< R e f e r e n c e > Partial damage to temperature gauge cables for the Reactor Pressure Vessel (RPV) found during the Fukushima Daiichi NPS Unit 3 Primary Containment Vessel () internal investigation November 30, 2017 Tokyo Electric Power Company Holdings, Inc.
Situation of temperature gauges at the bottom of the Unit 3 RPV based on the internal investigation (1/3) There are two routes for temperature gauges at the bottom of the RPV, one inside the pedestal (12 temperature gauges ) and one outside the pedestal (6 temperature gauges ). Three along the inside route and six along the outside route are used to monitor the limiting conditions of operation stipulated in the implementation plan. Damage to the inside pedestal route (12 temperature gauges) was seen in images taken during the internal investigation. Damage was found on cables inside the between the relay terminal platform and measuring points Diagram of temperature gauge locations RPV ( cross section) Damaged temperature gauges temperature gauge cables leading outside the pedestal penetration Location of damage (enlarged) water level temperature gauge cables Pedestal Relay terminal platform Terminal platform inside the penetration Damage was found along the route inside the pedestal temperature gauge cables temperature gauge cables leading inside the pedestal Temperature gauge cable condition inside :Temperature measuring point the pedestal before the accident :Temperature measuring point (implementation plan) 1
Situation of temperature gauges at the bottom of the Unit 3 RPV based on the internal investigation (2/3) <Assumed causes of the damage> temperature gauge cables found damaged are made of a metallic sheath and wires that have a melting point of more than 1,000. It is assumed that the cables experienced thermal cutting by high-temperature molten objects that fell onto the cables from the RPV bottom. The 12 temperature gauges along the route leading inside the pedestal are considered to be broken because thermal cutting of the cables has rendered them unable to measure the temperatures at the bottom of the RPV. <Status of reactor cooling> There are many uncertainties surrounding the location of fuel debris, however it is assumed that during the accident, molten fuel debris fell to the bottom of the RPV or and landed on structures located between the bottoms of the RPV and during the fall. This fuel debris is being kept cool in a stable manner by continuously injecting enough cooling water to sufficiently offset the amount of thermal heat being generated by the fuel debris. It has been confirmed from not only temperature readings taken at the RPV and, but also temperature gauges installed in the after the accident, gas management equipment dust monitors and the measured concentrations of hydrogen and xenon 135 that the overall state of cooling is being maintained and that the fuel debris is cool. 2
Situation of temperature gauges at the bottom of the Unit 3 RPV based on the internal investigation (3/3) <Temperature gauges other than those found damaged> It is assumed that the RPV was damaged after the depressurization of Unit 3, and thought unlikely that melted fuel spattered to the height that the temperature gauge cables outside the pedestal are located and reached temperatures of more than 1,000. Therefore, it is assumed that temperature gauges along the route outside the pedestal were not damaged by thermal cutting. Diagram of temperature gauge locations water level penetration Due to the location of installation it is thought that the state of cooling is confirmed, but a further investigation will be performed. Damaged temperature gauges Relay terminal platform Terminal platform inside the penetration Thermal cutting found here We will further confirm the state of cooling by measuring the resistance of temperature gauge circuits and looking at past temperature data for RPV and temperature gauges. We will also deliberate how to check the state of cooling going forward based on the results of the internal investigation etc. 3
Reference: Temperature measurements along the route inside the pedestal Temperature readings that have been obtained from the 12 temperature gauges along the route inside the pedestal infer the following in regards to the current circuit configuration and temperature measuring points. Insulation reduction 1 point Thermal cutting point 1 It is assumed that there is an insulation reduction point at the terminal platform inside the penetration. Circuit configuration diagram for temperature gauges with cables leading inside the pedestal Outline of temperature gauge positions Thermocouple temperature gauges can measure temperatures only when the copper and constantan wires form a closed circuit. The temperatures in the RPV (measuring points) cannot be measured because the cables were severed by thermal cutting. Insulation reductions inside the caused by the accident have been confirmed. Temperature readings can be taken at locations of insulation reductions because the insulation reductions form thermocouple circuits. This means that temperatures in the 2 are being measured. Damaged temperature gauges water level 2 Scope of measurement of temperature gauges with cables severed by thermal cutting Temperature measuring point Relay terminal platform Terminal platform inside the penetration penetration 4 :Temperature measuring point :Temperature measuring point (implementation plan)
Reference: Trends for current temperatures at the bottom of the Unit 3 RPV 60 RPV RPV 底部ヘッド上部温度 WALL ABOVE BOTTOM (TE-2-3-69H1) HEAD (TE-2-3-69H1) RPV RPV 底部ヘッド上部温度 WALL ABOVE BOTTOM (TE-2-3-69H2) HEAD (TE-2-3-69H2) 12 Temperature( ) 50 40 30 20 RPV RPV 底部ヘッド上部温度 WALL ABOVE BOTTOM (TE-2-3-69H3) HEAD (TE-2-3-69H3) スカートジャンクション上部温度 ABOVE SKIRT JOT (TE-2-3-69F1) (TE-2-3-69F1) スカートジャンクション上部温度 ABOVE SKIRT JOT (TE-2-3-69F2) (TE-2-3-69F2) スカートジャンクション上部温度 ABOVE SKIRT JOT (TE-2-3-69F3) (TE-2-3-69F3) RPV RPV 下部ヘット 温度 BOTTOM HEAD (TE-2-3-69L1):broken RPV RPV 下部ヘット 温度 BOTTOM HEAD (TE-2-3-69L2):broken RPV RPV 下部ヘット 温度 BOTTOM HEAD (TE-2-3-69L3):broken CST coolant line average temperature whole injection FDW flow CS flow 11 10 9 8 7 6 5 4 Injection flow(m 3 /h) 3 10 2 1 0 0 11/28 11/27 11/26 11/25 11/24 11/23 11/22 11/21 11/20 11/19 11/18 11/17 11/16 11/15 11/14 11/13 11/12 5