EJERCICIO LVI

The nuclear power plant and the nuclear materials used will be protected from illegal activities, such as vandalism and sabotage. Threats caused by terrorism or other illegal activities will be addressed through continuous implemen- tation of comprehensive security arrangements. They will supplement the protection provided by the sturdy structure and protection of sensitive components required by the plant’s basic safety planning.

The backgrounds of the personnel working at the nuclear power plant either permanently or during annual maintenance outages will be checked, and employee access in the plant area will be restricted, with the help of access permits of different levels, to areas necessary for the perfor- mance of their work tasks. Preparation for external hazards will also cover situations in which the threat is presented by a person or a group of people who work regularly or tem- porarily at the plant and have access permits.

4.4 Verification of nuclear safety

and authority supervision

The Ministry of Employment and the Economy requires that the Radiation and Nuclear Safety Authority (STUK) assess the safety of the AES-2006 pressurized water reactor plant. The implementation of the safety solutions will be described in detail in conjunction with Fennovoima’s sub- mittal of a construction license application for the nuclear power plant in accordance with the Nuclear Energy Act. The licensee and STUK will continue the supervision of the implementation of safety solutions throughout the project’s construction period. The solutions implemented, and the results gained from pre-operational testing, will be assessed as a whole when Fennovoima applies for an operating license in accordance with the Nuclear Energy Act.

Supervision of the use and safety of nuclear energy is the responsibility of STUK, and the safety of the nuclear power plant will be controlled by means of various author- ity inspections. STUK will determine, and record in the plant-specific operational inspection program, the inspec- tions to be periodically performed at the nuclear power plant. Additional inspections performed at the plant will include those required in the YVL Guides. To support the supervision operations, STUK is to be provided with peri- odic reports as well as reports on any transient situations. The radiation exposure of the local population caused by the nuclear power plant, the health impacts of radiation, and the emergency and rescue operations relating to the operation of the nuclear power plant are discussed in more detail in Sec- tion 4.5.3. Radiation monitoring is discussed in Chapter 10.

4.5 Management of abnormal

and accident situations

For nuclear power plant design and safety assessments, potential nuclear power plant conditions are classified as follows:

1. normal operating conditions 2. anticipated operational transients 3. postulated accidents

4. severe accidents.

This chapter deals with the last three categories, i.e. condi- tions which constitute a deviation from normal operation.

4.5.1 Abnormal situations at nuclear

power plants and the related requirements

According to the Nuclear Energy Act, provision shall be made in the design of nuclear power plants for the possibil- ity of operational transients and accidents. A nuclear power plant accident does not necessarily constitute a situation in which the operating personnel of a nuclear power plant or the residents of the surrounding area are exposed to signifi- cant levels of radiation. The more severe the possible conse- quences of an accident are, the smaller its probability shall be. While the primary objective in the design of nuclear power plants is the prevention of accidents, practical measures necessary for the management of accidents and the mitigation of their consequences must also be imple- mented. Nuclear safety is discussed in more detail above in this chapter.

The Government Decree on the Safety of Nuclear Power Plants (717/2013) sets limits for the radiation exposure of the general public and radioactive emissions due to events con- stituting a deviation from the normal operating conditions of the nuclear power plant. This Decree superseded the pre- vious Government Decree (733/2008).

4.5.1.1 Anticipated operational transients

Anticipated operational transients are deviations from nor- mal operating conditions that can be expected to occur once or more often than once during hundred years of operation.

The annual radiation dose limit for individuals of the local population due to anticipated operational transients is 0.1 millisieverts. The nuclear power plant shall withstand all anticipated operational transients without fuel damage. Possible causes for operational transients include individual equipment failure, mistakes made by the plant operating personnel, and external events, such as disturbances in the electric power transmission network and exceptional weather phenomena (Sandberg 2004).

4.5.1.2 Postulated accidents

Postulated accidents are events used as design bases for the safety systems of nuclear power plants. The nuclear power plant shall withstand postulated accidents without severe fuel damage and without such radioactive emissions that would require extensive measures in the vicinity of the plant in order to restrict the exposure of the local population.

The Government Decree (717/2013) classifies postulated accidents into two categories as follows:

1. Accidents with an expected frequency of occurrence lower than once in a hundred reactor operating years

90 4 Nuclear safety

but equal to or higher than once in a thousand operat- ing years. In this case, the annual radiation dose limit for the most exposed individual of the local population is 1 millisievert.

2. Accidents with an expected frequency of occurrence lower than once in a thousand reactor operating years. In this case, the annual radiation dose limit for the most exposed individual of the local population is 5 millisieverts.

The nuclear power plant will be equipped with safety sys- tems that retain their capability of performing their tasks during transients and accidents even in the case of system failures or unfinished maintenance operations.

Government Decree 717/2013 defines the design exten- sion conditions, which cover the following:

a. An accident involving a combination of an anticipated operational transients or a Class 1 postulated accident and the occurrence of a common-cause failure in the safety system required for the performance of the safety function

b. An accident caused by a combination of failures iden- tified as significant on the basis of a probabilistic risk analysis

c. An accident caused by a rare external event.

The plant will be required to withstand design extension conditions without severe fuel damage. The maximum annual radiation dose for the most exposed individual of the local population is 20 millisieverts.

4.5.1.3 Severe accident

In accordance with the defense in depth principle (see Section 4.2), the nuclear power plant will make an effort to prepare for situations in which the management of an anticipated operational transient or a postulated accident fails to function as planned, potentially leading to a severe accident. In a severe accident, a considerable portion of the fuel contained in the reactor is damaged.

A severe accident shall not cause a need for extensive civil protection measures or long-term restrictions on the use of large land or water areas. According to the Govern- ment Decree (717/2013), in a severe accident, the limit for the release of cesium-137 into the environment is 100 tera- becquerels. The matter is specified further in the Radiation and Nuclear Safety Authority guidelines (YVL Guide C.3 and VAL Guide 1), according to which a severe accident shall not cause a need to evacuate the local population outside the protective zone (extending to the distance of some 5 kilometers from the plant) or a need to take shelter indoors outside the emergency planning zone (extending to the distance of approximately 20 kilometers from the plant). The evacuation will be implemented if the radia- tion dose is estimated to exceed 20 millisieverts during the first week. Taking shelter indoors is justified in the case that an unprotected person is estimated to be exposed to a dose of over 2 millisieverts over a period of two days. The possibility of exceeding the emission limits shall be very

small. Additionally, the possibility of a release occurring in the early stages of an accident and causing a need for civil defense measures shall be very small.

According to YVL Guide A.7 issued by STUK, the expected core damage frequency shall be less than once in a hundred thousand years. Since not all damage to the reactor core will cause large radioactive releases, the probability for such is even smaller. According to the same YVL Guide, the expected frequency of occurrence of a release exceeding the 100-terabecquerel limit set for a Cs-137 release shall be less than once in two million years.

4.5.2 The International Nuclear Event Scale

(INES)

The International Nuclear Event Scale (INES) is used to rate nuclear facility events and nuclear accidents. The scale was developed in international cooperation between the Inter- national Atomic Energy Agency (IAEA), the Organization for Economic Cooperation and Development (OECD), and experts from several countries. The scale has been in official use for nuclear power plant events since 1992, and it is cur- rently used in approximately 70 countries (STUK 2013d).

The INES scale has promoted communication on nuclear power plant events by standardizing the related terminology. It allows for less ambiguous specification of an event’s significance for nuclear safety. On the INES scale, nuclear power plant events are rated into eight categories, ranging from INES 0 to INES 7 (Figure 4-5).

The rating criteria are presented in the IAEA INES Man- ual (IAEA 2008), according to which the consequences of an accident are divided into three areas: off-site impact (environmental impact), on-site impact (radiation within the plant area), and impact on defense in depth (deteriora- tion of safety). Each of these areas is considered separately when determining the INES rating. If the rating can be determined on the basis of more than one area and differ- ent starting points result in different ratings, the highest rating shall always be chosen.

The INES scale has seven actual ratings, with levels 1–3 representing incidents (events that compromised safety). The lowest levels (1 and 2) are mostly applicable to technical failures that have compromised plant safety. Levels 4–7 rep- resent accidents of varying severity. An accident is rated at least at level 4 if any civil defense measures must be imple- mented outside the plant. In accident situations, the severity level of the accident is determined as soon as possible, and it can be reviewed later. Level 0 events are below the actual scale due to their minor safety significance.

The different nuclear power plant events used for the design of, and safety assessment relating to, the nuclear power plant can be roughly divided according to the levels of the INES scale as follows: anticipated operational tran- sients are rated at levels 1 to 3, postulated accidents and design extension conditions at level 4, and severe accidents at levels 5–7.

INES 0: A deviation that has no significance for nuclear

or radiation safety, and thus cannot be rated on the actual scale. An example of an event rated at this level is a reac-

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