Zirconium, cladding

Hulls Handling. After the fuel has been dissolved, the residual pieces of zirconium cladding, referred to as hulls, are rinsed and removed from the dissolver vessel. The decay of activation products provides sufficient heat to ensure drying of the hulls and preclude hydrogen formation caused by the radiolysis of water. 

Meanx hile, success in the development of the natural uranium fuelled CANDU concept had led to very low cost fuelling and effective utilization of uranium even without recovery through reprocessing. AECL therefore decided to set aside work on reprocessing and concentrate instead on the once-through fuel cycle with storage of the irradiated fuel. The evidence indicated that the zirconium clad UO fuel could be stored under water for many decades until a decision was needed regarding recycle or disposal. 

Uranium fuel preparation takes the UF6 and is converted to either (a) aluminum-clad uranium metal for the weapons plutonium production reactors or (b) to Zirconium-clad U02 for electricity production in the light and heavy water power reactor (see Fig. 21.13). 

This is a batch process that is made complex by the fuel cladding and radioactivity. A typical fuel assembly for a PWR contains about 250, 0.37-inch zirconium-clad rods 12-feet long containing in total about 1200 lb of U02. The assembled rods are mechanically sheared into 1- to 2-inch lengths and dropped into a perforated basket. This is placed in the dissolver tank where the uranium oxide is dissolved as discussed above. The basket then is removed from the dissolver, and the zirconium hulls and associated hardware therein are dumped into containers, compressed, sealed, and sent to transuranic waste storage. Ultimately, they will go to a disposal facility. 

From power reactors with no reprocessing, the high-level waste consists of assemblies of zirconium-clad spent fuel rods to be packaged in stainless steel canisters. If the spent fuel is reprocessed, then the high-level waste will be converted to a silicate glass form similar to that from defense operations. The uranium... 

A chemical analysis of a typical Florida zircon is given in Table 7.10 [B2]. In addition to the compounds listed in this table, zircon often contains a few hundredths of a percent of uranium and thorium. These elements must be removed from zirconium in subsequent processing because they would form fission products if present in zirconium cladding. 

The additional analysis should check compliance with the criterion that the mass of zirconium cladding that reacted with steam should not exceed 1 % of the total mass of fuel claddings in the core. This sets a limit to the release of hydrogen into the compartments of the nuclear power plant. 

As it follows from Fig. X-5, the evolution of this accident for the VKR-MT and a WER-1000 is essentially different. In VVER-1000, the temperature of zirconium claddings increases promptly due to high-temperature heat accumulated in the uranium dioxide pellets and due to heat removal deterioration. The VKR-MT core is practically not heated in the first seconds of the accident process, as the temperatures of micro fuel elements and the coolant in normal operation are different by a few degrees only. Later on, the temperature of micro fuel elements slightly increases due to residual heat up until the start-up of the emergency core cooling system (ECCS) operation. The accident is localized after the core is filled with the ECCS water. As the temperature of micro fuel elements is well below 1500°C, the release of radioactivity to the containment remains at the level of 10. ... 

It can be seen that the character of this scenario is also different for the two core types. For a standard WER-1000 with rod-type fuel elements, the decrease of power takes place very slowly because of the positive Doppler reactivity being inserted when fuel, which is at 1000°C in normal operation, gets cooled. Fission reaction is stopped after 1000 s, when nearly all primary-circuit water is released through the safety valves. In this, the temperature of zirconium claddings exceeds 1000°C after about 20 s after the accident start. 

Such an accident is never considered for the reactors with cores based on rod-type fuel elements, as core cooling with an acceptable temperature of zirconium claddings cannot be provided in this case. For VKR-MT, the rupture of reactor vessel bottom is considered as a beyond design basis accident. 

Reprocessing of zirconium clad fuel elements 650 °C hydrofluorinator molten fluoride/HF-corrosion... 

The reactor core is located in the lower part of the vessel-vault and is composed of 91 hexagonal fuel assemblies with fuel rods of the WER-440 type containing uranium dioxide fuel in a zirconium cladding. The structural material of the fuel assemblies is zirconium alloy. Fuel assemblies are placed in a triangular lattice with the pitch of 147 mm and form a regular and symmetrical system. The reactor core height is 1400 mm the equivalent diameter of the core is 1420 mm. 

Corrosion of Zirconium-Clad Normal Uranium Plates in Superheated Steam. 

Tests run on zirconium clad-uranium core plates in hot air and steam show that, at 1470F, clad failure begins in three hours and has completely exposed the uranium core by four hours. A further test, run on a sample of Zircaloy 2 in dry air, duplicating the calculated temperature-time curve for the cooling of natural uranium elements in 80F ambient air for a period of six hours indicate that Zircaloy 2 clad should remain reasonably secure for this period of time. 

A failure that res tilts in the loss of part of the core water and exposure of a major part of the fuel elements above the water could result in the melting of portions of the fuel elements and the splitting open of the zirconium clad of some others. Cooling of the vessel wall could make conditions even worse. This is because of what little cooling the exposed elements get, practically all is due to the steam formed by the vaporization of the water in the lower half of the core. This vaporization woxild be reduced because of the cooling of this water by the vessel wall. 

Results on the zirconium-clad, normal uranium plates tested to failure are given in Table XVII, The plates are shown in Fig. 75. 

Although all of the samples, both of Zircaloy 2 and zirconium-clad normal uranium, failed under these conditions, none showed any violent reaction. The moist air condition appears to be more severe than the superheated steam or dry air. 

CORROSION OF ZIRCONIUM-CLAD URANIUM PLATES IN SUPERHEATED STEAM... 

A typical fuel assembly for a PWR contains about 190 half-inch zirconium-clad rods 12 feet long containing in total 11401b of UO2. The assembled rods are mechanically sheared... 

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