Abnormal transients

Simulation tests of abnormal transients caused by the nuclear heat utilization system... 

After obtaining new licences from Japanese government, some simulation tests of abnormal transients caused by the nuclear heat utilization system which will be connected to the HTTR are planned in order to contribute the design of the nuclear heat utilisation system. The test results will be utilized for the validation of analytical codes as well as both of the HTTR-IS system design and the future VHTR design. Two kinds of simulation tests are planned. 

Criterion 2 applies to the Zone 1 and Zone 2A canyon/Room 109 confinement structures, the Zone 1 and Zone 2A ventilation exhaust ducting and charcoal filters, and HCF stack HEPA filters. These SSCs constitute the physical barriers that function to prevent the uncontrolled release of radioactive materials from the HCF during normal and abnormal (transient) operating conditions. Thus, these items provide a defense in depth function. 

ABNORMAL TRANSIENT OPERATING GUIDELINES AS APPLIED TO STEAM GENERATOR OVERFILL EVENT... 

The reactor is shut down with various trip signals that are indicative of the abnormal transient status of reactor. To avoid false trip two or more independent detectors for trip signal are used. Typical trip signals used in PWR are... 

To evaluate the cladding temperatures directly during abnormal transients, it was necessary to develop a database of heat transfer coefficients at various conditions of heat flux, flow rate, and coolant enthalpy. The database of heat transfer coefficients was prepared by numerical simulations that successfully analyzed the deterioration phenomenon itself. The database, Oka-Koshizuka correlation, has been used for safety analysis. 

Its fuel rod design also follows that of LWRs. The failure modes of fuel rods considered are over-heating, pellet cladding mechanical interaction (PCMI), buckling collapse, and creep rupture at both normal and abnormal transients. 

The four basic design criteria in the fuel rod design are as follows, for both normal and abnormal transients ... 

In BWRs, all stresses (pressure difference, hydraulic vibrations, contact pressure of spacers, etc.) are first evaluated and categorized into primary membrane stress, primary bending stress, and secondary stress. The maximum allowable stresses are set for each of these categorized stresses at both normal and abnormal transients. The maximum allowable stresses in the Super LWR fuel rod design are determined similarly. 

Safety criteria need to be defined for the same abnormal transients and accidents as those of LWRs. Abnormal transients are defined as abnormal incidents that are expected to occur one or two times during the reactor service life. The requirements are the same as those of LWRs no systematic fuel rod damage, no fuel pellet damage, and no pressure boundary damage. Abnormal incidents with expected frequency below 10 per year are further categorized as accidents as in LWRs. They are required not to result in excessive core damage. 

Table 1.10 Comparison of fuel rod integrity criteria for abnormal transients...

The initial conditions and criteria for MCST in abnormal transients and accidents are shown in Fig. 1.37. The maximum peak temperature at the steady state condition, 740°C, has changed with improvement of the core design method and data as already described in Sect. 1.3.4. But when 740°C is taken, the temperature difference between the limits, 110°C for abnormal transients and 520°C for accidents are the margins. 

For reactivity insertion accidents (RIAs), the pellet enthalpy criterion of 230 cal/g UO2 is taken. It is the same as for LWRs. For abnormal transients with reactivity insert over 1, the criterion is set as 170 cal/g, again taken from that of LWRs. However, this criterion has not been applied to the safety analysis of the Super LWR and Super FR because no transient is followed by reactivity insertion over 1. It should be considered in the future study whether the pellet enthalpy criterion for transients is necessary, as in LWRs, or not, as in sodium cooled reactors. 

Fig. 1.37 Maximum cladding surface temperature criteria and margins for abnormal transients and accidents...

Abnormal transients Decrease in core coolant flow rate Partial loss of reactor coolant flow Loss of offsite power Abnormality in reactor pressure Loss of turbine load Isolation of main steam line Pressure control system failure Abnormality in reactivity Loss of feedwater heating Inadvertent startup of AFS Reactor coolant flow control system failure Uncontrolled CR withdrawal at normal operation Uncontrolled CR withdrawal at startup Accidents... 

The reactor coolant flow abnormality is important for the Super LWR because maintaining the core coolant flow rate is the fundamental safety requirement It should be noted that there are two types of reactor coolant flow abnormalities with and without reactor scram before events the former are abnormal transient types... 

BWR safety design. But the power rise is mild for the Super LWR because of the smaller water density change than that of BWRs due to the high pressure. How stagnation occurs and increases density feedback. It also mitigates the power rise. The pressure change itself is also small at the supercritical pressure [65]. The abnormal transient and accident analyses are summarized in Sects. 6.7.1 and 6.7.2, respectively. 

The change of cross flow within a subassembly may occur during transients. The MCST may change from the result of the single-channel calculation. A transient subchannel analysis code was developed and the safety analysis of a Super LWR was carried out [70]. The temperature rises from the steady state value are about 20°C at the abnormal transients and about 130°C at accidents. The maximum values still stay below the MCST criteria for transients and accidents. The development and application of the transient subchannel analysis code are sununarized in Sect. 6.8. 

S. Bcejiri, Y. Guo, Y. Ishiwatari, Y. Oka and T. Sawada, Research and Development of a Super Fast Reactor (6) Analyses of Abnormal Transients, Proc. 16th PBNC, Aomori, Japan, October 13-18, 2008, P16P1295 (2008)... 

The criteria for abnormal transients to ensure the fuel integrity are very important. They limit the maximum allowable coolant temperature and the choice of the fuel cladding material to be used at high temperature. So, to maximize the economical potential of the Super LWR, and minimize the research and development efforts, the criteria need to be rationalized based on detailed fuel rod analyses. 

Principle of Ensuring the Fuel Integrity at Abnormal Transients... 

---