Reactivity initiated accidents

Reactivity initiated accidents include events such as uncontrolled withdrawal of CPS rods, erroneous refuelhng and voiding of the CPS cooling circuit. 

Withdrawal of a group of rods from the core causes a great variation in reactivity with the ensuing early generation of emergency protection signals. 

Erroneous reloading of a fuel assembly is typically considered as an event in which a fresh fuel assembly is placed in a cell adjacent to a fuel channel that has low burnup and, therefore, high power. Such insertion of reactivity due to 

The addition of positive reactivity due to voiding of the CPS cooling circuit will not be fast due to the fact that, even in the case of the largest possible pipe break in the lower part of the circuit, it will take several tens of seconds for the water to leave the core. The emergency signals of the protection systems will be generated within this time interval. Compliance with the safe operating limits or acceptance criteria for fuel rods should be verified. 

Analysis should verify the effectiveness of the reactor protection systems. 

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Fuel rod failures, associated with the thermomechanical interaction of fuel with claddings, are mainly typical of reactivity initiated accidents. The... 

Computer codes must be qualified. The adequacy of the nodalization should be demonstrated as recommended, for instance, in Ref. [15], Integrated computer codes are preferable. For instance, analyses of reactivity initiated accidents with spontaneous control rod withdrawal should employ a three dimensional (3-D) neutronic code with a built-in multichannel thermohydraulics code in order to determine the distortion of the neutron field and the redistribution of thermohydraulic parameters in the group of fuel channels affected by the distorted power density distribution. 

The reference (initial) core state needs to be established for the analysis of accidents, especially reactivity initiated accidents. 

With regard to the CPS design, such an accident is hypothetical and its probability is practically negligible but, as it is the gravest of all conceivable reactivity-initiated accidents, its consequences actually define the nuclear hazard limit of the plant. 

The elimination of soluble boron control together with the adopted parameters of the fuel lattice provide negative reactivity coefficients on the fuel and coolant temperature negative steam and integral power coefficients of reactivity in the entire range of operating parameters, which altogether secures inherent safety features of the reactor core. These inherent safety features ensure power self-control in a steady state reactor operation, power rise self-limitation under positive reactivity insertions, self-control of the reactor power and primary coolant pressure and temperature self-limitation in transients, as well as the limitation of the heat-up rate in reactivity-initiated accidents. 

P. E. MacDonald, et al.. Assessment of light-water-reactor fuel damage during a reactivity-initiated accident. Nuclear Safety, Vol. 21 No. 5, September-October 1980. USNRC Information Notice 86-58, Dropped Fuel Assembly, July 11,1986. 

The elimination of a reactivity initiated accident (RIA) induced by a CR drop or ejection because of application of the internal upper entry CRD. 

These safety calculations for ADSs have since been complemented by a study of reactivity insertion accidents. For an assumed subcriticality of - 3, reactivity ramp rates of 170, 6, and 0.1 /s were introduced, leading to a total reactivity insertion of about + 3. These calculations showed an initially benign behaviour of the ADS (this important safety feature of an ADS had already been found earlier with simpler calculations). However, after tenths of seconds a limited steady state type overpower condition was predicted by the present calculations. In particular the slowest ramp led to a longer-term overpower condition of about 1.5 times nominal. If the accelerator is not switched off or the proton beam interrupted, this overpower will eventually lead to some pin ruptures and fuel sweepout which will stabilise the behaviour of the ADS at a low overpower. This core damage could be avoided by selecting a lower subcriticality of the ADS. 

In the design basis reactivity insertion accident (RIA) [XXX-25], the maximum reactivity insertion in the MSR corresponds to the drop of one graphite control rod into the core. Since the worth of a single graphite rod is only 0.06 %5K/K and less than one effective delayed neutron fraction, such initiating event does not result in any prompt criticality of the FUJI. 

Recent experimental data suggest that high bumup fuel may be more prone to failure during design-basis transients and reactivity insertion accidents than previously thought. Tests on the relationship between fuel failure enthalpy and bumup for pressurized water reactor fuel rods indicate lower failure initiation enthalpy thresholds (measured in differential calories/gram) than was crmsidered in the evaluation of currently approved fuel bumup limits. 

Compared to a standard PWR, the safety of SCOR is facilitated by the elimination of some initiating events at the design stage. Among them are large breaks in the primary circuit and reactivity insertion accident as initiated by control rod ejection. 

Reactivity insertion accident (RIA) since the innovative hydraulic control drive for the FSS and the adjust and control system is located inside the RPV, the control rod ejection accident is avoided only inadvertent control rod withdrawal transients are postulated. Two scenarios considering FSS success and FSS failure with SSS actuation were modelled, assuming a conservative hypothesis. The results of simulation show that safety margins are well above critical values (DNBR and CPR - critical power ratio) and no core damage is expected. Moreover, as there is no boron in the coolant, boron dilution as a reactivity-initiating event is precluded. 

Research indicates that the OSHA Integrated Management Information System (IMIS) identified 70 percent of the reactive incidents in Section 3.3, but none were tracked as reactive incidents. Only 25 percent of the reactive incidents that occurred from June 1994 through June 1999 were reported to EPA. These reports are contained in the RMP 5-year accident histories sent to EPA prior to the June 1999 deadline for initial submissions. 

The concept of a safety case comes from the requirements of the European Union/European Community (EU/EC) Seveso Directive (82/501/EC) and, in particular, regulations that the United Kingdom and other member states used to implement that directive. United Kingdom regulations (Control of Industrial Major Accident Hazards [CIMAH], 1984 replaced by Control of Major Accident Hazards Involving Dangerous Substances [COMAH] in 1999) require that major hazardous facilities produce a safety report or safety case.64 The requirement for a safety case is initiated by a list of chemicals and a class of flammables. Like the hazard analysis approach (Section 8.1.2), experts identify the reactive hazards of the process if analysis shows that the proposed process is safe, it may be excluded from additional regulatory requirements. 

The category "hazardous materials" includes a group of chemical substances called "reactive chemicals". A reactive chemical initiates reaction by itself or with other substances. These reactions are often exothermic (heat releasing) or produce flammable gases or explosive materials, which may in turn trigger accidents. 

Sensitivity to moistiwe 1 kg of water could increase die system reactivity up to -. 5 peff- 20 kg could lead to accidents widi initiation of spontaneous chain reaction (K,ff>l)... 

It has now been established that, under the circumstances prevailing at the time of the accident with the control rods withdrawn much too far from the core, inserting the control rods would initially have added reactivity to the reactor instead of reducing it. This would have occurred whether the control rods were manually or automatically inserted and could have triggered the power rise or made an existing power rise worse. The designers have now imposed control rod withdrawal limits which would prevent this situation from arising. 

Recent calculations indicate that given the conditions in the core at the time that the rods were dropping, the initial location of the rods in their withdrawn position, and the design of the control rod assemblies involving an absorber section and a nonabsorbing graphite follower, the control rod insertions actually added reactivity and contributed to the initiation of the accident. 

Criterion 20 - Protection system functions. The protection system shall be designed (1) to initiate automatically the operation of appropriate systems including the reactivity control systems, to assure that specified acceptable fuel design limits are not exceeded as a result of anticipated operational occurrences and (2) to sense accident conditions and to initiate the operation of systems and components important to safety. 

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