Irradiated fuel

Storage

Subsequent to its discharge from the reactor, most magnox fuel is stored on site in a water-filled cooling pond prior to its despatch to the chemical processing plant of BNFL. (The exception to this ‘wet’ storage is the ‘dry’ storage area of Wylfa power station.) The elements are stored under 5-6 m of water which provides the cooling medium for heat removal and also acts as a radiation shield for the highly radio­active fuel elements. The elements are stored for a sufficient length of time to allow the fission products to decay and for the associated heat produced to dissipate and to reduce to an acceptable level for transport requirements.

The elements are accumulated on receipt at the ponds in a skip, which is an open-top box fabricated from mild steel. On discharge, the elements are com­plete with the splitter assembly or with lugs depend­ing on the type of fuel. The presence of these items results in a poor packing factor for the fuel. Later, these items are removed. Since with magnox fuel there are no criticality considerations to be taken into account, the fuel may be moved as found necessary within the pond.

Cooling pond management

The principal objectives of cooling pond management are firstly, to satisfactorily store the fuel pending its despatch off-site and secondly to ensure its safe re­moval from site including its transportation to the processing plant. These objectives are further amplified as follows:

• To preserve the fuel cladding by minimising cor­rosion and handling damage.

• To minimise radiological hazards arising within the

pond area.

• To ensure that adequate cooling is achieved before

despatch.

• To control fuel movements into, within and from the

pond area.

The element canning material is a magnesium alloy and as such is a highly reactive metal which is readily corroded by water, unless a protective oxide/hydroxide film can be maintained over the whole of the can surface. This film, which is initially formed in the reactor environment, continues to grow during storage in the cooling ponds. The film may be damaged by mechanisms which are either chemical or physical in origin.

The chemical preservation of this protective film is effected by stringent chemical control of the cooling water. The film is maintained by employing a high pH level between 11.5 and ll.7.

Demineralised water is used in the ponds and a concentration of 200 mg/kg NaOH achieves the target pH level. To maintain a higher pH, say 12.5, would require a tenfold increase in the NaOH concentration, which would result in loss of pond water treatment plant availability due to increased resin regeneration times. Atmospheric carbon dioxide dissolves in the pond water and this results in a lowering of the pH by it reaction with the sodium hydroxide to produce sodium carbonate. At a pond water pH of 11.5-11.7 the sodium carbonate concentration will be in the range 100-250 mg/kg. The presence of anions, in particular chloride and sulphate, enhances the rate of corrosion of magnox. Thus anions in the pond water are controlled with a target of less then 0.5 mg/kg and an upper limit of 1.0 mg/kg.

A water treatment plant (Fig 3.43) is used to adjust the chemical conditions of the pond. Between З^о and 5% of the pond volume is passed through the plant per day. After filtration to remove particulate matter, the water is passed through a cation resin to remove the sodium ions leaving the hydroxyl ions as water and the carbonate ions as carbonic acid in solution. The latter is then scrubbed out of solution in a ‘decarbonating’ tower. The water is then passed through a mixed bed resin unit to remove all further ions. Finally, the water pH is restored to the required level by sodium hydroxide injection. It will be noted that the mixed bed resin will remove many of the ions associated with radioactive products arising in the pond. These are derived from the activation pro­ducts of the canning material and from fission pro­ducts leached from the uranium bar via leak paths produced by corrosion or mechanical damage to the canning.

It usually provides a special resin bed to cater for the removal of fission products caesium-137 and caesium-134. The resin, Lewatit DN, has the ability to perform efficiently at the pond pH levels. However, since sodium is also removed by this resin, a bed volume capacity of 12 000 to 15 000 is to be ex­pected irrespective of the caesium activity in the pond. Thus, this bed is not employed before the caesium has risen to a predetermined level and then its use is discontinued when the target operating level has been restored.

The fission product decay heat from a large number of elements can raise the pond water temperature by an appreciable amount. A maximum of 30°C is a practical limit which is reasonable to achieve with efficient coolers in the summer. Some stations have installed pond water chiller plants to maintain the water temperature at 10°C. This has the advantage of considerably reducing the rate of corrosion since chemical reactions are temperature dependent. Civil engineering requirements dictate that the rate of change of water temperature and the temperature distribution throughout the ponds shall be at a minimum and con­stant to avoid stresses in the pond structure.

Pond water invariably holds suspended particulate matter which in settling forms a sludge, the presence of which increases the possibility of corrosion. The paniculate matter is removed by filtration.

The skips in which the elements are stored are painted in a durable paint and it is essential that the paint surface is maintained in good condition. This is to avoid the possibility of enhanced corrosion of the fuel brought about by the production of galvanic couples between the bare mild steel and the cladding material.

It has been indicated that mechanical damage to the protective film can precipitate corrosion. To this end. elements are handled as little as possible even though the tools used for this task are purpose de — ‘mned. Similarly, the removal of the splitter cage by J ram and die process and the cropping of lues from lerringbone elements is delayed until the time of despatch ot that fuel off-site is imminent.

Before despatch, the fuel has to be cooled to re — ULj»e the heat burden of a road transport flask to an -^ptable lev el. Contractually, the CEGB is required

Л00′ tuel for a period of 90 days. This is re — 4mred so that the release of iodine-131 in BNFL’s 1 r{XeS4’nS plant is kept to a level suitable to their
operational requirements. Observance of this 90-day limit is obtained in the first instance by administra­tive control. This control is reinforced by the use of a device known as a ‘short cooled element moni­tor’. This device is employed at the displittering/ delugging stage of element handling. In use, the ele­ment is presented to the instrumentation which is designed to identify the lanthanum-146 at 1.6 MeV in the spectrum of energy emissions from the element. This peak is no longer identifiable after 90 days cooling when the fission products associated with the lanthanum will have decayed to a low value of activity.

Whilst a 90-day cooling period is a contractual requirement, the heat burden of a potential skip of fuel for despatch has to be below that required by transportation regulations. Whilst the heat burden varies with the type of fuel and transport flask design, it is of the order of 4.5 kW for magnox fuel.

On discharge from the reactor, the continued decay of fission products results in the production of heat on a decreasing scale. In addition, elements from dif­ferent areas of the core generate varying amounts of heat. Referring to Fig 3.44, it will be noted that at the end of 90 days cooling the heat burden of 200 elements from the flattened zone is still above the

acceptable level for transportation. Indeed, some 130-140 days cooling are required in this case. At the same time, fuel from the unflattened zone and from the edge of the core reaches a satisfactory level within 25 days. Even so this fuel would not have completed the 90-day contractural cooling period. Thus before despatch, and even before desplittering, it is necessary to compute the heat burden of the pro­posed full load. This load may be made up of fuel from various areas of the core because a skip of undesplittered/delugged fuel is 120-140 elements whilst that of fuel ready for despatch is 200-225 elements. This being so, the mixing of elements is unavoidable. To ensure that the selection of fuel for despatch is optimised, it is necessary to have a comprehensive recording and control system.