Reactivity balance

A difficult challenge in developing ARO reactions with carbon nucleophiles is identifying a reagent that is sufficiently reactive to open epoxides but at the same time innocuous to chiral metal catalysts. A recent contribution by Crotti clearly illustrates this dehcate reactivity balance. The lithium enolate of acetophenone added in the presence of 20 mol % of the chiral Cr(salen) complex 1 to cyclohexene oxide in very low yield but in 84% ee (Scheme 10) [23]. That less than one turnover of the catalyst was observed strongly suggests that the lithium enolate and the Schiff base catalyst are not compatible under the reaction conditions. 

The bumup reactivity swing has been deduced from the reactivity balances at different moments during the operation of the reactor. Control rod insertions as well as temperature increases have been taken into account to deduce the measured values (using the S curves of the control rod system). Advantage has been taken from the fact that the reactor has had periods without operation to separate the reactivity variation with time into two components the loss of reactivity caused by bumup (heavy nuclide transmutation and fission due to the mnning of the reactor under power), and the loss of reactivity due to the natural " Pu decay. The experimental results used for the comparison with calculation are taken from the first period of the life of the reactor (82.3 FPD over the first 40 months). 

Taking into account the above sections, the typical reactivity balance of a PWR could be similar to that shown in Table 4-2. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

Th potentiml for power axcurilons in %hf NFR, their consequences and the effectiveness of remedial measures are discussed in this section Power excursions are directly related to nuclear excur-sions They are induced by an upset in the reactivity balance In the reactor ich can be brought about by either esnrors or accidents involving the control systems or possibly by other accidents ich can alter the neutron balance. 

To ensure a 30-year period of core operation without refuelling, the plan is to replace CPS rods, as their 10-year lifetime expires, for redundant rods located in cells of the radial blanket. The reactivity balance in refuelling of the core and blanket fuel assemblies is given in Table XVI-8. 

TABLE XVI-8. REACTIVITY BALANCE DURING REACTOR REFUELLING... 

The quasi-static reactivity balance theory of designing for passive safety response to ATWS initiators has been applied in the design of STAR-H2. The efficacy of this design approach is confirmed by the evaluation results to be presented next. 

The passive power regulation system, described above, is on itself capable of shutting down the reactor. In addition, the CHTR has been provided with a secondary shutdown system. Under normal operation this system has a set of seven shut-off rods held on top of the reactor core by individual electro-magnets, which are passively released under abnormal conditions when the temperature of the core goes up. The shut-off rods are lifted up by active means. The requirement for these rods was that, when inserted in the fuel tubes, they should be able to bring the reactor to a subcritical state with necessary margin, even when the initial reactivity balance in the reactor is at its maximum. The maximum (with Keff= 1,111) is reached in an uncontrolled cold state therefore, the required worth of the shut-off rods should be evaluated namely for this state. 

Every time the area physicist makes a prediction, he makes a reactivity balance. In order to be Just critical, all the reactivity effects must add up to zero. Since the effects are really Just parts of kexcessi or keffective "1/ oH add up to zero, keffective -1 must be zero, so that ve will have a keffective of 1.0, which Is, of coixrse, critical. In an operating reactor, the balance might look like this ... 

Nov to make a prediction Since the reactor Is shut down there are no temperature effects so the reactivity balance looks like this ... 

These features include the charging and discharging of fuel from the bottom of the reactor vessels where the temperature is lower, the provision of separate fuel-handling machines at the top of the vessels if required and the provision of remotely adjustable rods for the maintenance of reactivity balance. 

An example of the approximate reactivity balance in a research reactor is illustrated in Figure 6.1. 

Shutdown margin, SDM, is the amount of negative reactivity that would be inserted into a reactor core if all rods were dropped from critical height. Technical specifications for a reactor specify the minimum value of SDM if all rods are inserted and if one rod remains stuck in the fully withdrawn position. Technical specifications also specify the maximum excess 3eactivity permitted in a core. Thus, as illustrated in Figure 6.3, a reactivity balance exists between the minimum required SDM and the maximum allowed In practice, the minimum SDM... 

TABLE XVn-1. REACTIVITY BALANCE FOR COLD SHUTDOWN STATE... 

Absorber rods in the shutdown systems are standardized in terms of absorber element diameter and number in the shroud tube and also in terms of the dimensions of the shroud tube with rods developed for the BN-800 reactor. The effective density of the absorber material, boron carbide, is also standardized. Five rods are intended for reactor emergency protection, five rods for compensation of the reactivity effects (all rods with B enrichment of 60%) and two rods with natural boron carbide for power control. The reactivity balance during reactor refuelling is given in Table XXI-3. 

In DHRUVA, on-line computation of important physics and process parameters has been achieved. The parameters selected are reactor thermal power, reactivity load due to Xenon, core reactivity balance, heavy water system inventory and performance monitoring of shut-off rods control valve and dump valves. Also off-line application for fuel management, failed fuel detection and location, and stores inventory management have been implemented. 

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