Reactor depressurization

Ernest, J.B. and C.A. Depew, 1995, Use of Dynamic Simulation to Model HPU Reactor Depressuring, Hydrocarbon Processing, January 1995, p. 72. 

Reactor depressurization (for GDCS operation) is provided by six squib depressurization valves. 

An industrial ammonia MGA unit for ammonia recovery has been installed at an Aliachem dye intermediate production plant in Pardubice (Czech Republic). The plant produces batches of dye intermediates in pressurized reactors. Ammonia is used as a reactant in the reactor. Depressurization of the reactor at the end of each batch results... 

During the TMI accident, one of the strategies unsuccessfully tried by the operators to regain control of core cooling was to depressurize the reactor system. The reactor was not designed for that operation and the manoeuvre did not succeed. A reactor depressurization system would probably... 

The Automatic Depressurization Subsystem (ADS) consists of the eight SRVs and six depressurization valves (DPVs) and their associated instrumentation and controls. The ADS quickly depressurizes the RPV in sufficient time for the Gravity-Driven Cooling System (GDCS) injecting flow to replenish core coolant to maintain core temperature below design limits in the event of a LOCA. It also maintains the reactor depressurized for continued operation of GDCS after an accident without need for power. 

The ADS consists of redundant logics capable of opening selected safety relief valves, when required, to provide reactor depressurization for events involving small- or intermediate-size LOCAs if the HPCI system is not available or cannot recover reactor vessel water level. 

The long and narrow design of the reactor allows for optimal passive heat removal from the core even under conditions with no coolant flow and the reactor depressurized. Heat flow through conduction and radiation to the RPV, and subsequent removal through the passive heat removal system in the reactor cavity, will limit the maximum fuel temperature and the vessel temperature so that both remain in the safe region. 

The power cycling behaviour of SGHWR fuel is of particular Interest in terms of future requirements for commercial reactors. Experimental irradiations have concentrated on the endorsement of fuel for the base load operation of the Winfrlth Heath Prototype, in which a typical history will Involve some 20-30 major power cycles during the fuel life. This number of cycles, of which about 10 may Involve reactor depressurization, have been successfully achieved by reference design fuel (0,025 in, (0.625 mm) cladding) and... 

Fig. 1.34 Coolant flow during reactor depressurization. (Taken from ref. [56] and used with permission from Korean Nuclear Society)...

In the other types of abnormalities, the event classification follows those of LWRs because the components such as the valves and the control rod drives are expected to be similar to those of PWRs or BWRs. In the category of the reactivity abnormality, the incidents related to the control rods are taken from those of PWRs. The loss of feedwater heating is taken like BWRs. Most of the incidents of the pressure abnormality are taken from BWRs because the Super LWR also adopts the direct steam cycle. The reactor depressurization is taken from PWRs. The abnormalities categorized into the inadvertent start or malfunction of core cooling system are taken from those of PWRs or BWRs. The inadvertent startup of AFS of the Super LWR corresponds to the inadvertent startup of ECCS of PWRs. The core coolant flow control system failure is the same as the feedwater control system failure for the Super LWR while the two incidents are different in BWRs due to the recirculation system. All the accidents categorized into the loss... 

The CR assembly misalignment and drop is representative of the radial power distribution abnormality. However, it cannot be analyzed because only the single channel model and point kinetics are used here. The change of the radial power distribution is expected to affect the cladding temperature distribution as it affects the departure from nucleate boiling ratio distribution of PWRs. The depressurization of core cooling system transient is not analyzed because it will be almost the same as the pressure control system failure transient It should be remembered that reactor depressurization does not threaten the Super LWR safety as is described in Sect 6.3. 

SPRAT-DOWN-DP Stea(dy-state at supercritical pressure Blowdown at large LOCA or reactor depressurization... 

Fig. 6.53 Comparison of reactor depressurization behaviors with large and small density coefficients...

Although the frequency of the small LOCA is more than twice of that of the large LOCA in the Super LWR, the CFD from the small LOCA is much smaller than that of the large LOCA. This is because the reactor depressurization is not necessary to avoid core damage as long as the RPS and RCPs work. Also, the unavailability of the ADS at the small LOCA is much lower than that at the large LOCA as shown in Table 6.27. The most dominant sequence in the small LOCA is SRV where the RCPs fail, then the ADS is successfully initiated but the LPCl fails to cool the core at low pressure. It is the sixth most dominant in the total CDF ranking and occupies less than 3% of the total CDF as shown in Fig. 6.79. 

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