Shutdown Heat Generation

Volumetric heat generation increases with temperature as a single or multiple S-shaped curves, whereas surface heat removal increases linearly. The shapes of these heat-generation curves and the slopes of the heat-removal lines depend on reaction kinetics, activation energies, reactant concentrations, flow rates, and the initial temperatures of reactants and coolants (70). The intersections of the heat-generation curves and heat-removal lines represent possible steady-state operations called stationary states (Fig. 15). Multiple stationary states are possible. Control is introduced to estabHsh the desired steady-state operation, produce products at targeted rates, and provide safe start-up and shutdown. Control methods can affect overall performance by their way of adjusting temperature and concentration variations and upsets, and by the closeness to which critical variables are operated near their limits. 

The basic requirements of a reactor are 1) fissionable material in a geometry that inhibits the escape of neutrons, 2) a high likelihood that neutron capture causes fission, 3) control of the neutron production to prevent a runaway reaction, and 4) removal of the heat generated in operation and after shutdown. The inability to completely turnoff the heat evolution when the chain reaction stops is a safety problem that distinguishes a nuclear reactor from a fossil-fuel burning power plant. 

During normal operation, the main circulator transports hot helium at 1266°F (686°C) from the bottom of the core to the steam generator which, in turn, produces superheated steam at I005°F (541 °C) and 2500 psia. The cold helium at 496°F (258°C) is returned to the top of the reactor core. During normal shutdown and refueling, the non-safety auxiliary shutdown heat removal system removes core afterheat if the main heat transport system is not operational. 

FUNCTION Control Heat Generation This function refers to the necessity to / control the heat generation of the reactor so that fuel temperatures are not excessive. Since Criterion II limits exothermic chemical reactions, the sole requirement of this function is to assure reliable reactor shutdown. 

Using, the equation for decay given above, the heat generated in the tank after shutdown, assuming no losses from the tank, is given by ... 

From the shove it is found thaf at the instant of shutdown the reactor heat, fftlls from 30,000 to 2400 kw, of whi-ch 400 icw is in the beryllium reflector. The.reactor active lattice heat development at shutdown falls from full power of 28,800 to 2000 kw or about a 1/15 reduction. Hence, although the water flow through the active, lattice at shutdown can be reduced about 1/15, the water flow through the beryllium reflector can be reduced only 1/3 sincd its heat generation, falls only from 1200 to 400 kw. Depending on the ... 

The core reactivity is controlled by control rods in the core and reflectors. A completely independent and redundant reserve shutdown system provides a diverse reactivity control capability using boron pellets stored in hoppers above special channels in the core. The inherent features that control reactivity and thus heat generation, include a strong negative temperature coefficient, and the single phase, neutronically inert cool. ... 

Many separators are designed to have the physical characteristics that endow them with a shutdown function, a safety feature in which the polymer melts to close the micropores and prevent ion transport between the electrodes in the case of abnormal heat generation caused by short circuit or other reasons. This function prevents battery overheating and therefore greatly improves battery safety. 

The RHR system removes residual heat generated by the core under normal (including hot standby) and abnormal shutdown conditions. The LPCl function of the RHR system is an integral part of the ECCS. The design objectives of the system follow ... 

Vertical movement of the annular reflector during plant operation, including the start-up and shutdown, is the only mechanism for reactivity control provided for in the 4S-LMR. The reflector is installed inside the reactor vessel and heat generated in the reflector is removed by sodium. 

Of ioqportance are the various sources of nuclear heat generation in the reactor and as veil the geometrical distribution of such sources. The heat generation during equilibrium reactor operation and after shutdown are both of Interest and must be considered. 

Hhe beta and ffluiuna heat (Generation rates after shutdown are plotted In Figure 9 3 2e3 3 for 1000 hour HER operations The toteil... 

The rated thermal output of MONJU [5.63, 5.64] is transported through the primary heat transport system (PHTS) and intermediate heat transport system (IHTS) loops to the steam generators. Shutdown heat removal is normally by forced circulation (FC) provided by pony motors associated with each of the loop pumps. Heat is rejected to air at the air blast heat exchanger of the intermediate reactor auxiliary cooling system (ACS) which branches off from each IHTS loop. Thus the auxiliary cooling system (ACS) of the Monju reactor is coupled with the secondary system which also has the role as decay heat removal system. 

Primary pump drive pony motors will normally not be used during an outage however, some units may be placed on electric drives to conserve steam Automatic actuation will normally be effective if the loop la on recirculation since drive unit speeds will be in excess of 900 rpm Shortly after a nuclear shutdown residual heat generation rates will be low and the emergency raw water system may be bypassed and moved from service to minimise the chance of raw water Introduction Into the primary system. Manual actuation will provide backup If normal cooling la lost and fuel overheating la approached. 

The decay heat generation is represented versus time in Fig. 7.15. The MSFR design implies that FPs are present in two different places when the reactor is stopped. Some are in the liquid-fuel salt and some in the gas processing unit. Approximately one-third of the heat is produced in the gas processing unit and two-thirds in the liquid fuel. The power of both heat sources decreases rapidly (by a factor of 10 in 1 day) from the value at shutdown, which depends on the history of power generation. The total amount of power at shutdown is approximately 5% of the nominal power. This value is lower compared with solid fuel reactors because FPs are continuously removed in this concept. 

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