Subcritical state

In the subcritical state, P < Pc, and consequently just a small fall in strength or none at all is expected in a brittle surface. Just before the critical load is reached, a limited stable expansion of surface flaws is likely to occur, but it will be imperceptibly small and thus obscured by fluctuation in the size of the flaws found during a typical strength test. 

Biodiesel fuel was prepared by a two-step reaction hydrolysis and methyl esterification. Hydrolysis was carried out at a subcritical state of water to obtain fatty acids from triglycerides of rapeseed oil, while the methyl esterification of the hydrolyzed products of triglycerides was treated near the supercritical methanol condition to achieve fatty acid methyl esters. Consequently, the two-step preparation was found to convert rapeseed oil to fatty acid methyl esters in considerably shorter reaction time and milder reaction condition than the direct supercritical methanol treatment. The optimum reaction condition in this two-step preparation was 270°C and 20 min for hydrolysis and methyl esterification, respectively. Variables affecting the yields in hydrolysis and methyl esterification are discussed. 

At the point C exists an intermediate state between a liquid and a gas called a critical state, or, if sufficiently removed from C, a supercritical state. This state is obtained at temperatures and pressures superior to those of the critical temperature and critical pressure, respectively. If one of these parameters falls under its critical value while the other remains superior to its critical value, we are talking about a subcritical state. 

As fluids in a subcritical state retain most of the advantages of supercritical fluids, some extractions can be undertaken under these conditions. 

Solvothermal Decomposition of Alkoxide Followed by Removal of Organic Media in a Supercritical or Subcritical State... 

Another purification method consists in dissolving the crude material acid in water in a supercritical or subcritical state, and recrystallizing. A two-stage condensation process is simpler, in which an oligomer is produced that is purified before the second condensation to the high molecular product. ... 

The influence of any parameter causing reactivity changes being more perceptible at low power, the reactor was kept in subcritical state at 180 and 400 C for about two weeks for observation but no reactivity transient was observed. 

Godiva is a very fast experimental reactor with I 10 sec. In the pulsed operation, it is quickly brought from a subcritical state to a state where kp > 1, in the presence of a very weak source. In such an operation, the equations for the case of no delayed neutrons are approximately valid but with k replaced by kp. 

TABLE 4. Energy consumption of a pump process. Throttling to the subcritical state. Isobaric heating after the pump. Supercritical solvent CO . Conditions of the extraction 40 MPa, 339 K Conditions of the regeneration 6 MPa, 301 K. 

This corresponds to an unlimited exponential increase of N (number of neutrons and reactor power). The behavior is shown in Fig. 57.6 for k= 1.1 and may be compared there with the behavior in the subcritical state k = 0.5 and 0.9). 

Outside the inner region (D2O) a subcritical assembly of fissile material in the form of a melt of fluorides (LiF/BeF2 containing UF4 or fluorides of other fissile nuclides at a temperature of 500/700° C inlet/outlet) is circulated producing the thermal energy required to drive the turbines. The operation of the core in a subcritical state (k = 0.97) increases the safety of the reactor. As it has been shown before (see Sect. 57.3.6), the reactivity window for safe operation of a critical reactor is given by 1 + (where fl is the fraction of delayed neutrons). 

Each of the systems ensures reactor shutdown and is capable of keeping the cold unpoisoned reactor in a subcritical state. 

On the other hand, the results of the pulsed-ncutron source experiments show a marked tendency to saturate in the subcritical state of keff < 0.95. Since the saturation is due mainly to the variation of , a proper estimation of is required for this technique. 

An important advanta of the method is that only known or measured properties of the fts e system at the subcritical state of interest are required to interpret the measured quantities. Alternative methods fix inferring reactivity Usually require calibration at the delayed critical condition, which may not be possible or desirable in many situations. In addition to the neutron multiplication factor k, the -driven method inherently yields additional useful properties of the fissile system, such as the prompt neiutron decay constant and the inverse neutron count rate, which is related to the source neutron multiplication. 

The core remains subcritical for a long time. Core behaviour is similar to the situation of core start-up with varying reactivity. Therefore, safety measures provided for the start-up mode, i.e., actuation of power doubling period protection, are required. As shown by the analysis, when the core is coming out of the subcritical state, minimum power doubling periods occur at the end of the transient and make up 19 s and 13 s for the reactivity coefficients of 30X10 1/°C and 60x 10 1/°C, respectively, while the reactivity increase rate is 7x QT p/s. This is an acceptable value that meets the current nuclear safety regulations. 

A soluble poison injection system designed to introduce boric acid solution into the reactor to shut it down and keep it in a subcritical state under a failure of the CPS... 

Mechanical system 104 electromagnetic drives with 12 boron carbide based control rods per each drive Transfers core to a subcritical state without cooling down the reactor... 

The fuel is contained in a sealed module inaccessible to outsiders for a long period of operation without on-site refuelling. It can only be manipulated at the factory. Only under design operating conditions the fuel remains in the core and the reactor becomes critical under any other situations, the fuel leaves the core and is stored in a subcritical state. 

Types of reactivity control systems Two independent protection systems with mechanical drives of reactivity control rods Each system can shut down the reactor and maintain it in a subcritical state... 

The transient characteristics are shown in Fig. XVII-6. An automated reactor start-up was initiated from a subcritical state by the insertion of reactivity at a constant rate (0.0069 cents/s). About 2 hours after the start-up, the reactor power comes to a peak (12% overpower). Quick LEMs are actuated by this overpower to counterbalance the LRMs positive reactivity addition. Then, the net reactivity is kept slightly positive and the reactor power gradually approaches the nominal value. 

Start-up strategy. Demonstration is required of the approach by which the reactor is taken from a deeply subcritical state with external heating sufficient to maintain the Pb coolant in a molten state to the nominal fission power level and primary coolant natural circulation flow ... 

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. 

---