Primary depressurization systems

A first cornerstone of the defence strategy, already incorporated in the AP600 design, is the voluntary depressurization of the primary system in case of the danger of inadequate core cooling. A feature of this type was proposed and thoroughly studied for the first time at the start of the 1980s for pressurized reactors (see Appendix 10 on Primary Depressurization Systems). 

The importance of a voluntary primary depressurization system in a PWR has been stressed many times in this book. It is an absolute requirement in a BWR in order to cope with the loss of the main condenser, given the fact that steam release to the outside is excluded for the radioactivity content of the reactor water. 

This system was not only a primary depressurization system, as it also included a subsequent passive water injection function in the primary circuit (low pressure and small flow rate) for the long-term refrigeration of the core. 

The concept of primary depressurization systems for PWRs has become ever more popular with time. 

The concept of passive safety meant the use of systems based on simple physical laws more than on complex equipment. One example is represented by safety injection systems on water reactors which use gravity as a motive force and not pumps. This principle was, for example, adopted in the passive PWR AP600, certified by the NRC in 1999. It comprises a voluntary fast depressurization system of the primary circuit and the provision of a water reservoir in the containment located at an elevated position with respect to the reactor vessel. Passive cooling of the containment was also incorporated in the design. Evidently, however, neither of these new concepts nor the industrial weight of the NRC certification are sufficient to immediately convince the investors because, up to now (2005), no new AP600 has been ordered. 

The primary depressurization eliminates at the source, all the severe accident sequences with a pressurized primary system (i.e. direct containment heating, destructive reaction forces due to perforation of the vessel, etc.). Moreover, in case of malfunction of the high pressure cooling systems, it allows the cooling of the core by intermediate pressure accumulators and low pressure systems. 

A series of three compressed gas accumulators and borated water at low pressure (1.8 MPa, relative), each connected to one of the three cold legs of the primary system. The volume of each accumulator is of about 333 m, 250 m of which are occupied by borated water at 2000 ppm boron. These accumulators are normally isolated from the primary system by non-retum valves only, as for the intermediate pressure (4.2 MPa, relative) accumulators commonly installed in PWRs. The connection lines with the primary system are of small diameter (approximately 50 mm), sufficient to supply, in case of primary depressurization, a slow and durable injection of borated water (indicative duration in typical cases >10 hours). Borated water injection performs the double function of maintaining the reactor sub-critical in the long term and of refrigerating it by flooding and vaporization (feed and bleed). 

Alternative OHR for PWRs remains controversial in the USA nuclear technical community. One PWR manufacturer rliminaTPrt prrsxiirirer blowdnwn valvpc (power operafe d relief valves) although the ACRS considered primary depressurization to be a.useful.methpd of decay heat removal [2]. Another designer included "bleed and feed" primary blowdown for an alternative method for decay heat removal. Outside the USA, G. Petranwii of ENEA (Italy) proposed a "core rescue" primary blowdown system [3],... 

The loss of water from the primary system can be simulated by an efflux from a depressurization system and from a hypothetical break in the primary system itself. 

It is assumed that the opening for the primary depressurization is located on top of the pressurizer (i.e. at the highest point of the system) so liquid efflux will occur only if the program detects a situation where the water volume in the primary system is equal to or higher than the volume of the primary itself. 

The version of the program described here does not foresee the study of Anticipated Transients Without Scram (ATWS) or the calculation of the pressure in a water tank where the primary liquid from the depressurization system is discharged. For additional calculations of this type, the following notes and formulae may be useful. 

More data and formulae for thermo-hydraulic calculations in the primary system and in the depressurization systems can be found in (Petrangeli, 1983). 

In the subsequent years, the technical thinking on the accident at ENEA-DISP led to the development of a proposal for the Core Rescue System (CRS) (see Appendix 10) based on the voluntary depressurization of the primary system and on the injection of cooling water by passive systems (Petrangeli et al., 1993). This type of system was subsequently adopted in various new reaetor designs (e.g. on the AP 600 Westinghouse reaetor). In particular, the voluntary depressurization system of the primary circuit, publicly proposed for the first time (for pressurized reaetors) in the eourse of the mentioned studies in Italy, has beeome a permanent feature in the new PWR plant designs. 

Automatic Depressurization System a Actuation mode (manual/automatic) - Automatic b Side location (primary/secondary circuit)- Primary... 

Primary circuit pressure control Pressurizer depressurization system Active ... 

A spurious primary coolant system dump during equilibrium power operation is a credible but unlikely Incident. The primary loop has been designed to withstand the thermal and vibrational shock of at least six dump transients. The Incident is chosen for aiscusslon because it represents the most severe of a class of incidents Involving rapid depressurization of the system and reliance on the emergency once-throuc raw water system without a preceding major line falliure. 

The use of narrowing devices in the nozzles connecting the primary circuit systems to the reactor, to limit blowdown flow rate of the water coolant, and selective placement of these connecting nozzles to provide a fast transition to the steam blowdown of the primary coolant under depressurization of the pipelines ... 

Primary depressurization Residual heat removal system... 

The first condition may be met simply by a depressurizing system for the containment for primary loop protection that, on the other hand, is already foreseen to follow the primary loop pressure in case of depressurization. 

Manifold barriers confine the radioactivity to the 1) ceramic fuel pellet 2) clad 3) cooling water, as demonstrated by the TMI-2 accident 4) primary cooling loop 5) containment and 6) separation from the public by siting. Further protection is provided by engineered safety systems pressurizers, depressurization, low pressure injection, high pressure injection and residmil heat removal systems. 

Rupture of a large-size pipe, capable of causing complete loss of coolant and depressurization of the primary system... 

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