Climate change

March 15, 2011

Further technical information on Fukushima reactors

Filed under: IFR (Integral Fast Reactor) Nuclear Power, Japan Earthquake — buildeco @ 11:33 pm


by Barry Brook

Below is edited material sent to me in confidence from some colleagues in the professional nuclear engineering and research community. It provides some further insight into what is going on at Fukushima, and what is still unknown.

The main document was written by one nuclear engineer; the quotes are the response from another engineer.

——————————

1. Likely timeline of incident is:

a. Reactors 1, 2 and 3 were in operation at Fukushima Daiichi nuclear power plant when the earthquake struck.

b. all three reactors were shut down and control rods were inserted when earthquake struck.

c. Cooling was maintained to remove decay heat

d. decay heat drops rapidly on reactor shut down (e.g a 3GW reactor will reduce to 200MW decay heat after 1s and 50MW after 1 hour… But takes long time (3-6months!) to reduce to negligible levels)

e. sometime (≈1hr) later tsunami struck and mains power was lost to coolant circuit on Unit 1

f. Diesel generators also failed when tsunami hit so cooling was run by backup batteries for 7-8 hours

g. Other emergency diesel generators brought in but insufficient to run pumps

h. loss of coolant leads to fuel rods no longer being cooled by two phase flow (it is a Boiing water Reactor) and eventually get hot enough to recat with steam to produce Hydrogen.

{While this is plausible it would suggest massive loss of Pressure Vessel (PV) generated steam beyond the containment boundary (since the explosion did not disrupt the containment). In my view a far more plausible explanation is that hydrogen routinely injected in the Make Up Water System to control the corrosives (mainly O2) produced by radiolysis was released suddenly and catastrophically from outside the containment and within the reactor building. In reacting with oxygen from the atmosphere within the building at the correct concentration of hydrogen (4-74%) only a spark is required to detonate a hydrogen oxygen explosion.

By contrast the spontaneous splitting of water to hydrogen and oxygen requires a temperature of greater than 2000 C in the absence of a catalyst. The normal radiolytic environment of a BWR core produces an excess of oxygen, not hydrogen. Given that the explosion reduced the source term, by all accounts, the reaction with hydrogen for Make UP water injection is a likely scenario and potentially the most plausible explanation for such an explosion at present. This could be even more plausible and verifiable if the Daiichi plants were not retrofitted with Noble metal corrosivity reduction systems (Pt) which have been implemented in many BWRs to reduce site hydrogen inventory. Even with noble metal systems there is still a significant hydrogen inventory external to the containment in BWRs to manage the corrosivity of PV water}

i. Gas pressure in steel reactor pressure vessel rises when coolant systems are not active and is vented to reactor building by engineers. {In my view there is not yet plausible evidence that the temperature of the PV water was sufficiently high to spontaneously split water}

j. The hydrogen in the reactor building is ignited in an explosion which blows out the walls but is not likely to have damaged Steel pressure vessel or concrete containment.

{this is true of both scenarios –but the source of the hydrogen is also external to the PV and containment in my explanation, overcoming the problem of why there was not a hydrogen explosion within the containment and outside the PV!}

k. Operators are now flooding reactor with borated sea water. Would only do this of they had run out of demineralised water – and were sure that water was not spontaneously being split in the PV.

2. Other relevant observations

a. These reactors are old generation I GE BWR’s nearing the end of their useful life so economic loss would not be large: but decommissioning and decontamination costs may be significant.

b. Some reports say 50-100cm of fuel was above coolant. Not necessarily serious in BWR but hydrogen explosion suggests some of the core got very hot

{wrong explanation in my view}!

c. No evidence of catastrophic loss of fuel element integrity yet. BWR fuel often gets small leaks and fission fragments enter water! Only relatively low levels of Cs and I reported so far suggesting fuel integrity still OK.

d. Increase in Cs and I would accompany fuel element breakdown and eventual meltdown.

e. A parallel series of event may now be happening in Unit 3.

f. MOX fuel may have been in one of these reactors

g. to a first approximation there is no difference on the present context between conventional and MOX fuel

h. If anything, the MOX should be marginally more benign for a radiological point of view than UO2 based fuel. It may have a higher toxicological risk however.

3. It is expected that enough coolant and power will be found to avoid meltdown. In which case it could be argued that this is a reasonable result for 50 year old Gen I Reactors exposed to the worst earthquake and tsunami for 100+ years!

4. If meltdown does occur fuel should melt through to concrete basement spread and eventually cool. Not a good result but hopefully, predictable.

5. These events would not happen in modern reactors which are designed to be cooled on shutdown by natural convection.

——————————

Response: H2 (production due to oxidation of Zircalloy by high temp steam has to be seriously considered in all full or partial LOCA (loss of coolant accident) scenarios, so I remain conservatively suspicious.

Reply: You then still have to explain why the primary event chain for PV generated H2 and O2 which is 1) PV to containment, then 2) containment (no explosion in the containment) prior to 3) venting to atmosphere!!! (which would in any event be to the stack via filters not via the reactor building)!!! The video footage of the explosion was clearly a fast H2/O2 (shockwave evident) and asymmetrical event– blast leaves to screen left – which implies a concentrated source of H2 mixing with oxygen (Make Up water inventory) rather than a diluting source into atmosphere via the stack which would have taken out the filters at least and increased rather than deceased the site boundary source term.

But I agree very hot Zircalloy in steam is a plausible low temperature route to H2/O2, but remember that the temperature is decay heat not fission generated so radiolysis to produce extra O2 will be many orders of magnitude below that of an operating BWR. I’ll regard a PV chain as more plausible if you can explain how hydrogen (very light stuff) got from containment to reactor building – possible with certain piping configurations…I guess.

Unit 1 was the GE BWR design. These references might assist:

Radiochemistry in Nuclear Power Reactors (1996) Commission on Physical Sciences, Mathematics, and Applications

BWR water chemistry – a delicate balance (2000) British Nuclear Societ

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