Reactivity insertion

CH3)3Si)2N]2U(CH2Si((CH3)2)N(Si(CH3)3)) Generally, uranium metaUacycles are quite reactive inserting a host of organics, ie, CO, secondary amines, nitriles, isonitriles, aldehydes, ketones, and alcohols. 

Large reactivity insertion (experimental loop rupture) SL.4 j... 

The mean core damage frequency from all internal events is 1.8E-4/yr, with an error factor (95% percentile divided by the median) of 5.0. The percentage contributions to the core dai e frequencies are Large Reactivity Insertion 28% Large LOCA, 28% Reactivity Insertion L P ramp, 12% Spurious/normal shutdown, 11% Loss of commercial power, 10% and others, 5 ... 

The purpose of this test was to ascertain the hazard that would result from a rapid reactivity insertion into a Kiwi reactor. The test provided an occasion to study radionuclide fractionation in debris from a reactor excursion. Because fractionation processes distribute hazardous radionuclides among debris particles in different manners, their effects require documentation and study. Chapter 17 by Crocker and Freiling in this volume will provide background for the reader who is unfamiliar with fractionation phenomena. 

Scheme X. Phosphido-bridge reactivity insertion into the bridge.

The safety demonstration tests in the HTTR are conducted to demonstrate an inherent safety feature, that is an excellent feature in Shutdown of the HTGRs, as well as to obtain the core and plant transient data for validation of safety analsis codes and for establishment of safety design and evaluation technologies of the HTGRs. The safety demonstration tests consist of Reactivity insertion test - control rod withdrawal test and Coolant flow reduction test as shown in Figure 6. In the control rod withdrawal test, a central pair of control rods is withdrawn and a reactivity insertion event is simulated. In the gas-circulators trip test, primary coolant flow rate is reduced to 67% and 33% of rated flow rate by running down one and two out of three gas-circulators at the Primary Pressurized Water Cooler without a reactor scram, respectively. 

To demonstrate characteristics of HTGRs under Reactivity insertion and Cooiant fiow reduction Reactivity insertion Test ... 

Inadvertent withdrawal of the control rod at reactivity insertion rate of 2 cents/second was assumed for the simulation of UTOP As expected, the reactor power reaches an asymptotic level higher than that of the initial steady state due to negative feedback effects As shown in Figure 5, the Doppler effect is an instantaneous and important feedback for UTOP and the net reactivity increases imtially and then decreases to negative values due to feedback effects... 

From these results, we can conclude that the seismic isolation provides great reductions of the impact loads as well as the number of contacts at the load pads of core assemblies at the former ring. This can allow the simple design of a core control system. And it is expected that the requirements of the core compaction and reactivity insertion problem can be easily satisfied when an efficient seismic isolation is adapted for the LMR design[9]. 

The largest reactivity change occurs during plant start up. The reactivity decrease from criticality at zero power under cold temperature conditions to full power is generally above 1. The worst case is reactivity insertion under cold temperature conditions. 

The basic dynamic characteristics of the core under various reactivity insertion conditions are shown in Fig. 8. The power transient reflects the super prompt critical condition when a large reactivity insertion occurs. On flie oflier hand, the power transient is small for the 4S during potential reactivity insertion at the plant start up phase. 

Fig. 8 Reactor Power Transients for Various Reactivity Insertion with p =100 /s...

Fig. 9 Reactivity Insertion when Lifting Partial Segments of Reflector up to 1.5m... 

The plant starts up by heat entering from the primary pump and the system temperature rises to 350°C from the cold shutdown state. Under this condition, all parts of the system, including the recirculation line in the water system, are uniformly heated. Then, a neutron absorber at the center of the core is withdrawn. At temperatures below 350°C, the neutron absorber cannot be withdrawn by the self-connected mechanism using the thermal expansion difference between the stainless steel and Cr-Mo steel (Fig. 14). After withdrawal of the neutron absorber, the reflector is lifted up by the hydraulic system to reach critical condition at 350 C. A ficzy control system is employed for this approach and a fully automatic operation circuit is provided because no malfunction causes severe reactivity insertion as described previously. 

As noted above, if the time of power increase is in seconds rather than microseconds, the thermomechanical fuel cladding interaction becomes weaker and the limiting enthalpy approaches its value at the beginning of fuel melting. As the typical time of reactivity insertion in RBMKs is measured in seconds... 

In this test the total reactivity insertion was -14 pcm and rod insertion was 85 mm. 

Core support structure design with square grids at the central portion and radial stiffeners around the periphery has been recommended. Structural integrity assessment has demonstrated that even with the absence of the most critical radial stiffener, the reduction of rigidity of the structure is within the acceptable limits from reactivity insertion considerations. 

Preliminary analyses have been performed for the ATWS events in order to evaluate the inherent passive safety performance and to assess the safety margin of uranium metal cores. Results show that the temperature limits are met with margins for the core, which has inherent passive means of negative reactivity insertion, sufficient to place the reactor system in a safe stable state for these ATWS events without significant damage to the core or reactor system structure. 

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. 

Fig. 3.9. Thermal power and average core temperature following a 0.4 reactivity insertion.

The stochastic aspects are of most concern when the reactor is being started up with a very small neutron source. In the operation of Godiva [4] as a pulsed reactor, the power pulse occurs at various times, up to several seconds, following the reactivity insertion. This behavior is related to the problem of finding Pn t) for large n from Equations (10) and (11) in the case that 8 is small. A more practical problem, perhaps, is the similar one which takes into account the delayed neutrons and refers to the start up of power reactors with very weak sources. This problem does not usually arise because strong sources are used and the reactivity is inserted slowly. 

A generally valid result due to Wilkins [11] states that for any constant rate of reactivity insertion, dpjdt = ajS,... 

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