Information on the implications of the Fukushima Nuclear Accident

Transcription

1 Information on the implications of the Fukushima Nuclear Accident The National Nuclear Laboratory and the Dalton Nuclear Institute at The University of Manchester offer the following information in support of Mike Weightman s report on the implications of the Fukushima nuclear accident. What you know or understand about the facts surrounding the events in Japan? Detailed and accurate information regarding what happened at Fukushima Dai ichi was not available at the time of writing this document. All factual information was therefore based on what are believed to be open source sources (mainly JAIF and World Nuclear News). The account below is not exhaustive. Information regarding recorded doses and leaks has been omitted. Earthquake and subsequent tsunami with a height in excess of those anticipated in safety case. According to WNN, design basis for Fukushima Dai ichi was 5.7m. Unconfirmed reports that the wave was over 10m high. The three reactors (Units 1, 2 and 3) operational at the time were automatically shut down. Remaining three units (Units 4, 5 and 6) were undergoing maintenance and were already in cold shutdown state or fully/partially unloaded. Damage primarily to seaward side of plant due to tsunami wave resulting in loss of on site diesel generators and residual heat removal capability (all 6 units) It is believed that batteries were used initially to power site following tsunami wave. Once depleted all 6 units lost ability to actively cool the reactor cores and spent fuel ponds situated on the same level as the pile cap Accumulation of hydrogen gas in all units. Accumulation resulted in an explosion within unit 1 reactor building which removed the external façade on 12 th March. Damage to the reactor containment including primary circuit believed to be minimal. It is not known whether the hydrogen accumulated was as a result of steam/zircaloy reaction in spent fuel pool or within reactor. Subsequent explosion in Unit 3 on 14 th March due to accumulation of hydrogen gas in reactor building. Further explosion in Unit 2 occurred on 15 th March. At the time, it was believed the explosion occurred within the suppression chamber (i.e. within the reactor containment). The external façade of the building visually remained intact. 1

2 Loud explosion heard from within Unit 4 (de fuelled but with significant amounts of spent fuel in spent fuel storage pond) resulting in damage to reactor building façade. Multiple fires originating from spent fuel pool. Cooling ponds believed to have been boiling and may have become completely dry. During this time, site had no access to fresh water and in order to cool the reactors and spent fuel storage ponds, decision was made to pump sea water into reactors using fire suppression system and into the cooling ponds using fire trucks and other similar vehicles. During this time, water temperature in Units 5 and 6 rose but cooling function believed to have been regained before any major damage to stored spent fuel. What you draw from these facts about the lessons that might be learnt to enhance the safety of nuclear facilities in the UK or any prospective new nuclear power stations? The following areas should be considered in respect of UK nuclear facilities. We note that the following does not imply that action is necessarily warranted since existing design and systems may already be adequate. However, this should be confirmed. Spent fuel management National issues regarding spent fuel management may have led to an increased risk associated with the spent fuel ponds at Fukushima. For instance, the accumulation of large quantities of spent fuel at power station sites due to, for example, the ability to make or carry through policy decisions; delays at the Rokkasho fuel recycling plant, or shipping spent fuel to UK or France for reprocessing. There are parallels in the UK where, for example, the country has accumulation of 100 tonnes of plutonium. We note the recent Smith School of Enterprise and the Environment, Oxford University, report on A Low Carbon Nuclear Future: Economic Assessment of Nuclear Materials and Spent Nuclear Fuel Management in the UK in this regard, which assesses four possible scenarios regarding how UK might address its spent fuel and plutonium stockpile. Related issues that should be considered include: How did the spent fuel storage ponds lose water so quickly? What about the long storage times needed for spent fuel before disposal from UK new nuclear build; up to 100 years for uranium fuel and substantially longer for spent MOX? Do we really understand the implications for such long storage times, and how confident are we that the fuel will be in good condition at the end? Are current and proposed spent fuel storage ponds an "Achilles heel" in the UK? Is the safety case for spent fuel ponds adequate both at reactor sites and at Sellafield? Should there be more "defence in depth" for fuel storage ponds? Are there any implications for the Highly Active Storage Tanks (HAST) at Sellafield? 2

3 Accident response Fukushima was a beyond design basis accident compounded by the rapid sequencing of severe events. It would be useful to review the severe accident scenarios identified for UK plant and to assess the plant response to rapid sequencing of events that compound the accident evolution. So called stress tests are being implemented in Europe, and it will be important to incorporate such scenarios within these studies. There are provisions in the UK for off site emergency response and it is unclear whether there are similar provisions in Japan. Given the scale of events in Japan and the associated huge demands on the emergency services in the period immediately following the earthquake/tsunami it would be too easy to criticise the emergency response. However, it is worth setting out what might be expected in the case of a major event. First, one would expect the off site response team to be giving detailed advice to the staff on site about actions to be followed. Second, one would expect the off site team to be concentrating on the provision of supplies of coolant and power as discussed above: for example looking at alternative supplies of water by tanker, reconnection of grid supplies, supplies of diesel, movement of pumps and generators from other sites. Finally, one would expect the emergency response team to be providing a clear summary of the emergency to the media: from a description of the plant, to the current state of the plant, and of course any implications or advice for the general public. However, there remains a need to demonstrate how the UK nuclear industry would respond to a catastrophic event or events and bring the plant back under control. One way to demonstrate this would be to plan emergency exercises that start with a catastrophic event and the team has to take the necessary steps to bring the nuclear plant back under control. The scale would have to approach that experienced at Fukushima where response measures may have to be of a national scale. The exercise could also involve the media as keeping the public informed is a key part of an emergency these days. We are unaware that this scale of exercise is practiced except we suspect in a limited manner with respect to a security event such as a dirty bomb in the centre of a city. Lessons learnt from such exercises would identify what would need to be mobilised at a national scale and would provide the required re assurance to the public. Related issues regarding severe accidents include: Was "defence in depth" adequate at Fukushima? Same question for t he UK. Were enough emergency exercises conducted at Fukushima? Same question for the UK. Was the emergency control centre at Fukushima adequate? Same question for the UK. Probabilistic Safety Analysis (PSA) Issues regarding PSA include: Were the seismic event ratings and frequencies accurate for Fukushima? Same question for UK plants. Were the tsunami event ratings and frequencies accurate for Fukushima? Same question for UK plants. Are the other accident ratings and frequencies for UK plants robust? 3

4 Reactor engineering and fuel design Without detailed inspection data it is difficult to comment on the robustness of the reactor and containment design to the beyond design basis loading experienced due to the earthquake, tsunami and explosions. We therefore raise the question, was the containment system at Fukushima adequate? Same question for UK plants. The requirements for and provision of back up supplies of coolant and power. Clearly, systems which have minimal demand for coolant and power following automatic, safe shutdown have inherent safety advantages. However, where there are requirements then back up systems need to be robust. Availability of fresh water needs to be considered when siting a new reactor. The use of saline water at Fukushima will lead to future complications and if the utility had access to water from a local fresh river or stream, the situation might have been slightly better. Thus, for a water reactor, one would expect emergency cooling water (purified) storage as the first back up in the event of a loss of coolant followed by mains (town) water supply (less pure) followed by other supplies (not purified; river, lake, sea) as a last resort. Other supplies (boron, etc.) also need to be considered. In the event of loss of electricity grid supplies, then one might expect a large plant to supply power from reactors that continue to operate to those that shut down. Essential diesel generators would then be the back up supply if all reactors shut down and there is no grid supply. Clearly, maintenance, testing and security of diesel generators (and the essential systems that they supply) are key issues and of course an adequate supply of diesel. Use of Zircaloy cladding and the subsequent steam Zircaloy exothermic reaction. Is Zircaloy the most ideal material for fuel cladding? Are there other material choices that (a) will behave better in an over heating accident and to a lesser extent (b) will not severely impact economics Silicon carbide, stainless steel (will result in economic penalty), more resistant zirconium based cladding material. Related issues regarding reactor engineering include: Would a water gravity feed system (from a seismically qualified water tower that avoided the need for pumps or sea water) have helped? Backup instrumentation and surveillance The instrumentation at the Fukushima plant has been designed for normal operation, and is now being used to manage the plant under severe accident conditions. This leads to two issues that could usefully be highlighted. Since many of the sensors failed due to the severe conditions, the issue of backup sensors should be considered. Understanding the status of the field, RPV, dry well and spent fuel ponds is difficult given the lack of surveillance equipment. This second aspect would help to manage the plant under severe accident conditions. Inspection and maintenance frequencies Issues regarding inspection and maintenance frequencies include: 4

5 Was the safety inspection and review regime at Fukushima adequate? Same question for the UK. Were the Fukushima reactors left operational for too long?/was enough safety upgrading done over time? Same question for the UK. Data management and communication. The collection, collation, QA and release of data is an area that should be assessed. The ability to find factual and unbiased information regarding Fukushima after the event was difficult. For example, the I 134 mistake arose from misidentification of a photopeak in the gamma spectrum, these days often automatically by computer, with nobody with real knowledge checking it. It would be helpful to consider an approach to international best practice for the generation and communication of information. We suggest the availability of detailed information regarding the state of the reactor prior to any accident should be kept by an internal agency (perhaps IAEA) and more importantly be kept up to date. This information could then be circulated to relevant national agencies in the event of a severe accident ( INES Level 5) to help assess their decision making process. Information such as the number of assemblies in the cooling ponds, estimated EOC date, BOC date, etc. could be made available which would help to determine a reliable source term for use in environmental modelling simulations. International response The events across Japan due to the earthquake and tsunami meant that the country had to deal with major loss of life and basic infrastructure whilst managing a major nuclear incident. A mechanism by which the international community was able to respond rapidly to help advise Tepco in how best to manage Fukushima would have been helpful. A coordinated international approach and preparedness for severe nuclear incidents would be useful. The INES scale could perhaps provide the basis for international engagement in a nuclear incident, e.g. severe accidents up to INES Level 4 could be handled nationally, while Level 5 requires international advice. This could be organised through IAEA or WANO? Siting and co location of nuclear power stations. Multiple reactors at same site and evidence of direct knock on effects. Unconfirmed reports that the explosion at unit 3 impacted the bespoke cooling function of unit 2 at the time. The explosion at unit 3 might have directly contributed to the explosion within the suppression pool at unit 2. Issues arising from the presence of shared facilities, for example control rooms; the difficulty of managing two different accidents from the same room; the risk of losing control rooms for two reactors simultaneously. Common mode failure it is believed that the backup diesel generators were used by multiple units. It is believed all diesel generators were situated on the seaward side so were not geographically separated 5

6 Impact of an accident in one reactor on others close by, for example through explosion damage (as is alleged for Unit 3 on Unit 2 at Fukushima) or through contamination limiting man access. Retro fitting of passive and additional safety systems It has been noted that the Fukushima nuclear accident was initiated when the tsunami flooded the backup diesel generators, making them inoperable. If a coastal site, diesel generators should not be situated on the coastal side but shielded from the sea by turbine hall / reactor building. The EPR includes two additional diesel generators, a supplementary back up system to the four main back up diesel generators. They are located separately from the main generators to eliminate, as far as possible, common cause failures between the two systems. For all nuclear plant, should be located in such a manner as to minimize the impact of earthquake or flooding damage. Both the EPR and the AP1000 include passive recombiners that catalytically combine hydrogen with oxygen to form water, thereby minimizing the risk of hydrogen detonation. These systems should be tested robustly, and retrofitted to existing plant where appropriate. Existing and more robust venting strategies should be assessed and explored to ensure the risk of hydrogen build up is mitigated. Recommendations regarding the retro fitting of such passive and additional safety features where appropriate on existing plant would be useful. For example, provision for the ability to connect more generic external power supplies to the main bus in order to power safety systems in the event of a prolonged system blackout and multiple connections to the grid. Consideration should be given to the provision of multi option safety systems which can respond flexibly to severe and compounded accidents as they evolve. For example, the provision of multiple options for power to the plant, locating pure water tanks for cooling of reactors and spent fuel ponds, additional pump systems etc. Issues regarding safety systems include: Was it necessary to place emergency equipment in the way of a large tsunami? Was the emergency equipment (e.g. diesel generators) adequate for the job? Why was hydrogen allowed to accumulate in buildings? the roofs of the outer reactor Is hydrogen appropriately dealt with in the UK? Public Understanding and media presentation of nuclear The public will have seen a NPP explode and then seen consequential health measures in handing out of iodine tablets and the Japanese public wearing masks. The public will ask questions; Could this happen in the UK with the technology and siting of our old plants and the planned new ones? How would the UK respond to a similar disaster? Do we practice our emergency response at this scale of event? 6

7 The public will be looking to Government for answers, and the manner of the answers will dictate the level of trust it will deserve. The industry, regulator and Government need to answer these questions openly and honestly. If current response measures fall short then you should say so and identify what steps will be taken and the timescale they will be taken over. The Japanese event provides a real opportunity to demonstrate the safety of existing UK plant and their siting and the steps that are taken to provide defence in depth against external events. Radioactive release and protection Issues r elating to radioactive release of protection include: Was sufficient and adequate protective clothing and equipment available at Fukushima? Same question for the UK. Were enough stable iodine tablets available at Fukushima, and in the right place at the right time? Same question for the UK. Was sufficient monitoring equipment available at Fukushima? Same question for the UK. Does more need to be done to capture any volatile elements that are released? Contact details Professor Andrew H. Sherry Director, Dalton Nuclear Institute The University of Manchester Professor Paul Howarth Managing Director, National Nuclear Laboratory National Nuclear Laboratory 7