From the beginning of commercial nuclear power plants, wet stores of limited capacity were planned and built at-reactor (AR) sites since the strategy for spent fuel management involved reprocessing, as described in the previous section. When reprocessing fell out of favour and disposal became a preferred option it became obvious that spent fuel storage capacity would need to be increased as well as storage duration. Dry storage technologies were soon developed and away-from-the-reactor (AFR) stores were built. At the beginning of the twenty — first century, spent fuel storage technology is a mature industry that can respond to the needs of nuclear operators on a commercial basis. During over more than 50 years of nuclear power plant operations, significant experience on spent fuel storage, either dry or wet, has been collected.

15.5.1 Wet storage of spent nuclear fuel

All UO2 based water reactor spent fuel is, after being removed from the reactor, stored in wet storage at the reactor (called the spent fuel storage pool or the irradiated fuel bay in Canada). There are also some centralized wet storage facilities where fuel is transferred for long-term storage. One example of such a centralized storage facility is CLAB in Sweden.

These wet storage systems aim to:

• Cool the spent fuel (heat removal system).

• Provide a biological shield for workers.

• Contain and remove contamination radioactivity that may be released from the fuel (all wet stores have a water purification system).

• Maintain the high clarity of the water to maximize visibility for remote handling of the fuel.

A spent fuel cooling and purification system uses demineralized water and typically contains pumps, filters, ion exchangers and heat exchangers. Typically, all components have a minimum of 100% redundancy, which is placed in parallel to enable the independent operation of each component. In most cases there would also be skimmers for removal of oils or other substances floating at the surface of the water.

Figure 15.9 shows a typical spent fuel storage pool. Pools are usually lined for water tightness either with stainless steel plates, or are coated with

water-resistant paint. At the bottom of the pools there are storage racks, the design of which is dependent on the fuel type, facility type and the facility operator.

After cooling the fuel for 5 to 10 years, it can be transferred to another wet store or to a dry storage facility (such as a centralized AFR store). The purpose of wet storage is the same as for AR spent fuel storage.

The fuel rod is the primary barrier for radionuclide containment purposes so that it is essential that fuel failures are reduced to the utmost. However, retention of the overall fuel assembly structure integrity is, if anything, even more important given that the fuel will at some point need to be retrieved. Consequently, the chemistry in spent nuclear pools must be carefully controlled to prevent corrosion of the fuel cladding and the structural elements of the fuel assembly.

There are variations in the chemistry of spent fuel pools and we will mention some key parameters that are typically controlled. The pH varies from the acid range (pH 4.5) to the basic range (5) depending on whether borated or demineralized water is used in the pools. For Magnox fuel, the pH is typically in the basic range (11.5-13) to prevent corrosion of the magnesium alloy by maintaining a magnesium hydroxide film on the cladding, which would dissolve in pure water. For stainless steel cladding, the pH is also maintained in the basic region. Although stainless steel is more susceptible to general corrosion than zirconium alloys, it is still low enough that is not expected to create problems during 100 years of storage. Conductivity is also a controlled parameter wherever possible. It has to be as low as possible under specific chemistry parameters (depending on the chemical additives; i. e. nuclear fuel from PWRs is stored with boric acid as a neutron poison. The acidity of boric acid affects the pH and conductivity that can be maintained). Chlorides, sulphates andfluorides are also maintained at a level as low as possible as they may trigger some specific types of corrosion. Sodium and calcium are also controlled in some pools.5 The level of radioactivity in pools is also monitored, primarily as a means of monitoring fuel failure and to indicate whether the purification system needs to be brought into action. Usually, the total radioactivity of the water is measured in addition to monitoring characteristic radionuclides like Cs-137 and Co-60.

One of the problems encountered in spent fuel storage pools comes from the growth of algae and bacteria. These can sometimes cause problems with the clarity of the water, which is essential for handling spent fuel during storage. Monitoring of water turbidity can provide an early indication of problems with bio fouling. Various chemicals are available that can remedy or eliminate problems of bio-logical activity in spent fuel pool water.

A number of potential degradation mechanisms have been investigated for fuel cladding under wet storage conditions: [27]

• Uniform (aqueous) corrosion — the corrosion of zirconium alloys in the spent fuel pool conditions is extremely slow.5

• Galvanic corrosion — zirconium alloys are near the noble end of the galvanic series and corrosion could occur through contact with Al. In this contact Al would be oxidized and Zr hydrided. Nevertheless, galvanic corrosion is prevented by the passive effect of the oxide layer on, which zirconium is generated during reactor operation (or even in some cases deliberately deployed as a thin layer). Galvanic corrosion due to contact between the Zr alloy and passivated stainless steel has not been observed.

• Pitting, and microbially induced corrosion — these are only possible if some specific conditions are present and that is avoided by the pool chemistry control.

• Hydriding — hydriding is to some extent prevented by the passive effect of the oxide layer on the zirconium. Hydrogen taken up by the cladding during reactor operation would under normal storage conditions precipitate as hydride platelets. Redistribution of those platelets could cause some loss of strength and trigger damage to the fuel element, but investigations so far indicate that this cannot occur under the conditions in wet storage.5

Another, safety concern in wet spent fuel storage is the reaction of Zr alloys with oxygen and steam considered for hypothetical accidents when the level of water in spent fuel pools decreases, leaving some of the surface of the spent fuel in contact with air. This would cause temperatures in the fuel assemblies to rise, accelerating the corrosion of the zirconium alloy cladding. The following chemical reactions with Zr can occur:

reaction in air Zr + O2 ^ ZrO2 reaction in steam Zr + 2H2O ^ ZrO2 + 2H2

Both reactions are strongly exothermic, which means they release large quantities of heat that can further raise cladding temperatures. These reactions can then become autocatalytic at high temperatures and explosive. Fortunately such autocatalytic reactions can happen only at temperatures that are very much higher than the temperature of boiling water (900-1000 °C).