The idea of “containment venting” has been front and center in discussions about the Fukushima Dai-ichi nuclear accident and what the NRC wants plants to do to improve their vents. But to most people outside the nuclear industry vents are the things in our houses that hot or cold air flows through. So here’s a little background.
The accident in Japan involved what’s called a Mark I boiling-water reactor. Mark I designs have a relatively small structure, or “containment,” to hold in steam and radioactive material if an accident occurs. If pressure inside the containment gets too high during an accident, the reactor’s safety systems will have trouble pumping water into the core to keep it cool – which will make the accident much worse and possibly lead to high levels of radiation escaping into the environment. Part of this accident scenario also involves hydrogen gas building up inside containment. As we saw at Fukushima, if hydrogen is not allowed to escape, it can explode and damage the reactor building, which also could lead to radiation leaking into the environment.
This is where vents come in. They can be used to reduce pressure in containment so that water can still be pumped through to cool the fuel rods. The vents can also safely release built-up hydrogen to prevent explosions.
Decades ago, U.S. Mark I plants installed vents, valves and piping, but the circumstances in the Fukushima accident suggest the vent designs should be improved. The NRC is also considering whether the vents should have filters to capture any radioactive material in the vented gas
On March 12, the NRC issued an Order to all U.S. Mark I plants, as well as similar Mark II reactors. The Order requires Mark I plants to ensure their vents are hardened and reliable, and it requires Mark II plants to install hardened, reliable vents.
“Hardened” means these vents must withstand the pressure and temperature of the steam generated early in an accident. The vents must also withstand possible fires and small explosions if they are used to release hydrogen later in an accident. The vents must be reliable enough to be operated even if the reactor loses all electrical power or if other hazardous conditions exist. The NRC staff will issue, later this summer, specific guidance on the requirements for containment vents.
In order to ensure these vent improvements are properly designed and installed, the NRC has set a deadline of Dec. 31, 2016, for the Mark I and II plants to comply with the Order.
Scott Burnell Public Affairs Officer
U.S. NRC Blog 
The NRC updated the Order requiring hardened vents in 2013:
https://public-blog.nrc-gateway.gov/2013/03/19/nrc-commission-approves-more-post-fukushima-upgrades-to-nuclear-plants/
The updated Order’s compliance requirements are:
Phase 1 (severe accident capable wetwell venting system): no later than startup from the second refueling outage that begins after June 30, 2014, or June 30, 2018, whichever comes first.
Phase 2 (severe accident capable drywell venting system): no later than startup from the first refueling outage that begins after June 30,2017, or June 30,2019, whichever comes first.
Scott Burnell
Information on each U.S. reactor’s progress in meeting the NRC’s post-Fukushima Orders is available on the NRC website:
http://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard/japan-plants.html
Where are we regarding the compliance date in December, 2016? Also, is this date for modifications to be fully completed or is this some “nebulous” standard of compliance in which if a complete power loss occurred two days after that date we’re still no safer than the day before?
They were part of the US TMI process. Had Japan followed our recommendations, we would have read about the Fukushima restart instead of their accident. Cheap – Fast – Quality… pick two
please read about reuse of fuel.. even for that ,matter the nuclear reactor operation… mostly the fuel reused are superior in utility (more fissile fuel after re-processing) to their fresh version. Anyways its not a coal based pant…
What? Umm, no.
I feel as though these precautions should have taken place before disasters such as the Fukushima Dai-ichi nuclear accident happen. Where were the environmental risk management programs? Glad to see some effort is being made, but we could have and should have done better.
Sir, i have a doubt . If the fuel is reused for the second time, its efficiency is reduced and the amount of sludges and vent dusts may increase. At that time if the same ventilation mechanism is used, can those ventilations survive otherwise what is the solution?
The real question is what is really being done about our safety as citizens. Whatever the vents cost or the process needed to make them the safest is the method need to be used
Managing the proper pressure levels in our own water,gas,oil systems is already challenging enough, with the use of various valves, gates, stainless steel and what not. Imagining containment of such a tragedy is difficult. I am familiar with industrial supplies for pressure management, ball valves and such, but would they have much play or usage in this type of situation.
The comment about CANDU safety is misguided. The end-point (reactor meltdown) for any Generation II plants would be similar under station blackout and lack of ultimate heat sink situation, with varying degree of release due to containment designs. The slight negative containment pressure during normal operation neither can be sustained nor would be a match to the energy release during extended station blackout, therefore is irrelevant in this case.
No reactor is designed perfect. Nothing in engineering is made to perfection.
All reactors HWR or LWR reactors can suffer a LOCA event. The advantage of small modular reactors SMR’s is they’re gravity passive fuel dump safety features plus the nucleotide dynamics are an added layer of safety. Besides the aggregate event fail is smaller than multi-gigantic light water NPPs.
The boiling-water reactors used at Fukushima are of an inferior US design. Had the reactors been of superior Canadian CANDU design, the Fukushima incident would have been a non-event. Canadian CANDU designed reactors have multiple safety systems including a Negative Pressure Containment Vacuum Building which prevents the escape of radioactive materials in the event of a Loss of Coolant Accident. Canadian designed CANDU reactors are the safest most reliable reactors in the world!
Severe accident response for water cooled water moderated reactors
Aladar Stolmar 11-14-24-2011
Summary
Considering the TMI-2, Fukushima Daiichi 1, 2 and 3 reactor severe accidents and the Paks-2 fuel washing vessel incident, also the SFD and other related nuclear reactor fuel severe damage experiments it is evident that the ignition and firestorm of the Zirconium-steam reaction occurs several hours after the severe reduction in cooling capability arises. This solution utilizes this time gap and proposes the equipment and response modifications aiming the elimination of ignition of Zirconium-steam reaction in PWR and BWR reactors. Also the solution deals with the results of such ignition and Zirconium-steam reaction in the maximum extent, in case the ignition despite the efforts occurs.
Aiming the public safety in presence of nuclear power plant two questions have to be answered: Do You prevent the ignition of Zirconium in the steam? And Do You protect the surrounding of the plant from radioactive releases in case the entire Zirconium inventory burned in a firestorm? The presented here solution gives positive answers to both questions.
The key element of this solution is the rapid reactor depressurization using a top vent from the reactor head. The same will provide a controlled routing for the Hydrogen generated in case the ignition of Zirconium-steam reaction still occurs.
Recognizing the need for response
It is very important to recognize that the condition for a severe accident response developed. The key characteristic for such a condition is the loss of reactor fuel cooling capability. The best demonstration of this is provided by the Paks refueling pond fuel washing vessel incident. The turning off the circulating pump required the switching on the natural circulation by removing the vessel head. Failure in the removing the washing vessel head after turning off the pump equals to loss of cooling capability. In TMI-2 the loss of cooling capability was caused by the loss of primary system coolant and hence the interruption of circulation through the core. The indicators were the temperature measurements from the core exit, – unfortunately ignored by the operators. In Fukushima Daiichi the loss of outside heat sink connection and continued heat transfer to the torus in form of exhaust from the turbine driven pumps caused overheating of water reserves and cavitations of the pumps and failure of coolant supply into the reactors. (In this later case the venting of exhaust from the turbine pump drives outside the torus also would increase the time, but there are reports of control failures as well due to loss of battery power.)
It is necessary to develop a plant specific list of critical indicators, the detection of which will require to initiate the severe accident response.
The guideline for developing such plant specific severe accident response trigger indicator list should start from inability to detect the reactor condition, go through the reactor circulation interruption and end with the loss of connection to the heat sink. Any of these should require immediate activation of Severe Accident Response.
The means of Severe Accident Response
It is a very simple high pressure vent line from the top of the reactor vessel to the outside environment, to the top of the vent stack or other high point. It must be equipped with a primary or inside the containment valve, which is locked open for the reactor operation and closed only for the reactor head removal and vent line disassembly during refueling operations; a Severe Accident Response Valve, which is in normal operating condition the only closing element between the top of the reactor and the environment and opened in response to the severe accident trigger indicator; a low pressure burst disc or blow away cap protecting the internal volume of vent pipe from the environment on the top end and a separator vessel with a float type condensate drain. The high pressure separator vessel should have a volume at least 10 times the internal volume of vent line and the incoming line should be the half diameter of the exhaust vent line, for example 100 mm reactor side and 200 mm vent stack side OD lines (also 100 mm OD separator drain) would be sufficient for a 500 MWe reactor system, 125 and 250 mm OD for 1000-1500 MWe.
The location of the Severe Accident Response Valve should be close to a continuously manned operator workstation and the operators should be trained to open the Severe Accident Response Valve immediately, when instructed by the Unit Chief Shift Operator to do so. At the fill-up and pressurization (or cold shutdown depressurization) of reactor a manual 12 mm OD air-vent bypass line should be operated to assure the fill-up and drainage of the vent line before the Severe Accident Response Valve. The vent line between the reactor and Severe Accident Response Valve should drain back to reactor and drain to the separator after that.
Borated water reserve and passive injection system is a standard element of nuclear power reactors, but a review of its operability in case of Severe Accident Response Valve activation and reactor system full depressurization should be mandated. The guideline for the acceptability of the system would be that a reactor from full power should be brought to cold shutdown using only these reserves as passive injection after opening the Severe Accident Response Valve. It is likely that addition of atmospheric pressure reserves located at higher elevations will be required in most of the NPPs.
Normal response and recovery
In case of the Severe Accident Response Valve activation even from a full power initial condition the reactor system would reach atmospheric pressure and the core would be continuously flooded by borated water, no extensive stagnant steam bubble formation in the core volume and ignition of Zirconium-steam reaction is expected. When the normal cooling and circulation is restored the Severe Accident Response Valve could be closed and after a detailed review the normal operation could be resumed. In case that an elevated coolant activity indicates some fuel cladding damage, the refueling and detailed fuel condition check should be performed before the normal operation recovery.
Severe fuel damage and response
In case the same severe fuel damage as in Fukushima Daiichi 1, 2 and 3 or TMI-2, – the ignition and firestorm of the Zirconium-steam reaction, – occurs the already open or subsequent opening of Severe Accident Response Valve will provide a vent line for the generated Hydrogen, where it most certainly will ignite and burn off. (Igniters could be provided to assure this.) The possibility to close the Severe Accident Response Valve after radioactivity detected in the vent line separator will allow a reduction of the radioactive releases into the environment. Periodic opening and closing of the Severe Accident Response Valve will provide means to reduce the containment pressure to safe levels for the expense of controlled radioactive releases.
Additional safety concerns
The routing of primary system outside the containment to the Severe Accident Response Valve could be viewed as a violation of the safety concept that the entire primary system should be enclosed in the containment. Providing an additional volume enclosing the vent line in a secure and controlled manner, which may include addition of isolator valves before and after the Severe Accident Response Valve will allow the sealing of primary system from the environment in case of leakages through Severe Accident Response Valve and a sealed volume connected to or separate from the containment will be formed what would resolve this concern. Indeed, this line should be treated as the most vital pipeline in the reactor system and should be provided with all the necessary anti-earthquake and other protections, most particularly freeze protection for the gas venting possibility. The operation of the reactor system could be allowed only when this system is fully operational, all isolator valves locked open. The sizing of this line should be based on water plug jets driven under the pressure difference from an explosive ignition and firestorm of the Zirconium-steam reaction.
Civil defense and Severe Accident Response Team
The actuation of Severe Accident Response Valve should be a trigger for the civil defense evacuation order in a predetermined extent and sequence, considering the downwind direction. The operators automatically become Severe Accident Response Team with a dedicated Recovery Outfit. In case the lost cooling capability and circulation through the core is verifiably restored, the head of the Severe Accident Response Team may order to close the Severe Accident Response Valve, which automatically switches the operation to a normal cold shutdown. The activated civil defense evacuation could be interrupted or ceased only based on the radioactive releases.
Periodic tests
The Severe Accident Response tests should be provided at least yearly for the operators and for the civil defense personal as mock tests, but a real test from 50% power also should be performed on each reactor after a full core reload and restart every 5 years. In order not to endanger the fuel integrity, the interruption of cooling is not required for the opening of Severe Accident Response Valve, only when the changing conditions in the reactor system require that. However, if elevated radioactivity of coolant would indicate fuel damage even in these conditions, a review of operating license is warranted. After the live tests the burst disks or blow away caps on the top of the vent pipe should be replaced!
Rational of the severe accident response proposal
The proposed manual valve operation in case of danger of severe accident in the nuclear power reactor system may be questioned based on the fact that we are deliberately venting into the air radioactive primary coolant or even fuel fragments containing Hydrogen reaction product in case the severity of the accident reaches the ignition of the fuel cladding and fuel bundle material, Zirconium ignition and complete burn off.
The rational of the proposed response is based on the fact that in TMI-2 accident as well as in Chernobyl-4 and Fukushima Daiichi 1,2 and 3 reactor accidents the burn off of the Zircalloy was very extensive and the produced Hydrogen in TMI-2 did not cause the demolition of the containment, but in other two nuclear power plants – four reactors! – the uncontrolled evacuation of Hydrogen and detonation of Hydrogen-air mixture destroyed the reactor buildings, making very difficult the localization and recovery. A timely opening of the manual response valve and fast depressurization of reactor system has a good chance to prevent of the ignition of Zirconium in the steam at all. Also the possibility of closing the same valve in case the radioactivity release detected provides new means of reducing the environmental releases, prevents the building demolition by allowing the lowering of pressure, venting the generated hydrogen. In short the rational is in the localization of consequences of the severe accident by preserving the integrity of reactor containment and preventing the severe accident at all by the actions of operators and for the expense of limited radioactive releases.
The argument that by adding such a response valve and its tests endanger the closure of radioactivity within the fuel bundles have to be rejected based on the requirement to the fuel cladding. It has to withstand such loads. And it has to be periodically demonstrated, as proposed. If there is damage to the fuel as a result of the periodic tests, the fuel supplier has to be responsible.
An obvious question that since we are adding new means, a new vent line from the reactor why not equip this line with all the necessary filtration for the worst case scenario? For new plant design such a consideration is valid, however this solution is general, for existing plants as well and the cost implications for a 5 or 10 year remaining life of a plant does not necessarily justify such expenses for a system, which will never be used, except the test.
Scenarios
Periodic test
After the first time the 50% of power achieved on a new or fully reloaded reactor a full scale test of the Severe Accident Response have to be performed, including the Civil Defense evacuation procedure. It is necessary to demonstrate that the reactor, the plant personnel and the public are capable to handle the most severe conditions possible and the severity for the environment is under control. This test is performed with manual reactor shut down (SCRAM) and Severe Accident Response Valve opening. The test is successful if the atmospheric pressure is achieved in the reactor and the atmospheric borated water injection starts. At that point the Severe Accident Response Valve could be closed, the normal operation of the reactor resumed after the blow away cap or burst disc replaced on the vent line. During the test the normal pressure relief and make-up and drain lines could be operated to speed-up the pressure reduction and cool down, and this is a test of all emergency cooling systems as well.
Inability to detect the reactor condition
An unlikely condition when the information from the reactor telemetry is lost or corrupted. The response is performed with manual reactor shut down (SCRAM) and Severe Accident Response Valve opening and Civil Defense alarm. The Severe Accident Response can be interrupted after the reactor condition detection is restored and the circulation through the reactor core is verified or the cold shutdown is achieved. The rational to use the Severe Accident Response Valve to control the reactor is the fact that the opening of the Severe Accident Response Valve reduces the pressure inside the reactor core which causes the boiling of the cooling water and the steam replacing the water within the core reduces the energy loss of neutrons, therefore reducing the reactivity on the thermal neutrons and shuts down the chain reaction, even if the SCRAM would fail for any reason in the PWR and BWR reactors. Also the opening of the Severe Accident Response Valve and venting the steam from the top of the reactor vessel organizes a flow stream through the core from the bottom to the top, mimicking the circulation in normal cooling condition, even if that circulation is interrupted. The differential evaporation of boric acid assures a safe shutdown on its own, but the quick reduction of pressure to the actuation of accumulators with elevated Boron concentration finishes the shutdown of chain reaction by flooding the core with cold borated coolant. A prolonged cooling of the core is assured by the atmospheric borated emergency coolant reserves injection into the core under the gravity.
Reactor circulation interruption
In the event the forced circulation through the core is lost a stagnant steam bubble can form in the core and lead to the Zirconium ignition and complete burn off. The response is performed with manual reactor shut down (SCRAM) and Severe Accident Response Valve opening and Civil Defense alarm. The Severe Accident Response can be interrupted after the circulation through the reactor core is verified or the cold shutdown is achieved. The rational to use the Severe Accident Response Valve to control the cooling of the reactor is the fact that the opening of the Severe Accident Response Valve vents the forming steam and reduces the pressure inside the reactor core which causes the boiling of the cooling water and the steam replacing the water within the core reduces the energy loss of neutrons, therefore reducing the reactivity on the thermal neutrons and shuts down the chain reaction, even if the SCRAM would fail for any reason in the PWR and BWR reactors. Also the opening of the Severe Accident Response Valve and venting the steam from the top of the reactor vessel organizes a flow stream through the core from the bottom to the top, mimicking the circulation in normal cooling condition, even if that circulation is interrupted. The differential evaporation of boric acid assures a safe shutdown on its own, but the quick reduction of pressure to the actuation of accumulators with elevated Boron concentration finishes the shutdown of chain reaction by flooding the core with cold borated coolant. A prolonged cooling of the core is assured by the atmospheric borated emergency coolant reserves injection into the core under the gravity.
Loss of connection to the heat sink
As a lesson learned from the loss of Fukushima Daiichi 1, 2 and 3 reactors an event when the cooling with an outside heat sink is ceased to function is considered. The response after the reactor is shut down is performed with manual Severe Accident Response Valve opening and Civil Defense alarm. The Severe Accident Response can be interrupted after the connection to the outside heat sink is restored and the circulation through the reactor core is verified or the cold shutdown is achieved. The rational to use the Severe Accident Response Valve to control the cooling of the reactor is the fact that the opening of the Severe Accident Response Valve vents the forming steam from the top of the reactor vessel and organizes a flow stream through the core from the bottom to the top, mimicking the circulation in normal cooling condition, even if that circulation is interrupted. The differential evaporation of boric acid assures a safe shutdown on its own, but the quick reduction of pressure to the actuation of accumulators with elevated Boron concentration finishes the shutdown of chain reaction by flooding the core with cold borated coolant. A prolonged cooling of the core is assured by the atmospheric borated emergency coolant reserves injection into the core under the gravity.