Overwrite isoxml file if already present

        p.wait()

        return p.returncode

    def process_xs(self, overwrite=False, particle='neutron', cwd=None):

    def process_xs(self, overwrite=True, particle='neutron', cwd=None):

        """

        Runs ISOXML on the current set of cross sections

            "doesn't exist: {}".format(cwd))

            raise ValueError(msg)

        # run isoxml executable on the current xs file

        cmd = [str(self.config.isoxml_exec), p_flag, str(self.xs)]

        p = subprocess.Popen(cmd, universal_newlines=True, cwd=str(cwd))

        p.wait()

        # construct the name of the new xs file and return it

        name_parts = str(self.xs).split('.')

        name_parts[-1] = 'xml'

        xs_name = '.'.join(name_parts)

        xs_path = cwd / Path(xs_name)

        if overwrite and xs_path.exists():

            warnings.warn("ISOXML output file already exists. Removing...")

            os.remove(xs_path)

        # run isoxml executable on the current xs file

        cmd = [str(self.config.isoxml_exec), p_flag, str(self.xs)]

        p = subprocess.Popen(cmd, universal_newlines=True, cwd=str(cwd))

        p.wait()

        return xs_name

    def generate_mesh(self, overwrite=False):