measurement system is needed to accurately assess whether a given pebble has reached its End-of-Life burnup limit and thereby provide an on-line, automated go/no-go decision on fuel disposition on a pebble-by-pebble basis. This project investigated the feasibility of using the passive neutron counting and active neutron/gamma counting for the on line fuel burnup measurement for MPBR. To investigate whether there is a correlation between neutron emission and fuel burnup, the MPBR fuel depletion was simulated under different irradiation conditions by ORIGEN2. It was found that the neutron emission from an irradiated pebble increases with burnup super-linearly and reaches to 10 4 neutron/sec/pebble at the discharge burnup. The photon emission from an irradiated pebble was found to be in the order of 10 13 photon/sec/pebble at all burnup levels. Analysis shows that the neutron emission rate of an irradiated pebble is sensitive to its burnup history and the spectral-averaged one-group cross sections used in the depletion calculations, which consequently leads to large uncertainty in the correlation between neutron emission and burnup. At low burnup levels, the uncertainty in the neutron emission/burnup correlation is too high and the neutron emission rate is too low so that it is impossible to determine a pebble’s burnup by on-line neutron counting at low burnup levels. At high burnup levels, the uncertainty in the neutron emission rate becomes less but is still large in quantity. However, considering the super-linear feature of the correlation, the uncertainty in burnup determination was found to be ~ 7% at the discharge burnup, which is acceptable. Therefore, total neutron emission rate of a pebble can be used as a burnup indicator to determine whether a pebble should be discharged or not. The feasibility of using passive neutron counting methods for the on-line burnup measurement was investigated by using a general Monte Carlo code, MCNP, to assess the detectability of the neutron emission and the capability to discriminate gamma noise by commonly used neutron detectors. It was found that both He-3 and BF 3 detector systems are able to satisfy the requirement on detection efficiency; but their gamma discrimination capability is only marginal. Even using thick gamma shielding, these two types of detectors shall deteriorate in performance after a limited period of operation time because of excess accumulative gamma exposures. Thus, two or more detector systems must be used alternatively for continuous measurement. On the other hand, fission counters were found that they can effectively discriminate gamma interference for this on-line application even without using gamma shield. However, detection efficiency of fission counters is low; thus a multi-fission-counter system (using at least 12 commercially available fission chambers) must be used to achieve the required detection efficiency. Overall, passive neutron counting could be used to provide an on-line, go/no-go decision on fuel disposition on a pebble-by-pebble basis for MPBR, if the detection system is well designed. Using active interrogation methods for this on-line burnup determination of fuel pebbles was also briefly studied. Results show that both active neutron counting and active gamma counting do not work for this application because the induced fission neutrons are overwhelmed by scattered source neutrons and the prompt fission gamma emission rate is much less than that of the decay gammas. In terms of gamma spectrometry measurement, the prompt fission gammas are stronger than the decay gammas in the high energy ranges; however, due to the extremely strong gamma emission at low energy and low gamma detection efficiency at high energy, it seems (though not confirmed) that active gamma spectrometry measurement is not promising either for this on-line burnup determination application.
Dr. Bingjing Su (Advisor)
114 p.
Pebble Bed Reactor; on-line fuel burnup measurement; passive neutron counting; MCNP simulations

Recommended Citations

Citations

  • ZHAO, Z. (2004). INVESTIGATION ON USING NEUTRON COUNTING TECHNIQUES FOR ONLINE BURNUP MONITORING OF PEBBLE BED REACTOR FUELS [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1096914339

    APA Style (7th edition)

  • ZHAO, ZHONGXIANG. INVESTIGATION ON USING NEUTRON COUNTING TECHNIQUES FOR ONLINE BURNUP MONITORING OF PEBBLE BED REACTOR FUELS. 2004. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1096914339.

    MLA Style (8th edition)

  • ZHAO, ZHONGXIANG. "INVESTIGATION ON USING NEUTRON COUNTING TECHNIQUES FOR ONLINE BURNUP MONITORING OF PEBBLE BED REACTOR FUELS." Doctoral dissertation, University of Cincinnati, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1096914339

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Cincinnati and OhioLINK.