Inference Generation and Reading Comprehension

4. FUTURE MISSION AND OUTLOOK

The focus of reactor safety research at GRS so far was on the development and validation of simulation programs for the current reactor generation and will be in future extended to the next reactor generation including SMRs. Newly added are issues such as spent fuel rod behaviour during long-term interim storage as well the aging of the storages beyond the planned and approved periods. Germany also intends to continue its reactor safety research in an international context, which requires necessarily an appropriate knowledge and a preservation of nuclear know-how. That’s why the German government funds the further development and validation of the GRS nuclear simulation chain for the safety assessment, introduced in this presentation. The simulation chain covers all relevant phenomena of reactor physics, thermal hydraulics and core meltdown as well as structure mechanics. GRS is also faced with challenges that stem from the fact that many of the codes are legacy codes. They retain code segments and structures that follow the good practices of the 1970s and 1980s and are not always suitable for the current programming approaches and tools and extension towards application for next generation reactors. GRS therefore needs to pursue the re-factoring of these codes to migrate them to recent programming standards and make use of up-to-date methods. Finally, GRS is also challenged in finding new staff with prior experience in programming languages that are less common nowadays and a solid understanding of the underlying physical phenomena. With its on-going development and validation programme and on-going recruitment and internal development of new staff, GRS is successfully addressing these challenges.

ACKNOWLEDGEMENTS

The development and the validation of simulation codes at GRS are funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) on the basis of several resolutions of the German Bundestag.

ensure the safety of nuclear facilities. The OECD/NEA FIRE Database (1979-2012) has indicated that such HEAF events occurred in 48 cases out of the total 415 fire events [1].

FIG 1. Damage to metal-clad switchgears caused by the Onagawa HEAF event [1]

A HEAF event occurred in the medium-voltage (6.9kV) metal-clad switchgears (M/C) at unit 1 of Onagawa Nuclear Power Station (NPS) of the Tohoku Electric Power Company Co., Inc. due to the Great East Japan Earthquake of March 11, 2011. Figure 1 shows the damage to M/Cs of the Onagawa HEAF event [2]. Not only the affected M/C but also connected cables and other components were damaged due to the arc thermo-mechanical energies and an ensuing fire that subsequently spread to 10 other M/Cs via cable duct. As a result, two residual heat removal pumps were stopped for a short period and consequently influenced the safety function of the unit. Similar HEAF events, although their impacts are different from each other, have occurred in the electrical equipment and components at NPSs worldwide [1-2]. Therefore, from the point of view of the safety regulation, it is necessary to evaluate the influences of HEAF events on the safety functions of NPSs.

This paper summarizes the information on the S/NRA/R test results such as knowledge about how HEAF events develop, how much arc discharge energy generates, and under what conditions a fire occurs, which are important factors to prevent ensuing fires and were used as the basis of the new HEAF requirements.

2. EXPERIMENTAL

S/NRA/R HEAF tests used three types of electrical cabinets such as M/C (around 7,000 V), distribution panel (DP (480 V)) and motor control center (MCC (480 V)). There is a need to set a short-circuit current as the target current value by which arc discharge is generated in electrical cabinet in a HEAF test. The short-circuit value of each electrical cabinet is set when the electric system of the nuclear power plant is designed, and it is calculated based on the impedance of the upstream trans-former and the rated current of the secondary-side transformer. The HEAF tests were conducted under the following test condition on the electrical cabinets such as M/C (around 7kV, 22.3kA), DP (480V, 52.3kA) and MCC (480V, 63.5kA).

3.RESULTS AND DISCUSSION 3.1 M/C HEAF test results

Figure 2 is a group of photos illustrating the HEAF test of the M/C. Photo (1) shows the M/C before the test. Photo (2) shows the moment when arc discharge occurred (after 0.2 seconds). Part of the arc discharge and metallic fumes came out of the switchgear from the opening on the top. Photo (3) shows the continuous arcing and generation of large quantities of metallic fumes, etc. (after 2 seconds). Photo (4) shows the ensuing fire (after 10 minutes). The ensuing fires occurred several minutes after the arc discharge, which caused spreading fires to the adjacent M/C and cables in the vertical tray. As seen in these photos, HEAF consists of two phases: the first phase is an explosive fast energy release (photo (2), (3)), and the second phase is ensuing fire resulting in the damages to the neighboring cabinets and cables (photo (4)).

3.2 Relationship between Occurrence of Fire and Arc Energy

Thirteen HEAF tests were conducted: six tests with M/C, three tests with DP, and four tests with MCC.

An ensuing fire occurred in six of the thirteen tests: four with M/C and two with a DP. Figure 3 shows the relationship between the arc energy and arc discharge duration required for causing ensuing fire. The arc energy increases with the arc duration. The tests where an ensuing fire occurred are marked with red. Ensuing fires occurred at conditions with higher arc energy and longer arc discharge duration.

The values of arc energy which can cause ensuing fires were between 26.3 and 28.6 MJ for the DP and between 42.6 and 57.2 MJ for the M/C.

From these test results, it is said that an ensuing fire can be prevented by decreasing the arc discharge duration and the arc energy. The ensuing fires caused by HEAF events did not occur immediately after an arc discharge was generated but were observed several minutes after an arc discharge was generated. An ensuing fire occurs presumably because the generated arc energy heats up the cables and other components in the electrical cabinet, thereby causing flammable materials to catch on fire. The arc energy necessary for causing an ensuing fire differs between the DP and the M/C. As mentioned regarding the relationship between the occurrence of a fire and the arc energy, M/C and DP have different chassis sizes (internal volumes) and different levels of containment such as tightness

and chassis strength, etc. Therefore, the arc energy necessary to cause an ensuing fire is thought to depend on the internal volume and openings (containment level) of each electrical cabinet. For example, an electrical cabinet with a smaller internal volume will accumulate the arc energy shortly, resulting a higher internal temperature and is more likely to cause the cables and other flammable materials in the cabinet to burn at a lower arc energy than electrical cabinets with a larger internal volume. An electrical cabinet with a higher containment level releases less energy and is more likely to cause the cables and other flammable materials in the electrical cabinet to burn at a lower arc energy than electrical cabinets with a lower containment level. These facts are clearly shown in the test results for the M/C and DP in Fig. 3. That is to say, because the M/C have a larger internal volume than the DP and have a lower containment level with vents in their tops, more energy is needed in the M/C to cause an ensuing fire than the DP by at least 20 MJ. Therefore, in evaluating the generation of an ensuing fire, it is required to properly take the internal volume and containment level of each electrical cabinet into consideration.

Accordingly, the results of the HEAF tests show that an ensuing fire can be prevented by decreasing the duration of arc discharge or decreasing the arc energy.

4. HEAF PROTECTION MEASURES (MEASURES FOR PREVENTING ENSUING FIRE) FIG. 2. HEAF test of M/C [3].

FIG. 3. Arc energy required for causing fire [3].

HEAF is a phenomenon where a large current arcing occurs, resulting a rapid release of energy (explosion) accompanying heat, light, metal vaporization, and a pressure increase in the first phase, and a fire may break out due to the accumulation of heat in the second phase.

Although the detailed understanding of the phenomena and evaluation methods of explosion in the first phase are still under study in safety research, the knowledge about fire occurrence in the second phase has been accumulated by the HEAF tests, and as a result, for example, it is becoming clear that if arc duration can be shortened by operating the protective relay of a power supply board in a short time or by other ways, as a measure for preventing fire, it is possible to prevent fire and to decrease the impact of explosions. One of the possible counter measures is replacement of analog type over-current relays (OCR) to digital type OCR. The response of the digital type OCR is much faster than that of old analogy ones.

5. HEAF REGULATORY REQUIREMENTS

The concept by which regulatory requirements for strengthening protection for damages of electrical cabinets due to a HEAF according to the abovementioned background is as follows [4].

1) Objectives

To prevent fire and to decrease the impact of explosions due to a high energy arcing of concerned electrical cabinets.

2) Concerned facilities and equipment

Electrical cabinets of commercial nuclear power reactor facilities, research reactor facilities, and reprocessing facilities (hereinafter referred to as "commercial power reactors and other facilities") 3) Requirements

Regarding the concerned electrical cabinets, it is required to decrease the consequences of an explosion and to set the cut-off time of upstream breakers appropriately so that an ensuing fire does not break out.

6. SUMMARY

The HEAF tests were conducted in order to obtain technical knowledge about the development of high energy arcing fault (HEAF) events, the arc energy level at which an ensuing fire occurs, and the impact of arc discharge. The technical knowledges were utilized for developing new HEAF regulation.

The approximate arc energy necessary for causing an ensuing fire was confirmed. The values of arc energy which can cause ensuing fires were between 26.3 and 28.6 MJ for the DP and between 42.6 and 57.2 MJ for the M/C. The energy necessary for causing an ensuing fire is thought to depend on the internal volume of each electrical cabinet and containment level.

The knowledge about ensuing fire occurrence in the second phase of HEAF has been accumulated, and as a result, for example, it is becoming clear that if arcing time can be shortened by operating the protective relay of a power supply board in a short time or by other ways, it is possible to prevent fire and to decrease the impact of explosions. This assessment results gave a significant contribution to prepare new regulatory requirements for the HEAF.

In the new Japanese HEAF regulatory requirements, prevention of ensuing fire and mitigation of explosion are required. Amendment of the regulatory requirements were issued on August 8, 2017 and enforced on the same day. One of the possible counter measures is replacement of analog type OCR to digital type OCR.

The response of the digital type OCR is much faster than that of old analogy ones.

In addition, concerning the degree of HEAF impact and other factors, safety research and investigations are to be continued, and when new knowledge is obtained, the results will be further reflected in the regulatory standards, as necessary.

REFERENCES

[1] OECD FIRE Project - TOPICAL REPORT No. 1, “Analysis of High Energy Arcing Fault (HEAF) Fire Events”, NEA/CSNI/R (2013)6, 2013:

https://www.oecd-nea.org/nsd/docs/2013/csni-r2013-6.pdf

[2] OECD FIRE Project - TOPICAL REPORT No. 3, “Event Combinations of Fires and Other Events,” NEA/CSNI/R (2016)7, 2016;

https://www.oecd-nea.org/nsd/docs/2016/csni-r2016-7.pdf

[3] H. Kabashima and S. Tsuchino, NRA Technical Report Series NTEC-2016-1002, March 2016.

[4] Nuclear Regulation Authority (NRA): NRA’s public document, Reference material 2, February 22, 2017, Tokyo, Japan. (in Japanese); https://www.nsr.go.jp/data/000179744.pdf

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