Handling restrictions and criticality assessments

Decay heating In practice, the timing of the various irradiated fuel movements and related handling op­erations is rigidly controlled by post-discharge heating considerations. During their residence in the reac­tor, AGR fuel stringers generate powers ranging from about 6.5 MW to typically 4 MW at end of life. These are figures which should be compared with peak mag — nox channel powers of perhaps 270 kW. Coupled with the high irradiation achieved at discharge, the increased ratings experienced by AGR fuel gives rise to very high heating levels following its discharge from the reactor. Referred to as decay heat, this is the energy released by the continued radioactive decay of fission products and heavy isotopes.

There are many reasons why the safe handling of irradiated AGR fuel needs to be linked to decay heating. The implications of stringer or element over­heating arising from various postulated fault condi­tions has been rigorously studied and, together with a general requirement to restrict fuel tempermures at other stages during handling, has led to the s.;ict ap­plication of maximum heating limits to the individual operations. Of all the likely fault conditions which are thought could arise in the fuel route, the most severe is that of a dropped irradiated fuel stringer in which gross damage to some of the tuel elements could be compounded by fuel can melting and the consequent release of radioactive fission products. Al­though such an occurrence, either in the reactor or at any of the fuel route facilities, is considered to be extremely unlikely, it is nevertheless necessary from a safety viewpoint to be able to guarantee that, even if it were to happen, an unacceptable release of radioactivity to the environment would not take place.

In this context the withdrawal of an irradiated stringer is forbidden even from a shut down reactor unless the total stringer decay heating is less than predetermined limits. The absolute values depend upon the available cooling capacity provided by the charge machine during the discharge process. The most com­mon off-load refuelling regime adopted involves com­pliance with a maximum stringer heating of around 40 kW, which in practice results in post reactor shutdown delays of up to 12 or 15 hours before refuelling can begin. A similarly derived limit is de­signed to protect against the consequences of a dropped stringer within a buffer storage decay tube. Other potentially serious implications of overheated fuel have been considered. Within the irradiated fuel dismantling (IFD) cell, for example, dismantling is not permitted unless the total stringer decay heating is less than 40 kW; this ensures that fuel temperatures do not reach levels which would significantly enhance the oxidation rate of any exposed UO2, at possible pin failure sites, to U3O8. UjOg would be produced as a fine powder, therefore constituting a ready source of cell contamination.

Routine estimation of AGR fuel decay heating is a highly complex matter. Since the very high fuel ratings and irradiations produce considerable levels of post-discharge heating, it is important that changes in stringer conditions during service are properly ac­counted for in the calculational process, so that heating levels are neither under nor over estimated. Therefore sophisticated techniques have been developed which allow the retrospective examination of ratings seen over the entire operating histories of individual stringers, prior to their removal from the reactor. By this means, recommended minimum cooling periods can be provided in order to ensure that the various heating limits are not exceeded in practice.

Criticality considerations In discussing the accidental criticality of AGR fuel, reference was made to pes­simisms included within the criticality safety assess­ments and also to allowances made for the possibility of flooding occurring under freak accident conditions. The latter holds special significance for the handling of irradiated fuel since some of the facilities involved are associated, directly or indirectly, with the pre­sence of large quantities of water. In the case of decay tube storage, for example, studies have assumed that the dropped stringer accident, referred to in the pre­vious section, causes a breach of the cooling water jackets with consequential flooding of the fuel and debris, and yet even if this were to happen, the con­clusions are that safety from criticality would be as­sured. No criticality hazard is foreseen in the IFD either, mainly because of the small amount of fuel involved. Provision has been made for deliberately flooding the cell with an emergency supply of boro — nated pondwater in order to provide cooling for the fuel following a postulated dropped stringer accident.

The cooling ponds are of special importance and criticality safety assessments have considered accident situations, such as overturned skips and fuel element oser-stacking. It has been demonstrated that under normal circumstances in which skips are properly loaded with fuel and stored in the pond bays only in the approved manner, criticality cannot be achieved, even if pure water were used. However, in order to provide further safety margins against skip accidents which could conceivably lead to damaged fuel be­coming reassembled in a more reactive array, the pondwater is dosed with a soluble neutron poison — boron, in the form of boric acid. The skip insert, which is welded to the main skip body, is made of boron steel (see Fig 3.55) and, together with good working practices which have evolved from the criticality assess­ment work, therefore provides for additional safety margins.