Leaker diagnostics

Estimating the occurrence of a failure

The onset of a fuel element failure is usually detected by monitoring the gamma activity level associated with specific radioactive fission products in the reactor coolant or off-gas.

Fission product data (in reactor coolant or off-gas) measured at regular intervals and trends throughout the operating cycle allow for a determination of failure times, and an assessment of further operational behaviour.

When a cladding breach occurs along a fuel rod during normal reactor operation, coolant can enter into the fuel to cladding gap and fission products (i.e., notably the volatile species of noble gas and iodine) will be released into the primary, causing a sudden increase in activity levels. With the entry of high pressure coolant through the defect, fuel may be oxidized, which can potentially enhance fission product release.Iodine release can also occur upon reactor shutdown, when the temperature in the fuel to cladding gap drops below saturation, permitting liquid water to dissolve the soluble iodine species in the gap and resulting in an ‘iodine-spiking’ phenomenon.Iodine rich water remaining in the gap can also be released upon subsequent startup, since the size of the gap is reduced due to fuel expansion.

Any significant change in fission product activity levels should be analysed to determine if it is due to a new fuel failure or an expansion of previously known fuel failures, or whether it can be attributed to other factors such as changes in reactor power, the letdown flow rate or the degassing procedure.

The best early failure indication is an increase in ¹³³Xe activity. Any significant increase above the tramp level or any permanent increase in the steady level should be viewed as a potential failure. The interpretation of corresponding changes in iodine activity is less reliable because small, tight defects cannot induce a measurable change in the steady state level of iodine concentrations. Alternatively, the absence of iodine spiking following a power transient is a reliable indicator of a defect free plant. Iodine spiking following a rapid power change indicates that the core contains one or more leaking fuel rods.

Moreover, the INPO/WANO fuel reliability indicator (FRI) provides an estimate of fuel reliability based on fission product activity present in reactor coolant. This indicator includes an approximate correction for tramp uranium activity and can be used to establish a threshold value below which a unit has a high probability of operating without defects.

Monthly indicator value is calculated using the following equation [4.5]:

FRI = [(A₁₃₁)_N-k(A₁₃₄)n*[(L_N/LHGR)*(100/P₀)]^1.5 where:

(A₁₃₁)_N is the average steady state activity of ¹³¹I in the coolant, normalized to a purification constant of 2 E-5 s^–1; k is the tramp correction coefficient (a constant of 0.0318 based on a tramp material composition of 30%

uranium and 70% plutonium);

(A₁₃₄)_N is the average steady state activity of ¹³⁴I in the coolant normalized to a purification constant of 2 E-5 s^–1; L_N is the linear heat generation rate used for normalization (5.5kW/ft);

LHGR is the actual average linear heat generation rate at 100% power (kW/ft);

P₀ is average reactor power in percent at the time activities were measured.

Another useful tool for the analysis of coolant activity during steady state operation is a plot of the release to birth rate ratio (R/B) of measured gas and iodine isotopes against their decay constant. This plot is simply generated by measuring the coolant activity for each isotope and dividing this by its fission yield fraction to provide an effective R/B. Recoil is the dominant mechanism of fission product release from tramp uranium. When fission occurs in tramp materials, fission product nuclides are released directly into the coolant with no waiting time for decay. Thus, the R/B for fission product nuclides from tramp materials in the first approximation would be independent of nuclide half-life. Plants free of defect will exhibit horizontal slopes in R/B plots versus decay constants  as shown in Fig. 4.2(a).

Alternatively, as soon as a defect appears, the slopes in these plots become strongly negative, as shown in Fig. 4.2(b). The slopes of the plots are heavily dependent on the operating power of the failed rods and the defect sizes. Also, a comparison of the R/B of iodines and gases provides an indication of the portion of iodines ‘trapped’

in the fuel–cladding gap.

In addition to R/B plots, different isotopic activity ratios provide an indication of the presence of failure. The presence of failed rods in the core modifies the distribution of long lived versus short lived isotopes in the primary coolant. In the absence of fuel failure, short lived and long lived isotopes are released into the primary coolant from tramp uranium without delay. When a fuel defect is present, the release of isotopes into the primary coolant is delayed by their diffusion in the fuel pellets, the pellet–cladding gap then through the defect. This delay results in a

FIG. 4.2. Typical curves of R/B versus l: (a) core with tramp uranium only; (b) core with defects.

ratios utilized as indicators of fuel failure are: ¹³³Xe to ¹³⁵Xe, ¹³³Xe to ¹³⁸Xe or ^85mKr to ⁸⁷Kr. A significant change in the value of such ratios is a clear indication of fuel failure.

Estimating the release of fission products from tramp uranium versus defective fuel: Fuel degradation monitoring The ability to distinguish between releases from tramp uranium and defective fuel is essential in assessing the in-reactor performance of fuel. For example, an increase in the release of fission products from both tramp and defective fuel elements may indicate the appearance of a large open defect releasing fuel to the coolant. Similarly, a constant tramp release accompanied by an increase in the release of the longest lived soluble and gaseous fission products would indicate growth in the number of small defects.

Experience has shown that the yield corrected release of a short lived isotope such as ¹³⁴I or ¹³⁸Xe is a good indicator of tramp uranium buildup in the core. Conversely, the release of the longest lived isotopes is mainly due to fuel failure, and uranium contamination effects can be ignored for these isotopes if no large open failures are present in the core.

Thus, degradation or uranium release can be monitored by observing the trends of short lived fission product gamma activities, such as those for ¹³⁴I or ¹³⁸Xe. A slowly increasing trend at constant reactor power is an indication of uranium release and deposition within core boundaries, whereas a stepwise increase generally indicates a sudden increase in hole size.

As indicated in Ref. [4.6], the measurement of fission products such as ⁹¹Sr and ⁹²Sr, or fuel activation product

239Np can also provide a good indication of fuel dissemination into reactor coolant. However, ²³⁹Np cannot be used for fuel degradation assessment in a BWR reactor applying hydrogen water chemistry. At reducing conditions, transuranic nuclides will form an insoluble complex and consequently, its concentration would be largely underestimated.

In addition, the presence of transuranic isotopes (actinides) in the primary coolant is an indication of the presence of fissile materials in the primary coolant due to erosion of fuel pellets through large defects. The most significant actinides are:

— Neptunium ²³⁹Np;

— Plutonium ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu;

— Americium ²⁴¹Am;

— Curium ²⁴²Cm,²⁴³Cm, ²⁴⁴Cm.

In CANDU-6 reactors, a delayed neutron (DN) system measures delayed neutron activity in the coolant of each channel. This activity comes from short lived fission products ¹³⁷I and ⁸⁷Br, whose activity concentrations are also proportional to tramp uranium levels in the core. When the average DN signal begins to increase, uranium release to the coolant is occurring. When this happens, the suspect channel — identified by having the highest DN signal count rate — is placed at the top of the refuelling list [4.7].

Identification of the fuel failure mechanism

Continuous activity measurements can also provide early information on the fuel failure mechanism. Indeed, some of these failure mechanisms have somewhat typical activity release characteristics which can be utilized in estimating the types of failure present in the core. Some examples of the correlation between fission product escape and fuel rod defect type are provided in Ref. [4.8].

Table 4.1 of Ref. [4.9] lists the general activity characteristics for various fuel failure mechanisms. This table has been compiled from industry experience and is intended to demonstrate a typical situation; actual numbers may vary depending on individual conditions.

TABLE 4.1. TYPICAL CHARACTERISTICS OF VARIOUS FUEL FAILURE MECHANISMS [4.9]

Handling damage 0–60 days Similar to grid fretting Not experienced

End caps Any time Progressive increase Low 0.6–0.8 Low >1.0

Pellet–cladding

a Typical relative level of noble gases measured in off-gas system, per failed fuel rod. Low means sum of six gases less than 1000 mCi/s per leaking fuel rod; medium means 1000 to 5000 mCi/s per rod; high means over 5000 mCi/s per rod (1 Ci = 3.7 × 10¹⁰ Bq.).

b Refers to typical range of slopes for linear fit to log (release to birth) versus log (decay constant) for the six gases. Isotopic release should be corrected for pre-existing tramp fission gas sources.

c Typical ¹³¹I activity in coolant per failed fuel rod. Low means less than 3 × 10^–3 mCi/g per leaking fuel rod; medium means 3 × 10^–3 to 5 × 10^–2 mCi/g per rod; high means over 5 × 10^–2 mCi/g per rod (1 Ci = 3.7 × 10¹⁰ Bq).