atw Vol.

atw Vol. 63 (2018) | Issue 8/9 ı August/September

446

OPERATION AND NEW BUILD

commercial delivery, while improving

the quality of the data and resulting

design models used to describe the

fuel. This new approach would be a

significant improvement compared

to the current, largely empirical

approach, which requires years to

obtain limited data from a very expensive

test reactor(s), as well as for the

fabricating, testing, cooling, transportation

and post-irradiation examination

of samples. To reduce the

licensing timeframe for EnCore Fuel,

Westinghouse plans to utilize:

• Atomic scale modeling:

• By utilizing first principles to

determine physical properties

of irradiated materials

• By leveraging Westinghouse

involvement in the Nuclear

Energy Advanced Modeling &

Simulation (NEAMS) Department

of Energy (DOE) program

on basic property prediction

• By leveraging Westinghouse

involvement in the Consortium

for Advanced Simulation of

Light Water Reactors (CASL) –

Virtual reactor design

• By continuing to utilize MedeA

and Thermo-Calc software

• Real-time data generation to verify

the atomic scale modeling:

• Poolside data generation

Gamma emission tomography

based on gamma-ray spectroscopy

and tomographic reconstruction

can be used for

rod-wise characterization of

nuclear fuel assemblies without

dismantling the fuel to detect

pellet swelling, pellet- cladding

interaction and pellet cracking

Potential use of a spectroscopic

detection system to select

different gamma-ray emitting

isotopes for analysis, enabling

nondestructive fuel characterization

with respect to a variety

of fuel parameters (fission gas

release)

• Wired or wireless transmission

technology for measuring

Centerline temperature

Fuel rod gas pressure

Swelling of fuel

In addition to saving time and cost, with

this approach Westinghouse hopes to

achieve, an increased confidence by the

U.S. NRC due to the predictability of

performance that can be obtained since

the performance models will have a

theoretical basis in addition to an

empirical basis. There should also be

reduced time and effort due to the reduction

in the number of submissionreview-revision-

submission cycles. This

should remove the review process from

the critical path to commercialization.

Communication with the U.S. NRC

Commissioners, and coordination

between the DOE, NRC and industry

for licensing of ATF, are in progress

and continuing.

7 Conclusion

Westinghouse and its partners are

continuing to make good progress on

U 3 Si 2 fuel, SiC cladding, and chromium-coated

zirconium cladding. These

new designs will offer design-basisaltering

safety, greater uranium efficiency,

and significant economic

benefits per reactor per year for PWRs.

While all testing and development to

date has been engineered for LWR

designs, Westinghouse believes the

technology could provide some of the

same safety and economic benefits to

CANDU and other reactor designs.

Fuel and accident modeling with

other types of reactor systems will be

required to evaluate the actual potential

for these benefits. This, together

with more beneficial power peaks,

lower impact of the transition cycles

and reduced dependence on uranium

price assumptions, make adoption of

the Westinghouse ATF, in conjunction

with a transition to 24-month cycle

operation, the recommended path

forward for implementation of the

Westinghouse ATF, EnCore Fuel.

References

[1] Gordon Kohse, MIT, 2016.

[2] Ed Lahoda, Sumit Ray, Frank Boylan,

Peng Xu and Richard Jacko, SiC Cladding

Corrosion and Mitigation, Top Fuel 2016,

Boise, ID, September 11, 2016, Paper

Number 17450, ANS, (2016).

[3] Jason Harp, Idaho National Laboratory

preliminary photographs.

[4] Lu Cai, Peng Xu, Andrew Atwood,

Frank Boylan and Edward J. Lahoda,

Thermal Analysis of ATF Fuel Materials

at Westinghouse, ICACC 2017, Daytona

Beach, FL, January 26, 2017.

[5] E. Sooby Wood, J.T. White and A.T.

Nelson, Oxidation behavior of U-Si

compounds in air from 25 to 1000 C,

Journal of Nuclear Materials, 484,

pages 245-257 (2017).

[6] Eugene van Heerden, Chan Y. Paik,

Sung Jin Lee and Martin G. Plys,

Modeling Of Accident Tolerant Fuel

for PWR and BWR Using MAAP5,

Proceedings of ICAPP 2017, Fukui and

Kyoto ,Japan, April 24-28, 2017.

Authors

Gilda Bocock

Robert Oelrich

Sumit Ray

Westinghouse Electric Company

5801 Bluff Road

Hopkins, SC 29061, USA

Analyses of Possible Explanations for the

Neutron Flux Fluctuations in German PWR

Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov

Revised version of a

paper presented at

the Annual Meeting

of Nuclear Technology

(AMNT 2017), Berlin.

During the last 15 years the neutron flux fluctuation levels in some of the German PWR changed significantly. During

a period of about ten years, the fluctuation levels increased, followed by about five years with decreasing levels after

taking actions like changing the design of the fuel elements [1, 2]. The increase in the neutron flux fluctuations resulted

in an increased number of triggering the reactor limitation system and in one case in a SCRAM [3].

There exist different possible explanations

how neutron flux oscillations are

caused by physical phenomena inside

a PWR. Possible explanations can be

based on complicated interactions

between thermo-hydraulical (TH),

structural-mechanical and neutron

physical processes (see Figure 1).

Yet, no comprehensive theory

exists, which can explain the neutron

flux fluctuation histories observed

in German PWR based on first

physical principles. Therefore, GRS

has started investigations to

explain the observed neutron flux

Operation and New Build

Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov

atw Vol. 63 (2018) | Issue 8/9 ı August/September 446 OPERATION AND NEW BUILD commercial delivery, while improving the quality of the data and resulting design models used to describe the fuel. This new approach would be a significant improvement compared to the current, largely empirical approach, which requires years to obtain limited data from a very expensive test reactor(s), as well as for the fabricating, testing, cooling, transportation and post-irradiation examination of samples. To reduce the licensing timeframe for EnCore Fuel, Westinghouse plans to utilize: • Atomic scale modeling: • By utilizing first principles to determine physical properties of irradiated materials • By leveraging Westinghouse involvement in the Nuclear Energy Advanced Modeling & Simulation (NEAMS) Department of Energy (DOE) program on basic property prediction • By leveraging Westinghouse involvement in the Consortium for Advanced Simulation of Light Water Reactors (CASL) – Virtual reactor design • By continuing to utilize MedeA and Thermo-Calc software • Real-time data generation to verify the atomic scale modeling: • Poolside data generation PP Gamma emission tomography based on gamma-ray spectroscopy and tomographic reconstruction can be used for rod-wise characterization of nuclear fuel assemblies without dismantling the fuel to detect pellet swelling, pellet- cladding interaction and pellet cracking PP Potential use of a spectroscopic detection system to select different gamma-ray emitting isotopes for analysis, enabling nondestructive fuel characterization with respect to a variety of fuel parameters (fission gas release) • Wired or wireless transmission technology for measuring PP Centerline temperature PP Fuel rod gas pressure PP Swelling of fuel In addition to saving time and cost, with this approach Westinghouse hopes to achieve, an increased confidence by the U.S. NRC due to the predictability of performance that can be obtained since the performance models will have a theoretical basis in addition to an empirical basis. There should also be reduced time and effort due to the reduction in the number of submissionreview-revision- submission cycles. This should remove the review process from the critical path to commercialization. Communication with the U.S. NRC Commissioners, and coordination between the DOE, NRC and industry for licensing of ATF, are in progress and continuing. 7 Conclusion Westinghouse and its partners are continuing to make good progress on U 3 Si 2 fuel, SiC cladding, and chromium-coated zirconium cladding. These new designs will offer design-basisaltering safety, greater uranium efficiency, and significant economic benefits per reactor per year for PWRs. While all testing and development to date has been engineered for LWR designs, Westinghouse believes the technology could provide some of the same safety and economic benefits to CANDU and other reactor designs. Fuel and accident modeling with other types of reactor systems will be required to evaluate the actual potential for these benefits. This, together with more beneficial power peaks, lower impact of the transition cycles and reduced dependence on uranium price assumptions, make adoption of the Westinghouse ATF, in conjunction with a transition to 24-month cycle operation, the recommended path forward for implementation of the Westinghouse ATF, EnCore Fuel. References [1] Gordon Kohse, MIT, 2016. [2] Ed Lahoda, Sumit Ray, Frank Boylan, Peng Xu and Richard Jacko, SiC Cladding Corrosion and Mitigation, Top Fuel 2016, Boise, ID, September 11, 2016, Paper Number 17450, ANS, (2016). [3] Jason Harp, Idaho National Laboratory preliminary photographs. [4] Lu Cai, Peng Xu, Andrew Atwood, Frank Boylan and Edward J. Lahoda, Thermal Analysis of ATF Fuel Materials at Westinghouse, ICACC 2017, Daytona Beach, FL, January 26, 2017. [5] E. Sooby Wood, J.T. White and A.T. Nelson, Oxidation behavior of U-Si compounds in air from 25 to 1000 C, Journal of Nuclear Materials, 484, pages 245-257 (2017). [6] Eugene van Heerden, Chan Y. Paik, Sung Jin Lee and Martin G. Plys, Modeling Of Accident Tolerant Fuel for PWR and BWR Using MAAP5, Proceedings of ICAPP 2017, Fukui and Kyoto ,Japan, April 24-28, 2017. Authors Gilda Bocock Robert Oelrich Sumit Ray Westinghouse Electric Company 5801 Bluff Road Hopkins, SC 29061, USA Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov Revised version of a paper presented at the Annual Meeting of Nuclear Technology (AMNT 2017), Berlin. During the last 15 years the neutron flux fluctuation levels in some of the German PWR changed significantly. During a period of about ten years, the fluctuation levels increased, followed by about five years with decreasing levels after taking actions like changing the design of the fuel elements [1, 2]. The increase in the neutron flux fluctuations resulted in an increased number of triggering the reactor limitation system and in one case in a SCRAM [3]. There exist different possible explanations how neutron flux oscillations are caused by physical phenomena inside a PWR. Possible explanations can be based on complicated interactions between thermo-hydraulical (TH), structural-mechanical and neutron physical processes (see Figure 1). Yet, no comprehensive theory exists, which can explain the neutron flux fluctuation histories observed in German PWR based on first physical principles. Therefore, GRS has started investigations to explain the observed neutron flux Operation and New Build Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov

atw Vol. 63 (2018) | Issue 8/9 ı August/September | | Fig. 1. Possible causes for neutron flux oscillations. fluctuations and amplitude changes [4]. Characteristics of neutron flux fluctuations in German PWR The neutron flux level during full power operation is measured by inand ex-core detectors sensitive to thermal neutrons [5]. In German Vorkonvoi and Konvoi type PWRs in total 16 ex-core detectors (ionization chambers) are located at four azimuthal positions (see Figure 2) and at four different axial heights outside the RPV wall within the biological shield. The signals of the upper two and lower two are combined. They measure the neutrons released from the core. Inside the reactor core eight measurement rods are located (see Figure 2). Each measurement rod consists of six self-powered neutron detectors (SP- NDs) located at different elevations. The neutron flux measurements used in the following analyses have been provided by an operator of a German Vorkonvoi PWR. The data were sampled with 250 Hz after they had been low-pass filtered with a cutoff frequency of 100 Hz. Figure 3 (left) shows the power spectral densities (per Hz) of the neutron flux signals of ex-core detectors measured in a Vorkonvoi PWR. The power levels measured at the four different azimuthal positions show no significant differences. The highest power spectral density of the neutron flux is measured at low frequencies up to 1 Hz. Figure 3 (right) shows the distribution of the time-dependent spectral power density. For each time step, a Fast-Fourier-Transformation was calculated, using the following parameters: sampling frequency = 250 Hz, numbers of samples = 4096, Hanning window function. The time steps in Figure 3 (right) are separated by 14.3 s, which is 7/8 of the length of a single FFT window (16.4 s). For each point of time and frequency the spectral power level is color coded. The spectral power density changes over time in a “chaotic” way. This means that the frequency of the maximal power density changes over time. This observation does not change if the number of samples used for the FFT or the time resolution used for the calculation spectrogram is reduced or increased. The top row of Figure 4 shows the measured coherence and the phase angles of two combinations of two different ex-core detectors each. The coherence was calculated by dividing the absolute value squared of the cross correlation of the corresponding two detector signals by the autocorrelation of the signals. Both detector combinations show a strong coherence at 1 Hz. The phase of the complex valued frequency dependent cross correlation was used to calculate the frequency dependent phase shown in Figure 4 (converted into units of degree). The two detectors located at perpendicular horizontal positions relative to the core center (at 45° and 135°) exhibit constant phase difference in the frequency range up to 1 Hz. In contrast, the two detectors located at opposing sides of the core center (at 45° and 225°) show a nearly constant phase difference of 180° in the frequency range up to 5 Hz. This phase difference of 180° can be found for all detector combinations calculated by the cross correlation for two detectors placed at opposing sides relative to the reactor center. The bottom row of Figure 4 shows the relative signal strengths over time of six in-core detectors located at different axial heights on the Co4 measurement rod (see Figure 2 for the positions of the detectors). Even though the amplitude at different elevations shows some differences (higher amplitudes at middle elevations than at lower or higher ones) the temporal OPERATION AND NEW BUILD 447 | | Fig. 2. Horizontal positions of in-core (marked light blue) and ex-core (marked red) detectors within the core shroud and outside the reactor pressure vessel. | | Fig. 3. Power spectral density (left) and spectrogram (right) of ex-core detector measurements in a Vorkonvoi PWR. Operation and New Build Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov