Localization of fuel failures

Sipping is the most common technique used to locate fuel failures in both PWRs and BWRs. Identification of fuel rod failure is based on the detection of fission product activity released through defects during sipping. The more common radioisotopes measured are xenon and krypton, and caesium or iodine in water samples.

Various versions of sipping have been used to detect leaking fuel assemblies, depending on:

— The details of configuration and system;

— The physical phenomenon used to promote fission product release, i.e. application of vacuum, heat, elevation (p).

These different techniques are known as vacuum sipping, wet sipping and in-mast or telescope sipping, depending on the physical phenomenon and configuration used.

FIG. 4.3. Typical ¹³⁴Cs/¹³⁷Cs activity ratio versus burnup.

Vacuum/wet sipping

Wet sipping uses the increase in internal pressure of failed rods due to heating of water to expel fission products. Vacuum sipping uses the decrease in external pressure created by a pumping station to expel fission products.

Following these techniques, a fuel assembly is placed either in a sealed container in the spent fuel pool or below a sipping hood for BWR assemblies which have a flow channel. In this last case, sipping can be performed in the spent fuel pool while assemblies are still in the core. The sipping hood is placed over several fuel assemblies and filled with air to isolate the top of each assembly. The air’s presence restricts coolant flow to the assemblies and causes a temperature increase which expels fission products. Each assembly is tested individually. In PWRs and WWERs, wet sipping is performed in a cell located in the spent fuel pool. In-core sipping equipment has also been developed for the WWER-440 Paks power plant (Hungary), and the Loviisa plant (Finland).

For sophisticated installations, the assembly is placed in an enclosed canister and activity concentrations in sampling water and cover gas circuits are monitored on-line. A gamma detector is used, or in the case of gas circuits, a beta-type scintillation detector. Water samples are collected for further evaluation and precise determinations of activity concentrations are made with a Ge detector.

Descriptions of typical sipping equipment are given in Refs. [4.30–4.32]. An example is shown in Fig. 4.4.

Each method has its advantages and disadvantages. However, vacuum sipping — due to its accuracy — was the most frequently used technique in BWRs.

In-cell sipping is the most reliable testing method (more than 99% sure). However, flushing of the equipment is extremely important to avoid cross-contamination between assemblies, and the method is still time consuming.

In-mast or telescope sipping

In-mast sipping has been widely developed and used in French PWRs, as shown in Fig. 4.5 [4.33]. The system, installed on the refuelling machine in the reactor building, is designed to identify irradiated leaking fuel assemblies during core unloading or shuffling operations.

FIG. 4.4. Sipping equipment for a fuel storage pool [4.32].

The on-line system uses a gas sipping method which provides inherently greater sensitivity than conventional water sipping, and is unaffected by reactor pool water contamination. When the core is unloaded or shuffled, each assembly is raised from the core to an upper position inside the machine mast, and the differential pressure caused by the change in elevation promotes the release of fission products from the defective rods. An air stream is continuously injected under the assembly and entrains the gaseous products. This steam is collected above the pool level and routed to a gamma activity measurement unit, where activity is permanently recorded. In order to provide the best signal/background ratio, counting is performed during the gamma peak of the ¹³³Xe isotope for two minutes with the fuel assembly in the upper position within the mast before further movement of the refuelling machine.

From French operating experience, in-mast sipping provides a highly reliable and easy to operate technique for identification of leaking fuel assemblies. Feedback based on experience shows that a failed rod which exhibited a low release rate of fission product activity during the operating cycle, or rods with significant damage, are clearly detected using this technique. Feedback has also shown that more than half of the under calls made were due to poor operation of this device. To remedy this problem, a functional requalification of the device is now being undertaken by the sampling injection of active gas into the sky of the mast.

Similar devices have been developed to detect defective fuel leaks in BWRs [4.34] and now in PWRs. During transportation, water is sucked from the fuel assembly into a hose mounted along or inside the telescope mast. The water is led to degassing equipment placed on the fuel handling machine or on the service room floor. Fission gas is separated from the water and measured on-line with a beta sensitive detector. Soluble fission products in the water can be measured either on-line or in a plant laboratory.

The main advantage with the last two solutions is that sipping can be performed parallel to fuel handling during shuffling or unloading of fuel without significant loss of time or outage schedule impact. Due to its satisfactory reliability, in-mast sipping is now the main technique used to identify failed fuel in both BWRs and PWRs.

FIG. 4.5. In-mast sipping [4.28].

4.3.2. Locating and identifying failed fuel in CANDUs

As a result of the CANDU design, it is relatively easy to detect defective fuel during operation. Two systems have been developed for locating fuel defects in the core: the delayed neutron (DN) and feeder scanning (FS) systems.

The first system is used when the reactor is at-power. It locates defective elements by scanning coolant activity immediately downstream of the fuel channels [4.35]. On each side of the reactor, sampling lines carry coolant away from the outlet end of each fuel channel to a common room within the reactor building (see Fig. 4.6).

The presence of delayed neutron emitting fission products (¹³⁷I and ⁸⁷Br) in sampling lines is detected in this room using BF3 detectors. If neutron activity in a coolant sampling line is higher than normal, the corresponding fuel channel is suspected of containing a defective element. The DN system has sufficient sensitivity to locate fuel defects with very small holes. The Bruce and CANDU-6 reactors employ the DN system.

The second system is used when a reactor is shut down. It locates defective elements by scanning activity on the inside surfaces of the outlet feeders connecting the fuel channels to a common outlet header [4.36]. The presence of gamma emitting fission products is detected by Geiger–Müller detectors which move within guide tubes that transverse the outlet feeders. If gamma activity in a specific feeder is higher than normal, then the corresponding fuel channel is suspected of containing a defective element. The FS system only locates defects that have deteriorated to the point of releasing uranium and fission products which deposit immediately downstream of the defect. The Darlington reactors employ the FS system.

The Pickering reactors have no failed fuel location system. Normally, a fuel defect cannot be located in the core, but it can be detected during discharge from the core.

Fuel removal confirmation

After a suspect channel has been refuelled in a CANDU, the defective fuel element is confirmed to have been discharged by various methods, again depending on the station:

— Inspecting the discharged bundles in the bay which, of course, corresponds to LWR practice (see Section 4.3);

— Monitoring gamma activity near the spent fuel handling system when bundles are en route from the reactor to the fuel bay (wet or dry sipping) and/or when they are residing in the fuel bay (wet sipping);

— Monitoring delayed neutron activity of the coolant at the outlet end of the channel during refuelling.

FIG. 4.6. Schematic diagram of a scanning room for the delayed neutron system in a CANDU-6 reactor.

The first method provides direct confirmation that a defective bundle has been removed. Portable underwater TV cameras are used at multiunit stations operated by Ontario Hydro, whereas periscopes are used at single unit CANDU-6 stations [4.36]. Photographs of the defective elements confirm that a defective bundle has been removed.

The second and third methods provide indirect confirmation that a defective bundle has been removed by monitoring gamma activity near the spent fuel handling system or in fuel bays. At Bruce, dry sipping techniques [4.37] are used to monitor airborne gamma activity in spent fuel transfer mechanisms where bundles are transferred from the heavy water to the light water environment of the fuel bay. A higher than normal signal that lingers after bundles have been transferred usually indicates the presence of a defective element. Two other techniques have been developed at CANDU-6 stations to confirm that defects have been discharged from the core. One technique depends on the radiation levels of fission products in the heavy water which is inside the fuelling machine. Before discharge of irradiated fuel to a bay, fuelling machine heavy water is transferred to a nearby drain tank. The presence of a defect is indicated when gamma fields near the tank trigger an area alarm gamma monitor. Another technique developed in the inspection bay at Point Lepreau is based on ‘wet sipping’, or measuring the gamma activity of water samples near recently discharged bundles. Again, a defect is present if gamma activity is unusually high.

Monitoring delayed neutron activity at the outlet end of a fuel channel during refuelling also provides some confirmation that a defect is being discharged from the core [4.38]. At CANDU-6 sites, special refuelling procedures are sometimes used which involve slow displacement of the fuel column while monitoring the DN signal of the channel. When the signal drops to below ‘pre-defect’ levels, the defect has been pushed outside the core boundary.

4.3.3. Locating and identifying failed fuel in WWERs

Two different systems are currently used during scheduled outages to control cladding integrity in Russian Federation NPPs; a ‘sipping’ system and a DAD (defective assembly defection). The ‘sipping’ system, used at Russian Federation plants [4.25], is based on registration of FPG activity releases through a defect in a leaker cladding within a tested fuel assembly placed in a reloading machine bar. FPGs are released as a result of a decrease in coolant external pressure when a tested fuel assembly is raised to the top transportation position. The resultant drop between internal and external pressures leads to gas and fission gas products being dissolved from a leaker into surrounding coolant.

The DAD system comprises a case into which a fuel assembly to be tested is placed, a system of pumping a coolant (pure water or a boric acid solution containing water) and of changing pressure inside the case. At the expense of a pressure rise in the case, water enters a defective fuel rod and compresses gas volumes inside that failed fuel rod. After a pressure drop, compressed gas volumes push out an excess volume of coolant with fission gas products dissolved in it. Spectrometric measurements of the samples taken register nuclides that have left a leaker.

4.3.4. Ultrasonic testing

Ultrasonic testing (UT) is a technique in the industry well known for locating failed assemblies and rods in PWRs [4.39]. The concept was originally developed by Brown Boveri and company in the early 1970s. Failed fuel rod detection systems can be based on different techniques; the pitch and catch, the through transmission or the pulse–echo.

All of these systems inspect fuel rods either circumferentially or radially for the presence of water in the pellet–cladding gap. The loss of energy at the inside diameter when water is present inside a rod is sufficient to provide a measurable difference in the amount of energy transmitted through the rod or reflected from the incident surface and arriving at the receiver. An array of probes mounted on flexible blades are inserted in the spacing between the rows of fuel rods in a fuel bundle. These blades are moved by a remotely controlled automatic manipulator that is positioned under water on the spent fuel storage racks.

Until recently, UT was the most common technique used in the United States of America to identify leaking

80% to 90%), it has its limitations, which have led to occasional under calls and over calls. To detect failed rods, the presence of water at the probe/transducer location is necessary. If water is not present at the location being tested, the rod will not be identified as a leaking rod. Furthermore, if water is not present around the entire rod circumference, a degraded signal may result which is difficult to interpret. To improve system reliability, several scans at different levels may be useful. Other effects, such as heavy crud on a rod surface, thick oxide, or pellet–clad bonding can also attenuate the UT signal and lead to misinterpretations. The effects of crud and pellet–cladding contact become more of a concern at higher burnup levels.

Nevertheless UT remains very useful for failed rod identification in failed fuel assemblies before fuel failure cause evaluation or repair.

4.3.5. Flux tilting (BWRs)

Flux tilting is used to locate fuel failure in BWRs during operation. If the number of failed rods is believed to be small, the operational procedures of flux tilting are used with some success to identify suspect regions or fuel cells prior to the end of a cycle, thus reducing the number of assemblies that need to be sipped.

The procedure involves creating a local power change through sequential insertion of control blades while monitoring changes in off-gas activity. If a failed rod is influenced by a power change, there will be an increase in the release rate of fission products from the defect. Increased fission gas release can be recorded in the off-gas system by using existing current measuring systems, but accuracy can be significantly improved by on-line measuring with Ge detectors for nuclide specific off-gas analyses.

The most common practice, described in Ref. [4.40], consists of fully inserting single or multiple control blades at reduced power (<65% of rated power) to exclude extra-secondary damage to failed fuel by utilizing severe local power changes.

The typical procedure consists of:

— Reducing core power;

— Fully inserting single or multiple control blades;

— Leaving the blade(s) inserted for a sufficiently long time to observe a change in off-gas activity;

— Extracting the control blade(s) to their original position;

— Waiting for a sufficiently long time to observe a return to unperturbed activity levels;

— Repeating the sequence with the next control blade(s).

The authors of Ref. [4.41] recommend carrying out flux tilting investigations at as high a reactor power level as possible, with moderate control rod movement, and only 10% control rod insertion. This makes use of the fact that fission product release is higher at high fuel temperatures, thus making flux tilting as sensitive as possible and, at the same time, reducing the transient necessary to produce a significant signal.

Data provided by these procedures can reduce the scope of sipping campaigns, but this approach is subject to the risk of missing some failures. Despite this limitation, as a result of increasing pressure to reduce plant operational and maintenance costs, methods such as these are becoming more widespread in use. Some years ago, flux tilting was widely used to locate leaking Zr liners or other sensitive fuels as early as possible; severe degradation could be prevented by shadowing leaks with fully inserted control blades.