The complete cycle for nuclear reactor fuel (the fuel cycle) is illustrated in Fig­ure 7.6. As will be seen, storage and transport of irradiated fuel play an impor­tant role in this cycle.

As we saw earlier, nuclear reactor fuel continues to emit heat even after the fission reaction ceases, due to fission product decay heating. Figure 7.7 shows the heat release rate as a function of time for spent fuel from the various types of reactors. Clearly, the more highly rated the reactor (e. g., the fast reactor), the higher the heat release rate and the longer it takes for it to decay to a low value.

Figure 7.7 shows that the fission product heat release is most intense immedi­ately after discharge. This is why it is common practice to store the fuel in a cool­ing pond for a period of time to allow both the radioactivity and the heat release to decay before removing the fuel from the immediate environment of the reac­tor. It is usual to store the fuel at the reactor site in a pool of water (though not, obviously, for the fast reactor fuel), although some air-cooled and gas-cooled (carbon dioxide) stores have been designed and operated. Water pools are well suited for fuel designed for water-cooled reactors, but they present a difficulty for the storage of fuel whose cladding has been designed for satisfactory perfor­mance in a gas environment. For example, the immersion of Magnox fuel for long periods in water ponds allows a slow chemical reaction to occur between the magnesium alloy cladding and the water, and this leads to the generation < J

Tr^ansportof solidified heat ^generating iwaste. after cooling, to disposal

hydrogen and the formation of a potentially troublesome silt of radioactive mag­nesium hydroxide. If the can is severely corroded, fission products may escape from the fuel into the pond, giving environmental control difficulties. However, with good management of the ponds (including special encapsulation of fuel that is known to be damaged), these effects can be minimized.

As with all other aspects of nuclear power, consideration must be given to the safety of the operation of spent fuel storage ponds. This can be illustrated by considering P^WR fuel assemblies, which are unloaded from the reactor and may he stored in water ponds for many years. The decay heat levels of P^WR fuel assemblies are such that if the water is completely drained from the pool,

the fuel that has been out of the reactor for fewer than 150 days will melt. Loss of water from the pool could occur if the pool developed a leak or if the pool cooling system were turned off, leading to water evaporation. Both of these events are extremely unlikely. However, the defense-in-depth strategy is con­tinued at the storage stage by either placing the store within the reactor con­tainment (as is done in the German PWR designs) or by providing it with its own containment, including ventilation and filtration systems (the U. S. ap­proach). The pond water is cooled by passing it through heat exchangers, and failure of this cooling system is perhaps the most likely failure mechanism for the ponds. However, it is unlikely that the operators would not notice a gradual fall in the water level in the ponds over a period of about 2 weeks, which would be required to uncover the fuel by evaporation due to the heat input from the fuel itself. Thus, loss-of-coolant accidents in fuel ponds are considered minor contributors to the overall risks of nuclear power.

In designing storage ponds for nuclear reactor spend fuel, consideration must be given to the problem of criticality, that is, the possibility that the pond itself would act as a nuclear reactor. With natural uranium fuel (Magnox and CANDU) there is no criticality problem in storing the fuel under water since the natural uranium-light water system does not become critical. For PWR, BWR, and AGR spent fuel, it is hypothetically possible to have a nuclear reaction, with

the fuel placed in a water pool. Thus, the pools must be designed with suffi­cient distance between the fuel elements to guarantee that no reaction occurs. The distance between the fuel elements in the store can be reduced if neutron­absorbing material is interspersed between the individual subassembly chan­nels, allowing a much higher packing density in a pool.

From a typical 1000-MW(e) PWR, about 25 tons of fuel are discharged every year, contained in about 60 fuel assemblies. About 8000 tons of spent fuel are removed from power reactors each year in OECD countries and some 150,000 tons of spent fuel are currently in storage ponds. With this rate of discharge, it is obvious that after a number of years the storage facilities at reactor sites will become full and fuel will have to be transported either to an alternative storage site or to a reprocessing plant.

Spent nuclear fuel is transported by placing one or more fuel assemblies in a transport flask, in which a large number of assemblies are transferred in a water-filled basket. A typical transport flask (or cask in U. S. terminology) for water reactor fuel is illustrated in Figure 7.8. Figure 7.9 illustrates the spent fuel flask used for Magnox fuel; the fuel is contained in a water-filled box (skip) sur-

rounded by the flask shielding. The fuel is placed in a steel basket inside the flask, which is then sealed with a cover as shown. The flask wall has a series of layers as illustrated in Figure 7.8 with a 12-l4-in.-thick outer steel layer and inner layers of depleted uranium and/or lead to absorb the gamma radiation and of water to act as a neutron shield. A flask for road transport might weigh about 20 tons and contain one or two elements, whereas a flask for rail trans­port might be much bigger, weighing up to 100 tons and able to carry 10-20 fuel assemblies.

During transport, heat must be dissipated from the outside surface of the cask. Typical heat dissipation rates would be about 10 kW for a road transport cask and 50-100 kW for the large rail transport cask. There are two main steps in this heat transfer process. First, heat is transferred from the fuel to a fluid within the flask (usually water), which circulates by natural convection around the fuel. The heat is then taken from the water into the flask wall and out to the atmosphere. The flasks normally have steel fins on the outside to assist the heat — dissipation to the air.

A variety of accidents involving transport flasks can be postulated. First, they may be accidentally dropped during transfer from the storage pool to the vehi­cle. To withstand such an impact, the flask must be designed to survive a drop of 30 ft onto an unyielding (e. g., concrete) surface without any impairment of its integrity and also survive a 40-in. drop onto a 6-in. spike. Second, the flask may become involved in a fire, and prototypes of a given design of flask are subjected to tests in which they are placed in a fire at 1000°C for a period of 30 min. Survival of these stringent tests is a necessary condition for licensing. Apart from these standard tests, demonstrations have been carried out by CEGB in the United Kingdom and at Sandia Laboratories in Albuquerque, New Mexico, in which simulated accidents have been staged. For instance, the effect of a low — loader truck with a transport flask on it, stationary on a railway crossing, being hit by a locomotive traveling at 100 mph. has been examined. The fact that the flask survived such dramatic impacts unscathed (although the locomotive did not!) has inspired great confidence in the safety of transporting spent nuclear fuel in this way.