atw 2018-09v3


atw Vol. 63 (2018) | Issue 8/9 ı August/September



corium resulting in a porous rubble

structure that mainly consists of fuel

and control rods. If this porous structure

enters in contact with the right

amount of water acting as moderator,

there is a potential for recriticality.

In order to avoid recriticality and its

adverse consequences, a criticality

evaluation of the debris bed needs to

be carried out.

The conditions of the debris bed

can be very diverse and strongly

depend on the accident scenario. The

criticality safety control of the fuel

debris is a challenge principally due to

the large uncertainty of the fuel debris

conditions (location, geometry, composition,

temperature, etc.). Severe

accident codes are able to simulate the

accident progression and can be used

to estimate the debris bed conditions,

however, an adequate observation,

sample taking and analysis of the real

fuel debris are crucial to perform an

accurate criticality evaluation.

Due to the high uncertainty of fuel

debris properties, it is necessary to

prepare a comprehensive and extensive

database, which embraces criticality

data of any possible debris bed.

The main factors on the criticality

evaluation of the fuel debris after a SA

are listed below:

• Total amount of corium

• Composition of corium

• Fuel debris geometry

• Coolant conditions

3 Calculation model

3.1 Geometrical model

of the debris bed

Figure 1 shows the conceptual

geometric model of the debris bed

for the Monte Carlo criticality calculations.

The innermost region of the

model represents the debris itself, as a

porous structure consisting of fuel

| | Fig. 1.

Geometric model of debris bed.

Parameter Range Boundary value

Particle size 1 to 14 mm 10.7 mm

Porosity 0.32 to 0.8 Optimum Porosity

Water void fraction 0 to 90 % 0

Fuel burnup 0 to 60 GWd/t HM

25.8 GWd/t HM

(accident conditions)

Debris bed size 10 to 200 cm 200 cm

Water boration 0 to 2,000 ppm B 0

| | Tab. 1.

Criticality parameters and ranges.

particles and water. For conservative

results, the shape of the debris was

spherically arranged minimizing the

neutron leakage and the critical mass.

Surrounding the fuel debris there is a

water reflector of effectively infinite

thickness (approx. 30 cm). Such configuration

was already used for a

criticality safety evaluation for the

TMI-2 safe fuel mass limit [5].

Debris beds comprise particles of

different shapes and sizes, which are

chaotically arranged in the space. In

order to reduce the computational

effort for the criticality calculations,

some simplifications have been

applied to model the porous structure

of the debris: the particles were

assumed to be spherical, all the particles

were assumed to have the same

size and the particles were assumed to

be regularly distributed in the space

following a Body Centered Cubic

(BCC) lattice [6].

3.2 Corium composition

In this study, the Unit 1 of Fukushima

Daiichi NPP was used as reference

[7, 8].

Conservatively, it was assumed

that there was nothing present in the

fuel debris but fuel pellets and water.

Thus, the negative reactivity effects

due to the possible presence of cladding,

fixed absorbers and structural

materials are ignored. As boundary

conditions, room temperature and a

fuel density of 10.4 g/cm 3 are considered.

ORIGEN 2.1 [9] was used to calculate

the radionuclide inventory for

different average burnups, from fresh

fuel up to a burnup of 60 GWd/t HM .

The average burnup in the reactor of

Unit 1 at the moment of the accident

was calculated to be 25.8 GWd/t HM

[8] and was used as reference model.

To perform the burnup calculations,

fresh fuel UO 2 with an initial enrichment

of 3.7 % wt 235 U was irradiated

considering a specific power of

20 MW/t HM in the reactor.

3.3 Coolant composition

Light water is used as moderator. The

density of the water (or void fraction)

was varied to analyse the influence on

the neutron multiplication factor.

Additionally, boron was added in

every scenario in order to know the

required concentration that guarantee

a subcritical condition of the

debris. Room temperature was considered

for all the calculations.

4 Criticality calculations

Criticality calculations have been

performed for multiple scenarios

using the calculation model described

before. Six parameters have been considered

for these calculations: particle

size, porosity, water void fraction, fuel

burnup, debris size and water boration.

The parameters and ranges of

variation are resumed in Table 1.

In order to analyse all the possible

dependencies between these parameters,

they all have been combined by

pairs, resulting in 15 possible combinations

or calculations sets. In each

calculation set, the paired parameters

have been varied over their whole

ranges, giving to the rest of parameters

a boundary value. The neutron multiplication

factor k eff was then calculated

for all the possible combinations. All

the boundary values have been chosen

to be conservative, except the burnup,

where the value at the moment of

AMNT 2018 | Young Scientists' Workshop

A Preliminary Conservative Criticality Assessment of Fukushima Unit 1 Debris Bed ı María Freiría López, Michael Buck and Jörg Starflinger

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