Hi,

I am trying to compute the Eigenvalue for a U235 sphere inside a given water sphere with the “search_for_keff” function.

My problem is that I got it working once with a bounding box out of 6 planes and “reflective” boundaries respectively.

Every time I use “vacuum” or “transmissive” boundaries I get a “f(a) and f(b) must have different signs” error. This problem persists also with the spherical surrounding water sphere(even with reflective boundaries).

Below is my source code for clarification. Thanks in advance for the interest or support.

```
import matplotlib.pyplot as plt
import numpy as np
import openmc
import openmc.model
def build_model(r_crit):
fuel = openmc.Material(name='U235')
fuel.set_density('g/cm3', 10.31341)
fuel.add_nuclide('U-235', 1.)
water = openmc.Material(name='water')
water.set_density('g/cm3', 1.)
water.add_element('H', 2.)
water.add_element('O', 1.)
materials = openmc.Materials([fuel, water])
fuel_radius = openmc.Sphere(x0=0., y0=0., r=r_crit, boundary_type='reflective')
moderator_radius = openmc.Sphere(x0=0., y0=0., r=15, boundary_type='reflective')
fuel_cell = openmc.Cell(name = 'U235 fuel')
fuel_cell.fill = fuel
fuel_cell.region = -fuel_radius
moderator_cell = openmc.Cell(name = 'Moderator')
moderator_cell.fill = water
moderator_cell.region = +fuel_radius & -moderator_radius
root_universe = openmc.Universe(name = 'root universe')
root_universe.add_cells([fuel_cell, moderator_cell])
geometry openmc.Geometry(root_universe)
settings = openmc.Settings()
settings.batches = 300
settings.inactive = 20
settings.particles = 1000
bounds = [-1., -1., -1., 1., 1., 1.]
uniform_dist = openmc.stats.Box(bounds[:3], bounds[3:], only_fissionable=true)
settings.source = openmc.source.Source(space = uniform_dist)
settings.output = {'tallies': False}
model = openmc.Model(geometry, materials, settings)
return model
r_crit, guesses, keffs = openmc.search_for_keff(build_model, bracket=[7., 11.], tol=1e-2,\
print_iterations = True)
print('U235-sphere critical radius: {:1.2f} cm'.format(r_crit))
```