Corresponding author: Peter M. Kalinichev ( kalinichev@triniti.ru ) Academic editor: Yury Korovin
© 2018 Peter M. Kalinichev, Igor A. Evdokimov, Vladimir V. Likhanskii.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Kalinichev PM, Evdokimov IA, Likhanskii VV (2018) A technique for detection of WWER fuel failures by activity of Xe radionuclides during reactor operation. Nuclear Energy and Technology 4(4): 263-270. https://doi.org/10.3897/nucet.4.31889
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Fuel failures during operation of Nuclear Power Plants (NPPs) may lead to substantial economic losses. Negative effects of reactor operation with leaking fuel in the core may be reduced if fuel failures are detected in due time of the cycle.
At present time, the ratio of the normalized release rates of 131I and 134I is used to detect fuel failures in WWERs during steady state operation. However, based on the activity of iodine radionuclides, it is not always possible to detect the fuel failure. This situation may occur in case of a small defect in cladding of a leaking fuel rod or for high burnup fuel if the defect is overlapped by the surface of the fuel pellet. If it is so, fuel deposits may be the dominant contributor to iodine activity, and the fuel failure may not be noticeable.
In PWRs, fuel failures are detected by activity of radioactive noble gases. Noble gases are not adsorbed on cladding inner surface, as distinct from iodine radionuclides. Release of noble gases from the leaking fuel rod may be considerable even though defect in cladding is small.
In this paper, a technique is proposed for detection of fuel failures at WWER reactors by activity of radioactive noble gases in the primary coolant. It is shown that radioactive noble gases may be a more sensitive indicator of fuel failures than iodine radionuclides. Detection of fuel failures is based on monitoring of the ratio between 133Xe and 135Xe activity. Some examples of practical applications are given.
WWER, fuel rod, fuel failure, fission products, technique, coolant activity, iodine radionuclides, radioactive noble gases
Fuel failures may occur during operation of Nuclear Power Plants (NPPs). This may lead to substantial financial losses. Monitoring and analysis of primary coolant activity is used to ensure the radiation safety of power units and to mitigate the adverse effects of fuel failures (
According to the regulatory document (RD) (
Activities of radioactive noble gases (NGs) are used for more reliable detection of fuel failures in PWR reactors. Noble gases do not interact with the fuel cladding (unlike surfactant iodine radionuclides). If much of the iodine is adsorbed on the inner surface of the fuel cladding, the release of the iodine radionuclides into the coolant, with all other things being equal, turns out to be smaller than the release of noble fission gases. The release of noble gases from a leaking fuel rod is detectable even when its hydraulic resistance is high.
In France, the fulfillment of one of the following conditions is taken as the criterion of the fuel failure during steady-state operation of PWR reactors: 1) the absolute value of the 133Xe activity is higher than 106 Bq/kg, or 2) the ratio of the 133Xe and 135Xe activities is higher than 0.9 with the absolute activity of 133Xe being in excess of 1.85×105 Bq/kg (
The criterion given in (
Below, a technique is proposed which makes it possible to detect fuel failures by NG activities during operation of WWER power units. The advantages of this technique are discussed against the criterion in (
The criterion of fuel failure detection using iodine radionuclides under steady state operation conditions (see RD (
A * = A (λ + η)/(λY), (1)
where A is the measured activity; λ is the nuclide decay constant; η is the rate of the radionuclide removal from the coolant by the letdown filters; Y is the cumulative radionuclide yield per fission which is composed of the intrinsic yield y and the cumulative yield of the precursor nuclide Yp.
According to RD (
The key concepts which make it possible to obtain the criterion of type (
Behavior of iodine radionuclides in the primary circuit. Balance equations (
Here, R and R are the rate of the FP release into the coolant from fuel deposits and leaking fuel rod, respectively; NI is the number of the iodine radionuclide atoms in the primary coolant, and NTe is the number of tellurium atoms in the primary circuit. Tellurium is more superficially active than iodine, so it is assumed in equations (2), (3) that tellurium is adsorbed on the structural surfaces in the reactor core and is not removed from the primary circuit as the coolant flows through the letdown filters.
The presence of the tellurium precursor is important to take into account for 132I and 134I. It can be taken for 131I, 133I, and 135I that the precursor decays practically instantaneously.
If the reactor operates in a steady-state mode, steady-state solution of equations (2) – (3) can be used. In these conditions, the normalized release rate (1) defines the intensity of the FP source in the coolant. For each iodine radionuclide, it is equal to the release rate into the coolant divided by the probability of its formation (cumulative yield) per one fission of heavy nucleus.
Release of iodine radionuclides from fuel deposits. With no fuel failures in the reactor, the coolant activity is defined by the FP release from fuel deposits. Fuel deposits are uranium dust that builds up on the outer cladding surface during fabrication and/or is accumulated inside fuel assemblies when fuel is washed out from leaking fuel rods during operation at NPPs.
One of the key mechanisms for the nuclide release from fuel deposits is the direct escape of fission fragments into the coolant. Another mechanism occurs if the fuel deposits include particles with a size larger than the fission fragment range in UO2. In this case, fission fragments can stick in fuel and escape further into the coolant by the radiation transport mechanism. For the reference iodine radionuclides, this mechanism is reduced to “radiation-induced” diffusion (
R = Rr + RD. (4)
Particles of fuel deposits may have a developed surface (this is possible, e.g., for pieces of high burnup fuel pellets that have entered the coolant) (
With regard for the results in (
R = YJf (h), (5)
J = erf SgF, (6)
where rf is the fission fragment range in fuel; F is the fission rate in fuel deposits in the core; e is the release efficiency; and Sg is the “geometrical” area of the fuel surface (with no regard for open pores). The value e is defined by the geometry and lies in a range of 0 < e £ 0.5 (
f (h) = 1 + k1StDc/(SgRc) + k2Sth/(eSgrf). (7)
The second term in the right-hand side of (7) describes the increase in the release rate due to the fission fragment “sticking” in open pores. Here, St is the total area of the exposed fuel surface; dс is the mean pore diameter in fuel deposits; rс is the fission fragment range in the pore-filling fluid; and k1 is the coefficient allowing for the pore geometry. For a spherical pore with k1 = 2/3, the value rс depends on what is filling the pore. If the pore is filled with water with a pressure of 160 atm and a temperature of about 600 K, the fission fragment range is about 3·10–5 m. If the pore is filled with gas, the fission fragment range will be larger.
The closing term in the right-hand side of (7) describes the diffusive release from fuel deposits. Here, k2 is the coefficient of the order of unity taking into account the geometry of the fuel particles; h = (D/l)1/2 is the diffusion length; and D is the diffusion coefficient. According to (
The normalized release rates of 131I, 133I, and 135I can be represented as
A * = R/Y = Jf (h). (8)
In the case of “even” iodine radionuclides, the precursor lifetime cannot be ignored and the release of 132Te and 134Te needs to be considered separately from the iodine release. The release of tellurium is described by expression (5). For 132I and 134I, the intrinsic yield y is used instead of the cumulative yield Y to describe the direct escape, and the diffusion release rate has the form
RD = k2StFh (y + Yph/(h + hp)). (9)
In this case, the function g (h, hp), similar to the function f (h), may be introduced, and the rate of the radionuclide release into the coolant can be written as
R = yJg (h, hp), (10)
g (h, hp) = f (h) + k2hStYph/(erfSgy (h + hp)). (11)
The normalized iodine release rate, with regard for (2), (3), and (10), has the form
A *=(R+Rp)/(y+Yp) = Jf (h)+Ypk2Sthp2/(YeSgrf (h+hp)). (12)
In the event of a “short-lived” precursor (hp >> 0), equation (12) goes over to (8). If the diffusion length hp cannot be neglected, as compared with h, then, as can be seen from (12), the contribution of the precursor nuclide can increase the normalized release rate markedly.
If fuel deposits consist of particles smaller than the fission fragment range, the radionuclide release can be considered in a “monolayer” approximation (
If fuel deposits consist of large-size particles, the ratios of the normalized release rates (12) due to the diffusive release may deviate markedly from unity. RD (
An analog of the dependence from RD (
Analysis of iodine normalized release rates and its constraints. If there is a fuel failure in the reactor, the source of each radionuclide Rc can be written as
Rc = R + R = R + Rf μ/(λ + μ), (13)
where Rf is the FP release rate from fuel pellets to voids inside the fuel rod; and µ is the escape rate coefficient for the given radionuclide from fuel rod into the coolant. The escape rate coefficient relates the rate of the radionuclide release into the coolant to the total inventory of this radionuclide inside the fuel rod (
When plugging expression (13) into equations (2), (3), and (1), one can see that a fuel failure may cause the ratio of the normalized 131I and 134I activities to increase markedly. This circumstance is used to detect the fuel failure in the reactor.
According to RD (
Differences in xenon and iodine behavior. It follows from equation (13) that the contribution of the leaking fuel to the cumulative FP source in the coolant is defined by the ratio between the decay constant λ and the escape rate coefficient μ. The 133Xe and 131I decay constants are close (Tasaka Kanj et al. 1990). If chemical properties of xenon and iodine were similar, a marked deviation of the 133Xe and 131I activity from the background during fuel failure would be observed simultaneously. However, due to iodine adsorption on the cladding inner surface, in a certain range of the failed fuel characteristics it is possible that 131I activity is still defined by its release from fuel deposits but 133Xe activity already exceeds the background level significantly. The width of this range is the larger the more intensively iodine is adsorbed inside the leaking fuel rod (the temperature near the defect is assumed to be higher than the water saturation temperature; if the defect is in the fuel’s “cold” zone and is covered with water, iodine can dissolve in it and be carried out of the failed fuel rather intensively). Thanks to such peculiarities, the NG activities can be a more sensitive failure indicator than the iodine activity.
For efficient detection of fuel failures based on the ratio of the NG activities, it is desired to use a pair of gaseous FPs with the following properties.
The pair of 133Xe and 135Xe satisfies to these conditions among the NGs whose activities are monitored at NPPs.
Release of xenon radionuclides from fuel deposits. To describe the activities of 133Xe and 135Xe in the primary circuit, it is required to take into account the behavior of the precursor iodines, 133I and 135I respectively. The balance equation for the xenon radionuclides in the primary circuit has the following form
Designations as in equation (2) are used here; the coefficient B allows for xenon which enters the core due to decay of precursor iodine in the letdown filters. It is assumed that the ion-exchange filters and the degasser are connected in series in the letdown system, and the coolant flow rate through these is identical. The cleaning rate ηXe can be written in the form
ηXe = Qα/M, (15)
where Q is the mass flow rate of water to the ion-exchange filters and the degasser; M is the mass of the water circulating in the primary circuit; and α is the degassing efficiency assuming the values of zero to unity. The coefficient B relates to the degassing efficiency a as B = 1 – α.
The behavior of the precursor iodine in (14) is described by equation (2). If the reactor operates in a steady-state mode, a steady-state solution can be used for equation (14).
For the xenon radionuclides, as well as for iodines, the rate of release from fuel deposits can be described using the functions f and g (see above). It follows from equation (14) that the ratio between the 133Xe and 135Xe activities in the reactor, when there is no fuel failure, has the form
The right-hand side of equation (16) may be dealt with as a family of curves G (ηXe) depending only on the xenon removal rate. The largest value of the function G, with the fixed value ηXe, is reached if the “diffusion” terms in the functions f and g are the predominant contributors. The parameters, with which this condition is fulfilled, are described above. The solid line in Fig.
The solid line in Fig.
Determination of the NG removal rate. The absolute values and the ratios of the gaseous FP activities depend on the degassing rate.
No measurement of degassing parameters is stipulated by the actual WWER regulations. The degassing parameters can be determined by measuring the NG activities downstream and upstream of the degasser. In this case, the degassing efficiency a in (15) is calculated from the formula
α = A2Xe/A1Xe, (17)
where A1Xe, A2Xe are the activities of xenon downstream and upstream of the degasser respectively.
The NG activities downstream and upstream of the degasser are measured, e.g. at the Temelin NPP (twice a month).
A technique to detect fuel failures based on the xenon radionuclide activity. Fig.
The fuel failure detection procedures can be formulated as follows.
Constraints. The ratio between the 133Xe and 135Xe activities after the fuel failure depends non-monotonically on the escape rate coefficient m for the leaking fuel rod. After the fuel failure, this ratio starts to grow if m increases. The growth continues for as long as the relatively long-lived 133Xe releases predominantly from the fuel, and the 135Xe activity is still defined by the background. As the extent of damage to fuel cladding progresses, the time of the radionuclide “delay” inside the leaking rod prior to the release into the coolant starts to decrease. When 135Xe starts to go out of the leaking fuel in a noticeable quantity, the ratio between the 133Xe and 135Xe activities starts to diminish.
In the event of a severe failure (e.g., when secondary defects are formed in the failed fuel cladding), the time of “delay” may become smaller than the lifetime of 135Xe. In the limiting case of severe cladding degradation, the 133Xe and 135Xe release from the leaking fuel rod will be such as if there is no cladding at all and there is a direct contact of fuel pellets with the coolant. If it is so, there will be no difference between the FP release from the leaking fuel with a low power and the FP release from a large mass of fuel deposits. For high power leaking fuel, when diffusion is a major contributor to the FP release from fuel pellets, the ratio between the 133Xe and 135Xe activities in the coolant may also fall within the “uncertainty zone” shown in Fig.
Table
Table
Average ratios of the 133Xe and 135Xe activities in different WWER-1000 power units for cycles with no fuel failures
Unit No. – Cycle No. | A-1 | B-1 | C-1 | D-1 |
A 133Xe/A135Xe | 0.08 | 0.1 | 0.11 | 0.07 |
Maximum weekly average ratios of the 133Xe and 135Xe activities in different WWER-1000 power units during steady-state reactor operation in cycles with fuel failures
Unit No. – Cycle No. | C-2 | E-1 | F-1 | G-1 |
A 133Xe/A135Xe | 8 | 10 | 3.7 | 6.5 |
One of the examples to illustrate higher sensitivity of the proposed procedure is cycle N-1. Fig.
a). Ratio of the 133Xe and 135Xe activities (the vertical axis on the left) during steady-state reactor operation in the course of cycle N-1. b). Threshold value corresponding to the fuel failure criterion for NGs in the zero degassing rate approximation (the vertical axis on the left) and for the iodine radionuclides (the vertical axis on the right). c). Ratio of the normalized release rates for 131I and 134I
It can be seen from Fig.
One leaking FA was found after cycle N-1 was over. Therefore, the analysis of the NG activities made it possible to detect the fuel failure promptly.
The paper proposes the technique to detect fuel failures in WWER units based on the activity of radioactive noble gases in the reactor primary circuit. It has been shown that the activity of noble gases can be a more reliable indicator of a fuel failure than the activities of iodine radionuclides. The proposed fuel failure detection criterion is based on analysis of the ratio between the 133Xe and 135Xe activities.
The procedure has a low sensitivity to the primary coolant sampling technique and to the extent of the water degassing during sampling, as well as to the actual nuclide composition of fuel deposits.
Examples of several fuel cycles show that, when there is no fuel failure in a WWER-1000 NPP, the ratio of the 133Xe and 135Xe activities is below the boundary as it follows from the obtained criterion. If there is a fuel failure, the ratio of the 133Xe and 135Xe activities turns out to be above the criterion boundary in most cases.
It has been also demonstrated that the proposed criterion is capable to detect fuel failures during steady-state reactor operation even if the 131I and 134I activities do not make it possible to make a firm conclusion on the presence or absence of leaking fuel in the core.
The authors express their gratitude to Oleg Vladimirovich Khoruzhy for the valuable discussion of the findings.