IN-CORE COOLANT BOILING DETECTION IN VVER REACTORS BY THE
ANALYSIS OF NEUTRON FLUX NOISE CAUSED BY FLUCTUATIONS OF
COOLANT PARAMETERS.
Semchenkov Yu.M., Milto V.A., Pinegin A.A.1, Shumsky B.E.1
1
Russian Research Centre “Kurchatov Institute”, Moscow
Moscow Engineering and Physical Institute (State University), Moscow
ABSTRACT
The paper discusses investigation results of SPND signals for noises caused by
fluctuations of coolant parameters. The dependence of neutron flux amplitude oscillations
(local power) in SPD locations on the value and frequency of fluctuations of coolant
parameters (pressure, flow rate, temperature) is analysed in the paper.
Detection Tasks of Coolant Boiling in VVER Core
Long time boiling of coolant in the core results in depositions that lead to significant
deterioration of heat exchange. Moreover, active mixtures (lithium, chlorine, and fluorine)
accumulate in cladding pores during boiling, which intensifies corrosion rate of zirconium
and niobium (1-%) alloy. Since the departure from nucleate boiling ratio is determined by
calculations, and the adopted calculation techniques are based on a big amount of
experimental data, which always contain some uncertainty, for the purpose of soon
diagnostics of departure from nucleate boiling it is required to develop instrumentation
methods for detecting local surface boiling. Moreover, the monitoring of boiling for initial
moment will allow to specify the realistic conditions of heat removal in a certain core, and
also accurately determine design limitations for the most stressed cores.
Implementation of In-Core Noise Diagnostic Systems at NPPs
Recently the in-core noise diagnostic systems (ICND) have been implemented at three
VVER-1000 power units (Kalinin NPP Unit 3, Tianwan NPP Units 1 and 2) as an integral
part of MCDS. System ICND covers 54 fuel assemblies. It receives the signals from 54
assemblies, each equipped with 7 SPN detectors (totally 378 SPND signals), which are evenly
distributed in the core.
The task of in-core noise diagnostic system is to detect the locations of coolant
boiling in the reactor core.
Coolant always shows the fluctuations caused by the following factors:
- rotation of MCP fins;
- temperature difference in coolant loops of the primary cold legs;
- inhomogeneous mixing of coolant in the lower plenum;
- acoustic waves, etc.
fluctuations always exist in the coolant. The fluctuations lead to coolant density fluctuation in
the core, and further, to neutron flux fluctuation in FA.
Methods of boiling diagnostics are based on the behaviour evaluation of relative
sensitivity of neutron flux fluctuations along FA height to fluctuations of inlet coolant
parameters in every monitored fuel assembly. Fluctuations of coolant parameters are
evaluated on the basis of inlet neutron flux fluctuations in FA.
Values of neutron flux fluctuations are effected by the following factors:
- burnup in FA;
- cycle moment;
- burnup in SPND;
- ratio between oscillations and amplitudes of various coolant parameters (flow,
temperature, pressure), etc.
Behaviour evaluation of relative sensitivity, namely, allows to neglect the
dependence upon the above mentioned parameters.
Calculated and measure data that lay the basis of in-core noise diagnostic system
For the purpose of monitoring the coolant boiling by instrumentation, and
developing the methods we have conducted a series of experimental studies.
Since SPN detectors with rhodium (Rh) emitter are the regular detectors of in-core
monitoring, we investigated the probability of their application in diagnostics of coolant
boiling. Figure 1 shows AFR of SPND. It is visible that up till 0.05 Hz the dominating
contribution is made by the main activation component of SPND current (half life period of
the main activation component makes about 44 seconds).
After the point of 0.05 Hz the amplitude and frequency response of SPND becomes
linear and allows registering neutron flux fluctuations within broad frequency range. An
instant component makes about 7% of the value of the main SPND current.
SPND (Rh) Magnitude Response
1.1
1
0.9
0.8
Magnitude
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0
Frequency (Hz)
Figure 1. Amplitude and frequency response of SPN detector equipped with rhodium
(Rh) emitter.
Neutron flux fluctuations are effected by coolant density fluctuation caused by the
changes in moderating properties of the coolant. In this case the amplitude of neutron flux
fluctuations will be proportional to average neutron flux value. Neutron flux fluctuations
correspond to the variable component of SPND current, while the average value of neutron
flux corresponds to the main component of SPND current. For the purpose of criterion of
coolant fluctuations we selected a mean square deviation of variable component of SPND
current normalised by averaged SPND current (Rms-Spnd-Norm).
N
(x
n 1
Rms _ Spnd _ Norm 
n
 x) 2
N 1
x
,
where xn is the value of SPND current,
N is the number of data points in equal interval data retrieval,
The selected normalised parameter makes it possible to avoid the influence of
average value of neutron flux, and the influence of decreased SPND sensitivity to neutron
flux during the detector depletion.
It has been proven by the experiments, that SPND with rhodium emitter makes it
possible to register the starting time of coolant local boiling. The possibility of boiling
registration by SPND was demonstrated during the experiments on diagnostic fuel assembly
at Rheinsberg NPP (Germany). Figure 2 shows the pattern of SPND arrangement (E8, E10,
and E11) adopted in the experiments at Rheinsberg NPP, and the curve of Rms-Spnd-Norm
dependence on the margin to outlet saturation temperature in FA.
Figure 2. Pattern of SPND arrangement (E8, E10, E11) in the experiment on diagnostic
fuel assembly performed at Rheinsber NPP (VVER-70), and the curve of Rms-Spnd-Norm
dependence upon underheating in FA.
As reflected by Figure 2, detector E11 shows a notable growth of Rms-Spnd-Norm
when the subcooling was shifted from 14 degrees to 10 degrees. This time is close to the
calculated starting time of surface bubble boiling.
Moreover, a special experimental test was conducted at reactor VVR-2 to study a
fuel pin surface boiling. A fuel pin of ~ 90 % enrichment was fabricated on purpose to obtain
a required heat flux. Cable thermocouples of 0.5 mm in diameter were built in the pin walls.
SPND with rhodium emitter was arranged along the pin. As a result of testing we obtained the
dependence between pin wall temperature and Rms-Spnd-Norm on reactor power (see Figure
3).
N is Rms-Spnd-Norm.
ТC1 is pin wall temperature measured close to the centre of FA height.
ТC2 is pin wall temperature measured close to the outlet of FA.
Figure 3. Dependence between pin wall temperature and Rms-Spnd-Norm reactor
power. Experiment conducted at VVR-2.
Figure 3 explicitly shows the growth of Rms-Spnd-Norm (N) with approaching to
the temperature of saturation in the pin wall. The spread of Rms-Spnd-Norm values at low
power is stipulated by low SPND current, and reactor unsteadiness in such conditions.
For the purpose of evaluating a probable Rms-Spnd-Norm level we conducted
dynamic, neutronic and thermal hydraulic calculations of neutron flux fluctuation versus
amplitude and frequency of pressure fluctuations, coolant flow and coolant temperature in
conditions without boiling.
The fluctuations were simulated in the form of harmonic oscillation of coolant
parameters at the inlet to reactor core.
Coolant parameters (temperature, pressure and flow) were perturbed individually at
a frequency ranging within 0.2 Hz and 5 Hz, and the amplitudes ultimately close to realistic.
The investigations were performed for the reactor state with nominal power and nominal
coolant parameters. Two specific cycle moments in terms of reactivity coefficient were
considered: beginning of fuel cycle and the end of boron control (end of fuel cycle).
The calculations were performed by computer code NOSTRA1 intended for 3D
simulation of in-core processes of VVER reactor.
Evaluation of outcomes allowed to estimate the value of Rms-Spnd-Norm in SPND
locations. The axial core arrangement of Rms-Spnd-Norm was revealed in conditions of
temperature, pressure and coolant flow perturbation. Calculated Rms-Spnd-Norm values of
SPND registered were obtained for perturbations of various coolant parameters. Figures 5 and
6 present the graphs of extreme Rms-Spnd-Norm values for every discussed option of coolant
parameters fluctuation at the beginning and the end of fuel cycle.
9.0E-04
8.0E-04
Rms-Spnd-Norm
7.0E-04
6.0E-04
L1
L2
5.0E-04
L3
4.0E-04
L4
3.0E-04
L6
L5
2.0E-04
1.0E-04
0.0E+00
0.1
1
10
Frequency (Hz)
L 1 - a 0.2° temperature amplitude at inlet to seven FAs
L 2 - a 1° temperature amplitude at inlet to seven FAs
L 3 - a 0.2° temperature amplitude at inlet to one FA
L 4 - a 1° temperature amplitude at inlet to one FA
L 5 – amplitude of flow of 175 kg/s
L 6 – pressure amplitude of 0.079 MPa
Figure 5 – Extreme Rms-Spnd-Norm values for every discussed option of coolant parameter
fluctuation. Beginning of fuel cycle.
1.6E-03
1.4E-03
Rms-Spnd-Norm
1.2E-03
L1
1.0E-03
L2
L3
8.0E-04
L4
L5
6.0E-04
L6
4.0E-04
2.0E-04
0.0E+00
0.1
1
10
Frequency (Hz)
L 1 - 0.2° temperature amplitude at inlet to seven FAs
L 2 - 1° temperature amplitude at inlet to seven FAs
L 3 - 0.2° temperature amplitude at inlet to one FA
L 4 - 1° temperature amplitude at inlet to one FA
L 5 – amplitude of flow of 175 kg/s
L 6 – pressure amplitude of 0.079 MPa
Figure 6 – Extreme Rms-Spnd-Norm values for every discussed option of coolant parameter
fluctuation. End of fuel cycle.
Figures 7 through 9 give calculated values of Rms-Spnd-Norm for each SPND (axial
profile) with harmonic oscillations of various parameters at FA inlet.
4.0E-04
3.5E-04
3.0E-04
2.5E-04
2.0E-04
1.5E-04
1.0E-04
5.0E-05
0.0E+00
7.0E-05
6.0E-05
5.0E-05
4.0E-05
3.0E-05
2.0E-05
1.0E-05
Rms-Spnd-Norm
(5Hz)
Rms-Spnd-Norm
temperature fluctuation (A=10C)
0.2 Hz
0.25 Hz
0.(3) Hz
0.5 Hz
1 Hz
5 Hz
0.0E+00
1
2
3
4
5
6
7
number of SPND along the FA height
Figure 7. Calculated Rms-Spnd-Norm for each SPND versus harmonic fluctuations of
inlet coolant temperatures in FA. Amplitude of temperature fluctuation is 1 degree C.
Beginning of fuel cycle.
9.0E-04
6.9E-05
8.0E-04
6.8E-05
7.0E-04
6.0E-04
6.7E-05
6.6E-05
5.0E-04
6.5E-05
4.0E-04
6.4E-05
3.0E-04
2.0E-04
6.3E-05
6.2E-05
1.0E-04
6.1E-05
0.0E+00
6.0E-05
1
2
3
4
5
6
Rms-Spnd-Norm (5Hz)
Rms-Spnd-Norm
flow fluctuation (A=175kg/sec)
0.2 Hz
0.25 Hz
0.(3) Hz
0.5 Hz
1 Hz
5 Hz
7
number of SPND along the FA height
Figure 8. Calculated Rms-Spnd-Norm for each SPND versus harmonic fluctuations of
in-core coolant flow. Amplitude of temperature fluctuation is 175 kg/s. Beginning of fuel
cycle.
pressure fluctuation (A=0.079MPa)
Rms-Spnd-Norm
3.1E-04
2.9E-04
0.2 Hz
2.7E-04
0.25 Hz
2.5E-04
0.(3) Hz
2.3E-04
0.5 Hz
2.1E-04
1 Hz
1.9E-04
5 Hz
1.7E-04
1
2
3
4
5
6
7
number of SPND along the FA height
Figure 9. Calculated Rms-Spnd-Norm for each SPND versus harmonic in-core
coolant pressure fluctuations. Amplitude of pressure fluctuation is 0.079 MPA. Beginning of
fuel cycle.
As reflected in Figure 7, the influence of inlet coolant temperature fluctuation upon
neutron flux decreases with elevation (DPND No. grows). Such a decrease is a characteristic
feature of every frequency of coolant temperature fluctuation. The peak in the location of
SPND 4 is caused by the maximum neutron flux sensitivity towards the changes of coolant
density in the middle of FA height. Significantly smaller influence should be pointed out with
high frequencies (5 Hz) of modulation. Figures 8 and 9 demonstrate a gradual growth of RmsSpnd-Norm along the core height with identical amplitude of pressure and coolant flow
modulation. It is necessary to emphasise, that in actual operation conditions, coolant
temperature, flow and pressure fluctuations are interdependent, and all together (in-phase,
sometimes) produce the influence on neutron flux fluctuations.
For the purpose of estimating a realistic value of Rms-Spnd-Norm during pressure
generation in the coolant, we conducted calculations of neutron flux fluctuation, as influenced
by coolant temperature fluctuations. The calculations were performed by code NOSTRA,
various versions of which allow describing the process of non-equilibrium surface boiling.
Let us discuss the case of surface boiling in the top of fuel assembly (pin cluster).
Within the section between FA inlet and the starting place of surface boiling, coolant density
is only dependent on the pressure (axially drops) and the temperature (axially grows in the
heated channel). Coolant density gradually reduces almost in linear manner. After the curve of
coolant density passes the starting point of surface boiling, it will be dependent not only
pressure or enthalpy, but also the fraction of non-equilibrium steam. The rate of steam
formation depends on cladding temperature. In conditions of liquid temperature, which does
not reach the point of saturation, steam bubbles that appear as a result of surface boiling will
collapse in the coolant flow. The bubble lifetime much depends on coolant temperature and
pressure. These effects influence the steam volume fraction, and, consequently, the coolant
density.
Moreover, the fluctuations of coolant parameters and, further, the distortions in axial
profile of power density lead to an axial drift of start and end points of steam formation.
All these factors result in the growth of non-linear fluctuation of coolant density,
when surface steam formation starts. The amplitude of coolant density fluctuations, which is
caused by coolant parameter fluctuations, will dramatically increase with steam formation.
This effect leads to a significant sensitivity enhancement of neutron flux fluctuations
registered by SPND, to fluctuations of coolant parameters, and, as a consequence, notable
growth of Rms-Spnd-Norm values.
The case may be proved by simulation results of coolant parameter fluctuations at
the core inlet in the conditions either free from steam formation, or with surface steam
formation.
For the analysis of Rms-Spnd-Norm behaviour due to coolant parameter fluctuations
during steam formation, we simulated the following process by NOSTRA code.
We simulated a coolant temperature fluctuation of 1°С amplitude at the inlet to FA
of maximum power density for the conditions of nominal reactor power and dramatically low
pressure (Р = 13 MPa). We reduced pressure to initiate the process of steam formation. Steam
formation starts in the middle of FA, while the value of steam quality (~10%) is reached in
the location of detector 7.
1.4E-03
12.0%
1.2E-03
10.0%
8.0%
8.0E-04
L1
6.0%
6.0E-04
φ
Rms-Spnd-Horm
1.0E-03
L2
φ
4.0%
4.0E-04
2.0%
2.0E-04
0.0E+00
0.0%
1
2
3
4
5
6
7
number of SPND along the FA height
φ – steam quality in the location of SPN detectors, with pressure P=13 MPa
L1 - Rms-Spnd-Norm with steam formation (P=13 MPa);
L2 - Rms-Spnd-Norm without steam formation (P=15.9 MPa);
Figure 10 - Rms-Spnd-Norm in the location of SPND with inlet 1°С temperature
fluctuation and frequency 0.3 Hz for two pressure cases at reactor inlet: 13 MPa (with steam
formation) and 15.9 MPa. (without steam formation)
The analysis of graphs in Figure 10 allows drawing the following conclusion. The
steam formation in the top of FA results in the following:
- converts the axial profile of amplitudes from downstream to upstream along FA
height;
- Rms-Spnd-Norm dramatically grows (in our application, by 5 times) in the
location of SPN detectors at the top of fuel assembly;
With P=13 MPa, the drift of starting point of Rms-Spnd-Norm growth from the
point of appeared steam quality to the location of SPND 3 is caused by the propagation of
neutron flux perturbation. In order to specify the location of steam formation along FA height,
such drift may be adjusted by means of calculation.
Thus, it is possible to state, that during steam formation, the sensitivity of power
fluctuation amplitude towards the fluctuations of coolant parameters significantly increases,
which results in significant changes in Rms-Spnd-Norm distribution along FA height.
Meanwhile, the changes in Rms-Spnd-Norm distribution along FA height makes it possible to
identify steam formation by SPN detectors
Some ICND operation data from various power units
Figure 11 gives the curve of averaged spectral power density for every DPND signal
3.5E-21
3E-21
Amp2
2.5E-21
2E-21
1.5E-21
1E-21
5E-22
0
0.01
0.10
1.00
10.00
100.00
Frequency (Hz)
Figure 11. Averaged spectral power density for every DPND signal. Nominal RU power
of Unit 3, Kalinin NPP (before application of digit filtration within the band of 0.1 – 3 Hz)
It is visible in the graph from Figure 11, that the main weight of spectral power
density of noise component of SPND signal is located within the range up to 3Hz. The drop
pattern well agrees with the graphs from Figures 5 and 6. The rise at 0.37 Hz is caused by fuel
pin resonance properties of this range of frequencies. Figures 5 and 6 (calculated data)
demonstrate a rise of Rms-Spnd-Norm sensitivity towards coolant fluctuations in this range of
frequencies. The peak at the point of 16 Hz is caused by vibrations and pulsations of coolant
density due to rotation of MCP pins. It must be pointed out that the view of each SPND
spectral power density is similar to the view of graph of averaged spectral power.
Figure 12 presents the selected graphs of Rms-Spnd-Norm distribution along the
heights of various FAs (SPND Nos.).
8.0E-05
Rms-Spnd-Norm
7.0E-05
6.0E-05
5.0E-05
4.0E-05
3.0E-05
2.0E-05
1.0E-05
0.0E+00
1
2
3
4
5
6
7
Spnd number
Figure 12. Selected graphs of Rms-Spnd-Norm distribution along the heights of various
FAs (SPND Nos.) Nominal reactor power, Unit 3, Kalinin NPP.
The values of Rms-Spnd-Norm from Figure 12 lie within the anticipated limits of
normal reactor operation and well agree with the calculated data from Figures 7 through 9
valid for small values of coolant fluctuations.
As reflected by Figure 12, Rms-Spnd-Norm values monotonically grow along FA
height. The above calculations allow assuming that the fluctuations of SPND signals are
mainly caused by flow and pressure fluctuations. It has been found at various reactors
(Kalinin NPP Unit 3, Tianwan NPP Units 1 and 2) and under various operation conditions
that the distribution of Rms-Spnd-Norm values along FA height is close to a straight line,
while the coefficient of Rms-Spnd-Norm increment between the top and the bottom SPND
ranges within 0.9 – 2, however, the typical increment makes 1.4. The spread of increment
coefficients is rather small within the period of signal retrieval from all SPND strings (~30
minutes).
Figures 13 and 14 show the core distribution of Rms-Spnd-Norm values from the
lower SPN detectors installed in the monitored FAs.
8.0E-05
7.5E-05
Rms-Spnd-Norm
7.0E-05
6.5E-05
6.0E-05
5.5E-05
5.0E-05
4.5E-05
4.0E-05
1
21
41
61
81
101
121
141
161
FA number
Figure 13. Rms-Spnd-Norm values from bottom SPN detectors installed in the
monitored FAs. Nominal reactor power, Unit 3 Kalinin NPP
3E-05
4E-05
5E-05
6E-05
7E-05
8E-05
Figure 14. Core distribution of Rms-Spnd-Norm values from bottom SPN detectors
installed in the monitored FAs. Nominal reactor power, Unit 3 Kalinin NPP
Figure 13 shows that the spread of Rms-Spnd-Norm values from bottom SPNDs
amounts to two. Such spread is stipulated by different pattern of fluctuations at the inlets of
various FAs. Figure 14 shows that at the inlet to central FAs the amplitude of coolant
fluctuations is lower. The calculated data allow assuming that the difference in fluctuation
amplitudes of coolant parameters, as compared against the average value of coolant parameter
makes approximately 0.05%. In abnormal reactor conditions (disabled MCP) the spread may
increase by the order of 10.
Methods of coolant boiling diagnostics adopted in ICND system
The methods of boiling diagnostics are based on the change evaluation of relative
neutron flux fluctuation sensitivity along FA height towards the fluctuation of coolant
parameters at FA inlet.
Coolant fluctuations at FA inlet are considered to be basic fluctuations for the
selected diagnostic methods. The monitoring of relative sensitivity of neutron flux
fluctuations (Rms-Spnd-Norm) along the core height towards the basic fluctuations allows
detecting the process of steam formation.
The basic fluctuation value for the analysed FA (Base-Rms-Spnd-Norm) is
determined by Rms-Spnd-Norm values of the string lower SPNDs, where coolant boiling is
improbable. A limitation for Base-Rms-Spnd-Norm must be introduced in order to avoid a
diagnostic error, should any anomalies in FA bottom appear (such as boiling, whence the inlet
coolant flow is shaded by debris). For this purpose, using the calibration on the basis of
Chauvenet's criteria, the average value of Rms-Spnd-Norm shall be calculated, and also the
mean square deviation of Rms-Spnd-Norm values of lower SPNDs in all monitored FAs. On
the basis of average deviation and mean square deviation the limitation of Base-Rms-SpndNorm values shall be calculated.
In various reactor operation conditions the axial increment of Rms-Spnd-Norm
differs due to fluctuation relationship of coolant flow, temperature and pressure. According to
the above mentioned, the axial distribution of Rms-Spnd-Norm values is close to a straight
line, while the coefficient of Rms-Spnd-Norm increment of the upper SPND to the lower
SPND for the option without steam formation ranges within 0.9 – 2. In order to account this
magnitude, it is required to calculate a universal increment coefficient (elevation correction)
on the basis of the data from all FAs and using the calibration by Shovene criteria. In order to
avoid an error in boiling diagnostics a limitation for the increment coefficient shall be adopted
for any extraordinary cases (boiling in all FAs).
In every iteration of measurement, limitation values of Rms-Spnd-Norm shall be
determined for each SPND using Base-Rms-Spnd-Norm values, increment coefficient and
coefficient of Base-Rms-Spnd-Norm increment during steam formation. Such limitation will
define a permissible steam quality in the coolant as about 5 %.
Figure 15 demonstrates the above mentioned methods of coolant boiling diagnostics.
Rms-Spnd-Norm
2.5E-04
FA 1
FA 2
2.0E-04
Permissible limits for each Rms-SpndNorm determining steam formation
1.5E-04
Exceeding of limits determining steam
formation
Values Rms-Spnd-Norm
1.0E-04
Values Base-Rms-Spnd-Norm with
adjusted increment coefficient by the
assembly height FA
5.0E-05
Values of basic fluctuations at
assembly input (Base-Rms-Spnd-Norm)
0.0E+00
0
1
2
3
4
Spnd number
5
6
7
Limitation on value Base-Rms-SpndNorm
Figure 15. Demonstration of methods of coolant boiling diagnostics. The excursion of
steam formation limits is set conventionally.
Fuel pin thermal physical performances facilitate the decrease of sensitivity towards
the pulsations of flow and temperature, when perturbation frequency grows. The drop of
spectral power density due to the increased frequency of noise signal is most of all dependent
on thermal fuel pin inertia. The contribution of high-frequency pressure pulsations is typically
much lower the contribution of any other pulsation. Without steam formation processes the
useful signal of higher than 5 Hz frequency is comparable with background noises of
instrumentation equipment. The range of 0.1 – 3 Hz is selected as the principal range for
diagnostics of coolant boiling.
When steam formation starts (as per experimental data), the fluctuations
dramatically grow after 5 Hz, however, in commercial ICND systems, where a great number
of SPND signals must be analysed, (over 300), it is rather difficult to ensure a true signal. The
range over 5 Hz is used as a redundant range.
The algorithms of commercial ICND system are based on various methods of true
signal detection. One of the principal methods is based on the analysis of correlation
coefficients of axial SPND signals. The typical correlation coefficients exceed 0.8. The
evaluation of correlation coefficients of SPND string signals allows calibrating the signals and
adopting a correction coefficient to account the contribution of uncontrollable (background)
noise to Rms-Spnd-Norm value.
List of Nomenclature
NPP – nuclear power plant;
Fuel pin
FA – fuel assembly
MCP – main circulation pump
MCDS - reactor monitoring, control and diagnostics system
ICND – in-core noise diagnostics
SPND - self-powered neutron detector
SPND string – 7 neutron detectors established in one channel (tube) at certain axial
FA positions
AFR - amplitude and frequency response;
Rms-Spnd-Norm – mean square deviation of variable component of SPND current
normalised by averaged SPND current;
References
1.
Computer code NOSTRA (Version 5.0). Software Certification. Registered No. 167 of
23.12.2003. Russian GAN, Moscow, 2003.