Fig. 3-1 Cladding temperature changes during a LOCA Fuel cladding will overheat within several minutes unless it is quenched by the emergency core cooling system (ECCS). On quenching, severely oxidized and embrittled cladding might fracture due to thermal shock.

Fig. 3-2 Schematic of simulated LOCA test apparatus Heated by the infrared furnace, test rods mounted inside the quartz tube are subjected to a flowing steam condition. They are quenched by bottom-flooding water after being heated at 1200 to 1500K for 30 to 5500 s.

Fig. 3-3 Failure map relevant to relative oxide-layer thickness and initial H concentration of cladding tube The figure shows test results for unirradiated Zircaloy cladding tubes that were artificially hydrided. Cladding tube fractures on quenching increase with the relative oxide-layer thickness and also increase with an increase in the initial H concentration.

Longer-term use (burnup extension) of fuel, and uses of MOX (U-Pu mixed-oxide) fuel and improved cladding materials are being planned to realize an advanced use of light water reactors (LWRs). These advances in LWR fuel can be beneficial for the effective use of nuclear resources and reduction of radioactive wastes; however, they must not degrade the integrity and safety of the fuel. We are performing experiments simulating accident conditions to confirm the safety of fuel in accidents. A Loss-of-Coolant Accident (LOCA), one of the accidents postulated in a safety analysis, results in a loss of reactor coolant from breaks in the reactor pressure boundary. Fig. 3-1 shows an example of clad temperature change during a LOCA. The fuel clad overheats to about 1450K for a maximum of several minutes until it is quenched by the reflooding operation of the emergency core cooling system (ECCS). Since the fuel cladding is oxidized by steam at high temperatures and embrittled, it is subject to fracture due to thermal shock during the ECCS reflooding. Further, the increase in hydrogen (H) concentration with burnup extension generally enhances cladding embrittlement. Simulated LOCA tests were performed on simulated high burnup fuel cladding (unirradiated Zircaloy-4 cladding tube for PWRs that was artificially hydrided) in this present study. The apparatus used in the tests is schematically shown in Fig. 3-2. The tests revealed that the lowest fracture condition (fracture threshold) tends to decrease with an increase in the initial H concentration, as shown in Fig. 3-3. Tests are now being performed with fuel claddings irradiated at nuclear power plants (high burnup fuel cladding) to examine the influence of burnup extension and cladding material change on fracture threshold during quenching. The results obtained are to be used for regulatory judgment by the government.

Reference F. Nagase et al., Study of High Burn up Fuel Behavior under LOCA Conditions at JAERI: Hydrogen Effects on the Failure Bearing Capability of Cladding Tubes, 29th Nuclear Safety Research Conference, Washington, U.S.A., October 22, 2001, NUREG/CP-0176 (2002).

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