Boron dilution

To keep the core of a PWR subcritical during cold shutdown states, a certain boric acid concentration in the coolant is necessary. An inadvertent deviation from the prescribed concentration, with the tendency to loose subcriticality, is called a dilution or deboration event. The possibilities for diluting the coolant are generally classified as homogeneous (slow) or heterogeneous (fast) deboration. 

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An automatic signal to start up makeup pumps was removed to prevent boron dilution in case of loss of off-site power. Also, a new operational procedure for the interruption of natural circulation was implemented. 

The PHARE project 2.08/95 Prevention of inadvertent primary circuit boron dilution will start. The Paks NPP will be the main beneficiary, the Dukovany NPP as well as Bohunice NPP will also benefit from this work.. 

Safety analysis of shutdown states for the Loviisa reactors has been started with analysis of leaks and transient cases during hot and cold shutdown modes. Earlier safety related studies analysed boron dilution events and pressurized thermal shock at low power conditions. 

The initial design of French NPPs takes into account that a certain number of accidents can possibly occur during shutdown conditions (LOCA with RHRS connected, homogeneous boron dilution accident, etc.). Only a few years after commissioning the first unit of the industrial series of NPPs in France (TRICASTIN 1), the first Hl-2 (total loss of the heat sink) and H3-2 (Blackout) procedures specific to RHRS-connected states (post-TMI actions) appeared. The first incidental procedure I-RRA2 (Malfunction of RHRS), which results in the introduction of the standby RHRS was implemented at the same time. The first technical specifications related to cold shutdown modes were subsequently implemented since 1988. 

A full scope LPS PSA has recently started for a modern KONVOI-type 1300 MW(e) PWR. The reference plant is Neckarwestheim, Unit 2 (GKN-2). The puipose of the study is a systematic analysis of internal events in relevant operational states with probabilistic methods. Within the spectrum of lEs, special emphasis is placed on BEs with boron dilution. It has become apparent from the first study that for such BEs additional analyses with advanced tools are needed. The general steps in all studies correspond to the respective IAEA guidelines. 

Boron dilution prevention during reactor startup, including hardware and procedure improvements ... 

At present a project is being carried out to summarize all needs for deterministic analyses in shutdown modes, to evaluate the applicability of existing models and methods to the specified conditions, and to create a basis for systematic evaluation of Bohunice NPP safety in shutdown modes. As a result, an upgraded SAR will be provided, reflecting also results of relevant LPS PSA-PHARE 2.09/95, Boron dilution-PHARE2.08, etc. 

A detection of the deboration is only possible in an early stage if the circulation has not been started. If the circulation is started, the available time is too short for countermeasures. Therefore, attention should be focused on to preventing a slug formation and identifying situations when a slug can occur. Preventive measures include the automatic interruption of boron dilution if one or more MCPs are stopped and also a careful measurement of the boric acid concentration in systems which will be connected to the primary circuit after maintenance. 

In order to get a realistic picture of the nature and safety significance of a LPS event, it is sometimes necessary to do a very detailed and comprehensive analysis. A good example of this is a boron dilution event for which models of mixing and 3D neutron kinetics are needed in combination with modelling of primary circuit thermal hydraulics. 

Prevent boron dilutions by requiring specific boron concentrations during the shutdown operation. 

Some HI were implemented to reduce the likelihood of boron dilution events. Instrumentation with alarms and modification of the charging flow intakes are examples of possible solutions. Another plant installed various interlocks to prevent boron dilution in the RCS. 

Anticipated operational occurrences are off-normal events, usually plant transients, which can be coped with by the plant protection systems and normal plant systems but which could have the potential to damage the reactor if some additional malfunction should happen. Their typical frequency of occurrence may be more than 10 year Some of the anticipated occurrences (PIEs - postulated initiating events) are due to the increase of reactor heat removal (as might occur for an inadvertent opening of a steam relief valve, malfunctions in control systems, etc.). Some are due to the decrease of reactor heat removal (such as for feed-water pumps tripping, loss of condenser vacuum and control systems malfunctions). Some are due to a decrease in reactor coolant system flow rate, as in the case of a trip of one or more coolant pumps. Some are connected with reactivity and power distribution anomalies, such as for an inadvertent control rod withdrawal or unwanted boron dilution due to a malfunction of the volume control system for a PWR. Events entailing the increase or decrease of the reactor coolant inventory may also happen, due to malfunctions of the volume control system or small leaks. Finally, releases of radioactive substances from components may occur. 

At the first stage, number of modifications of schemes was made, with the purpose to prevent boron dilution in primary circuit, to reduce the single failure and common mode failure probability. The effectiveness of primary circuit emergency supply systems and spring system was increased. The reliability and effectiveness of the SG emergency feed water systems are increased. Reliability of the emergency power supply systems and control systems is increased. 

Reactivity and power distribution anomalies inadvertent control rod withdrawal boron dilution due to a malfunction in the volume control system (for a PWR) wrong positioning of a fuel assembly. 

Reactivity and power distribution anomalies uncontrolled control rod withdrawal control rod ejection boron dilution due to the startup of an inactive loop (for a PWR). 

Reactor start-up is easily performed by continuous boron dilution. The injection of pure water for boron dilution is performed by a pump which can be operated only when the MCP is operating. This design provides a fail-safe mechanism for avoiding the possibility of reactivity insertion accidents. 

Generic Safety Issue (GSI) 022 in NUREG-0933 (Reference 1), addresses the possibility of core criticality during cold shutdown conditions because of an inadvertent boron dilution event. 

Inadvertent boron dilution events have occurred at PWR s during maintenance and refueling periods. If the boron in the RCS is sufficiently diluted and the reactor core is near the beginning of life, there is the potential for core criticality with all rods inserted (i.e., during cold shutdown conditions). 

The NRC and others performed a variety of studies of the consequences of an inadvertent boron dilution event. The conclusions of the NRC assessment along with other studies were (1) that the consequences of an unmitigated boron dilution event, although undesirable, are not severe enough to warrant backfit of additional protective features at operating plants and (2) Standard Review Plan (SRP) Section 15.4.6 (Reference 2) is adequate for plants presently undergoing license review. 

The acceptance criterion for GSI 022 is that new plants shall minimize the consequences of inadvertent boron dilution events by meeting the intent of SRP Section 15.4.6. Specifically, when performing a safety analysis to evaluate the consequences of an inadvertent boron dilution, plant designers should consider (1) design limits for maximum RCS pressure and minimum DNBR, (2) moderate frequency events in conjunction with a single failure or operator error and their possible effects on fuel integrity and radiological dose calculations, (3) and time limits specified for each mode of plant operation, if operator action is required to terminate an inadvertent boron dilution. 

The System 80+ Standard Design includes a safety analysis which demonstrates that the consequences of an inadvertent boron dilution during cold shutdown are minimized. Furthermore, clear and concise indication and alarm instrumentation is provided to an operator via the Nuplex 80+ ACC which is considered in the safety analysis. Therefore, the intent of SRP Section 15.4.6 is met and this issue is resolved for the System 80+ Standard Design. 

Major accidents, not limited to the specific reactor type (e.g. post-Chernobyl boron dilution concerns for PWRs) ... 

RC 1 Inadvertent boron dilutions under low power and shutdown conditions (France, Germany, Japan, Republic of Korea, Spain, WWER countries)... 

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