atw 2017-12
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<strong>atw</strong> Vol. 62 (<strong>2017</strong>) | Issue <strong>12</strong> ı December<br />
Case<br />
no.<br />
A01<br />
A02<br />
A03<br />
A04<br />
A05<br />
1<br />
Onset<br />
of severe<br />
accident<br />
2.5 hr<br />
(9003 sec)<br />
2.5 hr<br />
(9003 sec)<br />
2.5 hr<br />
(9003 sec)<br />
2.5 hr<br />
(9003 sec)<br />
2.5 hr<br />
(9003 sec)<br />
2<br />
Depressurization<br />
timing<br />
3 hr<br />
(10804 sec)<br />
3.5 hr<br />
(<strong>12</strong>604 sec)<br />
4 hr<br />
(14405 sec)<br />
4.5 hr<br />
(16205 sec)<br />
5 hr<br />
(17926 sec)<br />
way to the reactor vessel failure. The<br />
fastest injection is initiated one hour<br />
following the SAM entry, to account<br />
for the time required to open the<br />
POSRV, confirm the RCS depressurization,<br />
and align the FLEX portable<br />
pump for the external injection.<br />
To determine the ranges of injection<br />
flow, Equation 1 is used to calculate<br />
the flow rates in con sideration<br />
of the heat removal by the vaporization,<br />
the saturation, and the subcooling<br />
of the injected water, respectively<br />
[17].<br />
(1)<br />
where W is the water injection flow<br />
rate, ˙Q 0 is the decay power level before<br />
shutdown, t is the time elapsed since<br />
shutdown, c Pw is the water specific<br />
heat, ∆T sub is the difference between<br />
the saturation temperature at the RPV<br />
pressure and the water injection<br />
temperature, and h is the enthalpy of<br />
gas (g) and fluid (f), respectively.<br />
The temperature of the injected<br />
water is assumed to be 311 K (100 F)<br />
as the initial condition. For the<br />
minimum flow rate, the injected water<br />
is assumed to change completely to<br />
vapor. If the subcooling of the incoming<br />
water in Equation 1 is ignored,<br />
about 10 kg/sec is determined as<br />
the minimum required value. Alternatively,<br />
if only sensible heat removal<br />
is allowed (ignoring the enthalpy<br />
change term in Equation 1), 30 kg/sec<br />
is required to cool the core by single<br />
phase flow. For sufficient subcooling<br />
margin, 50 kg/sec is considered as the<br />
maximum flow rate in this analysis.<br />
Referring to the document related to<br />
the in-vessel injection of FLEX from<br />
NEI, there is a commercial pump that<br />
can supply up to a range of 50 kg/sec<br />
[18]. It should be noted that for each<br />
3<br />
Onset<br />
of SIT<br />
injection<br />
3.2 hr<br />
(11404 sec)<br />
3.7 hr<br />
(13205 sec)<br />
4.2 hr<br />
(15005 sec)<br />
4.7 hr<br />
(16805 sec)<br />
5.1 hr<br />
(18226 sec)<br />
| | Tab. 2.<br />
Analysis Results according to Depressurization Timing.<br />
4<br />
Time of<br />
first<br />
relocation<br />
to RPV LP<br />
9.2 hr<br />
(33188 sec)<br />
6.3 hr<br />
(22748 sec)<br />
6.7 hr<br />
(24000 sec)<br />
6.5 hr<br />
(23393 sec)<br />
4.8 hr<br />
(17395 sec)<br />
5<br />
Time of<br />
vessel<br />
failure<br />
11.1 hr<br />
(39786 sec)<br />
7.8 hr<br />
(28097 sec)<br />
7.9 hr<br />
(28468 sec)<br />
7.8 hr<br />
(28045 sec)<br />
10.3 hr<br />
(37036 sec)<br />
of the assumed flow rates, the adequacy<br />
of onsite water resources have been<br />
checked for 72 hours.<br />
2.5.3 Assessment of phenomenological<br />
uncertainties<br />
The most important phenomenon<br />
during the in-vessel phase is the core<br />
melting and relocation process given<br />
their potential impact on reactor<br />
vessel integrity. Therefore, it is necessary<br />
to examine the uncertainty about<br />
the parameters affecting these phenomena.<br />
Two parameters are important<br />
in this regard, EPSCUT, which is<br />
the cutoff porosity below which the<br />
flow area and the hydraulic diameter<br />
of a core node are zero, and TCLMAX,<br />
which is the temperature that will<br />
lead to cladding rupture. MAAP5<br />
simulates the melting process as a<br />
progression between five different<br />
configurations (types), each having a<br />
characteristic shape, porosity, and<br />
surface area as shown in Figure 5.<br />
| | Fig. 5.<br />
Types of Solid Core Geometry<br />
during Melt Progression [15].<br />
6<br />
Maximum<br />
molten<br />
mass<br />
in core<br />
7<br />
Mass of<br />
total debris<br />
bed in RPV<br />
lower plenum<br />
8<br />
Period from the<br />
onset of severe<br />
accident to first<br />
relocation to LP<br />
40.7 ton 63.5 ton 6.7 hr<br />
(24185 sec)<br />
55.7 ton 104.4 ton 3.8 hr<br />
(13745 sec)<br />
65.6 ton 100.1 ton 4.2 hr<br />
(14997 sec)<br />
64.4 ton 102.4 ton 4 hr<br />
(14390 sec)<br />
43.4 ton 116.2 ton 2.3 hr<br />
(8392 sec)<br />
As the type number increases, the<br />
porosity and the flow area decrease.<br />
While type 1 denotes an intact fuel<br />
rod, type 5 means the molten pool has<br />
completely melted.<br />
When the core melting begins,<br />
the mass of each core node moves<br />
gradually to the bottom (candles<br />
down). When the porosity of the core<br />
node becomes 0.1, which is the default<br />
value obtained by benchmarking the<br />
TMI accident in MAAP5, it becomes a<br />
block node that is no longer mass<br />
transferable as type 4 [19]. Therefore,<br />
changing this cutoff porosity value<br />
(EPSCUT in MAAP5) can change the<br />
core melting and relocation process.<br />
The default value of EPSCUT is 0.1,<br />
the minimum value is 0, and the<br />
maximum value is 0.25 according to<br />
MAAP5 manual.<br />
The clad rupture also affects the<br />
core melting and relocation process.<br />
When the clad temperature reaches to<br />
2,500 K, the core clad rupture could<br />
occur. After the clad rupture, the mass<br />
relocation to the lower plenum of<br />
vessel throughout the bypass region<br />
or downcomer occurs radially in the<br />
blocked node as shown in Figure 6.<br />
| | Fig. 6.<br />
Breach of Side Crust of Molten Pool [15].<br />
9<br />
Period from<br />
the onset of<br />
severe accident<br />
to vessel failure<br />
8.6 hr<br />
(30783 sec)<br />
5.3 hr<br />
(19094 sec)<br />
5.4 hr<br />
(19465 sec)<br />
5.3 hr<br />
(19042 sec)<br />
7.8 hr<br />
(28033 sec)<br />
ENVIRONMENT AND SAFETY 749<br />
Environment and Safety<br />
Analysis of the In-Vessel Phase of SAM Strategy for a Korean 1000 MWe PWR ı Sung-Min Cho, Seung-Jong Oh and Aya Diab