Negative reactivity

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

Mandates that fluorescent and immunohistochemical stains must be checked for appropriate positive and negative reactivity each time they are used. 

Target value 2 dilution or positive or negative Target value 2 dilution or positive or negative Reactive or nonreactive Target value 3 SD Target value 3 SD... 

One is the secondary- coolant reduction test by partial secondary loss of coolant flow in order to simulate the load change of the nuclear heat utilization system. This test will demonstrate that the both of negative reactivity feedback effect and the reactor power control system brings the reactor power safely to a stable level without a reactor scram, and that the temperature transient of the reactor core is slow in a decrease of the secondary coolant flow rate. The test will be perfonned at a rated operation and parallel-loaded operation mode. The maximum reactor power during the test will limit within 30 MW (100%). In this test, the rotation rate of the secondary helium circulator will be changed to simulate a temperature transient of the heat utilisation system in addition to cutting off the reactor-inlet temperature control system. This test will be performed under anticipated transients without reactor scram (ATWS). 

Clearness of qualitative information (positive or negative, reactive or non-reac-tive, above or within normal range). 

Clear cell RCC is usually positive for low-molecular-weight cytokeratin (LMWCK) such as CAM5.2, cytoker-atin 18. CCRCCs are also positive for cytokeratins AEl/ AES but negative for CK7 and CK20. They are positive for EMA and vimentin.3 4,398 negative reactiv-... 

Proliferation index [Ki-67 (MIB-1) index] in bd-2-negative reactive follicles >60%... 

Trip on negative reactivity incorporated after the reactivity transients. 

LMRs with oxide-fueled core Models modified and newly developed mto the code so far mclude models for reactivity feedback effects and pool thermal-hydraulics In order to venfy the logic of the models developed, and to assess the effectiveness of the inherent safety features based upon the negative reactivity feedbacks m achieving the safety design objectives of passive safety, a preliminary analysis of UTOP and ULOF/LOHS performance has been attempted... 

Tnp of all pnmary pumps with coastdown and the loss of IHX heat removal capability due to sodium water reaction in the steam generator is assumed for the ULOF combmed with LOHS event Reduction of the core flow is due to the coastdown of pnmary electromagnetic pumps, and the reactor power decreases to about 6% of the rated power due to negative reactivities When there were no... 

Self-protection, self-regulation and self-limitation of power due to negative reactivity... 

IXX Safety Circuit aoraae the vertical safety rods (VSRU) and the horizontal control rods (HCR s)j however only the negative reactivity of the VSR s la counted for reactivity control for this safety system In most Instances (exceptions are noted In subsequent paragraphs )> relays are used In parallel vlth contacts connected In series to provide high reliability to trip on demand ... 

The Phenix Reactor Block is the integrated type, designed in the 1960 s. In order to extend operations with respect to the current dimensioning rules, a special approach was implemented, taking into account the possibilities for inspection of the structures in-situ. This approach took into account the extensive feedback from operations that were provided by the analysis of the negative reactivity incidents that affected the reactor in 1989 and 1990. The approach was structured into three levels of analysis conducted on the main structures making up the reactor block. These levels are described in the following. 

More complicated materials, especially uranium in this context, display non 1/u behaviour. This may be described as showing a marked preference or resonance for neutrons close to certain resonance energies. Such resonances may be fairly sharp so that only neutrons close to the resonance speed or energy react. Some resonances are associated with fission and produce more neutrons others simply lead to capture, removing neutrons. On the whole, the resonance effects tend to lead to a net loss of neutrons and thus a negative reactivity effect. 

Under stationary power operation conditions, the reactivity absorbed by xenon and samarium varies between two and three per cent. However, after shutdown, the reactivity of xenon may increase many times showing the well-known peak at about 11 hours. The negative reactivity due to samarium increases asymptotically up to a few per cent. 

For calculations of this type, the evaluation of the shutdown effect of the depressurization is interesting. The depressurization, in fact, causes a loss of primary liquid and a pressure decrease which increase the steam volume in the core (the void content of the core is increased) with consequent introduction of negative reactivity and shutdown of the chain reaction. These evaluations can be done taking into account that results consistent with refined calculations are obtained by assuming that the core shutdown occurs for an average void ratio a in the primary system of 30 per cent. The value of a can be calculated by the following formulae ... 

Fast shutdown Fast insertion in the nuclear reactor core of negative reactivity, thus causing the immediate stop of the fission chain reaction. 

System capable of shutdown the reactor immediately. The rest belongs to the Adjust and Control System capable to introduce enough negative reactivity to keep the reactor in shutdown mode, with appropriate safety margin, during all cooling conditions. 

VII.25-VII.27], and efforts have been made to validate computational methods using data selected from these compendiums [VII.27-VII.29]. The measured isotopic data that are available for validation are limited. Of farther concern is the fact that the database of fission product measurements is a small subset of the actinide measnrements. In addition, the cross-section data for fission product nuclides have had much less scrutiny over broad energy ranges than most actinides of importance in INF. Fission prodncts can provide 20-30% of the negative reactivity from bumup, yet the uncertainties in their cross-section data and isotopic predictions reduce their effectiveness in safety assessments with bnmnp credit. 

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. 

In case of sodium loss from core and axial blankets, the blankets shall contribute sufficient negative reactivity so that the overall effect on reactivity will be zero or negative. 

---