ML19294C076
| ML19294C076 | |
| Person / Time | |
|---|---|
| Issue date: | 07/12/1979 |
| From: | Bickel J Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML19294C077 | List: |
| References | |
| ACRS-SM-0154, ACRS-SM-154, NUDOCS 8003060647 | |
| Download: ML19294C076 (2) | |
Text
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UNITED STATES A7 gfjggp g/g Y
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July 12, 1979 I.
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ACRS MEMBERS TRANSMITTAL OF PRELIMINARY REPORT ON RADIOLYSIS AND RECOMB REACTOR COOLANT SYSTEMS In mid-May, Dr. Stephen Lawroski requested an evaluation of the potential rates of generation of combustible concentrations of Hydrogen and Oxygen gasses within the primary coolant system of pressurized water reactors as This memo a result of post accident radiolysis/ recombination reactions.
transmits preliminary information on the subject that will undoubtedly be revised when additional information becomes available.
STATUS OF EXISTING KNOWLEDGE Under the presence of a radiation field, reactor coolant system water will decompose with the following net reaction occurring as a result of a number of intermediaries:
2H 0 2H
+
0 2
2 2
and 0 has been found to be sensitive to The net equilibrium buildup of H2 3
impurity levels, coolant pH and temperature.
During normal operation of a pressurized water reactor, it is essential that the equilibrium concentration of 02 (dissolved in the coolant system water) be maintained as low as possible to inhibit the metallic corrosion rate (by Typical requirements on oxidation) and its associated crud buildup.
Thiscorrespondstoamaximumconcentrationof3.125x1 maximum 0 moles /
2 0.1 ppm.
concentrations are normally controlled by maintaining an over-liter. 0 2 gas which is significantly greater than the normal H 2
2 pressure of H In this manner, dissolved H2+ gas is employed as equilibrium concentration.
O --/ 2H 0.
anoxygenscavengerandpromotesthebackreactign2H2 2
2 H gas This is accomplished by maintaining 15-40 STD cm /kg.H O of 2
2 This overpressure yields dissolved in the reactor coolant system water.
e ncentrations in the range of 6.7 x 10~4 mole / liter to 1.8 x 10-dissolved H levels 2 Actual utility experience in this area indicates that H2 moles / liter.
as low as 5-6 STD cm /kg.H O will hold dissolved 0 levels at or below 0.02 ppm, 3
2 2
even'in the presence of Cl, F~ and natural metallic impurities.
~
Under an accident situation where boiling occurs in the core region, the existing dissolved gasses in the coolant can be stripped out of solution.
This occurs because the solubility of gasses in water (which are dependent 7
[ We on Henry's Law) decreases with increasing temperature. These gasses are then free to accumulate in high points of the reactor vessel and in the primary I
coolantsystembiping.
In the localized region where boiling occurs, the removal of thejexcess H2 gas will permit the net radiolysis to proceed in the forward direc1Aon and evolve more dissolved H and 02 gas. Recombination of 2
H and 02 gasses will occur in the vapor region and detailed evaluations of 2
this have been made for the U.S. Navy. As with recombination in the liquid phase, the presence of excess H2 gas will promote a rapid recombination of H and 02 gas. Certain accident conditions can lead to extensive generation 2
2 gas via high temperature oxidation of the Zircalloy fuel. Ehen boiling of H ceases, this excess H2 gas can return to solution up to the limits of solubility based on Henry's Law. The presence of this hydrogen would terminate any further net radiolysis of the coolant. Additionally, the excess hydrogen in the bubble at the top of the reactor vessel would promote removal of any oxygen existing there at the time. Thus, after oxidation of the Zircalloy core and generation of excess hydrogen, the coolant could be described as an oxygen starved system. The implication of this is that while excessive hydrogen may have existed it would appear oxygen could not be present in sufficient quantities to warrant concerns over flammability within the Reactor Coolant System.
FURTHER ITEMS BEING INVESTIGATED Conv "tions with personnel at Argonne National Laboratory (ANL) and the Knol
.tomic Power Laboratory (KAPL) have indicated the existence for over a decace of numerical codes for simulating the effects of radiolysis and recombination in reactor coolant system water. One of these codes: WR20 (described in ANL-7693) is in the public domain and has been benchmarked.
One of the items presently being investigated is a determination of possible gas generation and removal kinetics in the coolant system at TMI-2 based on the postulated radiation exposure the coolant received. This is being carried out by Klaus Schmidt and Sheffield Gordon of ANL and they have promised to keep me informed of their progress in this area.
As a personal opinion, it would appear to be useful if a determination were made of the existence of any possible scenario for core damage similar to, but more extensive, than TMI-2 which could lead to generation of flammable concentrations of H and 0. As a first approach it would appear that one 2
2 of the possible mechanisms would be failure modes which lead to the existence of large concentrations of CK - emitters dissolved in the coolant (extensively greater than sources available from soluble Boron). This is because high LET radiations produce higher microscopic yields of OH, H0 '
2 and H 0, radicals (which are more conducive to 07 generation during 3
recombihation). Such a determination would be potentially useful in evaluating the need for and desirability of remotely operated venting systems for the reactor coolant system.
p (1
s John H. Bickel ACRS Fellow
Attachment:
McCreless (for Branch circulation) cc:
Subject report McKinley (for Branch circulation)
I cv for ACRS Fellows circulation