Monitoring the thermal power of nuclear reactors with a prototype cubic meter antineutrino detector

A new power monitoring method applied to a pressurized water reactors designed by combustion engineering. The method estimate quickly and precisely a reactor’s operational status and thermal power can be monitored over hour to month time scales, using the antineutrino rate as measured by a cubic meter scale detector. Antineutrino emission in nuclear reactors arises from the beta decay of neutron-rich fragments produced by heavy element fissions, and is thereby linked to the fissile isotope production and consumption processes of interest for reactor safeguards. On average, fission is followed by the production of approximately six antineutrinos. The antineutrinos emerge from the core isotropically, and effectively without attenuation. Over the few MeV energy range within which, reactor antineutrinos are typically detected, the average number of antineutrinos produced per fission is significantly different for the two major fissile elements, 235U and 239Pu. Hence, as the core evolves and the relative mass fractions and fission rates of these two elements change, the measured antineutrino flux in this energy range will also change. It is useful to express the relation between fuel isotopic and the antineutrino count rate explicitly in terms of the reactor thermal power, Pth. The thermal power is defined as

Pth = ’ZiNlf. E{ (7)

where N[ is the number of fissions per unit time for isotope i, and E[ is the thermal energy released per fission for this isotope. The sum runs over all fissioning isotopes, with 235U, 238U, 239Pu, and 241Pu accounting for more than 99% of all fissions. The antineutrino emission rate Пу{ґ) can then be expressed in terms of the power fractions and the total thermal power as:

ns(t) = Pth{t)Y, ifjzr f Vi (Ev) dE„ (8)


where the explicit time dependence of the fission fractions and, possibly, the thermal power are noted. <p(Ev), is the energy dependent antineutrino number density per MeV and fission for the ith isotope. <p (E^) has been measured and tabulated. Equation 7 defines the burn-up effect. The fission rates N? (t) and power fractions /;(t) change by several tens of percent throughout a typical reactor cycle as 235U is consumed and 239Pu produced and consumed in the core. These changes directly affect the antineutrino emission rate n^(t). Reactor antineutrinos are normally detected via the inverse beta decay process on quasi-free protons in hydrogenous scintillator. In this charged current interaction, the antineutrino v converts the proton into a neutron and a positron: v + p ^ e+ + n. For this process, the cross section a is small, with a numerical value of only ~10_43cm2. The small cross section can be compensated for with an intense source such as a nuclear reactor. For example, cubic meter scale hydrogenous scintillator detectors, containing ~1028 target protons Np, will register thousands of interactions per day at standoff distances of 10-50 meters from typical commercial nuclear reactors. In a measurement time T, the number of antineutrinos detected via the inverse beta decay process is:

N*(t) = (^)Pth(t’) ^ ^ / a Vi є dE„ (9)

In the above equation, a is the energy dependent cross section for the inverse beta decay interaction, Np is the number of target protons in the active volume of the detector, and D is the distance from the detector to the center of the reactor core. є is the intrinsic detection efficiency, which may depend on both energy and time. The antineutrino energy density and the detection efficiency are folded with the cross section ff, integrated over all antineutrino energies, and summed over all isotopes i to yield the antineutrino detection rate. The SONGS1 detector consists of three subsystems; a central detector, a passive shield, and a muon veto system. Figure 10 shows a cut away diagram of the SONGS1 detector. Further information can be found in (Bowden, 2007) and (Bernstein et al., 2007).


Fig. 10. A cut away diagram of the SONGS1 detector (showing the major subsystems).

This prototype that is operated at 25 meter standoff from a reactor core, can detect a prompt reactor shutdown within five hours, and monitor relative thermal power to 3.5% within 7 days. Monitoring of short-term power changes in this way may be useful in the context of International Atomic Energy Agency’s (IAEA) Reactor Safeguards Regime, or other cooperative monitoring regimes.